G4 UPGRADE AND NEW PINIONS

[robert_cheal]

I recently installed the new G4 on my PRT and plan on ordering new pinions. The cuts are not as smooth on some large arcs cutting at 2ips as I would have expected and existing pinion wear is one likely factor but I am wondering about changing the actual number of teeth on the pinions. All motors on my PRT came with PK296A1A-SG7.2 / 1/4 step / 30t on X & Y 25t on Z.

My first G4 top jog speed was 7 ips (8 ips for one brief period. Light cuts 4ips.

Yesterday after putting a new USB 2.0 (Belkan)it now jogs comfortably at 8ips. No actual test cuts yet.

The question I have is with my current motors where is the point of diminshing returns and would changing the pinion size help or hinder cutting torque, resoution etc. ? I guess I am intersted in reasonable speeds with maximin smoothness.

Very recently I thought that I read some discusion on pinion size, speed, resolution, and torque but I cant seem to find it again.

Any advice before a I order new pinions would have my many thanks.

I should add that I am basically very pleased with the new improvments of the G4.

Thanks, Robert

[Brady Watson]

Robert,
Contrary to popular belief, the 30T pinions are actually smoother and longer lasting that the 25T ones. While you get a small hit in resolution (a mute point with the 4G now microstepping)...the 30T has more teeth engaged in the rack than the 25T ones do. This great reduces cogging compared to the 25T. Get yourself a new set of 30s. If you have an older PRT with angle iron stepper mounts on the X & Y, I highly recommend upgrading to the Alpha/3G PRT motors mounts. They are much stiffer than the other ones and keep the pinions squarely on the rack as well as reducing torsional loads on the pinions when switching directions.

-B

[richards]

Robert,
The 30-tooth spur gears that you have on your X and Y axes are probably already optimum.

Mathematically, you can compare speeds, torque and resolution. Given the fact that you have 2,000 steps per revolution, that the 20-tooth spur gear has a one-inch pitch diameter, and that you're using 7.2:1 gear boxes, here are some figures:

Distance per step
30/20 * 3.14 / 2000 / 7.2 = 0.000327 (30 - tooth gear)
25/20 * 3.14 / 2000 / 7.2 = 0.000273 ( 25 - tooth gear)
20/20 * 3.14 / 2000 / 7.2 = 0.000218 ( 20 - tooth gear)

Jog speed is determined by the pulse rate multiplied by the distance traveled per step. So, given the fact that you're getting about 8-ips jog speed now, that means that your controller is putting out about 24,000 pulses per second (8 / 0.000327 = 24,458 pps). A 25 tooth spur gear would reduce the jog speed to 24,458 * 0.000273 = 6.67 ips and a 20 tooth spur gear would reduce the jog speed to 24,458 * 0.000218 = 5.33 ips.

Whatever cutting speed you're getting with the 30-tooth spur gears would be slower with the 25 or 20 tooth spur gears. If you switched to 25-tooth spur gears you would multiply your current speed by 25/30 = 0.8333 to find the new new speed. For a 20-tooth spur gear, you would multiply your current speed by 20/30 = 0.6666 to find the new speed.

According to the charts (and the tech support people) at Oriental Motor, the 7.2:1 gearbox is putting out about 1/3 of the expected torque. For instance the PK296 motor without a gear box will have about 300 oz*in of torque at standstill. So, adding a 7.2:1 gearbox should produce 300 * 7.2 = 2160 oz*in of torque at standstill; however, the chart shows that the maximum torque available with the 7.2:1 gearbox is about 45 lb*in or 720 oz*in. But, to compare the different gear sizes, you would use this formula to find the reduction in torque when using a 25 tooth gear instead of a 20 tooth: 20/25 = 0.80. So, a 25 tooth gear would give you 80% of the torque that you would have gotten with a 20 tooth gear. A 30 tooth gear would give you 20/30= 0.6666, or 66% of the torque that you would have gotten with a 20 tooth gear. However, I couldn't pin anyone down to exactly what it is about the gearbox that limits available torque. So, you might get the same torque on the output shaft from a 30-tooth gear as you would get with a 20-tooth gear.)

Edited: Although it would seem that changing from a 7.2:1 to a 3.6:1 gearbox would double your speeds, decrease your torque by 1/2 and decrease the smoothness (resolution) by 1/2, other factors come into play. I don't think that the motors are beefy enough to give you much more speed than you're already getting. So, based on my test bench experiments, your current configuration is one of the best - unless you have too much play in the system or have worn components.

(I've misplaced your telephone number. If you'll email it to me miker@xmission.com (mailto:miker@xmission.com), I'd like to contact you when I'm in Logan in a few days to see the G4 up close.)

[robert_cheal]

Brady,
Thanks for your sound advice and the insite on using Alpha/3G PRT motors mounts. I will call ShopBot tomorrow hopefully they could send me some with the new pinions.

Mike,

Thank you for spending time with my particular setup I think that I am starting to understand. I still need to study your answer, you make it sound so easy. Perhaps I need a Rossetta Stone.

I would look forward to seeing you I 'll send you a email.

Thanks again Brady & Mike.

Robert