[CAM_MAN]

Please someone help me out. I have built the CadCut router which I finished about 3 months ago. Anyhow I need to resolve some gearing problems. I have three 1200 Oz/In steppers driving all three axis.

Holding torque-Nm (oz/in) +/- 10% - 8.7 (1208)
Rated voltage - 2.7
Rated current per phase (amps DC) - 6.0
Resistance (Ohms) +/- 10% - 0.45
Induction (mH) +/- 20% typical - 5.1
Motor length (mm) - 118
Rotor inertia (g-cm^2) - 2700
Weight (kg) - 3.2

60 Volt power supply with 3 Gecko 202's.

On my X axis I am using gear rack. It is 14-1/2 pressure angle 12 pitch rack with a 12 pitch 12 tooth pinion gear that has a 1" pitch diameter. There is a central single driveshaft that drives the pinions on both sides. On the driveshaft is mounted a 72 tooth timing pulley and on the stepper is a 16 tooth timing pulley. The X axis is capable of running 500 ipm without breaking a sweat. The problem is at low speed like 40 ipm the motor growls because it is turning so slow. I have adjusted the trimpot on my G202 the best I can to keep it to a low rumble.

On my Y axis I have a .631 diameter x .2 Pitch Nook rolled ballscrew. The ballscrew has a 42 tooth timing pulley and the stepper has a 30 tooth timing pulley.

On my Z axis I have a .631 diameter x .2 Pitch Nook rolled ballscrew. The ballscrew has a 42 tooth timing pulley and the stepper has a 30 tooth timing pulley.

On the Y and Z axis I can only get about 70 ipm before they start screaming. After running them at this speed for about 30 minutes they tend to stall out on me. Everything on the machine moves very freely. Hardly any resistance at all. I only built it the way the plans called for but now I realize that the gearing is not were it should be. The X axis is way off from the Y and Z. I would like to be able to find some kind of compromise between the X and the Y and Z. I would like to be able to have some good cutting force but not lose a lot of speed. I would like to have a 150 to 200 ipm out of all 3 axis if at all possible. I am good at machining and programming but don't have it in the gearing department. Could someone please point me in the right direction. It would be greatly appreciated.

[jcc3inc]

Sir,

Looking at your X axis, you travel 3.14159 inches per pinion rev x 16/72 =
.69813 inches per STEPPER rev. With 10x microstepping, this is a resolution of .0003491 inches per step. Assuming a max pulse rate of 30,000/sec, you can go at 10.47 inches/sec = 628 IPM.

Your Y and Z axes look like this: .200"/rev x 30/42 = .142857"/stepper rev. With 10x microstepping you have .0000714'/step. Assuming a max pulse rate of 30,000/sec you can go 2.1428 in/sec = 128 IPM.

Now considering your steppers at 1200 oz-in rating, if they are 2 stack motors similar to Pacific Scientific motors, the torque at 30,000 steps/sec
corresponds to 3000 full steps/sec, and it is reduced to about 1/3 max torque at this speed and 60 volts.

If you want similar performance on all 3 axes, you need gearing about the same so the distance/step is roughly equal. That means you should gear UP the leadscrews so that they turn faster than the steppers.

I would say, however, that your limitation is acutually the max frequency of the CNC controller you are using. Most controllers are in the 30-45 KHZ range.

To give you a better answer, more info is needed. Whose controller, table travel dimensions, weight, etc.

Regards,
Jack C.

[CAM_MAN]

The controller I am using is Mach3 running on a 650MHZ AMD Duron with 128MEGS. I have Mach3 set at 24000KHZ. The router is 96 x 48 x 12. The gantry weighs in at probably 150 - 200 lbs. I am using 3/4" linear round rails with ball bearing pillow blocks on all axis. Before I connected the ballscrews and the X axis pinion drive shaft I tested all axis for friction. With the push of one finger the gantry would fly all the way down the table. The same for the Y and Z axis. Everthing move very freely on this machine so I don't think friction is a problem. The mass of the gantry probably doesn't help though. I have my acceleration set very low to minimize the abrupt stops. So what would I have to do in way way of gear selection to bring them closer together. I am willing to buy new gears and belts for all axis if I have to. I still would like to have some cutting force but also achieve a little more speed. I would be totally happy with 200 ipm if that is at all possible. Appreciate the help.

[JerryFlyGuy]

One step I would take on the Y would be to switch the pulleys, put the 30 on the ballscrew and the 42 on the stepper, this will over drive the ball screw, but you've got a serious resolution difference between the X and Y axis.

Your only going to be as accurate as your weakest or least accurate axis. If your X was only [I say only as its the lowest resolution axis- but as far as DIY machines are concerned its pretty high] 0.000349" res, having 0.0000714" on your Y isn't going to help you much. If you overdrive you Y axis, you could get it up to 0.0014" per step or 0.00014 per 10x microstep. This is closer to the res on your X axis. It'd be a start anyway. This would allow you to get 4.2"/sec or 252 IPM. Not a huge increase but some..

To get it closer you'd have to use other pulley sizes.

Jerry

[ninewgt]

CAM

Hello - This is Tom - Designer of the Cad Cut machine. I had another builder email me with some questions who had said you had a few 'issues' with the machine so thought Id drop in to help.

Let me know however I can help

Tom

[ger21]

One problem could be that the PC you're using is too slow. I've read quite a few posts recently on the Mach Yahoo group from people having trouble at higher speeds but couldn't figure out why. Switching to a faster PC has fixed quite a few of those problems. Try either a faster PC, or maybe see if Mach2 will work better.

[jcc3inc]

Sir,

In order to approximate the slow axes speed to the fast axis, you would need a 3.5x speedup of the leadscrew compared to the stepper; this means 3.5x as many teeth on the stepper pulley as on the leadscrew pulley. Then each rev of the stepper will give 3.5 x .2 = .7" per stepper revolution. This is .7"/2000 or .00035"/microstep.

For the higher speeds, I'd prefer rack and pinion to leadscrew because of the critical speed limitation of a leadscrew. Also, I'd prefer 20dp rack and pinion, 20 degree pressure angle. The 20 degree PA runs more smoothly, and a 1' D. pinion has more teeth in contact with the rack than does a 12 dp pinion.

Regards,
Jack C.

[JerryFlyGuy]

For the higher speeds, I'd prefer rack and pinion to leadscrew because of the critical speed limitation of a leadscrew. Also, I'd prefer 20dp rack and pinion, 20 degree pressure angle. The 20 degree PA runs more smoothly, and a 1' D. pinion has more teeth in contact with the rack than does a 12 dp pinion.

Jack I totally agree with the changes to the pulleys to get a balanced resolution onthe ball screws. I to would think that rack/pinion is a great idea, however I think 20DP is going to cause problems, esp w/ a 1200 oz-in stepper. I'm working on building a rack/pinion system right now and I've had some serious issues getting my pinion gear into spec. The problem comes from the strength [rated] of the gear. I found that w/ my gearing of 5:1 on a 640 stepper, I was right at the rated torque limit for a 12 DP gear @1" dia.

I've since changed to a 10:1 gearing system and a 2"dia gear [ it works out to the same stepper speeds and forces as the 5:1 and 1" dia gear] This setup fits much better into the rated torque levels of the gear. I think the 1" dia [12DP] pinion was only rated at about 250 in-lbs of torque, where as the 2" is something like 600. Something to check when specing gears. 20 deg is the better choice as far as power transmition and backlash.

Cam_man has about a 4.5:1 reduction on his x axis so w/ a 1200 oz stepper this equates to 5400 oz-in [at stall], at the pinion [ 337.5 in-lbs] I don't have my spec's right here, but I'm pretty sure this will exceed most gears under several inches in the 20DP range, it exceeds the 1"dia [12 DP] spec's.

Boston gear has lots of info in their open gearing section, on their website.

Jerry

[CAM_MAN]

Thank you very much for your help guys. I have some new pulleys and belts arriving today from SPD/SI. Went ahead with the 3.5x speedup on the ballscrews, 42 tooth on the motor and 12 tooth on the ballscrew. Hope that this will help out by bringing them closer together resolution wise.