August 13, 2017

Reamer Regrinding

In the near future I'll need to make a bunch of things with tight-tolerance 22 mm bores, so instead of buying a 22 mm reamer, or just boring the holes like a normal person, I tried out re-grinding a 7/8" reamer (22.225 mm) reamer down to 20 mm by setting up my toolpost spindle as a toolpost grinder.

Here's the setup:

The reamer is set up between centers, and the toolpost spindle has a 4" grinding wheel on an arbor.  There's a mist-coolant dispenser to keep everything cool and lubricated.  The center on the chuck-side was turned in place out of some steel scrap, to ensure its concentricity with the spindle.


All together it turned out pretty well.  I turned down the shank and ground it as well, so it will fit in the lathe's drill chuck.

I overshot my target 22 mm by 5 microns, but that's not too shabby, considering the ticks on the lathe's cross slide are 25.4 microns each.  It seems to cut a tiny bit oversized, so it works out in the end to give me a very tight slip fit onto a 20mm ground shaft.

This method leaves no relief behind the cutting edges, so as you might expect, they drag a little bit.  The surface finish inside the bore is okay, but definitely not as nice as I'm used to getting with reamers.  Over all, this was definitely way more time and effort than just boring the holes like I normally would, but it was a fun exercise, and good to get some experience using a toolpost grinder.


  1. You can create the clearance by lowering the grinding spindle just a touch.

    1. Only if you index to each flute and move in z with the lathe spindle locked, and that would only work with a straight flute reamer. If the reamer is spinning, you will get no relief, regardless of the height of the grinder.