T-Slot cutter

I'm making a Chunn twin and need some special slot cutters to make the
space for the big-ends in the crankcase. Basically I need a 27mm dia
semicircular groove 4.75mm long but with the proviso that the neck of
the cutter will need to fit into an 8mm dia hole that will end up being
the crankshaft bearing. This means the neck of the slot cutter will
need to be around 7mm dia for about 15mm nearest the cutter head.
Casting material is aluminium.
There seems to be essentially zero chance of buying such a cutter and
as an alternative I see on Ishimura's excellent website at

there is a description of some home-made slot cutters and I have some
questions for the group about making some from silver steel :
Ishimura seems not to describe any relief on the back of the cutting
edges, though the photos do seem to show a amall amount, can anyone
tell me what the correct included angle of the cutting edge of each
tooth should be for aluminium ?
Should I provide some side clearance by facing the sides of the cutter
with the topslide over at say 5 degrees ?
Is it going to be possible to heat treat the finished cutter by holding
it over a domestic gas hob or won't that get hot enough ?
All other comments received with interest.
Many thanks,
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Hello Mike
There may be easier ways of doing this if I've understood it correctly. Why not just make a shank mounted disc cutter (like a flycutter) of the approprate diameter and mount a sideways-facing carbide tip or a bit of ground HSS to it? If you made a series of mounting faces at different depths around the perimeter this would allow the depth of cut adjustment.Or a home made old fashioned boring bar type cutter that you can manually adjust the depth of cut in steps? In either case you can make the shank fit the 8mm hole. I'm assuming here that you can't index the cutter sideways using the table traverse because of the restriction of the 8mm hole, and assuming that the 4.75 is the width of the groove rather than the depth? If I've completely misunderstood this and made an ass of myself let me know and I'll have a rethink!
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Peter Neill
Interesting bit of lateral thinking, thanks, a single-point tool never occurred to me.
Actually though, istm that by the time I've machined a stalk on a bit of bar of big enough diameter to take the toolbit and then drilled a couple of holes in it to take and secure the bit then I could have made up a slot cutter.
I misquoted the width of the groove it's 7.95 mm (forgot the width of the crank webs). But otherwise you've understood me I think.
In use a single-point tool is either going to need to be the plunge type or else will need to cut on both edges to allow it to be traversed. I think a slot cutter will be easier to use and I'm not sure it'll be much if any harder to make.
Thanks for the reply I may try that if a slot cutter turns out to be too hard, in the meantime though I'm still interested to know the answers to my other questions above.
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A disk turned out of silver steel with a bit of side clearance sounds fine to me. For a one off such as this I'd tend to knife and fork it and cut the teeth out with a hacksaw and file leaving just a small circular land that can be backed off with a smooth file. In alloy I'd go for 4 teeth, easy to mark and cut and you need plenty of chip clearance in soft metal.
May be touch and go with a gas hob to get hot enough, try burning the gas bill, mine should keep British Steel going for a month :-)
You want cherry red and quench into oil, it's not as fierce as water and I've found for soft metals you can do away with tempering.
If you can't get hot enough still go for it, remember you are only cutting soft material and a sharp edge and plenty of WS40 or paraffin will do this. Purists won't agree but as they never get off their arse's to do anything all they know is the sharp end of a printer ribbon.
Lick the 4 faces up on a grinding wheel if you can get in, if not stone then up to get a decent cutting edge and stone the top land.
That should get you going
-- Regards,
John Stevenson Nottingham, England.
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John Stevenson
hi mike, i have managed to succesfully make a dovetail cutter, th
procedure is basicaly the same, made from silver steel then brough upto dull/mid cherry red then quenched in ordinary vegatable oil, don sway the bit around just drop in the oil and leave for around 20 mins then give a final sharpen, you can see the tool i made on my websit at
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under "workshop tools" at the botto of the page.
-- blueswar ----------------------------------------------------------------------- blueswarf's Profile:
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Thanks all for the replies. I got a bit behind this weekend but i'll have a go next Friday.
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