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Tapping plastic (repair of cookie making toy)

This story is in many ways typical. My son bought a toy at a garage sale for 50 cents.

This toy is a extrusion device, it has a cylinder, a piston and a threaded rod/shaft that pushes the piston through the cylinder. It extrudes cookies or cream, shaped according to hole patterns on metal ends. A battery powered electric gearmotor turns the threaded shaft, pushes the plunger/piston, and extrudes dough.

This toy was missing the piston. I made it last night from a cutting board. Metal related aspect was that I made it using a boring head on a Bridgeport. The piston is round, 55mm in diameter, 8mm hole, and two

4mm notches on opposing ends.

Anyway, closer to my question. The piston goes on the end of the threaded shaft, which is fitting snugly with the 8mm hole on the end of the shaft. Shaft gets thicker after that so it can push the piston with a "shoulder".

I believe that I also somehow need to fasten the plunger to the shaft so that it does not come off when it is pulled (as opposed to pushed) back.

So, my first thought is to just use a bolt or screw that goes axially into the shaft and holds the piston. My question is what kind of bolt/thread to use to hold in plastic.

i
Reply to
Ignoramus26157
Loading thread data ...

Coarse thread is good.

Reply to
Rex

Same bolt thread you'd use in metal. One extra advantage to those dense and somewhat soft plastics is you can drill the initial hole slightly undersized before tapping and get an effect similar to a "Nylock" nut.

Reply to
Pete C.

Not an answer to what you asked, but is the diameter of the shaft where is pushes into the "piston" large enough so you could drill and tap it and use a stainless screw and washer to keep the piston in place on the "backstroke"?

(With maybe some Loctite or something to make sure the screw doesn't back out and end up in a cookie, breaking some kid's tooth. )

Jeff

Reply to
Jeff Wisnia

Jeff, thanks for your interest and the same to others as well.

Yes, that diameter (8mm) is big enough for a screw like 10-32.

Good point. What I am mostly worried about is that the shaft is made of plastic that just might crack if tapped or if a screw "spreads it apart".

i
Reply to
Ignoramus20511

I agree with Jeff, I suspect if you attempt to rely only on a metal to plastic connection this will not work too well. I would make the screw to be a shoulder screw that snugs the plastic when tightened but bottoms out and is tightened against the shaft.

Reply to
Roger Shoaf

There I go, cought on the horns of an assumption again....

When I read your OP and saw the words "threaded shaft" all my "mind's eye" could show me was a metal shaft. Guess that's because I don't see too many threaded plastic shafts. I'm trying hard to think of one I have seen somewhere, and can't.

But my approach to solving your problem would probably still be ok, but skip the tapping operation an use a self tapping screw like the zillions which hold together all the plastic stuff we buy nowadays.

Jeff

Reply to
Jeff Wisnia

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