I have been planning out my CNC conversion process, and have come to the motor selection process.
There are lots of choices between different kinds of steppers and servos. I’ve decided to go with Teknic ClearPath servos, which are considered to be drop-in stepper replacements with built-in drive modules.
A lot of what I talk about here can relate to other motor types.
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Fair warning – some of this is written in “thinking things through format,” as that’s what I did.
The ball screw conversion kit for the mill I bought (Precision Matthews PM-728VT) comes with NEMA 23-size mounts for X and Y axes, and a 34-size mount for the Z axis.
NEMA 34 motors are larger than NEMA 23 motors, and can (usually) handle higher axial and radial loads.
The Proper Way to Size a Motor
The proper way to do things is to calculate the load on the axes, resistive forces, and other motion characteristics, and determine the load inertia. Then, a servo or stepper motor can be selected based on its rotor inertia.
Oriental Motor has a post about this, where they say:
For AC constant speed motors, AC speed control motors, and brushless speed control motors, you will need to look at the permissible load inertia values. For stepper motors or servo motors, you will need to know the allowable inertia ratio each type of motor can handle.
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For stepper motors, the general guideline is to keep the inertia ratio (load inertia or reflected load inertia divided by rotor inertia) under 10:1, and 5:1 for faster motion profiles or smaller frame sizes than NEMA 17.
For closed-loop stepper motors, up to a 30:1 inertia ratio is recommended.
For auto-tuned servo motors, the inertia ratio increases to 50:1. For manual-tuned servo motors, it can increase to 100:1.
But, I’ve found it quite difficult to determine all that. Plus, the load is going to change depending on workholding equipment and the size and weight of the work materials. A 30 pound vise, for instance, is going to change things.
Internet Recommendations
According to online forums, for the mill I will be converting to CNC control, Teknic’s CPM-SDSK-2321S-ELS is a suitable choice for X and Y axes, and CPM-SDSK-3421P-ELS for the Z axis.
The Centroid Acorn controller I plan to use can take advantage of the enhanced (E) resolution option, and sealed shafts (S) are a good idea given the environment the motors will be used in.
For the motors, CPM denotes “ClearPath motor family,” SDSK denotes the “step & direction Stepper Killer” series and model motors. The first two numbers are the motor size, the next denotes the body length, and the last is for torque-speed characteristics.
After that, S is for series-wye for higher torque and lower speed, D is for parallel-delta for lower torque and higher speed, and P is for parallel-wye for a combination of characteristics.
For the NEMA 23 motor, the 2321S servo delivers 492 oz-in peak torque, 98 oz-in continuous torque, and 3,170 RPM max. These value depend on the input voltage, more on that in a bit.
For the NEMA 34 motor, the 3421P servo delivers 547 oz-in peak torque, 164 oz-in continuous torque, and operates at up to 3,070 RPM max.
Others who have converted their PM-728VT machine have shown these to work well, and a Teknic tech has also confirmed these recommendations online.
The ball screws in my conversion hardware kit have a 5mm lead, as per its spec sheet, which is around 0.200″ per turn, or 5 rotations per inch. This means that at 3,000 RPM, servos will move the axes at a rate of ~600 IPM (inches per minute).
From Teknic’s torque and speed diagrams, the torque capabilities drop off at higher RPM. This is typical for stepper motors as well.
Here’s Teknic’s chart for their 2321S servo motor.
Here’s the chart for the 3421P servo motor.
Teknic has selection tables and comparison graphs on their website, and engineers who can answer questions over the phone.
What do the Motor Torque Charts Mean?
Generally, these motors have two torque ratings – peak and continuous (RMS). Consider how strong you are. How far can you carry a 5 lb load? 50 lbs? 100 lbs? How far can you carry such loads when you’re running compared to when standing still or walking slowly?
Heat dissipation is also going to be a factor. Motor torque specs are usually given for specific operating environments and will be derated for higher temperatures.
The difficulty in selecting a motor is in matching its speed and torque capabilities to a machine’s size and the user’s preferences and requirements.
Do I need 3000 RPM on the Z-axis? Perhaps not, but I do need torque to be able to lift and lower the heavy motor head.
The Teknic tech remarked in an online forum that these servos, for the PM-728VT and Precision Matthews’ conversion kit, can deliver 600 IPM rapids and 1 G of acceleration on X and Y axes, and 500 IPM and 1/3 G acceleration on the Z axis.
It’s not clear to me how Teknic calculates their G acceleration estimates.
The max IPM travel speeds are calculated from the RPM and number of inches of travel per ball screw rotation.
It’s worth looking at the shape of the torque curve at different speed ranges.
Straying from Internet Recommendations
What if I add cooling mounts and other accessories to the Z axis in the future, or something a big heavier such as a powered drawbar?
Torque drops off at max RPM, but the machine won’t (usually?) be performing cutting operations at higher travel speeds.
I think I want greater torque reserve, for the sake of being more future-proof.
I should add that Teknic’s SDHP (high power) ClearPath servos are similar to the SDSK models I’ve been looking at, but with more linear peak torque curves at higher costs.
In case you were wondering, here’s what the torque and speed profile looks like for SDSK (Stepper Killer) vs SDHP (High Power) motors. The HP motors have a more linear peak torque vs speed relationship.
The peak torques line up for lower RPMs, and the continuous torque specs line up most of the way, with the SDSK motor torque starting to droop sooner than for the SDHP.
The price difference for this is substantial; in this example the SDHP motor costs 45% more than the matching SDSK motor.
The benefits look to disappear at 48V. Meaning, if you’re not using Teknic’s 75V power supply, and aim to run the motors using a 48V power supply, there’s no apparent benefit to the “high power” series.
I’m sticking with the less expensive SDSK series of motors.
Here’s a comparison between Teknic’s ClearPath 3421P and 3432P motors (for the Z axis). The 3432P motor is longer, heavier, more expensive, and consumes more power. It delivers greater peak and continuous torque, but has a lower max speed.
To simplify things, I have been looking at the torque for specific speed ranges, such as 0 to 500 RPM and at around 1000 RPM.
Here’s their comparison chart for Teknic’s 2321S and 2331S motors, which I’m looking at for the X and Y axes for my machine.
The peak torque starts to dip at around 400 RPM, which for my machine would be around 80 IPM. This is important for operations such as plunge cutting. If the feed rate is increased to say 750 RPM or 150 IPM, the peak torque will have been reduced by nearly 50%. The same is true for both motors.
There’s less benefit in going with the beefier motor even between 400 and 500 RPM (~80-100 IPM).
To be frank, I don’t yet know what the ramifications of this will be, but it’s something I will try to remain aware of.
What if I Choose Wrong?
Teknic has a 90-day exchange policy for under/over-sized motors. I’m a little hesitant, as I read that some of the motor faces need to be slightly modified (drilled through) to fit the machine mounts. But, Teknic looks to build their servo motors to spec for orders of 5 or less, and so they might take be able to swap a modified mounting plate to a new motor.
In this case, motor selection can be guessed, but requires testing to verify. ClearPath motors can/should be tuned via software, and that will tell me what their torque output looks like for different loads. But, this still requires choosing a trio of motors to start off with.
ClearPath servos are not inexpensive – they’re a couple of hundred dollars EACH – and so I’d like to get things right.
This isn’t a CNC router with travel distances that can be measured in feet, and so I might choose higher torque motors that give up a little on speed.
S vs P Torque Profiles
For the NEMA 23-sized servos, I’m sticking with the ClearPath “S” torque profiles, as they provide high low-end torque and reasonably high max speeds. For the NEMA 34-sized servos, I’m sticking with “P” torque profiles, as they provide high torque with relatively high speeds.
The “S” torque profiles in the 34-sized servo motors sacrifice too much speed. Maybe? Let’s think about that.
This is how the 3421P and 3421S motors differ with respect to torque and speed output. With the S configuration, you get around double the max torque but around half the max speed compared to the P configuration.
Peak torque is about the same between 500 and 1,000 RPM, and so the question is whether I will need higher low-speed torque or a higher max RPM ceiling.
Add in the 3432P motor, which is slightly larger, heavier, and more expensive, and you can see that it delivers more torque than the 3421P model, and without sacrificing as much speed as the 3421S configuration.
The 3232S motor sacrifices too much speed for me to consider it. But besides that, look at the range from 0 to around 250 RPM. Above that, or above around 50 IPM, the 3232S motor’s torque drops down rather quickly. In comparison, the 3221P and 3232P motors maintain their peak torque ratings until around 500 RPM, which would be around 100 IPM for my machine.
Peak torque comes into play when moving a stationary axis, or when doing things such as reversing from moving in one direction to the other. The continuous torque is what to look at for most machining operations, where the cutting action involves forces that resist movement.
The speed is important for machining and rapid travel motions, but its importance for rapid travel – in my opinion – depends on the size of the machine.
What do I want greater reserve or headroom of – torque, or speed?
1000 RPM would still provide for roughly 200 IPM travel speed with a directly-driven 5mm lead ball screw.
I’m concerned about material removal rates.
At the back of my mind, I’m thinking about the balance between speed and feed rates. The spindle speed of my machine reaches around 4,000 RPM max. Smaller end mills in softer materials – such as aluminum (which I plan to mainly work with) – tend to require higher speeds. Certain tooling tends to require higher speeds as well.
Plus, toolpath have evolved towards faster movements with less engagement. “Adaptive clearing” and “high speed machining” are terms I didn’t see thrown around the DIY and hobby space when I first started looking into CNC machines and tech 10+ years ago.
I’ll need fast movement speeds, up to what the rigidity of my machine will allow, leading me to want at least some reserve speed.
This has me looking at the continuous torque line up until where it starts to drop down with speed. Basically, I’m focusing on the zone where 100% of the continuous torque rating can be achieved. Leaving a little extra on the table provides for a margin of error, and should help to ensure I’m not maxing out the servos.
Still, I want some speed in reserve – and usable speed.
Max Speeds of Commercial CNC Mills?
How fast can expertly-designed CNC machines operate?
- Tormach 770MX
- X/Y: 300 IPM
- Z: 250 IPM
- Tormach 770M
- X/Y: 135 IPM
- Z: 110 IPM
- Syril X5
- X/Y/Z: 393 IPM
- Langmuir MR-1 gantry router/mill
- X/Y: 100 IPM
- Z: 40 IPM
- Shapeoko HDM gantry/mill
- X/Y/Z: ~197 IPM
- Bantam Benchtop mill
- X/Y/Z: 250 IPM
- AVID Benchtop Pro
- X/Y/Z: 200 IPM cutting speed, 300 IPM rapid travel
- HAAS Mini Mill
- X/Y/Z: 500 IPM max cutting, 600 IPM max travel
As an aside, I find it strange that Syril describes their X5 machine as a “hobby CNC mini mill,” seeing as how it’s priced at $30,000.
If the motion motors used for these machines have power and speed curves anything like the ClearPath servos or typical steppers, I’d assume – or at least hope – that the max feed rates correspond to the point at or before the continuous torque drops off.
So, if I’m ignoring all of the guidance I saw online for CNC mill conversions, and starting from scratch, I suppose I would look at other small vertical mills and aim for max rapid feed or travel speeds of maybe 250 to 300 IPM – at most.
Even that seems extremely ambitious for anything aside from rapid travel (as opposed to cutting).
Figuring Out the Specs I Want
The mill I’ll be converting has a total weight of around 370 pounds, and so it’s nowhere as rigid as larger machines or any of the commercial models mentioned. Rapid travel speeds over 300 IPM will be useful in a CNC router. But for a benchtop mill?
Also, what’s my target IPM for cutting aluminum? I’m not sure. The max spindle speed (~4000 RPM) will be a limitation, and the toolpath can require very different speeds depending on traditional or high-speed techniques (if even possible with this spindle speed).
Everything I’ve read suggests feed rates of less than 50 IPM for aluminum.
100 IPM and under seems most realistic, which would correspond to 500 RPM for the ball screws I’ll be using.
This suggests that, roughly, I’m looking at 0 to 250 RPM for operational torque, maybe up to 500 RPM, and up to maybe 1250 to 1500 RPM for max rapid or feed speeds.
The PM-728VT is a small benchtop mill. Even so, throwing the weight of its table and spindle column around at a couple of hundred IPM is going to require a very sturdy table.
Back to the Charts
The 2321S motor is considered to be sufficient for the X and Y axes. A fixture plate and low-profile hold-downs won’t add much weight. But what about a vise. Three vises? A vacuum workholding plate?
I’ve been thinking about the work I’ll want to do on the mill, and might need a second vise to hold longer workpieces.
Stepping up to the 2331S from the 2321S is a $20 to $23 difference (depending on shaft seal and resolution options), or $40 to $46 for both X and Y axes. Power consumption would be a little higher – 193W vs 173W each.
This would provide an additional ~26% in continuous and max torque. There’s a sharp changeover at around 2400 RPM. 2000 RPM would be around 400 IPM, at which point, according to the chart, the 2331S would still provide a slight torque boost compared to the 2321S.
How likely am I to run the machine above 400 IPM?
The extra torque seems unnecessary for the mill, where I am likely to exceed the unpublished max weight limit of its cross slide table before the NEMA 23 fails to move it. But I think I want the extra reserve anyway.
Is there a downside to greater peak torque at under 500 RPM/100 IPM, and continuous torque across the speed range I’m looking at? I don’t think so.
I’m also thinking that lower utilization during regular operations will help with keeping the motors cooler for when higher torque is needed.
Certain operations might require the motor to operate between continuous and peak torque limits. Even if two motors can both deliver the same torque for a certain operation, the one with the higher peak torque ceiling rating should in theory run cooler and longer.
It seems there’s no wrong answer to motor selection, at least until I can test things via trial and error. Unfortunately, at several hundred dollars per servo, I’m not willing to buy multiple sizes to experiment.
For the Z axis, there’s the question as to how much speed I’m willing to sacrifice. From 0 to 250 RPM, there are clear advantages in going with the 3421S configuration.
The torque will peak when accelerating or reversing the heavy Z axis. Here, the motor has to work against gravity on top of friction and inertia.
Roughing work will usually be done along X and Y directions, with lighter passes taken along the Z axis even for complex contours. Let’s say the mill is shaping a ball. The roughing steps would likely create stepped diameters before smoothing over with a ball end mill or similar.
In other words, the mill isn’t going to hog out material in the Z axis, at least not usually. Narrow holes can be accomplished with drill bits maybe, and larger holes will have short plunges.
The 3421S looks to start dropping off at just over 1000 RPM. It has a significantly higher torque ceiling up to maybe 400 RPM or so, after which it quickly matches the profile of the 3421P servo.
At 500 RPM, which would be around 100 IPM for the ball screws I’ll be using, the 3421S still provides more continuous torque, but drops below the peak torque of the 3432P motor.
The 3421S looks to drop-off in torque too early.
There are a lot of Z axis movements as a machine switches paths.
If the 3421P is considered adequate for the benchtop mill I’m converting, the 3432P is the next-best option with respect to torque reserve. There’s a boost in continuous and peak torque, which will be beneficial if I add to the Z axis over time, such as with a power drawbar, accessories, or maybe even a higher RPM spindle motor.
Compared to the 3421S, the 3432P can reach higher speeds. The continuous and peak torque curves for the 3432P line up at around 1750 RPM, after which they drop off slightly. For my machine, that would mean the 3432P should – in theory – sustain its continuous rated torque up to around 350 IPM, compared to roughly 400 IPM for the X and Y axes if I go with the upgraded NEMA 23 motors there.
The 3432P delivers higher torque, and sustains its power in the speed ranges it matters. Or at least, that’s my thinking. Remember, I’m new to DIY CNC mill conversion.
The price difference here is $39 to $45 depending on seal and resolution options.
There’s no realistic way to know how much torque overhead there is until everything is purchased, installed, and tuned via the ClearPath software.
Are there Downsides to More Powerful Servos?
The way I see it, stepping up to the next-level up servo motors results in higher cost (less than $100 more for servos that cost <$1100 as-is). They’re also a little larger, and consume more peak power. The peak power consumption would come into play when the motors are pushed to their limits, in which case the upgrade would be justified.
I should add that a lot of the same considerations are made regardless of whether we’re talking about servo motors or steppers.
There could be added strain the the motor mounts, but not any more than comparable stepper motors.
I’d be giving up some speed, but it’s not full-power speed. As mentioned, I don’t anticipate wishing I could step up from say 400 IPM to 500 IPM. With the motors I’m looking at for X and Y axes, the faster motor starts to lose torque at around 2400 RPM, compared to around 2100 RPM for the more powerful step-up model.
Max acceleration is a factor too, but every estimation approach I’ve researched requires measurements and motion specs I don’t have. That this can be adjusted and experimented with via software is part of why I’ve decided to go with ClearPath servos.
Is your head spinning? Sorry, mine is too. A lot of the CNC conversion and DIY motor selection advice I’ve seen online and in videos aren’t rationalized.
I feel that motor selection would be easier for a scratch-built linear assembly, as opposed to mill that’s being converted for CNC, as factors such as weight and friction can be more easily modeled or drawn from specs.
I have seen examples of what works for others, but couldn’t shake the idea that I could benefit more from added reserve torque compared to speed. And, I’m paying attention to the lower speed end of things, where my machine will be running for most processes. For this reason, I’ve been looking at the torque vs speed curves and how flat they are before drop-off.
If the torque drops off at too low of a speed, that’s something that is likely to impact cutting performance, as well as rapid movements during step or between different toolpaths.
The Dilemma
Do I go with the servos that maybe 2 people in an online forum mentioned having good results with, or step up to higher power that I might or might not need?
I have seen a couple of other builds with different ClearPath motor selection, and across a couple of other brands’ motor options.
That’s part of the challenge with CNC conversions and DIY projects, or the fun, depending on the person.
But, it’s not for naught. With this back-and-forth, I at least learned how to interpret servo and stepper motor spec charts.
See Also:
Addicted2Red
Wonderful post that details what goes on in engineering a solution.
Unless you’re doing production runs with this setup and its making you money then personally I would save the money and go with the lesser torque options. You’ll probably never need the theoretical higher speeds.
While I like clearpath and their core system. Look at Japanese YASKAWA for some potential cost savings.
Stuart
Thanks, I appreciate it!
To be honest, this feels more like a guessing game than proper engineering.
I looked at YASKAWA before, and the costs I saw were quite a bit higher.
The ClearPath servos seem like an easier solution compared to DMM and other brands, and the power supply operates at 120V, which simplifies the electronics setup a bit.
There are so many things to learn and research in parallel that I don’t mind saying I’m taking the easy path when I can.
With respect to torque, my concern isn’t about speed per se, but the ability to effectively move a heavily loaded table or column, and to move the table hard and fast enough for efficient chip creation. I’ll be using an air blast and maybe micro-droplet coolant in the future, as opposed to flood coolant or similar, and so getting as close as possible to recommended speeds is important.
The machine lacks the rigidity for high torque cutting, but I’m hoping high movement speed can make up for that a bit.
I’ve been trying to build my expectations off what more experienced hobbyists and small-scale machinists have shared online, but it’s always hard to tell when someone has thought things through and experimented thoroughly or is simply guessing with high confidence.
I’ve revisited the DIY CNC space quite a few times over the years, and a lot of times there are popular trends that don’t seem to be perfectly rooted into practicality. Sometime works, and then others run with it.
Addicted2Red
That’s the thing, it really is a guessing game. You’ve done your due diligence tracking down as many variables as you can and charting specs. But in the end until you have hardware on hand you wont know the real performance.
I once planned a robotics deployment and spent 10 months trying to track down the best nema 34/42 motor package for our needs. We had 2 mechanical engineers and an electrical engineer going over figures for months. After all that we put it together and could barely get it to move. We nearly had to double the nM rating to get what was first expected.
Mist/Air coolant is better for this type of mill so you’re on the right track there. Increased feed speed may help chipload but I doubt it at the spindle rpms you’re limited to. I haven’t done enough milling to know.
Stuart
I should add that I’ll be consulting with Teknic before spending any money, just to be sure I’m not over-buying.
TonyT
Yaskawa is definitely NOT cheaper than Teknic, and they, like most Japanese servo motor companies, like to sell complete systems so you have to use the Yaskawa motors with Yaskawa drives.
I’d go for spending a bit more for more extra torque; note that with longer motors you tend to have lower maximum speeds. Anyway, I’m a firm believer in applying a generous fudge factor, since I’ve been bitten before by calculations that must have ignored major factors, because the motor as sized couldn’t handle the load.
Daniel L
Good read, you’re definitely taking a similar tack to how I’ve been approaching my own CNC router build.
Granted, yours sounds like its going to be significantly beefier: I’m just putting together an open-builds style c beam unit for light aluminum machining and wood working.
IME, there’s still a pretty heavy emphasis on the commercial manufacturing application of CNC in online communities. I’m curious as to the impact of this, as in a machine shop will put a premium on speed: the more you produce, the more income the shop can generate. Speed requires a lot of power and ridgidity throughout the system…but even a cheap 3018 unit can be made to do some lightweight aluminum work.
It sounds like yours will likely be able to take on steel and (maybe) titanium though. Perhaps not at extreme speeds, but certainly capable.
I’m sure you already know the following, but these are the lessons I’ve learned from my own aluminum excursions:
–Workholding. Doesn’t matter how rigid your machine is if your piece doesn’t stay put. Chattering is the death of bits.
–A little lube can’t hurt, and helps keep the stock from welding itself to the bit.
–Use an air blast to clear those chips
–fewer flutes. Single flute cutters are great on aluminum.
I’m tricking mine out on a budget…homemade linear power supply, control cabinets with everything on DIN rail, hand/off/auto switches for the features I’m building in.
I’ll keep my eye on your progress, curious to see how it all turns out. Hopefully I’ll be able to get back to building mine once I’m settled in my new shop.
Stuart
Yes, there is still a lot of commentary and shared-about experiences online, but a lot of it relates to machinery an order of magnitude heavier and more expensive. To them, this isn’t a machine, it’s a toy. Tormach? Toy. Gantry-style machines? Toys.
I’ve been looking at coolant options, maybe something like the fogbuster. A vortex cooler would be great, but I still don’t have an air compressor large enough to power it.
MM
This is the classic problem you will run into with machine tools, and it’s not new. I remember learning how to run a milling machine in the 1990’s: the various tooling (drill bit, end mill, etc.) manufacturers would provide cardboard “calculators”–basically a slide rule, which you would use to determine feedrates or spindle speeds. You will find the same thing in machining textbooks and online “feedrate calculators”. CAM software does this too. The problem is that there are a zillion variables which affect these numbers, and all the calculators seem to assume full-size industrial machinery weighing over 10,000 lbs and being cemented to a foundation. Those numbers do NOT apply to a Bridgeport or a tiny desktop mill.
Best advice I can give? Leave the computer alone for a while and pause research. Instead, fire up your mill and run it manually for a few hours. You’ll learn more in your first hour manual machining than you will in a hundred hours doing online research or reading books on the subject. Want to know what power you need for your mill? Feel it with your hand, while you turn the wheels cutting something
Stuart
That’s great advice, thank you! I am planning to do exactly that.
There’s still a lot of parts to look into regarding the controller and electronic components, and I’ve been trying to make progress into that at night or when I’m too tired for hands-on work.
I found myself stuck at motor selection, and had to understand things better.
Regarding manual vs CNC, there’s still a disconnect between manual and automated techniques; I cannot control the hand wheels in ways modern toolpaths would. I’ve seen this in the difference between handheld and CNC routing.
MM
Clearly you won’t be able to duplicate the same toolpaths by hand that the CNC controller will do, but the main point of using the machine manually is that it will give you an idea of the numbers involved. Right now you’re comparing numbers on a page but I suspect they feel rather abstract to you. Once you try to machine something you’ll instantly get some real-world feedback regarding how much force it takes to turn the handles, what kind of loads the machine can handle before it starts to complain (noise, vibration, poor cut quality). I think you’ll find you need much less power/speed than you might think now.
Andrew
I had BIG plans for my CNC mill. Then I realized that I sat in front of a computer at work all day writing code, then came home and sat in front a computer all evening doing CAD/CAM work so that could make 1 of something, maybe 2. Maybe I should have thought this through a little more….
Sold the CNC conversion for a fraction of what I paid for it. A 3-axis DRO does almost everything that I need.
Stuart
That’s something I’ve considered as well, and it still seemed worthwhile to try.
Peter Fox
I can whole heartedly agree that some time spent using the machine manually is a worthwhile experience. Practical experience with how hard you can push a machine is really hard to duplicate and at least equally as valuable as doing the math.
One thing I don’t see in your numbers is any consideration for how fast it makes sense to move based on the cutter diameters and number of flutes and your max spindle speed. I have some suspicion that your limiting factor on how fast you may need or want to move be be keeping the chip load per tooth low enough. With a relatively slow spindle and small cutter diameters this a significant limitation. I have a Grizzly G0619 mill which is similar sized to the PM-728 but has a slower max spindle speed of 1800 RPM. Rarely can I cut faster than 15-20 IPM even in aluminum with 4 flute cutters as the chip load per tooth gets high fast. Just because a machine can move fast does not mean it can actually cut that fast in actual use.
Stuart
To be frank, I can’t say much until I gain experience through experimentation.
I have read extensively about the matter over the years, but it’s not a substitute for trial and error.
Because of that, I’m looking more at theoretical limits, with motor selection that dials back the speed. I’ve seen hobbyists showing off 500 IPM and even higher rapid movements on benchtop mills, and that seems completely impractical to me.
Ideally I would start with a set of motors, experiment, analyze, and determine whether they’re in the Goldilocks zone or if I need to select differently. But at $1000+ per set of servos, that’s not practical.
My goal is to settle on a set of motors that won’t be the weakest point in speed or feed determinations, and can suit at least 90% of my current and foreseeable needs.
MM
500ipm rapids are completely silly for a benchtop mill, that’s just hobbyists pushing extremes. Small high-speed machines absolutely are useful in industry but if the mill doesn’t have a tool changer then high speed rapids like that are pointless.
Gus
Consider adding an air-spring to the Z-axis to take some of the load off that servo, otherwise you’re always going to be burning power fighting gravity which isn’t necessarily bad, but less power means less heat and a longer life for the servo. An air-spring can help negate future additions like a power draw-bar (PDB).
You may also want to look into a power-off brake for the Z-axis if you don’t want the spindle to slam onto the lower mechanical stops when the servos lose power (like during an e-stop event). Teknic sells one; MPC034-24-003-T. They’re not that cheap but it could save your part and tooling if you need to e-stop while milling.
On the servos; there’s a temptation with these conversions to go a little oversized on every components but it’s a slippery slope. The 728VT is a small mill so there’s little benefit with going oversized on the servos or upgrading the spindle. It’s not the right base to start with if that’s what you want. I would suggest sticking with the cheaper options that complement the machines (lower) strengths.
Stuart
Thanks! I am planning for a brake for the Z axis.
What your saying makes sense. Most of my thinking is about this machine, but I’m also thinking that a step up can be more future proof and potentially more useful beyond just this machine should I change course or AW my eyes in something larger or different (such as a gantry mill/router). In that case, if a little overhead doesn’t hurt, under a $100 difference can potentially make the entire package a little more versatile for not just this machine. And if this is my forever machine, is there harm in reserve power?
Gus
Going with a bit more power because you can is a reasonable enough justification on it’s own in my opinion. Future-proofing isn’t really possible since you’ve stuck with the rigidity the mill, and if you built a different machine you almost certainly wouldn’t need the exact same sized servos, or you would end up making compromises on the design to fit the servos you have.
For a difference of under $100 it’s not really even worth the time researching or deciding which option is better.
I’m looking forward the the rest of this series! Thanks for taking the time to document your journey.
Stuart
I spoke with a Teknic engineer earlier; their standard recommendation for this machine is indeed 2321S for X/Y and 3221P for Z. With a reasonable load on the table (he mentioned 75 lbs) there’s still a ~40% torque reserve.
There’s no functional downside to stepping up, but it’s looking like it might not be necessary.
Vince
Talk with the Teknic guys. They are very knowledgeable in regards to motor sizing based on the application, motor setup and tuning. They will provide you recommendations on your needs, not what others on the internet have done. Not to take away what other have done, but if your able to directly talk with them I know you will feel much better based on their knowledge and understanding
I have first hand experience using Teknic hardware because in my day job, I design, build, & integrate automation equipment using their product. I can speak that the auto tuning works great for balls crews, so it lessens the learning curve and helps with second guessing. Their Clearview (GUI) software is easy to use, not buggy, and is pretty powerful if you know what your doing. They back their product up and have a no none sense warranty. Plus, all their stuff is designed and machined, assembled in house at their NY location. I this is a pretty big distention from all the other possible choices at or around this price point. I might sound like broken record….but after using them for 10 yrs in my day job, we are still using their stuff for existing and new R&D products.
One last comment, I know at one time they offered “discounts” on their products if it was going to be review and shared with others on the web. I don’t know details to the extent of the discount offered or any stipulations but it might be work worth looking into. I think Physics Anonymous on you tube was able to buy the clearpath motor at a reduced price for their PM940 cnc conversion
Stuart
Thank you!
I sent them an email last night, will see what they advise.
I don’t like to ask for discounts from brands I’m not already familiar and friendly with. I also didn’t think of that, as industrial companies traditionally have very different PR and media policies than consumer product brands. It might not hurt to try – money saved is money saved.
Robert
Will Faulkner would be proud. Though I doubt he was an engineer, much less abide by ASTM Y14.100.
Franco
Stuart, I do not know if you have a degree in this field or simply very well informed, but all of this is way over my head…just impressed in the detail and knowledge you put into this.
Good luck on your final choice.
Stuart
I have a working knowledge of DC motors, but approach CNC motion control servos and stepper selection from a near-zero starting point. That’s why I had to work backwards a little from online community recommendations.
The “what do I choose and why?” can be a stalling decision to make. My journey to answer this question in the context of tools is what set the foundation for ToolGuyd.
In this application, there are missing blanks, making it difficult for me to select parts from a spec sheet or ask manufacturers for guidance. Asking questions and examining on-paper differences made things seem easier to comprehend, and I figured – or hoped – this would help others facing similar half-informed motor selection decisions.
Plus, the act of laying it out in the manner above helped me process through things; it’s not just a reflection of my thought process, it *is* the thought process, but just tidied up a little.
Nathan
So very very interesting. and you eventually got to what my first thought would be.
How fast do you really need to go? While it’s been years since I was in engineering school I do remember materials and reading recommendations for machining and the beginnings of “high speed machining” where the goal was to go fast enough to create the desired finish and the final dims in one pass.
So in my mind I would do this. Max travel in X, Y Z = ? So if the max X travel is say 15 inches, then it would see that anything more than 100 IPM is way too fast. and you already mentioned it – but what materials will you run and what’s the recommended feed speeds as opposed to tool head speeds. and I supsect 100 IPM is the upper limit of what you need. I would double the recommendation – so you know you will always be able to hit it. but I wouldn’t run the machine much faster.
TLDR – I think you are already on this path but fundamentally why get the bigger motors.
Next bit why did you settle on 48V is that also due to your controller?
What are the next bits – also do you have any ideas on how heavy the maximum items you will machine are. I think you could SWAG that too so you have an idea of your mass movements. IE – volume of 90% X, 90%Y, 90% Z travels = ? cu mm – density of mild steel ….
I assume here the movement mass will have some factor in the setup. Where big machines will have a defined capacity.
anyway again very interesting. OH and what do you plan to make?
Stuart
From what I’ve read, high speed machining isn’t just about rough and finish cutting in a single pass, but more efficient cutting.
Let’s say you have a rectangular block of aluminum and need to remove 1/2″ depth.
Traditional milling and toolpaths might go back and forth, making straight cuts with stepped cutting. If say 1/8″ is removed with each pass, it would take 4 passes to get to depth. Instead, a mill might step down to create a circular pocket, and then expand that pocket outwards with even chip removal throughout.
Or, a rectangular pocket might be created by milling in circular motions rather than linear passes.
With a greater length of the end mill engaged with the work material, there’s less localized heating, and it can chug along faster.
My understanding is that the speed and engagement characteristics allow for faster processes. The toolpaths I’ve seen are very non-intuitive and machine-calculated rather than simply being automated and controlled emulations of manual movements.
I’m going with ClearPath’s 75V power supply. I looked at 48V to better understand differences. The point about the high power motors losing their benefits at 48V vs 75W was purely academic.
My goal was to understand theoretical expectations to help guide my expectations. If the torque to speed curve is flat up to say 500 RPM, then in theory the motor should handle any speed in that range.
This is all for no-load conditions. At say 10 IPM, there will be lower resistance forces due to cutting and tooth engagement than at 20 IPM. One can reduce the number of flutes or go with lower cutting penetration depth to lower the dynamic load.
It seemed worthwhile for me to start thinking about free travel speed limits first, at least to better understand things.
If there’s more torque available, an end mill can be pushed harder into work materials, but then there are other limitations such as spindle rigidity.
The dynamic load will vary greatly depending on weight on the cross slide table and spindle column, and also depending on the end mill or cutter selection, feed rate, and more.
I cannot model all that.
It seems silly to try to select motors without knowing more. Online community recommendations or popular selections were a good starting point, but I haven’t seen anyone mention torque utilization or anything of the sort. *That* would be a big help.
Larger machines will usually have a max table weight capacity, suggesting huge overhead and reserve torque. A 4-inch vise weighs 30 to 40 lbs, and industry-standard 6-inch vises can weigh 70 lbs or more. Industrial users might have several on the cross slide table at once, plus parts.
I plan to make robotic parts, mounting brackets for various equipment, and small functional things.
When I design and build test jigs, I often have to do so based on what I can buy to match specific hole t-slot extrusion spacings. Being able to design and machine mounting brackets or fixturing exactly to spec, rather than according to what I can source, will speed things up and save me money.
MM
There is an awful lot of theory in machining–you touched on a lot of it in that post. But as I wrote earlier, most of this theory is derived from massive industrial machines that are in a totally different weight class from yours. It doesn’t matter that some specific toolpath is more efficient than another if your tiny benchtop machine doesn’t have the stiffness to execute it. Every benchtop machine I have used has been very different than standard industrial machines that all this theory has been written about.
Instead of worrying about what the textbook or the manual says about optimal feedrates for most efficient chip removal you’re simply trying not to break cutters or doing whatever you can to get the vibration under control.
You will likely never figure this out by reading, there are just too many variables involved and many of them change depending on the part you’re machining, your workholding solution, etc.
You will figure this out very quickly operating the mill manually which is why I am so keen on suggesting it.
TonyT
Overall, great thoughts, just two additions:
— The one area I felt you needed to talk about is how long you can spend in the peak torque area. Normally, you can spend a longer time when you’re lower in the peak torque area; for example, the Copley drives I typically use have an I2T (current squared * time) algorithm, so as current goes higher, the time allowed over continuous current limit is reduced.
— Most servo torque curves I’ve looked at are much flatter than these ClearPath curves; I think the difference is the ~6:1 peak:continuous torque ratio. Note that the continuous torque curve is basically flat until close to max speed, while the peak torque curve drops a lot.
Stuart
I don’t know enough about the duration or duty cycle above continuous torque to be able to comment, other than to say it’ll be less than 100% in the “intermittent duty” range.
I believe that the ClearPath motors are heat-limited, due to their drive electronics being located within the motor housing, which is why the peak torque curves look like they do.
Are these the best servo choices? Probably not, but there’s a greater amount of hand-holding compared to my having to pick separate motors and drives.
TonyT
I believe the peak torque on normal motors is also heat limited, for example, if the motor gets too hot, you might demagnetize the permanent magnets (according to a note on the Emoteq QB23 series data sheet).
The ClearPath motors look like a good choice for your application, and are very competitively priced.
Nathan
yes typically peak torque at duration is limited to some form of a heat problem.
Leads me to a thought – you don’t think you need to actively cool the motors do you? I’m assuming here you won’t be running this 24/7. and even then I imagine if you stay in that continuous duty line. Provided you keep the motor within it’s environment window (temp at humidity) then they shouldn’t need anymore cooling. They are tested to live there.
but I liked the comment on chip removal.
James+C
Sounds like this thing is one of the deeper rabbit holes one could go down. Good luck!
Double0jimb0
I don’t have an answer on this, I’m in middle of determining the culprit on my overbuilt cnc router. I never came across this issue in my research, only have had it show up now that I’m making cuts. I’ve got the clearpaths, done all the auto tune, etc, same issue as here:
https://www.cnczone.com/forums/servo-motors-drives/367472-clearpath-servos-lock-like-stepper.html