"Tooling up" - Mill's Z axis

Mini-Mill digital Z axis conversion

Yesterday, I started work on a little side project that should have only taken a few hours, but instead ended up taking all day.  Don’t they all!

One of the things I wasn’t too happy about with my mini-mill, is the fact that the Z axis (vertical) has quite a bit of backlash due to the play in the gears, and this has a small but bothersome effect every time I use it. 

Let’s say for example that I have just lowered the mill head until the cutting tool barely touches the part, so that I can find and set the zero height position. Next, I would raise the tool to reposition it somewhere else on the part I am working on.  Backlash is such that it takes some turning of the knob in the opposite direction before the tool even starts moving up again. The same thing happens when I later want to lower the tool again to cut in the new position.

Backlash demo on converted mill

The only way to quantify how far up or down I have moved the tool, is by reading the graduations on the knob itself. Problem is that by now the numbering is all screwed up, and will give readings of a movement that hasn’t really happened, at least to the extent indicated on the dial.

To compensate for this, I have to first take up all the backlash every time I change the direction in which I turn the knob.  Then, make a note of when the head starts to finally move.  Finally, re-zero the indexing on the knob before its numbers can be trusted again. This is not always possible, or convenient all the time.

Backlash is a real pain in the rear.  It slows me down, complicates every thing I do, and leads to inaccuracy.  This troubling phenomenon occurs in different amounts on all three axes, but has been more of a problem in the vertical direction.

Fortunately for me, included in my lathe and mill purchase came a few extras, among which was a caliper-style digital measuring device.  I decided to use it, and modify the mill in such a way that any actual head movements correspond to a new caliper position.

The beauty of this method is that no matter how bad the backlash is, the caliper only moves when the head moves.  The end result is that I end up measuring only actual tool movements, and the backlash issue is eliminated altogether.

Unfortunately, I couldn’t just screw the caliper to the mill column, because it needed to clear a cylindrical part of the lift spring mechanism that protruded through the right side. So, I decided to make some aluminum standoffs on which to mount the caliper.

Here are a few pictures of the conversion in progress.

Aluminum from which the standoffs were cut, plus standoffs before and after

Caliper-style device on the standoff

Finished standoffs

Tapping the top side of the mill column

Top standoff bolted in place

Tapping the lower holes in the mill column

The cylindrical object in the middle of the column is the reason for the standoffs

The whole shebang in position

Tapping the hole on the head of the mill

Digital measuring device on the mill, ready for use