Michael Koblic wrote:

I am slowly going through the milling machine. I have cleaned it, greased
it, trammed it all successfully. I have adjusted the gibs on all axes.There
is some backlash on the X and Y but nothing to worry too much about. The
run-out is 0.001" on the spindle and 0.002" on the chuck (but the rod I used
is of unknown concentricity so I am happy).

The one thing that drives me nuts is the Z-axis feed: For a start the
backlash is almost the whole revolution (.050" on a 0.060" wheel).
Furthermore, it is not very crisp or consistent (it is hard to tell when the
drive is actually engaged). I can see no way to improve on this and the
manuals are no help at all (the manual that came with the machine is brief
and Chinese - or might as well be!). I have downloaded two others related to
similar equipment including the notes from the Little Machine Shop (which is
the best) but none of them deal with this.

One of the jobs I do regularly is drilling and tapping shallow blind holes
in thin plates (5 mm or so). It is important to get the hole as deep as
possible to get at least 2 threads of 32 pitch without breaking through on
the other side. I had great hopes for the mill to improve my accuracy with
this. At this point the procedure is:
1) Get the drill bit touching the plate.
2) Back off two full turns on the Z
3) Move off the plate completely on X or Y or combination of both.
4) Reverse two full turns on Z
5) Add the calculated depth (usually plate thickness minus 0.025")
6) Fix the Z stop plate at this level (NB if you lock the spindle it changes
the Z-position by about 0.010-0.015"! They have a sort of back-stop bar
which I suspect is an afterthought - it is not mentioned in any of the
manuals)
7) Disengage the fine Zdrive. Lift spindle
8) Relocate the hole on X and Y
9) Drill
10) Repeat 1-9 with a cut-off drill bit of the same size (gives me better
depth at sides without the point going through - I reckon worth about half
to one thread)
11) Tap

I am not at all sure that this is the right procedure but cannot think of
any other on this machine. I do not seem to be getting consistent depths.

With the drill press and a credit card I can do this much more quickly and
dare say consistently.

Should there not be a better way to pre-set the depth/Z-distance travel
without going through this palaver? Is this amount of backlash on the Z-axis
reasonable? Can it be corrected?

Thanks,


I think you may be doomed to frustration with trying to rely on the Z
axis drive.

You may be able to improve things with a stripdown and some fettling,
but probably not. It's the price of small and cheap tools, esp. machine
tools.

Consider setting yourself up with a solid, clamp-on stop, that you can
use to set the depth that the drill can reach to, if there is not one
built in. Once the depth of cut is set by some trial and error, you can
then get on with cutting all the holes required. Or use a dial
indicator, or one of the cheap digital calipers, to build yourself a
crude "DRO" to use when moving the head up and down, and thus separate
the movement of the crank, from the movement of the head, if that makes
any sense.

Cheers
Trevor Jones