Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: native34 on December 22, 2015, 05:00:53 PM

Title: Help with machined parts wrong size
Post by: native34 on December 22, 2015, 05:00:53 PM
Could someone help me understand why when I am machining something parts of the part come out the wrong size. I machined a part today that had an inside pocket and outter contour. The inside pocket came out to size but the outter profile came out almost .010" larger than it should.
Title: Re: Help with machined parts wrong size
Post by: magicniner on December 23, 2015, 08:39:48 PM
Were inside and outside cut with the same strategy, both with the same depths of cut, finishing cut and both climb or conventional?
If your machine has significant flex in it changing almost any parameter can change the path slightly,

 - Nick
Title: Re: Help with machined parts wrong size
Post by: native34 on December 24, 2015, 06:01:47 AM
I am using an industrial hobbies/charter oak automation mill. There should be plenty of rigidity, especially since I was machining plastic. For the inner pocket I used a 2d adaptive clearing strategy for roughing then did a contour cut the last .010" for finishing. For the outside profile I used a contour cut. All of these were at full depth. All cuts were climb, with the same 1/4" 2fl hss end mill.
Title: Re: Help with machined parts wrong size
Post by: magicniner on December 24, 2015, 06:25:38 AM
Did your outside profile have a finishing cut? If not try it with one of the size of your error, you could also try reversing the direction of your profile cut to see if it reverses the error to undersize, 

 - Nick
Title: Re: Help with machined parts wrong size
Post by: native34 on December 24, 2015, 01:12:14 PM
My outside cut didn't have a finishing cut as there was only about .050" material to be removed. I wouldn't think there would be any error. I set the path to leave no stock on profile which should have machined profile to size. Ok thank you I will give that a try.
Title: Re: Help with machined parts wrong size
Post by: native34 on December 24, 2015, 01:16:48 PM
If by reversing the cut the part comes out undersized. What would that imply?
Title: Re: Help with machined parts wrong size
Post by: BR549 on December 24, 2015, 01:41:41 PM
I would FIRST check the GCODE program for errors. Then work out from there.

(;-) TP
Title: Re: Help with machined parts wrong size
Post by: MuskokaMike on December 25, 2015, 05:40:14 AM
If by reversing the cut the part comes out undersized. What would that imply?

reversing the direction of the cut would indicate flexing of the gantry/z axis or any other distortion due to the forces on each piece being reversed....

I've noticed this to on my POS machine that uses plastic pieces as structural elements...I cut a LOT of letters and when I cut clockwise it cuts fine...when I cut in the opposite direction, it takes off about .01 more....It's not in my design software, it's not in machine code error, it's just a POS....so what I have to do is have a separate tool for reverse cuts....ie: I  use a .125 down cut spiral for the initial clockwise cut...and then "lie" and use a .127 for the reverse cut.....I still use the actual .125 bit, but I tell the software it's .127 in diameter.....

I do two passes because I cut a LOT of pine and in one direction: the bit doesn't shear off the fibres cleanly in all areas so a counterclockwise takes care of them...saves me a LOT of sanding.....
Title: Re: Help with machined parts wrong size
Post by: magicniner on December 26, 2015, 06:49:19 AM
If by reversing the cut the part comes out undersized. What would that imply?


A rotating cutter when engaged in a material and moving in a given direction will be subject to a force generated by the rotational movement of the cutter which can generate sideways take-up of any slack or flex in the system.

Test your outer profile cut with clockwise and anti-clockwise paths to see if there's a size difference and how much.

 - Nick
Title: Re: Help with machined parts wrong size
Post by: native34 on January 05, 2016, 05:12:21 PM
Ok, I was able to get the part machined using the recommendations of you guys regarding conventional milling strategy. Here are my results. The inside pockets are still .750 and the outter pockets are still .0055" larger than the dimensioned size of 1" x 2". I gave myself a +/- .005" but I'm trying to get this thing dialed in, as this is the part of my build where I'm calibrating the machine. What is Weird is that the inside pockets mill to within .0005" of dimensioned size but the outter profile is .0055" off. I can't make sense of it. So it appears that changing the cut from climb to conventional did not yield different results.
Title: Re: Help with machined parts wrong size
Post by: native34 on January 08, 2016, 06:48:36 PM
Anybody have any other ideas what could be the problem.
Title: Re: Help with machined parts wrong size
Post by: Overloaded on January 08, 2016, 06:56:03 PM
You never responded to TP from reply #6.

Post the DXF and the GCode file you created.

Gotta start there.

Russ
Title: Re: Help with machined parts wrong size
Post by: native34 on January 08, 2016, 07:44:10 PM
Posting the file and gcode I don't believe will be helpful. I've tried this with different parts in different cam software and the results are the same. I also did what was asked regards hanging from climb to conventional path and the results from that have not changed.
Title: Re: Help with machined parts wrong size
Post by: Overloaded on January 08, 2016, 07:45:30 PM
Very well .... carry on. Must be mechanical then.
Regards,
Russ
Title: Re: Help with machined parts wrong size
Post by: magicniner on January 09, 2016, 07:38:32 AM
Given it's not the things you say it can't be Russ is right, it looks mechanical

 - Nick