2-sided machining, any idea what could be causing this?

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ironspider
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2-sided machining, any idea what could be causing this?

Post by ironspider »

Greetings all, I have an AVID CNC 3hp CNC and I'm trying to do a 2-sided machining test. I tested the CNC today and it is square (did their 4 dowels in 4 corners and the diagonals measured exactly the same) and trammed within .001 so I feel like the machine is set up properly. So in this test cut I set up my project in vectric as a 2-sided job flipping on the Y axis. I am using a asymmetric locating dowel approach with 3x .25" dowels. But when I cut the bottom side, everything appears to be off by .025" in the X+ axis. The Y axis (I think) appears correct. I'll attach a pic and my vectric file if that helps.

But what could be causing this? Assuming the machine is square, and I was able to flip it and get the holes onto the dowels properly, shouldn't it be impossible for it to be off like this?
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2-sided_test_non-symmetric_2.crv
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IMG_8231.jpg

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martin54
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Re: 2-sided machining, any idea what could be causing this?

Post by martin54 »

Couple of questions, you say the machine is set up square & you have trammed the spindle but have you calibrated each axis to make sure the machine is moving the correct amount?
Not looked at the file, do you have any tool changes & if you do did you zero just the z axis on the tool change?

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Adrian
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Re: 2-sided machining, any idea what could be causing this?

Post by Adrian »

One possibility is bit deflection on the pockets. To eliminate that as an issue you could do a profile cut on the circle that barely touches the material and see if that is off.

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rscrawford
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Re: 2-sided machining, any idea what could be causing this?

Post by rscrawford »

Are your Z axis rails perfectly orthogonal to the spoilboard, or did you simply tram the spindle so its orthogonal? If the rails are slightly off, the spindle will change position as it moves up and down, giving you that problem (your dowel holes are cut into the spoilboard, but you are cutting the top of your material, which could be off 0.025 if the rails are 'leaning' in that direction).
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SteveNelson46
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Re: 2-sided machining, any idea what could be causing this?

Post by SteveNelson46 »

Try using a profile toolpath cutting on the inside of the line with a shallow pass depth instead of pocket toolpaths and see if it makes any difference.
Steve

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adze_cnc
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Re: 2-sided machining, any idea what could be causing this?

Post by adze_cnc »

Profile vs pocket fixes why the “counterbore” is not circular. But, I still don’t see how things have bee shifted 1/4” in the x axis.

I presume that the central hole is 1/4” in diameter but I don’t really see anything misaligned by that much. Having said this I haven’t had a chance to look at the CRV file so there could be something that I’m ignorant of.

ironspider
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Re: 2-sided machining, any idea what could be causing this?

Post by ironspider »

Thanks for the replies all. After reading the replies I switched from the asymmetric 3 locating dowels (which were 1/4" pockets cut with a 1/8" end mill) to doing 2 symmetric (centerline) 1/4" dowels with a 1/4" bit using a drilling toolpath. The cut does seem to be closer now. Maybe this lends itself to the possibility of deflection during the cutting of the locating pins using the 1/8" bit as a pocket?

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martin54
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Re: 2-sided machining, any idea what could be causing this?

Post by martin54 »

ironspider wrote:
Mon Jul 04, 2022 6:48 pm
Thanks for the replies all. After reading the replies I switched from the asymmetric 3 locating dowels (which were 1/4" pockets cut with a 1/8" end mill) to doing 2 symmetric (centerline) 1/4" dowels with a 1/4" bit using a drilling toolpath. The cut does seem to be closer now. Maybe this lends itself to the possibility of deflection during the cutting of the locating pins using the 1/8" bit as a pocket?
I never use a pocket toolpath for anything small like that, deflection & lost steps can be an issue when the machine is moving with the rapid changes in direction, I sometimes use the drill toolpath but most of the time I will use a profile toolpath cutting inside the line with a spiral ramp :lol: :lol:

wilkigr
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Re: 2-sided machining, any idea what could be causing this?

Post by wilkigr »

adze_cnc wrote:
Mon Jul 04, 2022 5:13 pm
Profile vs pocket fixes why the “counterbore” is not circular. But, I still don’t see how things have bee shifted 1/4” in the x axis.
In his original post, the OP says that the shift was .025" not .25". :D

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adze_cnc
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Re: 2-sided machining, any idea what could be causing this?

Post by adze_cnc »

0.025” offset right below 0.25” dowel ‘confoosed’ my brain.

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Adrian
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Re: 2-sided machining, any idea what could be causing this?

Post by Adrian »

adze_cnc wrote:
Tue Jul 05, 2022 12:03 am
0.025” offset right below 0.25” dowel ‘confoosed’ my brain.
Caught me out as well to start with. I spent more time than I'd like to admit looking at the file for inside/outside and tool diameter settings. :lol:

wilkigr
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Re: 2-sided machining, any idea what could be causing this?

Post by wilkigr »

Full disclosure .... In fairness to both of you, and FWIW, I read it as 1/4" too and was all set to post a question when I re-read it for about the fourth time and realized that my question would be stupid since we are talking about less than 1/32", not a 1/4". :D

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