Hello! This started life as part of a different project, but the hex hole cutting was off on a massive
tangent, so I’ve separated it out as a stand alone
film. The long and short of it was that I arbitrarily
decided I wanted a 4mm hex recess in the end of a
piece of rod even though a simple screwdriver slot would
probably done the job. Without really thinking it through, I set off making a broach, using some 8mm silver
steel (drill rod) and a hex collet block. Now my hex will be 4mm across flats, which
will be 4.62mm across the corners I began by turning off some of the excess
material, leaving it oversize at 5mm I made this little saddle for the vice, to angle my collect block to give a some relief
on the cutting edge. I just did this by eye probably somewhere between two and three degrees It took a few goes to work the bugs out of
this I tried horizontal, gripping the flats of
the collect block and taking cuts across the job That didn’t work very well I also tried having the block stood up vertically But that wasn’t very successful either The best accuracy and repeatability was with the block horizontal, barely gripping the corners This probably needs a smaller collect block
or a bigger vice! This time, cuts were made by advancing the
cutter along the axis of the rod. I took multiple passes. Rotating the job after each cut and then dropping the mill head, and going round
again. Take my advice: Rushing, by taking too much off at once, will
spoil it At this point the broach is still a little
oversize but because the tool tapers the across flats dimension of the tip can be reduced by making it shorter I’m going to use a ball ended milling cutter
to leave the end of the broach concave At first I used a small cutter but it left it too sharp with too little material leaving it weak A larger cutter leaves more meat at the end which is less prone to breaking I used the lathe to take a fraction at a time
off the broach until the end cross section was about 0.05mm
oversize I’d suggest erring on the size of a leaving
it a little bit big. A slightly loose fit on the allen key will
be preferable to being too small to fit, and then having to start over. My plan was to hold the broach in position
with a support tube add the pre-drilled shaft I wanted the hex
hole in then put them in the vice and squash them
together It was only at this point that I thought about
the actual dimensions Having spent more time and effort than I’d
planned making the cutter, I didn’t want to break it first go! Across flats the hole needed to be 4mm so that’s
the hole I’d drill. A hexagon centred on this hole will be 4.62mm
across it’s largest dimension That means the broach would have to cut almost a third of a millimetre of material in one pass. And that’s on each corner! A total of 1.86mm It might work in aluminium or plastic but wanted to be able to cut brass or even steel Even if the cutter survived being pushed in, there was a good chance it would break off when I tried to pull it out again. And I might get away with it once, but I wanted
to use it on at least five separate parts Over to Google for some advice… I came across Mikes Workshop, and his excellent
article on rotary broaching. It seemed quite by accident I’d already
made a cutter more or less exactly to his specification So it seemed sensible to adopt the rest of
his method. There’s a link in the text to his page and
his video Instead of just forcing the broach directly
in, it is allowed to rotate and pushed into the
spinning shaft at an angle. This means only a small part of the cutter
makes contact at a time. As I didn’t have any steel large enough so I cut a lump off some 2” square aluminium and used the four jaw to clean up the sawn
face Next I drilled 10mm hole through the centre so it would fit over my tool post stud After swapping the four jaw for the three jaw I used the lathe itself to drill into the
side of the block so it would necessarily be at centre height. It took several passes, and a swap to Morse Taper for the larger sizes. Over on the mill, one side of the block was trimmed at a one
degree angle to give me an alignment edge when setting
the tool. Back on the lathe, I drilled out a steel sleeve, a bit at a time, to 5/16 of an inch (7.94mm) Then reamed it to 8mm One end was tapped M10 to accept a cap head
screw. The cap head was drilled to accept a ball
bearing which was made captive with a centre punch. Parting off is usually a disaster for me, but this just goes to show it is sometimes
possible with a mini lathe, even in steel! A test fit with a piece of silver steel showed
everything looked ok Finally the sleeve was glued into place with
Loctite. I added a screw and spring to make the broach captive in use. This is brass so it won’t marr the broach. Any burs made inside the sleeve will make
it very difficult to remove. A recess was cut in the shank to match the position of the brass screw Next the broach was hardened I’m using Clickspring’s boric acid method to reduce the amount of oxidation on the part A cage was formed from steel wire, the cutter placed inside and then covered
in boric acid paste The part was heated to red Notice the green flame from the boron in the
boric acid paste? It was at this point I realised I was running
out of gas Then the part was quenched in water The broach was tempered in the oven for an
hour, at 225 centigrade Once cooled, it was given a quick polish in
the lathe to ensure a smooth action in the holder Here I’m preparing a brass rod for a test
cut After centre drilling, a shallow 4mm hole is drilled Then it’s countersunk to slightly wider
than the tip of the broach A drop of oil and the cutter is driven home Not a bad result 🙂 Whilst trying to broach steel the cutter failed. Probably a combination of too small a pilot
hole, too much cutting pressure and possibly too
much hardness in the thin neck of the tool. It took about three hours to get the first
useable cutter, but the second one only took about 40 minutes. This one was tempered to straw by heating
the shank, rather than in the oven. Considerably quicker and hopefully concentrated
the hardness at the tip, with a little more given further down. All told, an interesting side track. And that’s all for now. Thanks for watching.