On 10 Nov 2010 04:39:20 GMT, "DoN. Nichols"
wrote:
[...]
Windows Live Mail has done it again. Not only did it swallow my
original post but also your reply. Fortunately I now have two other
news readers.
Hmm ... sounds like a tramming error -- the axis of the spindle
is not perpendicular to the bed. And likely the column axis as well.
Your drill is longer than the center drill, so you have to move the head
up the column. If it is not perfectly vertical, this will drift the
center line of the axis to the side -- and front to back as well. The
column on a lot of the small mills tilts, so it provides adjustment for
the side-to-side part. However, the fore and aft takes a bit more
tricky work to get straight -- using shim stock is the usual approach.
Get a machinist's square (a blade with a heavy right-angle
piece) and resting that on the bed, check for a slight angle between the
blade and a piece of precision ground rod in the chuck. (or better, in
the collet, as chucks can introduce errors too.) (Actually -- get *two*
machinist's squares, and check them against each other as well. They can
come out of square to start with.)
Is yours one of those whose column has a pivot at the bottom, so
you can drill holes at an angle? If so, have you checked that the
column is truly vertical?
That's the one. Last time I did this it was within 0.001" from
end-to-end and front-to-back. I will do this again - but first have to
clear things off the table. Oh, joy! The opportunity to re-align the
vise. Again :-)
[...]
I tried a different No. 1, I tried both ends, same result. Looking at the
slowly rotating point with a magnifying glass it describes a small circle
which is not obvious when I bring it down on the metal.
That is either runout in the chuck, or a bent arbor for the
chuck. I forget -- does your machine use R8 collets? If so, how are
you holding your drill chuck? An R8 arbor, or a straight shank arbor
held in a collet? In either case, the arbor might be slightly bent.
Before I read your post I checked the run-out just under the chuck. No
dowel pins or drill blanks here so I used the drill shanks. I did two
just incase. Both (3/8" and 9/64") showed TIR 0.0005" about 3/8" under
the chuck. BTW my spindle is MT3. I have never had to take the chuck
off so I am not sure how it is attached at the other end. I just
change the whole thing for a collet chuck when I need to.
I was out of town today and took the opportunity to look for dowel
pins etc. No luck.
However, there is
perceptible vibration of the mill which is absent if I drill with the twist
drill. I interpret this that the mill head is doing the circles while the
point is embedded. If I had a more rigid set-up the circle would perhaps be
apparent.
If you had a more rigid setup, the center drill would simply
make the hole a bit oversized as it swept around. And depending on the
orientation of the flutes to the eccentricity, it would either rub hard,
or cut oversized like a fly cutter.
I tried the same experiment with a No.2 and No.3 - same result.
Did you try a runout indicator on the shank of the drill? How
about with a longer rod (best would be a drill blank -- or an unusually
long dowel pin, but you might get away with some drill rod if it has not
been bent.
I shall go through the drawers and see if I have any unusually long
drills or anything else which might do the job.
I thought I'd better find out which is the true center: The "center drill"
or the "twist drill" one. This was even more complicated than I expected. I
used two centere finders on small punch marks. They both showed center
differently! The centre found by the barrel-type coincided with the center
drill point, the wiggler type was quite significantly off (I use 10x
magnifying glass to get the best accuracy with both).
I presume that you started with the wiggler a bit off center,
and pressed a finger against the side until it stopped wiggling? *That*
is what sets it on center.
Yes, but then you have to bring the point over the mark. I find that
challenging without the magnification. Even using the other center
finder (this one, so there is no confusion:
http://www.busybeetools.com/products...RE-FINDER.html )
the alignment of the two parts of the barrel are tricky and much
easier with a magnifying glass.
BTW I thought that the wiggler should be more accurate. However, it
was the other one that was consistently agreeing with the center drill
entry point.
[...]
I was wondering about the way the drills are clamped in the chuck and I
tried different degrees of tightening.
The proper way is to tighten equally using all three holes, one
after the other.
I would go round al of them 2-3 times. With the No. 1 the feeling is
of never really tightening enough. With, say, 1/4" drill you go around
and you know when the thing is tight. There is a definite "stop". With
the No. 1 there is a sort of springy feeling even after you have been
around 3 times. That is what made me wonder about the possible
distortion caused by the chuck. I thought this was supported by the
fact (yet to be confirmed) that if I tightened only lightly the center
drill run straighter (see the last effort where the center finder,
center drill and the twist drill all managed to coincide).
[...]
First get the runout indicator and check for runout on a ground
hardened rod (e.g. a drill blank or a dowel pin) both just where it
exits the chuck and down closer to the end. If it is better where it
exits the chuck, then the arbor may be bent. If it is equally off down
the length of the rod, it is likely a bur on one of the jaws in the
chuck.
Lots of things to check.
Tomorrow is a good time. After 11 AM...
Michael Koblic,
Campbell River, BC