On Thu, 13 Nov 2008 18:33:07 -0800, Michael Koblic wrote:
[...]
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
[snip other 7 steps]

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?
....

(IIRC, your machine is somewhat like:
http://www.harborfreight.com/cpi/cta...emnumber=44991
)

I don't know for certain what the proper approach is for the
problem you describe, but here are a few observations that might
help until someone gives definitive answers.

-- Backlash will be less of an issue if you consistently pull down or
push up on the spindle head when you are using the fine Z dial to
lower the head. Otherwise, the spring counterbalance system lets
the head move or stick unpredictably in a fairly wide range.

-- Consider using a center-cutting end-mill from the start, rather
than a regular drill followed by a square-cut drill.

-- Use a collet or (preferably) an end-mill holder to hold the
end-mill; don't try to hold an end-mill in the chuck.

-- Attach a DTI or a digital caliper to indicate depth. Say you
want a 4mm or .157" deep hole in 5mm or .197" material. If you
zero the indicator when the tool tip touches the work, or set it
to -.001" when it touches a cigarette paper on the work, then
you will be at desired depth of cut when the indicator reads .157".

-- Make a 4mm thick shim. When the tool tip touches the work, set
the limit block (the Z stop plate) by putting the shim between the
limit block and the head. I think this will cut out a few steps
of your procedure, and should let you mill the holes without
engaging the find Z feed.

--
jiw