On 2016-12-01, Bob La Londe wrote:
Now I know the easiest way to do what I want is with a push broach and make
them as two pieces, but I want to make some specialty hex drivers with a one
piece body. That leaves me with rotary broaching. The thing is I am not
going to make a bunch of them so buying a rotary broach holder or spending a
day making one isn't all that wise of use of my time or money. However once
I was setup to do it rotary broach inserts are a lot cheaper than push
broaches.
Yes. Recent experience -- in MSC, a Dumont 1/4" square push broach
is $101.??, and a 1/4" rotary broach is about $60.00.
Recently I had need to do some cross drilling that would have been easiest
with a preset cross drill on the lathe. Its slow and tedious to center up
and drill the part on the mill, but of course I can do it. I don't have to
do it often so again making a cross drill for the lathe isn't good use of my
time.
Hmmm ... for cross drilling -- have you ever seen the tailstock
tool for the purpose? A Morse-taper mounted round pad with a V-groove
across it. Just slap the workpiece (assuming cylindrical) into the V,
and with a drill chuck in the headstock spindle, drill away. I've seen
them in old _How to use a Lathe_ books. Either the Atlas/Craftsman one,
or the South Bend -- if not both.
I would actually like to have one of those myself.
As for centering up on a mill -- the same trick as used for
aligning tailstock and headstock centers can be used. A piece of thin
material -- a 6" pocket scale, or a razor blade, and you can tell which
side is getting the center of the bit by which way the thin piece tilts.
Of course -- another trick which I have read about, but not yet
tried, is to drill the center of the round stock in the lathe with the
size bit desired, then part it off, and when dross-drilling time comes,
put the round stock in a vise with the round parted off part on top.
Use the hole in the parted off piece as a drill bushing to get the bit
truly on center.
Then I had an aha moment. Make it fairly robust and use it for rotary
broaching as well. Centering would be a little tedious, but getting my 1
degree back angle from center would be dead easy to do after that. Then
lock down all the gibs not in use and advance the carriage. I doubt it
would ever work on the mini lathe or the small lathe, but maybe the PM1440
could handle it.
Likely -- ideally with the compound removed and the tool bolted
directly to the cross-slide. This eliminates several locations for
play.
If I can use the same tool for both applications it almost makes it worht
making one. I might still know where to find it by the next time I need it.
Driven with hand held drill when cross drilling and of course driven by the
stock when rotary broaching.
I don't think I've ever cross tapped stock in the lathe, but if I don't see
why I could not add enough functionality to do that too if I need to.
For the drilling and threading, you would want the spindle
locked as you are describing it. Or -- if using a variant of the V-pad
in the tailstock, you want a releasing tap holder in the lathe chuck or
collet so you don't tap too deep.
So what's wrong with it? I know rotary broaching takes a lot of power. The
lathe is big, but its only 3HP. Flex in the compound, cross slide, and
carriage could be an issue. If a spacer is used internally in the cross
dril that rides on a thrust bearing I think the drill itself could take it.
I also considered bronze bushing with a 1/2 shaft collet chuck because the
bronze can take so much force.
I have no experience with the rotary broaching (though perhaps I
should get that). However, I just did some broaching in A2 drill rod
using a 1/4 square broach. After making a fixture to hold the drill rod
vertical and setting it up in a 3-ton arbor press (and broaching a hole
which was 3/4" long) I got about half way through before I could not put
enough weight on the lever (my 3-ton does not have the ratcheting lever
arm -- and one of my own arms is still recovering from being broken a
couple of years ago), so I had to move it to the 20-Ton Harbor Freight
Hydraulic press to finish. This with Molybdmium DiSulfide grease as a
lube.
So -- if the rotary broach takes more force, I don't want to
play. :-)
Enjoy,
DoN.
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