Toolholder unseating in the Dickson style toolpost
 

In the continuing saga of repairing the Clausing 5914, tightening
dovetails, et al, another chapter:

I am in the process of making the pivot axis upon which the
Dickson-style toolpost will revolve (unless prevented by insertion of
the 7mm diameter anti-rotation pin). This pivot axis is a thick steel
cylinder 1.260" in diameter with a 11/32 hole down the middle (clearance
for the 5/8-18 threaded rod).

The current operation is to make a 1.263" diameter recess about 0.125"
deep in the ridged spacer plate between tool post slide on the 5914 and
the bottom of the toolpost. Never having made such a recess before, I
started with a 3x3" piece of 0.5" thick scrap hot-rolled steel in the
4-jaw chuck.

First step was to drill a 11/32" hole in the center, using drill bits
held in the tailstock. After tightening the tailstock's bed clamp so
the tailstock wouldn't slide away from the headstock under drilling
pressure, no problem.

Now for some boring, with tool held in the Dickson toolpost. I first
used a stout boring bar with a 0.5" diameter round shank, held in a
toolholder with a V-groove intended for holding round shanks. The
boring bar was 3.25" overall length, with about 2.25" sticking out, with
a brazed carbide insert tip. This kind-of worked, but left a very
rough, chewed surface.

Then I noticed something odd. When the tip of the bar hit the bottom of
the recess, the whole holder+bar tilted, with the holder unseating
visibly from the toolpost, and returning to normal when the tool was
withdrawn, all with no apparent damage to anything no matter how many
times this was tried. Nor did the crosslide or toolpost tilt. I had
cranked the holder down to the toolpost quite strongly, and I was quite
perplexed. Then I noticed that the clamp bolt was hitting its stop,
rather than exerting full force on the toolholder, so the holder was not
necessarily held all that strongly to the toolpost.

Hmm. Maybe the clamp bolt is bent. Or something. Let's use a
different face of the toolpost, and thus a different clamp bolt. Done.
Made no difference whatsoever. Hmm. The holder is awfully high on the
toolpost, with part of the holder sticking out above the top of the
holder. How can a little overhang matter so much? It does seem to
matter, as the same holder held lower doesn't cause the clamp bolt to
hit the stop. No idea why. Disassembly required; later.

Changed to an ordinary 3/8" shank brazed-carbide bit held in the same
holders. The stick-out is about 0.75", and the holder is lower in the
toolpost. This worked, yielded a smooth machined surface, and no
tilting when the bottom of the recess is machined.

The cutting edge in the boring bar is about 0.25" above the bottom of
the tool slot in the toolholder, while the cutting edge in the 3/8"
lathe bit is 0.375" above, a gain o 0.125". What a difference. Could
not use a larger lathe tool bit without the heel of the bit skating on
the walls of the recess.

It would appear that the Dickson toolpost needs to be higher than it now
is, with the current 0.7" spacer made perhaps 0.25" thicker. I'm
assuming that the clamp bolts are not in fact bent, because they would
all have to be bent the same.

Nor do I like the damage inflicted by toolholder on the boring bar
shank. I think I will want a holder with a tight-fitting hole that
clamps the shank without setscrews clamping directly upon the shank.

Joe Gwinn