Thread: Internal vs External Cut Dimensions

I don't know Mastercam or any industrial cam software, my only experience is with our very GUI based setup on an Oce/Zund cutter.

Having said that it sounds as if there might be two different settings for tool diam/radius when offsetting the toolpath for internal or external cuts. If it was a single setting then your cuts would be off for both.

If your parts are coming out 4mm oversize then your toolpath is being offset 5mm instead of 3mm, so it thinks your running a 10mm diam bit instead of 6mm.

Under the "Cut Parameters" there is a "Compensation Direction" which has options of "Left" and "Right". So far I have been leaving it on Left, but I am guessing I need to change this to Right for outside cuts. I tried to experiment with it today but didn't get as far as doing the outside cut, so should be able to confirm my suspicions tomorrow.

The 2mm difference could be down to fine tuning of the axis. My circles were slightly under but my lengths were slightly over, so I have left it as is for now. If tomorrow I find the length is under, I will adjust the axis settings again.

My experience is with a multi tool machine. I have forgotten which software product we used to prepare the code, but I know that it had profiles for each tool and operation that it was used for. For example, different profiles for say a 3/8" compression bit for single pass full depth internal cut, single pass full depth external cut, multi pass full depth internal cut (1st pass cuts to 0.5mm above table and 0.1mm inside finished dimension, 2nd pass full depth and to finished dimension), multi pass full depth external cut (similar to multi pass internal except that the first pass was 0.1mm outside the finished dimension).

Can be cumbersome to set up all the different profiles for multiple tools initially, but very easy to modify all of the profiles in one shot if you change the physical bit diameter to a different size, or just have a bit resharpened with a minute change in diameter.

Joy is that once you have the profiles fine tuned, your only real issue is deciding what profile to assign to each tool path or operation, then let the software do it's bit. Concept works well, and produces consistent results over years of commercial operation. Sorry, I have no background with the software that you are using.

I used the "right" setting and on a 310mm width I got a measurement of 312mm. What I think I need to do is do a test with no offset so it follows the drawn line with the middle of the cutter. That way I just subtract 6mm from the size of the square / rectangle. I may still need to do some axis adjustments.

It might pay you to try and draw a square and a circle close to the size of your table with a pencil instead of a bit in the (stationary) spindle collet. You should then be able to compare the dimensions on them with those on the initial artwork so that you can calibrate the steps per inch/cm etc . Once you can get the scaling spot on, you can then concentrate on sorting out offsets for internal and external cuts etc. The machine I mentioned earlier had a pencil tool as one of the tools mounted, initially used to verify scaling and squareness, and subsequently used to write part labels on components. (We were using preprinted labels by the time I took over as operator).

Good idea, the pencil line will be thinner for more accurate measurements. I don't have changeable tools so don't have to worry about tool offsets at the moment. I would like an auto changer down the track but they are expensive and really not justifiable at the moment.

Working on building Mk2 of the Z axis at the moment, now I have the rack and pinion Z axis operational, I can use it to build my ball screw Z axis for greater precision.

Ok I think the compensation sussed...

I put the pencil in and drew 6 x 100mm squares, two with comp off , two with comp left, and two with comp right. I told the software I had a 20mm bit in for easier measurements.

As expected the comp left and right drew lines (as best as I can measure with verniers) 10mm either side of my comp off line. But where it got tricky was depending on the direction of cut, depended if Left and Right was inside or outside.

Drawing clockwise Left was to the outside and Right to the inside, but drawing anti clockwise Left was on the inside and Right was on the outside.

Having sussed this out and knowing my last outside job cut in 12mm alloy was Right and anti clockwise, my measurements should have been to size, but it was 312mm wide instead of 310mm. Not ready yet to blame the calibration, as the job might have moved.

I have some Styrofoam, I might do some test cuts on it tomorrow.

Originally Posted by HeadScratcher

But where it got tricky was depending on the direction of cut, depended if Left and Right was inside or outside.

Drawing clockwise Left was to the outside and Right to the inside, but drawing anti clockwise Left was on the inside and Right was on the outside.

Yep, the compensation is related to the direction you've gone to draw your shape.