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Offline wdp67

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Trouble cutting accurate parts
« on: December 11, 2007, 10:45:34 PM »
I am having more trouble. It seems that from day to day things change on my mini mill. I cut a part one day, and if I cut the same part the next day and do not change any settings, it comes out a different size, what am I doing wrong?? I set the steps in the auto setup, using a dial indicator, but then when I run a program, seems to just fall apart. Motors are not making any funny noise like they are losing steps or anything, but I can not for the life of me get any kind of consistancy.

Thanks
Walt

Offline jimpinder

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Re: Trouble cutting accurate parts
« Reply #1 on: December 12, 2007, 03:18:33 AM »
I take it this is why you are doing the home switches !!!

When you say "it comes out a different size" what are we talking about here - thousanths of an inch, or a factor or two, or what.
When you say "if I cut the same part next day" - do you mean 24 hours later, or just cut two consecutive parts.

The number of steps per unit set for your motors do not make any difference - even if they were wrong, they would be wrong all the time and the parts would come out the same size (albeit the wrong size). When you say "they just fall apart" do you mean the figures in the motor tuning alter, or that the motors just seem to do their own thing.

You only need to set the steps per unit once - finish - not every time you start the machine.

I don't know whether you are using steppers,  or motors with some sort of positional feedback.

If they are steppers, then they are accurate and (I would have thought) that the only problem you have is a mechanical one, where somehow, something is slipping, either the motor IS missing steps (in which case things would be consistantly smaller) or a belt or gear is slipping.  If you motors have a positional feedback, then maybe this is faulty.

Just make sure your acceleration and speeds are not too high so that you do not loose steps on acceleration, or overun when stopping.

The only other thing is "backlash" - have you got this set up. If you haven't, then your results may be indeterminate, depending on which side of the backlash you happen to be when you start.

The main thing with a CNC machine is consistancy - yes, that is what you are after, but you must be consistant as well - in that on a run of parts, YOU must do exactly the same positioning moves, so the machine is ALWAYS in the same position when it starts.

The ideal, I suppose, is to run the machine to the homes, put the work in a jig (so it is in exactly the right position) and set off from there, which, I assume, is what you are thinking.

Stick with it !!!



Not me driving the engine - I'm better looking.
Re: Trouble cutting accurate parts
« Reply #2 on: December 12, 2007, 04:59:37 PM »
There is a real chance that you are losing steps and not actually hearing it. Often times it can happen under a heavy cutting load. Other than that, your steps per unit & backlash settings could be off.

Offline wdp67

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Re: Trouble cutting accurate parts
« Reply #3 on: December 12, 2007, 07:32:29 PM »
Thanks for the replies!! What I am saying is I did check my steps per unit, but everything checks out ok. In set step per unit I tell it to move a set amount,and it does everytime within a .001. I am happy with that, I think. Then I go to cut a part. Mde a fence that fits in one of my table slots, then I cut the fence square to the x travel. I clamp my part to the table, which in this case is a peice of 1/8" plexi glass making sure it is square against the fence. I am using a 1/8 end mill to cut the outside profile of the part. In my drawing I have offset the outside of the part by .0625 and used that to create my tool path. I cut out the peice using 2 passes. When it is all said and done the peice is generally about .010-015 bigger than it is supposed to be and that seems to vary from day to day, or I should say evening to evening. One day I cut a few out and they were all .010 bigger, the next night I cut some more and they were .015 bigger.

Hopefully I am explaining myself clearer.

Thanks again for the help!!

Walt

Re: Trouble cutting accurate parts
« Reply #4 on: December 12, 2007, 10:19:31 PM »
Walt, it definately sounds like you are losing a few steps. It can happen for a number of reasons. One common reason is if your rapids are set too high. While you won't miss steps at jog speeds or feed speeds, often on a G00 move as when the tool returns to the origin for the next cut, it's quite possible for you to miss steps. Two things to look at, your motor tuning for one, try backing down the velocity. Secondly, what is often overlooked is the Rapids Speed setting in your post processor. Don't know what CAD/CAM you are using but double check your post processor and back the rapids down.

I had a similar issue a long time ago, and I could replicate it by using a 2" travel dial indicator. I could G00 2" and it would be off 9 times out of 10. Drove me nuts as I was thinking it was backlash but it was missed steps.

If the problem was consistant, i.e. always a few thou short or over of the required move then it would be leadscrew error. That can be tweaked in the Units setting for the axis.

Likewise backlash can show up in a similar manner. Backlash is tweaked with a dial indicator and jog stepping a thou in +/- directions until you get a thou movement  consistantly when you change directions.
Re: Trouble cutting accurate parts
« Reply #5 on: December 13, 2007, 07:08:01 AM »
Hi Walt

I started a thread called "is this a bug in Mach 3" as i was having a similar sort of problem. It's been deleted.

I was consistently gaining steps which it seems that you have the same problem.

I have now resolved the issue and it was down to one of three things.

1: I was using a BOB that had opto isolated outputs and my drivers had opto isolated inputs. IE two sets of opto isolators.

2: There was noise in the BOB board.

3: I hadn't wired the drivers correctly.

The problem is resolved due to a change of BOB that no longer has opto isolated outputs to the drivers

I also changed the wiring to the drivers the original wiring had a common ground the steps and direction were connected to the positive side of the drivers they are now wired as 5V common the negative now controls the steps/dir signals

Basically what was happening was on every direction change some extra steps were added.

It happened on every axis by the same amount.

HTH

Phil_H


« Last Edit: December 13, 2007, 07:36:40 AM by Phil_H »
The Good Thing About Mach3, Is It's very Configurable

The Bad Thing About Mach3, Is It's Too Configurable

Offline wdp67

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Re: Trouble cutting accurate parts
« Reply #6 on: December 13, 2007, 09:24:55 PM »
Ok I took out all the rapid moves and made them all feed moves and the results stayed the same, still .010-.15 larger than supposed to be, I will trymessing with the aceleration I guess.

Any other ideas?

Thanks
Walt
Re: Trouble cutting accurate parts
« Reply #7 on: December 13, 2007, 10:10:57 PM »
Is the problem consistant or random? If you set up a DI to measure your distance moved as outlined in the manual, is it consistantly .010" off or does it vary? Is the error on all axis or only one.?

Can you describe the configuration of your system. What CAD/CAM are you using to generate your code?
Re: Trouble cutting accurate parts
« Reply #8 on: December 13, 2007, 10:31:51 PM »
I do not wish to complicate matters, but...........
How is it possible to LOOSE steps and have the part come out BIGGER ? (Rectangular pocket or island)
This doesn't make sense to me, but it doesn't take much.      ::)
Re: Trouble cutting accurate parts
« Reply #9 on: December 13, 2007, 11:29:16 PM »
I do not wish to complicate matters, but...........
How is it possible to LOOSE steps and have the part come out BIGGER ? (Rectangular pocket or island)
This doesn't make sense to me, but it doesn't take much. ::)

Duh, you are right!

For some reason my brain was thinking 'smaller' as it's a more common situation. Larger would be either backlash setting being incorrectly set but more than likely, the steps per unit setting needs to be tweaked. Ball screws are not perfect and have error. Use a DI and a gage block to determine the travel distance and correct the steps per unit setting. I had to do that on all 3 axis of my machine to get it tweaked in. 

For example, I have 200 step/rev motors and I use 1/8 step so 200 x 8 is 1600 steps per rev. I have .200" per rev ball screws so 5 x .2" =1" of travel 5 x 1600=8000 steps per 1" of travel or 1"/8000 steps = .000125" per step.

If you have your steps per unit set at 8000 and you move 3" in Mach and the measured distance is 3.015" it means you have .005" error per inch in your ball screw.

So we take 1.005" divide it by 8000 steps and we get.0001256" per step. Divide 1" by .0001256"/step and we get 7962 steps per 1" which would be our correct steps per unit setting.