On 2010-11-12, wrote:
On 11 Nov 2010 05:50:14 GMT, "DoN. Nichols"
wrote:

BIG snip

So, here is the result of 3 hours' work:

1) I found a dowel pin. At least I am pretty sure it is a dowel pin.
It is pretty and shiny, 0.5" x 6", it has those black caps either end
and on it it says USA made, 1/2". I do not know how I got it.

2) In the chuck, using a different indicator from last time, the TIR
3/8" under the jaws, 2" and 4" respectively was 0.001", 0.003" and
0.006". Re-tightening made no difference.

O.K. The drill chuck, or its arbor is not true. Bent arbor, or
burred chuck jaw.

3) In a collet the figures were 0.001", 0.0015" and 0.002"

Better, but not perfect. How are your collets mounted? Since
you don't use R8, presumably you have a Morse taper shank socket and a
Morse taper holder for something like ER collets. (Or perhaps you are
using Morse taper collets?) I can't remember the details of your
machine at a distance like this. :-)

Anyway -- since there is runout with the collet, and still a
lack of parallelism to the spindle, check for burrs in the Morse taper
socket in the spindle. (Try spotting blue (Prussian blue) in a very
thin film on the Morse taper shank of the tool, then put it into the
socket lightly, twist a few degrees, and pull it back out. Check the
bluing for where it is rubbed off. If widely spread on one side, and in
a narrow location on the other side, then there is likely a burr in the
socket (which will need application of a Morse taper finishing reamer to
clean off). Or -- there *could* be a burr on the Morse taper arbor
instead -- patterns of blue built up around a clean spot could indicate
that, and require a little stoning to remove the burr. There could be
burrs on both the collet's arbor (assuming ER style collets) or the
individual collet (assuming a Morse taper collet).

4) Repeat test with the same 3/8" drill I used before the TIR under
jaws was 0.001"

5) I tried my 3/8" reamer. Under the jaws the TIR was the same but
2-1/4" down the shank 0.0115" !

Reamers (assuming a chucking reamer instead of one with a square
tap wrench drive on the end) have a soft shank which can bend, so you
can't trust that for indicating runout away from the chuck.

6) The No. 3 center drill runs out at 0.008"

Weird.

7) The No. 4 at 0.007"

Again weird. This is in the chuck, or in the collet?

8) I repeated the center finding tests. the two instruments varied
sometimes by as much as 0.02"

Still using the chuck -- or the collet?

9) As the punch mark have definite dimensions I tried just locating
crossed lines. This is quite difficult: Just because you can see the
lines on the bench does not mean you will see them clearly under the
spindle, paint etc. notwithstanding. The results, confirmed with
center drilling , were considerably worse than center-finding a punch
mark, however small.

One of the little pocket sized gooseneck LED illuminators is
nice for dealing with this. It has a magnetic base so you can stick it
nearby and bend the neck so it illuminates the cross scribe.

10) In view of the discussion spotting drills vs. center drills I
thought it made more sense to abandon using No.1 and No.2 and I used
No. 3 instead to make a small dimple with the pilot only. This should
approximate a starter hole produced by a spotting drill. This was a
definite improvement and the difference between the center drill hole
and subsequent 1/8" twist drill position was only 0.005" on the Y axis
only.

O.K. but note that you will have a chisel point on the tip of
the center drill (like on the normal jobber's length drill bits), while
the spotting drill comes to a sharp point.

It should be noted that I used my machine screw length drills so the
actual distance from the chuck jaws to the point was pretty much the
same for both the center drill and the twist drill.

O.K. The machine screw length drills may have split points, so
they can start more accurately even without a dimple or a spotting
drill.

11) I cleared the table and returned to the dowel pin in the collet. I
used the machinist square and could not see any deviation at all. The
two squares I have pretty much agreed with each other. Note I did this
along the X axis with the spindle in 4 different positions. The table
is too small to do this effectively along the Y axis.

O.K. The axis is pretty perpendicular to the table, then. Did
you have a light behind the square and pin?

12) Tramming:

a) The machine was out of tram to the tune of 0.008" over the X axis.
This was quickly corrected to 0.0005" with the head low. With the
head high this increased to 0.0035".
b) Moving the table along the Y axis produced no significant change. I
interpret this as the table being flat.
c) A spindle sweep in an arc from back to front, however, showed a
change of 0.005" over the 2" available in the Y axis. I interpret this
as the axis of the spindle being off in the Y axis plane.

Yes. And this can cause a drift in Y-axis position with
different length drills or mills.

Conclusions:

1) Do not use digital indicators for this sort of work.

You had been using a digital one? I don't remember you saying
this. *Some* digital ones are more accurate than some mechanical ones.
The Starrett "Last Word" mechanical if the bias spring is not applying
bias throughout the range. (A bump on the side of the point can cause
it to skip in the spiral, so there is a major deadband in the range.)

2) Change the center drilling procedure as outlined above.
3) Grind a spotting drill?

How about *buy* at least one, so you know what it needs to look
like? I recently got (from a sale flyer from MSC) a couple of 1/4" x 60
degree spotting/centering drills (MSC #FJ71332167). If the sale from
that flyer is over, the "FJ" won't do any good, and the price will be
back up to whatever it was.)

4) To locate cross lines on a work piece it seems preferable to use
optical punch first and then locate the center on the machine. This
probably makes center finder and spotting drill unnecessary and can be
done with a twist drill directly.

Probably -- especially with a drill with split points. For
larger standard bits, the chisel point may be wider than the dimple left
with the optical punch, in which case the drill is likely to walk away
from the punched center point.

5) Consider acquiring better center drills.

Consider acquiring at least one or two proper spotting drills,
so you can compare them with what else you have and what you make later.

6) Use collets whenever possible.

Yes!

7) Ideally one should try to correct the spindle axis. From what I
read of others' experience this is very difficult with this machine.

It is certainly a source of error.

Good Luck,
DoN.

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