Cutting a small slot?

I'm building a model engine that uses a dumb-bell shaped piston in the steam valve.

It's occurred to me that the valve linkages would be easier to assemble if there was a slot cut in the end of the dumb-bell to take the end of a small screwdriver. I could then screw the end of the valve rod into the connection to the eccentric by twirling the piston.

I can't cut a slot clean across the dumb-bell because the ends might align with the port and mess up the valve timing. How can I cut a slot like that shown in the picture that doesn't cross the full width of the piston? The piston is only 6mm in diameter, making the slot approximately 4mm by 1.5mm by 2mm deep.

Thanks,

Dave

Edited By SillyOldDuffer on 08/12/2017 18:21:23

If you don't need the slot to have square ends then easy enough with a 1.5mm dia milling cutter, could even come down to 1mm.

Other option would be a small dia slitting saw or single point tool and make a cut the goes almost all the way across with a concave bottom

J

PS that slot looks a lot less than 1.5mm wide if the end is 6.0mm dia

Edited By JasonB on 08/12/2017 18:28:21

Easy if you have a mill, just use a 1.5mm slot drill and leave the cutter radius in the ends. If you have the speed on a mill carbide cutters are cheap enough, some on this months MSC offer sheet.

'Lateral Thinking' time ...

Go for an existing recess [hex-socket; Torx; PoziDriv etc.] and make the item from a suitable bolt.

MichaelG.

If you don't want to mill it, two small holes and make a peg screwdriver.

Neil

I would go MichaelG's route and make it from a stainless hex socket bolt. But wait - this sounds like the ideal excuse for buying a rotary broach!! Or making one if you are into making your own tools.

Murray

Glue a small hex nut or the head of a suitable screw on to it.

bear..

Woodruff key cutter

or leave a square or rectangular male tab or tang sticking out of the end. If it must be a female slot +1 on the woodruff keyseat or two drilled holes and a pin spanner idea.

Course if you know anyone with an EDM it's a doddle to get a square ended slot.....my EDM is about 3000 miles from you, but if you were anywhere near Toronto Ontario Canada you'd be welcome to drop by and I would do it.

Can't see how you could make from a cap head or bolt as the bobbin is "dumbell shaped" how would you get the second large dia?

Edited By JasonB on 08/12/2017 19:41:41

.

Make it from a big grubscrew if you must.

... I couldn't judge all of the relevant dimensions.

The important suggestion was 'make from'.

MichaelG.

Perhaps use a countersunk-head stainless screw - the sockets in them are smaller than cap heads and may be smaller than the thread root diameter...

Ditto socket shoulder screw.

Murray

Wow I'm impressed. There I am completely stuck and the team come up with no less than eight suggestions!

1. Mill the slot with a small cutter
2. Cut a semicircular slot with a small slitting saw or a Woodruff cutter, or a Dremel cutting blade might do the job.
3. Use an existing bolt or shoulder screw to make the dumb-bell
4. Drill a pair of holes and make a peg screwdriver
5. Buy or make a Rotary Broach
6. Glue a nut on the end
7. Find a friend with EDM.
8. Do the slot in reverse by leaving a tab or tang sticking out the end

I'm entertaining the family today and will be able to ponder possibilities for tomorrow. At the moment I'm favouring #2 over #6 because steam might melt the glue. Even so I think I'll invest in some small milling cutters and the Hemmingway Broaching Kit as well. My only objection to #4 and #8 is the need to make a simple tool as well. Easy enough to do, but it's the sort of special I can't find when I need it.

Many Thanks,

Dave

I think the peg screwdriver would be the easiest route and probably quickest solution in the end. You don't even have to make the peg bit either, just slot across a wide enough screwdriver and space the holes to suit the resulting pegs.

Whether or not it satisfies the quest for the more technical solutions or not I don't know, but I am pretty sure JS would have given it his seal of approval while wearing his bodger's hat

Brian

Tick for the two holes and a peg driver.

Ian S C

I like the 'pigs nose' nickname for the peg spanner two hole solution.

Mike

Makes a change from my usual pig's ear approach

Neil

Forging ?? You could perhaps heat some oversize material and "bash" the slot into the end of the material with a screwdriver blade and a hammer. Once cooled, machine the piston to size.
BobH

Yes, they’re called spanner screws not “peg screws” although some yanks call them snake eyes for some reason.