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Reasonable Accuracy

Message 1 of 4
154 Views, 3 Replies

Reasonable Accuracy

I am not sure if this post belongs on this Forum so please disregard if you don't feel it applies.


I spent the last two years converting a small 2HP benchtop mill to cnc. I did not spend lots of money on high end ball screws or bearings as this is truly a fun hobby.

I spent quite a bit of time fine tuning the turns ratio for the ball screw as well as lash compensation. If I line up two 1x2x3 gauge blocks in measure their length, I consistently get results like 6.000 or 5.9998 inches. The mechanical backlash was .0008, .001 and .0012.

I recently milled a small aluminum block which is supposed to be .75" x .75" at the top profile.
The actual dimensions of the top of the block ended up at .7539" x .75375". I used a 3D adaptive tool path leaving .005 stock radially, then I used a contour path to remove .004 with a final finish pass of .001. I maintained the same stock position throughout the machining process.

Having no past machining experience and was wondering if results like .75339 and .75375 seem reasonable for a hobby machine?

Interestingly enough, I made a second block and the dimensions were .7546 x.7541

Thanks... Richard

Message 2 of 4

Some of this can be caused by tooling. Is it a new carbide tool, or a used HSS (high speed steel) tool? Is there runout in the tool (this would cause an undersize condition). How far out are you sticking the tool; the shorter the better. Do you take a "spring pass"?

Seth Madore
Customer Advocacy Manager - Manufacturing
Message 3 of 4

Thanks Seth.  I am using primarily YG1 aluminum specific tooling and the tool is new.  There is 1.65 inches of stick out on a 1.25" flute length.  I did take a spring pass.  Someone pointed out to me that I never measured the actual tool diameter.  I just plugged in .5 inches for a .5 inch endmill.  I think this may be reason for the oversized part.  


Thanks as always for your thoughts.  Very helpful.



Message 4 of 4

4 thou is normal for this sort of thing. Most tools, in a proper holder will cut about .0005" under nominal, minus deflection and wear. 

You'd either use cutter comp for this, or accept that the part most likely is still in tolerance for what youre doing. I don't know how youre getting all those extra decimals though. 

Please click "Accept Solution" if what I wrote solved your issue!

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