Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: Dan13 on May 31, 2009, 05:07:17 AM

Title: Inaccurate Cut Dimensions Weirdness
Post by: Dan13 on May 31, 2009, 05:07:17 AM
Hi,

I have a strange inaccuracy in the parts I cut. If I cut an outside profile, for example a rectangle, I get the dimensions about 0.06mm larger than programmed. If however, I cut an inside profile, for example a rectangular pocket then the dimensions are about 0.05mm smaller than programmed.

My backlash is less than 0.015mm on both X and Y. If it were a backlash issue then, the axes would move less than the programmed distance. But it is not the case when cutting outside profile - if it were backlash, the dimensions would have been 0.015mm less than the programmed, but instead they are 0.05mm bigger. Also in case of backlash, the pocket dimensions would have been 0.015mm smaller, but they are 0.05mm smaller.

I thought it might be a lack of rigidity on my little Emco, but trying to rerun the program to see if it removes more material, it only improved the dimension by 0.01-0.02mm, i.e. from 0.06mm discrepancy to 0.04mm. Subsequent rerunning the program doesn't seem to remove any material.

I was experimenting with aluminum and not cutting too deep, so really seems unlikely to be a rigidity issue...

Any thoughts are appreciated.

Thanks,
Daniel
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Hood on May 31, 2009, 06:36:20 AM
Have you done  a straight cut into a piece of alu and measured the width?
Hood
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: RICH on May 31, 2009, 07:29:50 AM
What Hood mentioned, namely actual cutting of the material to find a usable tool diameter, as compared to, measuring the end mill with v-anvil mic's and using those numbers in your code. Cutting with and without coolant and also direction has an affect. Just may add up to what your differences are in the end.
RICH
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Dan13 on May 31, 2009, 07:50:41 AM
No, I haven't tried this. But may I ask what's the logic behind this? I think that when making a cut like this, when the cutter is loaded from both sides, the width would be slightly bigger than the actual cutter diameter. Is it wrong?

Thanks,
Daniel

Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Hood on May 31, 2009, 07:52:46 AM
If you make a cut like that and if the cut is under by 0.05 or so then you have the answer to your question, OD bigger by 0.05 and pockets under by 0.05
Hood
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Dan13 on May 31, 2009, 08:04:00 AM
Hood,

I see. So will it mean that the cutter diameter is smaller than nominal or what? Don't think it could be the cutter diameter, since, first, it is Hanita brand cutter, second, I measured the cutter diameter and found no error and third, it happens with any cutter I use.

Daniel
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Hood on May 31, 2009, 08:09:21 AM
Then it must be something else and I have no idea.
Hood
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Dan13 on May 31, 2009, 08:11:25 AM
OK. I will make that trial cut you suggest anyway, and see what I get.

Daniel
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: RICH on May 31, 2009, 08:35:05 AM
Daniel,
There is a practical side to CNC. We would like to think that all involved with making a part is perfect.
Just hit the button and everything should come out just so. We loose sight of all the things that add
to not meeting some tolerance. Testing and knowing how things turn out using your machine along with different
machining conditions, applying that knowledge, is just a practical side that needs to be applied to the job.

If the combination always gives similar results, do the rough and add some tweaking to the finishing based on what you know.
RICH


  
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Dan13 on May 31, 2009, 08:50:40 AM
Rich,

OK, thanks! Just was looking for a logical explanation to this I guess...

I will do more testing and try to find any rules, that will allow me to correct the dimensions in the program.

Daniel
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: RICH on May 31, 2009, 09:46:14 AM
Hey Dave,
As you get down in the tolerances  the whole system comes into play. If It's big variations then maybe there is  one culprit to be found causing a problem. But  when you start cutting hairs sometimes it's difficult to refine the machine
 / system and frankly may be wasting your time.
Here is an example:
My old mill, which I CNC'd, will produce an egged hole when milling a circle to hold a bearing. The gain in the lead screw is 0.00025" / inch and that gain varies over a 12", but there a few sweet spots in it's travel, the ball screw is within spec. I can use the sweet spot, even adjust the steps, and do an extra step or two and produce a very close tolerenced hole which is not egg shaped if i want to take the time to do so.

My engraving machine is different in that the X & Y have phenomenal accuracy and the Z movement is great also.
So no need to even think about tweaks, but, the tool runout can become limiting. 
So there's were i come from in my response and logical to me ( FWTW ).  ;)
RICH
 
 
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Dan13 on May 31, 2009, 11:44:57 AM
Hi Rich,

It is true that as the tolerances get tighter more things come into play. But do you really think that 0.05mm can be referred as a tight tolerance in the above sense? I would think that it is 0.01mm and finer tolerances when things get tricky.

What does your gcode look like when you make those adjustments to achieve an accurate circle? I mean, if it's oval then tweaking the radius in the gcode won't do. I would think of using scale factors instead....?

What's your mill's backlash? And what kind of tolerance for the circles can you get?

Daniel
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: RICH on May 31, 2009, 03:49:24 PM
Daniel,
What you have is what you need to work with and do the best you can with it.
Comparing diffrent machines is useless.
No code changes are done.
RICH
 
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: vmax549 on May 31, 2009, 08:34:47 PM
IS the error the same no matter what the size of the rectangle is or does it grow the larger the size?

(;-) TP
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Dan13 on June 01, 2009, 12:55:44 AM
Rich,

Wasn't trying to compare machines ;) Wanted to get an idea of what kind of tolerance you can get on your machine with known backlash and gain if you work appropriately, and then work out from there of what can I get if I learn to work correctly with my machine.

So what do you mean by "adjusting steps" and "adding steps"?

Daniel
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: Dan13 on June 01, 2009, 12:58:13 AM
Hi Terry,

Yes, the error seems to be the same no matter what's the size of the rectangle. Were you thinking it was wrong steps per in motor tuning?

Daniel
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: RICH on June 01, 2009, 06:45:51 AM
Sorry I mentioned the "adjusting steps " since if I get into this kind of discussion folks will think it is good practice not realizing that something was done for a very specific understood reason and it's not normal to do so.
 
You calculate and use the steps per unit and check / reafirm as noted in the manual and NEVER change them.

There are books about maching techniques and suggest you review that information.
BTW, a few swipes with a piece of sand paper can go a long way in adjusting something to dimension.  ;)
RICH   
Title: Re: Inaccurate Cut Dimensions Weirdness
Post by: vmax549 on June 01, 2009, 09:04:57 AM
HI daniel, Yes that came to mind(;-).  You need to first do some dedicated testing to confirm the steps/per is correct.

SEEMS to does not count(;-). Test and confirm.   You start with the straight line test and then move to  square/round  cuts and confirm the diamensions..  

RUle is do the math for the steps value IF possible.  THIS should be the correct value. BUT you may find variances in the screw itself so you test across the range and confirm.

IF the math cannot be done (unknown values) then test and tune is appropriate. DO the mach calibration test and TEST TEST TEST until you have a repeatable value to work with. IT must repeat itself in ALL conditions to a very close +/- toerance. The key is repeat itself(;-).

With all that out of the way you are down to machine design / wear tolerances. SOme machines just cannot get tight tolerances do to design and or wear. Cutting flex CAN effect you as well. Sharp bit VS dull bit, there is a host of things to effect accuracy.

SO start at the top and work you r way down throught the list untill you find the culprit. THERE IS ALWYAS A REASON(;-)  Then repair it IF possible and then live with the rest as best you can.

Just a thought, (;-) TP