on requeston requestFor some time now I have wanted to make a set of broaches but like everything else that's not essential it remained as an idea for several years. For the last twelve months my workshop activities have been curtailed due to illness and so I spent some computer time designing a broaching tool with the hope that it would be a good project to re-kindle my desire to get back into the workshop.
The tool was designed to exploit the use of the lathe and mill and also
present a bit of a challenge. The basic requirements were that the broaches
(also to be home made) would be 'pulled' rather than pressed. Pressing delicate
broaches is a precarious affair and the result is more often than not a snapped
broach and a ruined job. As the broaches take some time to make I decided the
best approach would be to pull the broach through the work piece as this would
present the least amount of strain on the broach and unlike pressing,
the broach would be unlikely to 'bend'.
With this in mind the basic requirements for a broaching tool were sketched
out. The tool would pull the broach using a guide to prevent 'bending' and to
make things a bit more interesting the mechanism for the 'pull' movement would
be a square thread screw. The majority of the build is made up from hot rolled
50mm x 15mm.
In all the time I have owned a lathe I have never made a square thread of
any length and to go with it the need for a close fitting nut! The latter is
more of a problem and with hindsight I should have made a tap to finish the nut
thread, but I decided that as a challenge I could make it without a tap based
on the belief that working to close tolerances the two parts should fit
together easily. That is the theory, in practice the fitting was difficult to
say the least but eventually it mated and resulted in virtually no play. To
make things a little easier I made the nut from aluminium which was easy to cut
the internal thread and when it came to fitting I used a small amount of
lapping compound to get to the final size. The feed screw which was a little
over 7 inches long and 0.600" diameter with a 0.050" square thread
proved to be a time consuming job. The amount of 'cut' for each pass was around
a few thou' each time. As the cutting tool was also 0.050" wide it was
quite delicate and could have easily broken so for that reason I kept to the
few thou' in-feed. I might add that this approach was learned the hard way. The
first time I tried a 0.005" feed, the tool lost it's edge and then jammed!
Next thing, two gears in the lathe gearbox stripped! The gearbox was one I had
made some 15 years ago and the gears were standard Myford gears reduced to
0.3125" thickness. This had served me well for 15 years but as the gears
were made from cast iron and 20dp and 14.5 degree pressure
angle the tooth thickness was easily
overcome by my mishap! The job was put on hold and I sent of for a gear cutter
to make two more 25t gears only this time from mild steel.
The gears made and the gearbox back in action the second attempt at the feed screw was started only this time a gentle cut was made and eventually I was very pleased with the finished thread. The nut was made from aluminium using the same cutting tool so in practice it should all fit together. As I mentioned before, next time I shall make a tap to finish the nut thread to size.
To prove the tool I made a broach to form a 0.25" square hole. The broach would cut about 0.002" per edge. The broach was 'loaded' into the 0.25" thick work piece which had a 0.25" diameter hole and the feed screw rotated to start the 'pull'. I was surprised just how much effort was needed to pull the broach through the work piece but as it progressed I could see the slivers of chips coming off and soon the broach was at the end where it was at finished size.
On inspection the hole in the
work piece had been successfully transformed into a square hole and it was
exactly to size and each corner was sharp at 90 degrees. I was over the moon,
not only had the tool worked but the resultant job was exactly to size. I
cursed the fact that I had delayed making this tool for so long and previously
made round holes square by filing! This filing method did not produce a good
job and even after much effort the finished hole was not something to be proud
of. Working in brass was always a problem when it came to filing but my new
broaching tool made it all seem so easy and required no skill in use.
Next job is to make a full set of broaches from silver steel. I also plan to make broaches for finishing round holes to size, these are easy broaches to make. After that comes a broach for cutting key ways in gear teeth which will come in very handy if I manage to destroy yet more gearbox gears!
Update - A set of broaches to produce square holes has now been made giving 1/8", 5/32", 3/16" & 1/4". All of these were made from silver steel and hardened. In addition to the square broaches I have since made a broach to cut a 1/8" key way slot in a gear wheel. The first attempt making this was a disaster as the silver steel was badly deformed on hardening. Having read that a commercial firm is now making broaches from steel and case hardening I decided to give this a go. Using hot rolled base steel the cutting teeth were filed by hand and the broach case hardened. There was no discernible deformation and it performed equally as well as the silver steel variety. I think this may now become the preferred material for other than round broaches.
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