Why does the DRO value not match the cut.

[LROYSON]

I have a pm1228 lathe and I have not been able to get consistent results with with my DRO reading when making cuts. I measure the stock when I chuck it up and set the x value on the DRO. I then make a cut say .020 in depth and remeasure. The DRO then reads .0015 to .002 less than the measured reading. Is this to be expected with that depth of cut? If I take a real light cut then the DRO seems to be more accurate. I measured with 2 different calipers and a micrometer and they match.

 

[Aukai]

Deflection? If you are touching off to set zero, and leave a scratch your zero is below the surface level to add to the stacked tolerances. Backlash, if unused axis planes are not locked, or temperature changes from heat generated are another addition to stacked tolerances. I'm just thinking out loud, the real gurus will chime in.

 

[MrWhoopee]

If theory and reality always agreed, anyone could do it. It takes a machinist to understand the variables and allow for them BEFORE the part is scrap. Did you take a second (spring) pass at the same setting?

 

[Asm109]

What diameter stock?
How much stickout from chuck?
end supported by tailstock center?

 

[LROYSON]

The stock diameter was a little over 1/2 inch and the stick out was about an inch and no tailstock was used. It was chucked in an R8 collet.

Also I didn't do a spring pass.

 

[woodchucker]

So first make a cut, don't touch the cross slide measure, set the dro and now you should match after the next cut.

 

[jwmelvin]

So first make a cut, don't touch the cross slide measure, set the dro and now you should match after the next cut.

Just to expand, any cut will have some deflection. If you take cuts of about-equal depth, you’ll get about the same deflection each time. Predictable helps.

 

[38super]

Look into your DRO manual, there may be a correction factor. It involves making a cut, measure and compare measure to DRO. Input the correction factor, rinse and repeat to verify. Usually done in metric.

 

[Dyspnea]

Have you checked the fasteners which hold the scales?

 

[markba633csi]

Classic case of deflection (both workpiece and machine), that's why your light cuts show minimal error