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Author Topic: Inaccurate Cut Dimensions Weirdness  (Read 7289 times)

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Offline Dan13

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Inaccurate Cut Dimensions Weirdness
« 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

Offline Hood

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Re: Inaccurate Cut Dimensions Weirdness
« Reply #1 on: May 31, 2009, 06:36:20 AM »
Have you done  a straight cut into a piece of alu and measured the width?
Hood

Offline RICH

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Re: Inaccurate Cut Dimensions Weirdness
« Reply #2 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

Offline Dan13

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Re: Inaccurate Cut Dimensions Weirdness
« Reply #3 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

Offline Hood

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Re: Inaccurate Cut Dimensions Weirdness
« Reply #4 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

Offline Dan13

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Re: Inaccurate Cut Dimensions Weirdness
« Reply #5 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

Offline Hood

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Re: Inaccurate Cut Dimensions Weirdness
« Reply #6 on: May 31, 2009, 08:09:21 AM »
Then it must be something else and I have no idea.
Hood

Offline Dan13

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Re: Inaccurate Cut Dimensions Weirdness
« Reply #7 on: May 31, 2009, 08:11:25 AM »
OK. I will make that trial cut you suggest anyway, and see what I get.

Daniel

Offline RICH

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Re: Inaccurate Cut Dimensions Weirdness
« Reply #8 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


  

Offline Dan13

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Re: Inaccurate Cut Dimensions Weirdness
« Reply #9 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