[bakon_boy]

Hello all,

After spending quite a few hours lurking and researching CNC routers decided to take the plunge and start building my own. I have been working on the design for a couple of weeks and these are the specs of the machine I am planning on building. I am a mechanical engineer in training and have some experience designing the mechanical parts of numerically controlled machines. This is my first personal CNC build and first exposure to the electrical/software side of the CNC world.

Specs:

X Travel: ~48"
Y Travel: ~30" for both spindles
Z Travel: ~4 1/2"
Frame: 80/20 Aluminum
Desired Rapids: 400 ipm or greater
Desired cutting speed (plywood w/ 1/2" bit at 1/2D): 200 ipm or greater
Spindle 1: Porter Cable 890
Spindle 2: Bosch Colt
Linear Motion: Ball screws w/ 10mm lead from Linearmotion2008
Linear Guides: Supported Rails from Linearmotion2008
Motors X & Y: Stepper 1200oz-in from Automatedtechnology
Motors Z: Stepper 570 oz-in from Automatedtechnology
Drivers: 203V Gecko
BOB: PMDX-126 w/ Ethernet smooth stepper
PSU: KL-7220 72V 20 Amp

I think that covers most of the highlights at this point. The main use of the machine is for routering kitchen cabinets. Right now we are using a router table which consumes quite a bit of time for each setup. We are using jigs for the holes for adjustable shelves, roller mounting, and hinge mounting. This will hopefully take all of that and combine it into 1 operation, hence the dual spindle.

I will post some pictures of my design tonight.

Cheers.

[ger21]

You don't want 1200 oz steppers. 460oz Nema 34 motors will probably be at least twice as fast as the 1200 oz motors.

[bakon_boy]

You don't want 1200 oz steppers. 460oz Nema 34 motors will probably be at least twice as fast as the 1200 oz motors.

I have read that bigger doesn't mean better but i wanted to make sure I have enough torque available as I don't want to be losing steps. What makes them faster? Would I see a big speed difference between 460ozin and 570oz-in motors.


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[bakon_boy]

You don't want 1200 oz steppers. 460oz Nema 34 motors will probably be at least twice as fast as the 1200 oz motors.

I have heard that bigger is slower. But I also have heard if you don't have enough torque you will lose steps which isn't good either. What makes the NEMA 23's so much faster? Is it just inertia or is there other factors at play?

I figure my load torque will be about T(w)=2.1Nmm*w where w is RPM. From that I found at 1000 RPM both the load torque and the motor torque will be about 2.2Nm. Factor in the lead of 10mm and I figure that should be about 400ipm. Could I expect to get faster speeds with the NEMA 23?

[louieatienza]

200ipm in plywood is great if your spindle can handle it. And with a wood router, I don't think so unless the passes are pretty light. Trying to shove a router through material even at light passes is a great way to kill it. 8020 extrusion is not really that heavy, though with larger NEMA23 steppers (over 380in-oz) I'd stick with the G203V and get a larger PSU.

For example, Onsrud recommends a chipload minimum of .016" for their 3/8" compression spirals, which is probably what you want for through cuts in double-sided material. At 200ipn that gives you a spindle speed of 12,500rpm at 200ipm and 3/8"doc for a single flute. I have doubts the PC router can handle that for too long. With the money you save on big NEMA34 steppers and larger PSU you can use for at least a 3kW spindle.

As to NEMA23 steppers, part of their speed advantage is inertia, but a larger part is due to the smaller coils needing less current to "charge up." Higher end steppers from Sanyo Denki, Kollmorgen/Danaher, Parker/Pacific Scientific, Vexta can be rated at 3000rpm max with the right drive, controller, and PSU. Other stepers probably can attain that speed though the bearings are not as good as well as the motor wire insulation; better steppers can also run hotter more reliably. You can help keep the steppers run cooler by making proper stepper mounting plate or block (they have them on eBay really cheap out of solid billet aluminum.)

I don't think top speed should be a big concern here since there shouldn't be too many long rapids moves cutting cabinet partitions.

[bakon_boy]

200ipm in plywood is great if your spindle can handle it. And with a wood router, I don't think so unless the passes are pretty light. Trying to shove a router through material even at light passes is a great way to kill it. 8020 extrusion is not really that heavy, though with larger NEMA23 steppers (over 380in-oz) I'd stick with the G203V and get a larger PSU.

For example, Onsrud recommends a chipload minimum of .016" for their 3/8" compression spirals, which is probably what you want for through cuts in double-sided material. At 200ipn that gives you a spindle speed of 12,500rpm at 200ipm and 3/8"doc for a single flute. I have doubts the PC router can handle that for too long. With the money you save on big NEMA34 steppers and larger PSU you can use for at least a 3kW spindle.

As to NEMA23 steppers, part of their speed advantage is inertia, but a larger part is due to the smaller coils needing less current to "charge up." Higher end steppers from Sanyo Denki, Kollmorgen/Danaher, Parker/Pacific Scientific, Vexta can be rated at 3000rpm max with the right drive, controller, and PSU. Other stepers probably can attain that speed though the bearings are not as good as well as the motor wire insulation; better steppers can also run hotter more reliably. You can help keep the steppers run cooler by making proper stepper mounting plate or block (they have them on eBay really cheap out of solid billet aluminum.)

I don't think top speed should be a big concern here since there shouldn't be too many long rapids moves cutting cabinet partitions.

Thanks for the reply. I am trying to pullout the advice from your comment. As far as the spindle router debate at this point I think I will start slow and work up until I feel like the router is working too hard and back off a little. I am more concerned about the rapid speeds than the cutting at this point.

I feel like your agreeing I should go to a nema 23 but my concern is that the torque charts I have seen do not seem like there is enough torque unless my calcs are grossly overestimating loads. Like I said at 1000rpm using acceleration time if 0.1s I am getting a torque load of about 2.2Nm which lines up with the 1200ozin nema 34 torque chart. Are my numbers off from actuals?

You mention that 80/20 aluminum is not that heavy. What do you mean by that?

You also mention proper mounting blocks for the motors reduce heat. Is this strictly an alignment issue or what reason does the motor run cooler?


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[louieatienza]

Unless you are just jogging back and forth from one side of the machine to another I don't know if you should have much worry of rapids. It does look impressive, but a good blend of rapids and cutting speed (with torque) is probably better. This is the crux of using steppers, there will always be a tradeoff between torque and speed, as well as speed and resolution. If rapids speed is still the primary concern you might be better off with 2525 ballscrws and maybe 600-900in-oz steppers. So you get good rapids as well as torque at cutting speed. Mechanical resolution of just less than .005" is more than enough for kitchen cabinets (and probably better than what can be done efficiently by hand.)

I've seen quite a few torque charts as well, and they may not always tell the whole story since some do not show the PSU voltage used. My current machine uses 2 425in-oz steppers (on underpowered 48V PSU) on the long axis driving 1/2"-8, 8 start leadscrews with delrin AB nuts, pushing a gantry weighing close to 100lbs, and I can rapid at over 700ipm. (In fact I had a machine using one 425in-oz steppers running two leadscrews via timing belt and still managed over 300ipm.) I use the same leadscrew on the gantry and even the Z carriage with no problems, and I cut aluminum and fiberglass-based phenolic regularly. So I know for sure that the steppers are capable of more voltage, the friction on the leadscrews makes them about 60% efficient (as opposed to ballscrews which are over 95% efficient). Even with all that working against me my machine is still capable of over 700ipm (at 30in/s^2 accel.) Can I prove it?

DIY CNC Rebuild using 8020 Extrusions! 1000IPM Rapids - YouTube

This machine BTW uses extrusions from 80/20. This is their designated heavy-duty, which is nowhere near the mass of Bosch heavy-duty extrusions of the same series, which is less than Misumi heavy-duty extrusions (the wall thickness of Misumi heavy duty is probably 3-4 times that of 80/20.)

The motors run cooler because the motor mount acts as a heatsink. If you under-voltage your steppers or set your accel low you may never see the steppers get more than luke warm. But if you want performance the steppers will run hotter than you may think they should. Marriss of GeckoDrives explains that an efficient stepper motor will run hot though he does not give a number to this. I do know from research and experience that higher-end steppers are designed to run hotter, meaning you can supply them more voltage than the same size Chinese steppers which equals faster speeds and more torque at higher RPMs. I even plan to incorporate fins into my next stepper motor mounts to pull even more heat off the motor.

[bakon_boy]

First off thanks for all the advice. Alot to process. mcArch varying the distance between z's is not a big concern for me as it will be for routering and drilling but down the road if I see the need for that I see no reason a couple of mods couldn't achieve that.

Heres my problem with using 381ozin (2.7Nm) motors:
I have no problem believing that you can use them to get something going fast eventually. The problem I see is getting it there. At constant velocity the only forces you are overcoming is the cutting forces and friction. Which for a CNC router both would be fairly low. But when you need to change the speed you have to accelerate. Now not only do you have cutting forces and friction but you have inertial forces of the mass as well as all the bits spinning. Every torque curve I have seen for stepper motors drops off that just inheirantly a trait of stepper motors so you need to design around that. So the trade-off is this. Faster acceleration with lower top speed which costs more or lower accleration with higher top speed which costs less.

So my first question is for the machines that are a similar size/weight to mine what acceleration can they run before losing steps? I don't really need to have rapids over 400ipm (~1000rpm) but I would like to get there quickly. Simple math tells me that is would take about 7 seconds to traverse the cutting area which is fine considering that will not be a common occurance. Maybe the 381ozin motor is the right on, I am just skeptical because the holding torque isn't much higher than the torque I calculated I need to accerate the gantry. So what motor (if any) would get me about 1000rpm and good acceration?

With the G540 package can they be daisy chained together? I have a total of 5 axes and the standard G540 package can only handle 4.

As far as the machine side. I respectifully disagree with you ger21. Yes it is lighter than I have seen people build, but this is another one of those trade-offs. More mass should equal more stiffness if the mass is put in the right spots. But more mass equals less acceration as well. Now the only variable that I struggled with is the actual cutting forces, it does't seem to be a well talked about topic. Does anyone have any insight on this? I assumed 50lbs but maybe that is grossly underestimating it.

The way I designed it is I am trying to keep all mechanical deflections below 0.01" excluding tool deflection. Is the X-axis the places where I see deflection occuring is the motor mount plates, gantry about its weak axis, and the drop plate. The only place the y-axis will really deflect is the gantry side plates. The z-axis deflection should not be a problem as cutting forces will be minimal. That being said I am not a believer in just beefing up for the sake of beefing up. I still have a few more calcs to do but I havn't seen any deflections that will be a problem yet.

Ger21 you mention that the orientatation for the z-axis guides isn't the best. Is there a better guide to use? I don't really want to use profile guides as they are $$$. I went with an unsupported shaft becuase I thought that would be better than a supported shaft in that orientation.

[bakon_boy]

Full view of the machine.
Attachment 202896

Zoomed in on the dual spindles
Attachment 202898

The case I picked up for the electronics
Attachment 202900

Feedback welcome!

[ger21]

Unfortunately, it's almost impossible to know what a steppers torque will be at any specific rpm. Unless your using the exact same components and setup as the ones used in the torque curve your using to determine that torque.
But I can tell you that in the 10 years this forum has been around, there have been nearly no machines using 1200 oz motors. I've also seen many times where users switched to smaller motors and got drastically better performance.
The vast majority of members here are using 380oz Nema 23 motors and Gecko G540's. This combination typically gives nearly double the performance your looking for.

I'm very skeptical that the 1200oz motor you're looking at would have more torque at 1000 rpm than the 381oz motor, as torque curves on large motors fall of very rapidly, usually as low as 300-500rpm.

[Bob La Londe]

I like the video. Lately I've been using my servo controlled big mill a lot, and I forgot how much I like the sound of steppers.

[bakon_boy]

Another question for sizing PSU's. I noticed that the voltage output of a torrodial psu drops as more current is drawn from it. My question is when you use that formula to calculate voltage (sqrt(2.5mH)*32=51V should I just get a 48V PSU knowing that under load it might see more like 40V or should I go to a 60V so when under load it see closer to that 50V.

I also noticed that impedance values are +/- 20%. I imagine if anything they actual will be higher than stated so new voltage requirement could be as high as 55V which makes me think maybe a 60V is a better selection.

[louieatienza]

The voltage limit is set by the manufacturer, for the G540, 50V is the absolute limit; typically we'll use PSUs under that for a little safety. As to the impedance values, I'm not sure but you could be right about that.

[bakon_boy]

The voltage limit is set by the manufacturer, for the G540, 50V is the absolute limit; typically we'll use PSUs under that for a little safety. As to the impedance values, I'm not sure but you could be right about that.

Yes for the G540. but I am still planning on using G203V so both are well below the 80V maximum. So with that in mind with a motor that has a 3V rating with 2.5mH +/- 20% would you say play it safe with the 48V or try and get the most out them as possible and for with a 60V?

[louieatienza]

Yes for the G540. but I am still planning on using G203V so both are well below the 80V maximum. So with that in mind with a motor that has a 3V rating with 2.5mH +/- 20% would you say play it safe with the 48V or try and get the most out them as possible and for with a 60V?

It depends on the motor. Like I mentioned, the higher end stuff can be pushed harder. I probably wouldn't go max out either, though in reality you can throw as much voltage to a stepper that your drive will allow, as long as you keep everything from melting! You should be able to adjust a 60V PSU down a few volts via trim pot. What you don't want to do is over-current.

I've become a big fan of EdingCNC's USBCNC drive board, and they have a 5-axis version. Their interface is excellent as well as support.

[bakon_boy]

Been working on the electronic cabinet. I think the $20 computer case is quite a find. Got the control computer mounted in the case and most of the power cables lengthened. Just waiting on a few SATA cables from Ebay.

Attachment 204156

Front Panel

Attachment 204158

Mounted power supply

Attachment 204160

A bunch of fans and motherboard mounted.

Attachment 204162

Back panel. Computer cut-outs done. Waiting for the electronics stuff to arrive to start the motor cut-outs, limit switches etc.

[Dylwad]

Do the math for voltage. Square root of inductance x 32 if I remember correctly. Lots of info on geckodrives site about stepper motors.

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[louieatienza]

Do the math for voltage. Square root of inductance x 32 if I remember correctly. Lots of info on geckodrives site about stepper motors.

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This is a good point as using Marriss' formula, even the venerable G203V does not have a high enough voltage rating to run those 1200in-oz steppers efficiently. Leadshine makes drives up to over 300VDC. As to recommenfations, it hard to say, because it not really so much how much each axis weighs, as much as how much force is needed to move that mass, and with LM bearings, it may not be much. The 381in-oz is probably the largest stepper that can be run to its potential at 48v. I think the max using Marriss' formula is closer to 51-53V if I remember correctly.

To find the "ideal" voltage according to Marriss, you'll need to find the inductance of the particular motor in the wiring you choose (if you're using an 8-wire motor.) And the inductance of a motor can be an idicator of performance; the lower the number is usually better.

And yes, there are many good articles in the GeckoDrive website along with some rudimentary formulae you can use to help determine stepper size, PSU size, etc...

[Bob La Londe]

This setup has worked really well for a lot of folks on routers w/ 48VDC. 4 Axis Electronics Kit | CNCRouterParts Ahren has said the 320s out perform the 381s. I have two machines running his 381s though and they seem to work fine. I have had a Taig with 381s slinging a 50lb vise on the table around at 40-50 IPM on 20 TPI leads. (It took loose gibbs and nuts to do that) It was awesome to see, but the whole work bench (steel legs, and butcher block top) started walking across the shop. I am also running a smaller router (much smaller than your build) that I tested upto 600IPM with some stalls that I locked down at 300ipm for rock solid reliability with the same controller and motors.

The one thing that Automation Tech has that looks impressive though is this setup. NEMA23 Closed Loop Stepper Motor System-Hybrid Servo Kit | Automation Technology Inc . I don't know if those 282ozs will push your machine, but I have a buddy running a 36x48" router with some open loop 282s I gave him.

[bakon_boy]

What is everyone's thoughts on running 570 ozin 5 amp on the x and y and 381 ozin 3.5 a on the z axis?

Both have inductances of about 2.5 mH which works out to about 48V. Total of 2/3(23 amps)= 15 amps.

Anybody see problems with this. Still overkill? Scares me because my numbers are showing that there is not enough torque to run at 400ipm with 0.1s rise time but I have to trust experience over my numbers.


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