Re: Turning speeds re-visited

On 2008-12-06, Michael Koblic <mkoblic@xxxxxxxxxxxx> wrote:
Wild_Bill wrote:
The discussion went from turning the diameter of a 4" workpiece in a
lathe to the suggestion of utilizing a rotary table with a small mill.

Actually I was puzzling about the speed ranges of the various lathes
available and things went on from there.

Basically, the speed range is determined by the maximum swing
(the maximum diameter which can be rotated over the bed). It has to be
slow enough to produce a SFM (Surface Feet per Minute) rate within
reason for turning the workpiece material with HSS (High Speed Steel).
So this determines what is a reasonable minimum speed.

Maximum speed is determined by the smallest diameter workpiece
likely to be used, and perhaps boosted by using carbide instead of HSS
as a tool material.

As a result, large lathes tend to start with very slow speeds,
and go up to speeds which are scary with the size of chuck which they
will handle. (My Clausing will go up to 1600 RPM, which is quite scary
with a 10" 4-jaw chuck.) It goes down to 35 RPM.

A secondary factor making slow speeds very useful is when
single-point threading to a shoulder. While your reaction time can be
improved with training and experience, it is safer to start with a very
low speed with anything but the finest threads. (Perhaps 20 TPI on down
to 224 TPI or so you can use higher speeds, but when you plan to cut a 4
TPI thread, and are turning towards a shoulder, you really need to be
able to disengage the half nuts before the cutter crashes into the
shoulder.

A lot of the small import lathes do not offer speeds low enough
to handle coarse threading unless you already have good reaction times.

IMO, a 6" RT is too big for use with the mini mill you have. I have a
6" Phase II horizontal model that isn't particularly easy to mount to
a 6" x 16" table on a 12x20 3in1 combo machine.
Cranking a RT isn't fun either, so I added a small worm drive motor
to it, for making disks without center holes and/or large holes.
FWIW, the base of a typical RT is larger than 6", mine overhangs the
6" table I mentioned.

I suspect a 4" RT will overhang mine.

If it is mounted directly to the bed's T-slots, yes. If you
make a mounting plate, you can handle a larger one.

BTW How much space is there between the vertical dovetail on the column
and the back of the table with the table cranked as far back as it can
go? This can determine what overhang you can tolerate.

If one wanted to side-mill the circumference of a disk using a RT,
securing the disk to the RT becomes an obvious step. If there are no
holes in the disk (that can be used to mount the workpiece to the
RT), it's likely that some shop-made accessory will be required.
I've seen an example of a rigid truss-like bar over the workpiece
with a vertical screw/thrust bearing clamping device which acts like
a lathe tailstock pressing a disk up against a chuck or faceplate.
The truss-like bar needs to be secured to the machine table, elevated
over the workpiece on the RT.
A truss like this could allow some features to be machined on the
face of the disk too, just not near the center.

I was thinking a 3-jaw chuck - most of my plates have a hole in the middle.

A 3-jaw chuck can have the jaws expanded inside such a hole to
grip while turning the OD -- but the trick is how to make that hole in
the first place. If you use your older methods to turn the OD close to
size, you can then use reversed jaws on the 3-jaw chuck to grip it by
the OD while you machine the ID on the mill. A lathe would be better
for this.

The indexer type head you mention (no handwheel) is primarily used for
repeatability of indexing to even/odd locations of a revolution.. for
example, 2 opposed flats on a round shaft, 4 flats, hex, or sprocket
teeth, well you get the picture.
The rapid indexers I've seen generally use power to advance the
rotation to the next indexed position, as an automated setup, and
some are quickly advanced by the stroke of a pneumatic cylinder.

For milling in general, bigger (larger diameter) cutting tools
require more motor HP.
At some point, a small motor just can't make a large cutting tool cut
material.

And -- with the plastic gears, at some point a larger cutting
tool will result in striping the gears.

Get spare plastic gears *before* you need them or you may be out
of service for some time while they are ordered.

The problem I have with the plates is two-fold:
1) The edges. I can handle those quite well now by other means.
2) The faces - that is more tricky and a proper facing procedure with a
lathe would be useful and improve the product beyond what I am getting now.
That is the part I cannot see done on a RT. I was thinking a flycutter 360
degrees but I am told the rate of rotation would have to be very regular
else it will show in the surface finish.

O.K.

1) You can't use a fly cutter on one which is held by a 3-jaw
(or 4-jaw) chuck without risking cutting the jaws. (Unless you
set up a chuck with soft jaws, and mill a step shallower than
the thickness of the workpiece so you can access the entire
surface at once.

2) Using a fly cutter typically will show an artifact in the
finish a short distance from the edge as the cutter "rings" from
the impact as the cutter moves from air to cutting metal.

3) I would consider using a smaller cutter and making multiple
passes at decreasing radii until the entire surface is cut to
the desired thickness -- and then try the finish called "engine
turning" -- an abrasive embedded in the end of a dowel, or an
abrasive rubber spin in the chuck, brought down onto the
workpiece surface (making a series of concentric rings) then
lifted, and the table is rotated about half the diameter of the
rod and it is brought down again. Repeat until you have a
complete circle, then move in by perhaps 3/4 the diameter of the
rod and repeat. It can make a high-tech *looking* finish,
although you can do it with only a drill press and a rotary
table.

I don't think that you will be able to get a better finish than
that with a mill and a rotary table.

For reading dials, I've found it most important to know how much
metal is removed per tick on the dial (regardless of what the tick
supposedly represents). I can see 1/8" much more easily than I can
see thousandths of an inch. For an precision dimension, pausing to
take more actual measurements reduces errors. If it's an important
dimension, I'll sneak up on the last .001" more slowly/cautiously.

I was trying to determine a centre of a cylinder to drill a radial hole. I
was walking up to it form one edge. It seems that I screwed that up too -
off by 20/1000"!

Every time I see you display thousandths of an inch in fractional
form I get slowed down in my reading. The common way in machining to
show such dimensions is as a decimal fraction with a leading "0" if
there is no integer part -- so that would come up as '0.020"'. Among
other things which slow me down is having to make sure that there are
only three zeros after that one.

The leading '0' before a decimal point (if there is no integer
part) is to make it more difficult to miss that there is a decimal point
there at all.

You mentioned "howlers" in another article. This is not exactly
a "howler", but it is awkward at best.

Fractional notation is normally used for powers-of-two
fractions: 1/2", 1/4", 1/8", 1/16", 1/32", 1/64", 1/128" and multiples
of those. 1/128" is the finest that I have ever seen used in machining,
and that only with very old tools. And of course, since fractions are
more difficult to work with (just listen to the Metric advocates),
decimal fractions are preferred -- especially since the micrometers and
modern calipers read in decimal fractions. (Some recent digital
calipers have taken a step back -- approximating a reading as the
nearest fractional size as an option.)


I just enjoy converting good metal into chips, just for the pleasure
of it. I'm not required to achieve any high levels of precision, some
times it matters, most times it doesn't.

I agree, but there is a certain need to do things as well as possible given
the circumstances. If a machine will not cut to tolerances, well, so be it.
Live with it or buy a better one. If I the result is poor because I screw up
the math, that is less acceptable as it is totally avoidable.

If the machine will not cut to tolerances may *require* buying a
better one -- depending on how necessary the tolerances are. For your
project making sundials -- no real problem I think. For someone making
parts to repair or built a machine -- if it won't cut to tolerance, the
part you make is just so much scrap metal.

Good Luck,
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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