Re: Turning speeds re-visited

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 :-)?

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...

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.

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.

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...

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.

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...

>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.

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...

I
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.

--
Michael Koblic,
Campbell River, BC


.

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