Cutting a small slot?

[SillyOldDufferModerator @sillyoldduffer]

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

[SillyOldDufferModerator @sillyoldduffer]

[JasonBModerator @jasonb]

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

[Chris Evans 6Participant @chrisevans6]

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.

[Michael GilliganParticipant @michaelgilligan61133]

'Lateral Thinking' time …

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

MichaelG.

[Neil WyattModerator @neilwyatt]

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

Neil

[MuzzerParticipant @muzzer]

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

[Grizzly bearParticipant @grizzlybear]

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

bear..

[vintagengineerParticipant @vintagengineer]

Woodruff key cutter

[Jeff DaymanParticipant @jeffdayman43397]

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.

[JasonBModerator @jasonb]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 08/12/2017 19:41:23:

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?

.

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.

[Neil WyattModerator @neilwyatt]

Posted by JasonB on 08/12/2017 19:41:23:

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

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…

[MuzzerParticipant @muzzer]

Ditto socket shoulder screw.

Murray

[SillyOldDufferModerator @sillyoldduffer]

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

[Brian WoodParticipant @brianwood45127]

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

[Ian S CParticipant @iansc]

Tick for the two holes and a peg driver.

Ian S C

[Mike PooleParticipant @mikepoole82104]

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

Mike

[Neil WyattModerator @neilwyatt]

Posted by Mike Poole on 09/12/2017 12:48:38:

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

[Speedy Builder5Participant @speedybuilder5]

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

[VicParticipant @vic]

Posted by Mike Poole on 09/12/2017 12:48:38:

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

Mike

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

[Howard LewisParticipant @howardlewis46836]

Daft suggestion?

Turn valve head undersize.

Mill Slot

Press/ Loctite on a ring to bring O D back to original diameter.

Howard

[MeunierParticipant @meunier]

Posted by Vic on 09/12/2017 15:43:56:

Posted by Mike Poole on 09/12/2017 12:48:38:

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

Mike

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

I think because the two holes resemble the "2" side of a dice when thrown and have heard 'snake-eyes' called (on film)
DaveD

Edited By Meunier on 09/12/2017 21:48:28

[SillyOldDufferModerator @sillyoldduffer]

Just in case anyone's interested I set to, drilled holes in the piston head and made a pin spanner to fit. Works very well.

Much to my amazement the engine ran first time I connected air. That's a first! Still looks a mess though; I'm having real trouble getting parts to look good as well as function properly. Looks like there's still more learning to do. Gawd knows what will happen if I try painting it.

Thanks again for all the advice. I'm going to try some of the other methods later.

Dave

Edited By SillyOldDuffer on 10/12/2017 19:16:23

[Author]

Posts