using set screw or dowel pin/pin to hold on drill motor coupler thinggy

i was wondering what would be best to attatch the drill motor extension shaft to the motor shaft with (the design is similar to team 45’sa dual gear switching design)

should we pin it using a dowel pin, regular pin, set screw yuck or regular thru screw?


I know the technokats used to use a coupling to go around the drill motor shaft, and drill and tap through it, and screw in a #8-32 socket head screw. The coupling/extended shaft should be good enough to hold the drilled motor shaft together. And this screw should provide enough strength to hold the motor shaft and the extended shaft together.

That is not exactly the drill motor extension shaft the technokat’s have on their motor mount design, but the concept is the same. A pilot hole on the coupling/extension shaft will make the drilling much easier, because you know you are drilling the dead center.

It might be a little challenging to line up the pilot hole to be straight, but you can always just put a long pin with exact size as the hole, use the drill chuck to grab onto that, lock down the vice around that shaft and lock down the vice, and then pull away the pin and replace with a drill bit. Then you know you are drilling the exact center and the right direction.

If you don’t have the pilot hole on the coupling/extension shaft, I suggest you drill one first before you start drilling through motor output shaft.

And then it’s pretty tricky to tap through the drill motor shaft, just imagine how strong that metal is. Make sure you talk to a machinist about taping that hole, or consider using steel spring pins or steel dowel pin… Or else, you will break your tap in the drill motor shaft, and probably have to replace it. Use the tools suggested by Andy Baker in quote below.

A question to engineers: Is a steel 1/8" spring pin strong enough for this purpose?

Anyway, you know me and quoting. Here is quoting Andy Baker’s exact words:

"Here’s what we did, and by the way, this operation was made by a machinist, since no coach or student on our team has ever had sucess doing this operation without buggering it up. So, if you want to do this, get a professional’s help.

Put a 3/8" lock washer on the shaft, then thread the motor coupling on (yeah, the one provided by SPI). Screw it on until it bottoms out on the lock washer (lock washer holds it in place).

There is a 1/16" dia. pilot hole on the coupling… use a “spade drill” or a “carbide bit” to drill a pilot hole for a #8-32 screw, all the way through the coupling (this is hard to do).

Once your hole is drilled, use a titanium coated #8-32 tap… I think it’s called “titankote” (or something like that)… to tap all the way through the coupling, with the shaft included. Take your time as you tap… don’t break it or your gearbox is junk.

Then, put a #8-32 socket head cap screw in there, and voila, a killer coupling attachment.

No, the #8-32 screw is not too big, as long as you don’t drill your pilot hole too big."