Question Undersized Parts

I am working on getting my SC840 set-up and I'm cutting some test pieces, 32mm squares but the
parts are coming in undersized, about 31.86mm on all sides.

I have tuned the machine as best I can, and it runs 100% rapids in all axes. I have added in 0.2mm
backlash compensation in the X-axis. I had first thought I was using a 1/8th in. cutter but double checked
it as 3mm. It's the same dimension on all sides that has me stumped. Any help or advice would be great.

Did you tick the "enable backlash" box?

Hi Rory,

Yes I have it ticked. Just have backlash on the X.
Axis velocity is at 200 and comp. acc. is at 240 (+20%)

Just to add, as you may have thought it too but I'm using a
brand new Mitutoyo Digital caliper to measure, and I'm confident
it's not the measuring.

Evan

Could it be that the milling cutter is not running true - if my visualisation is correct a wobble of .07mm will reproduce what you are seeing. Line up a straight edge close to the cutter and rotate the spindle by hand. Looking at a light through the gap between the two would show up any such problem.

Is your spindle quite free of side shake, and is your collet holding the cutter correctly centred? Obvious points I know, but identical errors on both axes is extraordinarily suspicious - almost too good to be true!!

Peter.

Does sound like something like that Peter..

Evan - try milling a straight slot with the cutter - one slingle line profile and measure the width of the profile with the same calipers?

Tested the spindle for wobble with the straight slot and thankfully all is well, spot on 3mm.

I did however test the clamping of the kress in the tool holder, and I may have had it a little loose. Although I cannot turn the Kress in the holder by hand, I'm guessing vibration etc. might have caused the error.

At the moment my X-axis is nigh perfect 32.00mm but with 31.90 +/- 0.01 on the Y, so got a bit more figuring out to do.

Thanks for the support.

Added a backlash of 0.1mm to the Y and parts are coming out spot on, happy days.

Successful design is not the achievement of perfection but the minimization and accommodation of imperfection.
- Henry Petroski

Great to hear that - just shows how accurate the machine can be if properly set up and adjusted.