Confused about unit values and gear ratios

[Mayo]

In checking my unit values against the chart shown on the Shopbot wiki,
it appears that my settings do not match what I *think* my motors are.

I have a (No upgrades) 2001 PRT
The circuit board is labeled 2.07 and there are 4 drivers on it.
There is a paper label marked PRT 13101-00409
The software I'm using is Ver 3.4.27
Control box firmware ver = 106
Ini ver 4.27

The motors are Vexta PK296A1A SG 7.2 2phase .25/step with gear boxes built onto them as supplied by Shopbot
The motors each have 4 wires connected. Black, Green, Blue, Red connect to the Shopbot cables with like colors connecting and White on the Shopbot cables connecting to Blue on the motors.
The pinions on X and Y are 30 tooth
The pinion on Z is 25 tooth

My unit values are set at
X & Y 611.1552
This is what the chart says it should be for the 3.6:1 gear ratio and 30 tooth pinions.

Should I assume the Z is also a 3.6:1 gear ratio? If it is, the unit value that works is not what is listed on the chart for my 25 tooth pinion.
Is the chart correct? If it is, then I should be using 733.386 but that doesn't work.

Does the 7.2 label on the motors mean it has a 7.2 gear ratio?
The specs on Oriental Motor web site states they are 7.2:1 gear ratio motors

The Z unit value I am set at is 1833.4656

On the chart in the wiki, that unit value is for the 7.2:1 ratio and a 20 tooth pinion. My pinion on the Z is 25 tooth. I didn't count the teeth but I can see the stamp on it that says 25. The motor is the same as the others. If the motors are using a gear ratio of 3.6:1 why would the unit value not be 733.386? Are the gear ratios on the Z motors different than the X & Y motors even though they are labeled the same?

According to the chart, if I have a 7.2:1 gear ratio and 25 tooth pinion, the unit value should be 1466.772 but that does not travel the correct distance. If I have a 3.6:1 gear ratio and a 25 tooth pinion the chart says the unit value should be 733.386. This also does not work.

As much as I'm able to measure the Z travel, my unit value of 1833.4656 appears to move the correct distance, but it's possible that it's very slightly off - however that unit value is for a 20 tooth pinion and 7.2:1 gear ratio. So this really has me confused now!

All the driver chips on my circuit board are the same. They are marked IMS IM481H

[bob_dodd]

my old prt /indexer set up is using 733.386 as the unit value with PK296A1A-SG3.6 and 25 tooth gear for X , Y , Z
check with shopbot support

[tlempicke]

There is really a much easier and more accurate way to do it.

Relate index units to measurement.

I used a dial indicator to do mine. I would suggest getting in range with a steel ruler and doing the fine adjustments with an indicator.

For either the x or y put the carriage at the end of the ruler and do a "Z2". Issue the command MX1to move the carriage i inch in the positive direction. If that is hard to read use "MX10" to move it ten inches and then divide the distance off by ten.

Change the index units by something that will be comfortable to work with. Say 100 units. Do the test again. The difference between the first test and the second will tell you how may index units equals one inch, millimeter or whatever you are working with.

Once you have it as close as you can get it with the ruler set up a dial index to measure the last inch. You should be able to run the axis out and back by 20 inches or so and hit consistently within .002 of an inch.

Don't forget to keep a good record of what you did and how many index units per inch.

[dana_swift]

Mayo did you figure out the correct unit values? They can be calculated with precision if you are interested in doing that.

:D

[Mayo]

Unfortunately I don't have a dial indicator,
and yes I would like to know how to set the correct unit values with precision - that was the main reason I posted my specs above.
(If I can do it on a calculator - I didn't pay much attention during math class)

I just don't understand why the unit values I'm currently running are working reasonably close. According to the chart, it would indicate I'm running with two different gear ratios, which I don't think is normal. It would also indicate I'm using a pinion size that I'm not using.

Clearly something is wrong - hopefully it's just my understanding!

Is there a software setting in the Shopbot ver. 3.4.27 that is causing the discrepancy? Something that tells it what the gear ratio is or what the pinions are or that the motors are 1/4 stepping or whatever? Or do the unit values accomplish all that?

[Brady Watson]

Mayo,
Try this math & these numbers. They should be pretty close. Also, yes - you are correct in assuming that the 7.2 on the motor is referring to a 7.2:1 gear ratio. Some motors don't have the 7.2 designation, but tell you the step size, which in your case would be .25 degree step on a 7.2:1 motor and .5 degree step on a 3.6:1 motor (standard PRT).

It is safe to assume that if you are using a 7.2:1 motor on the Z, AND a 20T pinion, that the UV would be exactly double that of the stock 3.6:1 motors. 1833.4656 is correct for a 20T and 7.2:1 motor on a 1/4 stepping IMS drive.

Since you are using a 25T pinion, we can use some simple math to get you in the ballpark:

1833.4656 X 20T ÷ 25T = 1466.7725 for Z axis

If running 7.2:1 motors on X&Y with 30T pinions, we take the stock 3.6:1 ratio and double it to get: 1466.772, which is the value for a 7.2:1 motor with 25T pinion. You have 30T, so:

1466.772 X 25T ÷ 30T = 1222.3100 for X&Y axes

Since you have 2X the resolution with the 7.2s as 3.6s, it doesn't hurt to try to get some of that speed back with gearing. Since you are going UP in gear, your resolution will go down, and in turn, the number of steps it takes to go 1 inch will also go down. (Your UV)

Try those numbers out and see if they get you close. The IMS drivers were only used for a short time, and I believe there were 1/2, 1/4 and 1/8 stepping variants, so...see how you do with these & report back.

Just to proof my numbers, if you take 1466.7725 ÷ 1222.3100 = 1.20000040 & if you take the ratio of 30T pinion and divide by the 25T pinion (30 ÷ 25) you also get 1.2 - Should be good.

-B

[Mayo]

OK I tried the unit value of 1222.310 for X and Y
This moves the gantry very fast - in fact, I can't jog over 2.25 inches per second without it stalling and making horrible noise.
The drawback to that unit value is it travels 2X what it should.

On the Z unit value of 1466.7725 when I make a Z move that should be 2 inches, it actually comes out to about 1 and 19/32 of an inch.
Does this mean my drivers are set up with an incorrect stepping setting?

I looked up specs for the drivers here:
http://motion.schneider-electric.com/downloads/manuals/discontinued_prod/im481h.pdf
and it has a chapter titled Microstep Selection but I don't understand it.
It does say "The microstep resolution can be changed at any time."
Is there a setting in the Shopbot software that changes the microstep resolution or is it strictly how the motor is wired to the driver?

I suspect my drivers are set up in a way that might not be 100% correct for my motors and pinions. Why do I suspect this? Periodically having to reset the Z zero on X and Y from "drift" or cumulative error which seems very slight. There's also the issue of chattering and stepped looking cuts that should be smooth but I realize some of that could be attributed to mechanical issues other than drivers.

[Brady Watson]

Mayo,
There may be a selector switch on or near the driver to change from 1/2, 1/4 or 1/8 stepping. I've only worked on a few of these with the IMS drivers, so my advice would be to call ShopBot and ask them. It could be that the Z was setup at a higher microstep resolution than the other axes. Keep in mind, SB was still relatively new at the time and they were trying out a bunch of different drive and electronics combos. It appears from your link that step resolution is set via TTL hi/lo.

-B

[bleeth]

SB default Unit values for a current Alpha X/Y from SB3.6 or 8 setup are 2,482.8171, so I'm also assuming that the "latest" Alphas have a 30 tooth pinion on X/Y and with a unit value of 2,979.3805 the default number in SB3.xx the Z's are standard 25 tooth pinions, correct?

[Brady Watson]

Correct.

-B

[bleeth]

Thanks-
For a normal Alpha that primarily cuts plywood and melamine do you think it's better to go with the increased torque from 25 and 20/1's respectively? Cutting speed would still be limited by relatively small bit size (5mm and/or 1/4").

[Brady Watson]

No. If you go with the 25T, pinion life is drastically increased. UV @ 2900 is more than enough for resolving detail & Z torque on just about anything a rack & pinion tool is capable of resolving.

-B

[bleeth]

Ok-I imagine you mean "decreased".
Same rack for both?

[bleeth]

Hey Brady:
I've been having a very small undersizing issue and just did some checking into my control software and discovered that x and y unit values were set at 2482.8169 and set up told me I had a custom set-up. I can't for the life of me remember changing from default. Is it a safe assumption that the .0002 difference in what I had from the default settings would account for the parts being smaller? Or is it possible that they were changed to that to solve this issue earlier and it just didn't compensate enough?