Re: Turning speeds re-visited

On 2008-12-08, Michael Koblic <mkoblic@xxxxxxxxxxxx> wrote:
DoN. Nichols wrote:
On 2008-12-06, Michael Koblic <mkoblic@xxxxxxxxxxxx> wrote:

If it is going all the way through, I would call it a
"diametrical" hole or "cross hole".

OK. Or diametrical cross hole, to be sure, to be sure :-)?

:-) I think that one or the other will do.

O.K. First off, a cylindrical surface really cries for a
center punch mark to start the drill.

Using the method described I just could not see how I could use a centre
punch. I was hoping that a small centre drill would take care of things...

Unless you have located the spindle over the axis it probably
won't. And with the tapered workpiece, it is likely to encourage
skidding. With a totally horizontal surface (flat) you don't have as
much chance of skidding off center, and the center dill will work quite
well.

The other advice -- very shallow milling on the top surface
gives you both a narrow flat, so things won't want to walk, and makes it
easier to judge the true center. But your tapered workpiece with the
ability to tilt under pressure is a problem anyway.

As for ways to locate the hole -- there are several, one of
which you described above. But you don't mention whether you always
fed the same direction to remove the backlash from the leadscrew.
Even a new machine will have some significant backlash, except
perhaps for a CNC machine with ball screws.

Oh, yes. Big time! On my Z it is a full revolution.

Ouch! (But then, you aren't measuring from the Z here.)

Anyway -- one way to find the center of a cylindrical surface is
to lay a small steel rule across it, and gently bring the (non
rotating) bit down to touch the rule. View it from the side so you
can judge the tilt of the rule. Relax the pressure and crank the
feed a little and try again -- until the rule is as level as you can
tell.

Nice! Will try. I got one of them Y-shaped gizmos but found it worse than
useless.

The same trick can be used on a lathe to check whether the
tailstock is offset or not. Trap it between two hardened centers, one
in the headstock and one in the tailstock, and an angle will tell you
which way to correct. (You may be wondering *why* the lathe has the
ability to offset the tailstock. This is one way of turning tapers,
between centers. Lathes with taper turning attachments are
significantly more expensive. But -- if you are doing multiple parts,
you need to make sure that the distance between centers is the same for
each one, or the taper will vary.

Another very good one requires a lathe.

NO!!!

:-)

Mount some of the same
stock in the lathe (best with a 4-jaw chuck and taking time to center
it properly). Then face across the end of the piece, put a center
drill in the tailstock chuck, and lightly start a hole. Once it is
started, shift to a drill bit of the size which you plan to use for
drilling the cross (diametrical) hole, and drill in perhaps a little
more than 1/4". Then part the last 1/4" off, yielding a disc the same
diameter as the workpiece.

Then -- but the workpiece in the vise axis horizontal, and put
the disc on top of it, axis vertical. When you tighten the vise's
jaws, the hole in the disc will be centered above the diameter of the
workpiece, leaving you only the need to adjust the position of the
whole along the length of the workpiece. Then adjust the position of
the vise until the bit will feed easily through the hole, turn on the
drill's motor, and drill. (This is presuming a truly cylindrical
workpiece, if it is tapered you will have all kinds of problems.

Wow! That re-defines "umstandlich"! But if it works...

I've not yet tried it, because the ruler technique works well
enough most of the time, but it is really a good way. If you are doing
a lot of the same kind of part, you can either drill undersized, ream to
just barely clear the drill bit and harden, or drill oversized to accept
hardened drill bushings -- so the sides of the drill don't enlarge the
hole as you work, introducing the possibility of more and more error.

That's one way to do it -- but you need a scriber with a really
skinny point, and to be careful that it is angled so the point is
closer to the edge than the upper body of the scribe point.

Pretty much. The fact that the two methods coincided makes me think there
was another reason for the miss.

Yes -- tilted surface and perhaps the spindle not at true right
angles to the table, so a longer drill will touch down at a different
spot than a stubby center drill.

I should for sure. I got one of them hermaphrodites which are very
good for this. The one problem with them is that they are not
screw-operated and it is hard to get them adjsuted particularly for
the small parts.

Hmm ... what brand?

Ehm, the $4.95 one. The only one in store...

Hmm ... what *store*? One which would stock Starrett tools, or
one which would stock only the cheaper tools?

>The ones that I have (two different sizes)
were made by Starrett, and have a knurled nut screwed onto a stud
sticking up form the flat surface about 1/4 of the way from the joint
to the tip.

Hmm ... look at this web site:

<http://catalog.Starrett.com/catalog/catalog/groups.asp?GroupID=441>

and check the middle image. That is the:

"Lock-Joint with fine adjustment" No. 42 series

That looks like the answer.

It is. I use them for a lot of things -- including scribing a
line parallel to an edge of the workpiece.

Mine does not have the round point for the scribing point, but rather
a flat one -- they've changed over the years I guess. Anyway -- the
large knurled nut at the hinge locks most of the caliper while the
small knurled nut 1/4 to 1/3 of the way down one leg adjusts the
position of the point for about 3/16" of travel or so, depending on
the size, so you can set it very precisely.

I see that Starrett also makes the ones without the fine
adjustment feature. You can have those. :-)

Mammaries on a bull come to mind...

Well -- I once saw an excellent answer to that sort of comment.
This was in alt.folklore.computers, IIRC, and a young poster (who had his
girlfriend in his dorm room at college, and was posting between
sessions) commented that it was nice to have two more erogenous zones. :-)

Don't get me wrong: I am sure I will want a boring head before long.
But for a .251": Boring head $70, 'F' drill bit $7. Not including
time.

And then a "'G' drill for another $7.00 -- when you discover
that the next '1/4"' rod is just a bit bigger than the previous. :-)

Point taken.

But still get that first spilt-point drill -- just so you can
see how much nicer it drills. :-)

Enjoy,
DoN.

--
Email: <dnichols@xxxxxxxxxxx> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
.

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