Setting up drill offsets on the Tree using a concentric indicator

Determining the X offset for drills in the Tree (and other CNC lathes) can be a bit tricky, given that the drill is actually mounted in the turret (cross slide). For this to work without breakage, you need to determine the X coordinate of the drill. This is then entered as the X axis tool offset and ideally it would be constant for all of the drill / boring bar mounts on the turret.

There are a few ways to do this, such as trial and error (by eye etc) ie spinning a piece of stock and moving the turret in X until the tool (drill) tip scribes a point rather than a circle. Another way is to use a concentric indicator, as often used on non-CNC milling machines that don't have electronic probes.

I got one of these before Xmas. Not much more than an ebay / Aliexpress version (and almost certainly Chinese made) but perhaps subject to slightly better quality control.

In simple terms, you probe the outside or inside of a concentric, cylindrical part, then move the turret to minimise the indicated eccentricity. In my case, I can move the turret in X but not Y (ie vertically up and down). Given that there is no evident means of adjusting the turret in Y on the Tree, I have to hope any error isn't significant.

Let's go:

First, set up the indicator so that the main body is deadly concentric. Initially with this Kurt-badged Chinesium thing...

...then with a decent Mitutoyo DTI. I seem to have excelled myself here, as there is no discernible runout shown. That suggests it must be of the order of microns ie far better than this machine could actually attain.

Then, with the probe parallel to the X axis (ie horizontal in my case), spin the machine slowly (~60rpm) and adjust the X axis to ensure the needle is at the same position when the probe is at both front and rear of the bore.

That means the bore is on the spindle axis in terms of the X axis. But next, measuring the error in +/- Y shows that I have around 50-60um eccentricity on each side

Not ideal - but for now there's not a great deal I can do about it. Above all, I don't want to break drills. For accurate bore dimensions, I would normally expect to use a boring bar.

Ideally I would also save the G53 machine coordinate, as this should be a constant, regardless of the reference tool etc. 

Saving the drill offset:

This is tool #6:

Here's the absolute machine coordinate for X (-114.342mm):

With tool #6 and its offset selected (T0606) - and X axis zeroed, this is what you'd hope to see. For instance if you loaded a drill in tool #6 and asked for it to positioned in line with the spindle axis:

I may be forced to try out drilling at some point in the near future. No excuse now...