Re ATM Routing

["Archer Sully" <archer@meer.net>]

(Note: what follows are the observations of a guy who grew up in a
house where we played chess, go and bridge instead of building
things.  So there's probably nothing of interest except to others
who grew up playing chess, bridge and go ;-).

From: "Peter Jones" <pmjco@email.msn.com>
> Subject: Re ATM Routing
>
> I bought a router specifically for use on my 10 in Dob project. I
> experienced some difficulties with premature burning out of the HSS (high
> speed steel) which I reported to the list. The feeling was to use solid TC
> (Tungsten Carbide) cutters instead.
>

Absolutely.   The machinists at work tell me that TiCarbide is even
better (those are the gold-colord bits and cutters).

> To combat this I drilled an accurate perpendicular hole in the centre of a
> board and then routed half the depth from each side which I thought would
be
> OK - WRONG! I was left with a discernible step - I guess through cutter or
> bearing deflection. I removed the step by rotating the cut disc against a
> sanding disc held in a electric drill clamped in a workmate.
>

I didn't have too many problems once I did the following:

1.  Make sure that the center of curvature is firmly attached to the
work piece.   For upper rings, this was easy.  For altitude bearings,
this involved glueing a piece of wood to the work piece aligned with
the axis of the cut.  I have pictures still in the camera (film ;-(.  I'll
get them up when I can.  And, no, I don't call trunnions bearings ;-).

2.  I also used a 1/2" piece of plywood as a foundation.  I put a small
hole in it with a sleeve to accept the pivot pin.  This worked well, but
had to be cleaned regularly.  The pivot hole went all the way through
the work piece, and into the sleeve, rather than just using the work
piece itself as the pivot.

3.  My circle jig was made from some weird pieces of 1/4" Al I had
lying around from a surplus sale.  I think that they were used to hang
test equipment from the underside of bench hutches.   Again, pictures
when I can.  This jig gave me more stiffness and more accuracy than
I got from earlier attempts at all plywood jigs.

4.  Secure the work piece to maintain the relationship to the pivot
when the piece is flipped.  I used locating blocks and several clamps.
This worked well.  But I had to be very careful to measure and
place everything, including getting the locating blocks lined up
parallel to the axis of the cut.  This is very important, otherwise
you can end up with circles that are offset from each other, and
really awful stepping.  Maybe I'll post pictures of the piece I
did before I figured this point out  ;-).

5.  CUT SLOWLY!!!  When I tried to do as much as 1/3"-1/2" at a time,
I got burning and uneven cuts.  When I reduced the depth of the cut
to 1/4" or less, the cut was much more even, with no burning.  If your
jig is solid, and the work piece is well located, there should be little
or no stepping.  I'm reasonably happy with my rocker box.  Although
at this point I think I'm due for a new cutter, with all the wear it took
before I figured out the right way to do things  ;-).

> I don't know but I imagine that the moral is that 1/4 dia routers are
> suitable for light work only . I guess that there would have been less
> deflection of the cutter with a 1/2 router which also might have
> better/stronger bearings.
>

Yes.  But if you are careful, a good 1/4" router (mine is a small Makita)
with a good cutter can do the job, even on thick stock.

> I might try routing some 1/8 aluminium which I need to shape but I not
sure
> that 1/4 routers are man enough to do it.
>

I'd be leary.  A decent wood router spins at over 20,000 rpm (mine is
rated at 25,000).  I can imagine hearing damage at the least.  Better to
put good metal cutter in a variable speed drill press (yes, I know that
the bearings aren't designed to withstand lateral forces) and cut VERY
SLOWLY.   Or shop around for a used mill-drill, which is actually
designed for this job.  Or find a friend with a well equiped shop ;-).

Archer Sully
Boulder, CO