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Accuracy issues

I have been attempting to size an aluminum rod with a rough size of about 6.48 mm down to 6.28 mm to fit in a pinch-grip hole.  I have made over 10 attempts at this and always seem to come out undersized.

In the last trial, I had the rod in a 3-jaw chuck, exposed about 25 mm.  I wanted to try taking about .001 inch and used - .02 mm as my X P setting. On each pass I increased .02 to .04, and then 06 mm - assuming each pass would take an additional .02 mm (.000078 inch or .001).  After the second pass at .04 mm, the rod miked at .2325" or 5.9 mm.   I next added another .02 mm to have the X P at -.06.  After that pass the rod miked at .2261" or 5.7 mm - which is the .02 mm requested, but when I read it with the micrometer I'm getting a .006 difference in size:

.2325"  -- 5.9055 mm
.2261"  -- 5.7494 mm
------------------
.0064"    -- .1561 mm
I was expecting the .2325 to go down to .2315 or so...  Not 6 thousands!

Where am I going wrong here?  I'm using a conversion calculator to do my mm-inch-mm conversions so I should be good on that part.  Am I trying to hold too close a tolerance?

=Alan R.

To be honest, I cannot follow your numbers 🙂 Never understand why America measures in thumbs and feet when there are metric values

However, the Els is capable to cut down to ca. 0.01mm radius precision if everything is correctly set up.

I often cut rods to this precision.

First check if your screws are precise and if the correct values are entered  then measure the backlash and enter it  if you have imperial screws you should enter the pitch exactly in mm

then check the ways, drive X to the chuck  Zero it  Set X P to -50.00 and drive there until it stops at -50.00.  Measure with a caliper if it really drives 50.00mm

could also be a step loss due too high acceleration