Ball Screw Question

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justasking
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Ball Screw Question

Postby justasking » Sat Jun 24, 2017 10:43 am

All, I am fairly new to the CNC world and I have a small CNC lathe. The machine uses Mach4 with stepper motors and 16mm ball screw on the Z axis and 12 mm on the x axis. When I try to calibrate the axis's I get inconsistent measurements on the travelled distance. I usually move in .1000 increments with a dial indicator measuring the distance to compare what the machine think it did versus reality.
one movement of .1000" may be right on the next might be .003 over then the other may be .004" under. It is all over the place. Can ball screws be that far off? Could this be a problem with Mach 4, the motion controller or am being over critical.
I appreciate any insight. Thank You.

J

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SteveHGraham
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Re: Ball Screw Question

Postby SteveHGraham » Sat Jun 24, 2017 11:33 am

Have you looked into the issue of losing steps? I am having the same problem, and this is where I was told to look. I haven't found the bug yet.
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justasking
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Re: Ball Screw Question

Postby justasking » Sat Jun 24, 2017 11:51 am

I thought about loosing steps, but in some instances the distance grows and I would not expect that with loosing steps. Not much, but .002-.004 makes me scratch my head.

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Dave_C
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Re: Ball Screw Question

Postby Dave_C » Sat Jun 24, 2017 12:24 pm

Ok, in order to get Mach 3 or 4 to do a decent ball screw calibration you need to move at least 1.000"!

This will help take out some of the ball screw error as the resulting calculation will be over a larger area thus reducing some of the error.

I used Roton rolled ball screws on my lathe setup and was able to get my X axis to zero error over the full 8" of travel. The first 12" of Z is good as well but the screw has some error past that. Not a problem as any part over 12" usually isn't that critical on the length.

Then we need to talk about your setup choice. Did you check MM or Inch? If you set the software up at the start for MM, then from that point on, all entries have to be in MM including your tool setups. If you chose Inch, then all moves, tool setups and so one are in Inch even though you have Metric ball screws!

So here is the rub. If the default units are MM the ball screw calibration routine wants to know how many steps per unit. IE: how many steps to go one unit, in this case 1 MM. If you chose Inches, then the unit is one inch, not .100" and there in may be the cause of your problem!

So try moving one inch (1.000") and then enter how far it actually moved. Do it two or three times and it should be good from there on.

Dave C.
I learn something new every day! Problem is I forget two.

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SteveHGraham
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Re: Ball Screw Question

Postby SteveHGraham » Sat Jun 24, 2017 6:10 pm

So I have to get an accurate dial indicator that has a travel of more than an inch.
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Dave_C
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Re: Ball Screw Question

Postby Dave_C » Sat Jun 24, 2017 6:20 pm

SteveHGraham wrote:So I have to get an accurate dial indicator that has a travel of more than an inch.


There is another way to do it with gauge blocks! With your indicator in the quill, fasten a stop to the table. Zero your indicator against the stop block. Now make your move of 1 inch and then insert a gauge block of 1.000" against the stop and then see what your indicator reads. My .250" indicator will read about .015" on the low side if I start at "0" so you should be able to read a short move and a long move (within reason)

You can then move to larger amounts depending on how long your gauge blocks are. I can wring together about 4" accurately. (Checked with a dial mic)

If your off more than that you need to start over with the math.

Dave C.
I learn something new every day! Problem is I forget two.

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SteveHGraham
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Re: Ball Screw Question

Postby SteveHGraham » Sat Jun 24, 2017 6:29 pm

Man, that sounds like a complete nightmare. And do you really trust the indicator to be accurate when you're taking the pressure off of it and then reapplying it after moving the tailstock?
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SteveHGraham
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Re: Ball Screw Question

Postby SteveHGraham » Sat Jun 24, 2017 6:35 pm

I'm looking at the accuracy of dial indicators, and even when you don't remove the pressure and then reapply it, the accuracy is +/- 0.003". I would be better off using calipers or an outside micrometer.

I'm glad you were here to point this problem out. I've always used measurements of 1" or less, and I have never gotten anywhere.
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Dave_C
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Re: Ball Screw Question

Postby Dave_C » Sat Jun 24, 2017 6:45 pm

Here is the math for a metric ball screw with a 5MM pitch.

One Turn of the screw is .19685" In order to move one setup unit of 1" you need 5.08001 turns. (1/.19685)

A common steps per motor turn is 2,000 (200 steps with 10 micro steps) So the steps per unit in this case is 10,160.01 (5.08001 * 2,000) to move one inch.

Try starting with this number and let mach 3 or 4 calculate from there. Of course this assumes a one to one ration of motor and screw. Add an additional calculation if you have a different ratio.

Dave C.
I learn something new every day! Problem is I forget two.

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SteveHGraham
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Re: Ball Screw Question

Postby SteveHGraham » Sat Jun 24, 2017 6:57 pm

The problem is that when the correct math is used, incorrect movements occur. That's why I think I'm missing steps.
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Dave_C
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Re: Ball Screw Question

Postby Dave_C » Sat Jun 24, 2017 7:28 pm

Steve,

If you are missing steps the measurements you take should not repeat! Lost steps should give you random readings...

Of course there is also backlash issues, loose thrust bearings, bad (cheap) ball nuts and all that stuff. Without seeing some of the parts and the setup I can't really offer much else that will help.

Dave C.
I learn something new every day! Problem is I forget two.

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SteveHGraham
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Re: Ball Screw Question

Postby SteveHGraham » Sat Jun 24, 2017 8:13 pm

Don't exert yourself. I am not fooling with it right now, so even if you gave me the answer, I would not do anything with it right away. I appreciate the information.

RE random readings...this is exactly what was happening the last time I dealt with it.

If you are right about my measurements being too short, I need to fix that before I do anything else, and then I can worry about motor settings.
Don't trigger me, bro!


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