Most bolts are made by cold-heading and then thread rolled. A coil of
wire of the appropriate diameter is fed through from the back of a
collet type chuck which then grips tightly while a shaped heading
'hammer' whacks down onto the free end to hob the hex/philips/cheese
head form. The collet then releases and feeds the wire forward to the
required thread length, then a chisel type head comes in from the side
and shears it off.
The blanks are then rolled down an inclined channel lined with thread
chaser plates on each side which spin the shank and roll the thread
form onto it.
I saw this years ago in Birmingham when I was in the automotive
industry and was quite amazed by it, although I have to say that it
takes second place to the even more amazing automatic spring formers.
Don't know whether you could replicate this at home though, short of
turning a blank and then sparking the hex form in, or using a tiny
cutter and a dividing head to set the hex shape up.
Tony Phillips showed me a very interesting device for forming
hexagonal (or indeed square) blind holes in the SM&EE workshop at the
recent Midlands exhibition at the Fosse.
Held in the tailstock it consisted of a chuck free running on a thrust
race which was itself on a dovetail to adjust offset and mounted at a
slight angle to the axis of the tailstock so it pointed 'crossing the
axis'. In the chuck was a cutter whose cross section was hexagonal,
but tapering rearwards to give relief.
I didn't actually see it cutting, but as I understand it the work is
held in the headstock, drilled with a hole the a/f of the hexagon and
the tailstock device brought up and feed applied using the tailstock
feed wheel. The work rotates the free running chuck 'walking' the
cutter round the hole and cutting with a shaping action.
I made a mental note to get more details from Tony when he next comes
to Marshall House.
Many thanks for that - excellent description here:
That is the way I'd do it as I have a spark eroder but beware it will
set up stress risers due to minute cracking at the roots so the bolt
head won't be as strong as if it were 'cold headed'. The 'wobbly
broach' method has me intrigued, and certainly I'd like to try it !
I have no spark eroder, and have done a special this way: make a hex
punch or broach (from a bit of Allen key) and tap that into a round
hole, preferably using a staking tool to hold things in alignment. You
might need to drill or chisel out the chips from time to time.
BTW, I often work the other way round, using a caphead as a handy source
of high tensile material.
I made a one-off stainless steel cap head screw from a piece of rod by
heating it as hot as I could in a butane flame and ramming a suitable sized
alan key into it a few times, making sure to remove the alan key before the
metal cooled enough to grip it! Then I threaded the rod with a die - and it
On 27 Oct 2005 00:10:33 -0700, mangled firstname.lastname@example.org wrote:
David, there are three articles from 'Model Engineer' magazine,
written by Martin Cleeve, that deal with this issue.
Issues 02/Feb/56:200-202; 09/Feb/56:241-243; 16/Feb/56:271-273.
They are available online via one of the many Yahoo groups and the
tireless efforts of J W Early (unfortunately I can't remember which
I could email them to you if you wish (cut toenails to reply).
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