I’ve been boring holes through some 3/4″ thick MDF recently and having a little trouble with it.

The blanks are about 3″ x 6″ and I’m boring 50 holes in them. When I set them up I put a pair of parallels under them in the vice and tighten the screw.

The problem I’m having is that the first holes go all the way through, like I want, but when I get down to the other end of the block the holes are a little short. The depth that I’m boring them to is .790″, so what gives?

This evening I did some investigating and discovered a few interesting things.

The first thing I did was take out my dial drop indicator to check whether when I told the Z axis to move 1.000″ it actually moved 1.000″. It turns out it did not.

I set up the indicator vertically and placed the point on the top of the router. My indicator has a travel of one inch (as most do) so I set it at about .050″ in. Then I told the Z axis to move .900″.

The indicator read .888″. Ah-ha. I repeated the measurement several times to be sure. I was pleased to see that there is very little backlash in the Z axis. When I sent it back to zero, it read zero. When I told it to move .900″, it moved .888″ every time.

So now I have a correction factor for the driver. The setting that is mathematically computed using the steps per revolution and the screw pitch is 188.947 steps/mm. I multiplied that by my correction factor (.900/.888 = 1.0135) to come up with a new value of 191.500 steps/mm. I entered this into the GRBL shield and viola! When I told the Z axis to move .900″ it moved .900″. Huzzah!

Then I thought “better check the other axis while I’m at it.” So I did.

I took a picture of the X axis setup.

When I told the X axis to move .900″ it went .892″. So I followed the same method to generate a correction factor and fixed that.

Here is a very exciting video of the X axis traveling .892″

Then I did the same with the Y axis. Which moved .890″ when told to move .900″.

There is also not very much backlash in the X and Y axis either, but a little more than the Z. Still, hardly enough to worry too much about. I’m not making rocket engines. Yet.

So that solved part of my problem.

Next I decided to check and see if the Y axis was parallel to the table surface. I was pretty sure it was, but it never hurts to check again. So I hooked up my dial test indicator and ran it across the table. It was within .005″, so I called it good enough.

Then I put a blank into the vice and ran the test indicator across that. Ah-ha again. The indicator showed that the blank was about .025″ lower on the far end! That, plus the mis-callibration of the Z axis, would cause the hole boring problems.

So I grabbed some .010″ brass shim stock and put .020″ of shim under the low side of the vice and Bob’s yer uncle! Now the blank is within .005″ from end to end. I’ll take that.