Engraving and engine turning

Hello group,

Since things seem to be sort of quiet this week, and some of the recent
posts have touched on engraving, I want to talk about the project I want to
undertake. I want to set up a CNC mill that does engine turning and
engraving with a single point tool. I make clocks, and a number of clock
elements, particularly dials, are engraved with a repetitious wiggled line
pattern, cut with a fixed, shallow angle vee cutter. There are two types
of engine turning machine. One is called a rose engine, and it is a lathe
with a headstock on a pivoted arm so it is free to move back and forth. In
the part of the headstock where one would ordinarily expect drive pulleys,
there is a series of wavy edged cams, that depending on the feeler arm
engaged, produces different frequencies of wiggle patterns when the work
piece is faced with the cutter. There is a different machine, called a
straight line engine turning machine, in which the work piece is held in
place on a bed and the tool is moved by a linear cam, which come in a
series of patterns. Both of these machines are traditionally hand cranked,
and the depth of the tool is determined by hand pressure against a return
spring loaded slider. The depth of cut is determined by a burnisher type
"rubber" (It's steel, not rubber), so named because it rubs the work piece
in the next, uncut, position. This variable depth of cut is important,
because the flatness of the work piece may vary before cutting, and you all
know what happens when you cut the skin off of one side of a piece of metal.

Now here's the deal. A complicated dial may have a straight line center,
rose engine sectors or subsidiaries, as well as various knurled edges at
the edges of the engine turned fields, and in addition the numerals would
be hand engraved in the sectors. It involves lots of different setups, on
hard to get, expensive, and extremely crude, by modern standards, machines.
(Most rose engines have WOOD beds) It is extremely labor intensive. I
don't see any reason why the whole thing couldn't be done on a CNC mill
that was set up in a special way. The trick would be to have the spindle
be able to be rotated, controlled by a separate servo, and have the
software keep the spindle rotated so that the face of the tool was always
facing the direction of motion. In addition, there would need to be some
kind of indicator traveling in advance of the tool to indicate the depth of
cut. This would need to be rather accurate, as well as the motion that it
controlled, as the depth of cut on a .040 wide groove is only about 3 or 4
thou deep. I understand that the easy way to control the z-axis axis, from
the quill, is not particularly accurate, so I assume there would need to be
a servo on the knee.

This is comparable to a program for single point threading, where the
spindle needs to be rotated as the x and y-axis describe the circle to be
threaded, and the z-axis is in motion as well.

So, I guess my first question is about precedents. I know that machine
engraving with a rotating cutter is done on cnc mills all the time. Has
anyone done any fixed point "hand" engraving with computer control as I
have described?

I have a better than standard bought new low hours Oriental knee mill (BP
clone) with hardened ways and pretty good spindle bearings and .0005"
resolution Mitutoyo scales and DRO. Is it better to convert this or to
look for a mill with ball screws and servos in place, like a CNC with an
obsolete control? Any suggestions? Since there is at least a theoretical
way to make some money with this, or at least to have some serious fun, I
am not adverse to spending some money on the project.

Would the Linux/EMC system be adaptable to this? There would be at least
four axes, and to be able to index a rotary table or dividing head would be
a real advantage in some applications, for a fifth.

I am a clockmaker, with a pretty good machine shop, good CAD facilities
(Vellum/Mac) and a basic understanding of some of the principles involved.
I am no computer maven. How do I tell if I'm getting in over my head? I
would at the very least want to pay to have the ballscrews installed on my
mill, if I convert the existing one. I've had the table off this to move
it, and it's pretty intimidating to me.

I welcome all comments and suggestions.

A REALLY simple engine turning pattern can be seen on the edge of my #7
regulator uniblock, just basically a knurl, no wavy patterns:

http://www.arcadianclock.com/7/

David M. Munro