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Square hole for cotter wedge

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smfr:
I need to make a rectangular hole in the connecting rods for my Muncaster engine, for the cotter wedges, and I'm trying to figure out the best way to make this hole:



There's always filing, but only part of the hole intersects with the cut-out portion of the rod, so there's an awful lot of material to file away. Another option would be to use a broach, but keyway broaches I think are too deep to fit into the pre-broached hole. I have to start here with a drilled 3/16" hole; I'm not sure I can even widen the hole, since there isn't room for two holes that don't run into eachother.

The best I could do would be be to drill the hole against the left face (in the image) before cutting the bearing opening (so the hole doesn't wander), then cut the bearing slot, then widen the hole towards the big end. I suppose I could then file those two corners square, which leaves me with enough space for a broach 7/32" deep, which is thinner than anything I can find in duMont or Hassay Savage catalogs.

Any ideas, other than making my own broach?

Simon

Jasonb:
You could use a square broach to do the part nearest the rod, fit a packer and broach again to get the rectange. I'd drill out then mill with a 3/16" slotdrill and then file.

This was discussed on ME forum a couple of days ago and one suggestion to ease the amount of filing is to drill away metal from the fork end going towards the rod, that way you on/y need to file the entry and exit holes to shape.

To make your own turn some drill rod to a long taper that is 3/16 at the small end and the accross corners size at the  other then cut a series of teeth along its length. Transfer to the mill and mill 4 tapered sides, het treat and drive it through a 3/16" hole

J

Dave Sohlstrom:
I'm having trouble seeing how your cotter is going to hold the bearing in place with nothing to push against. Seems that your cotter should be on the open end of the slot so the bearing halves get wedged tight against the closed end of the slot.

Dave

Jasonb:
I expect there is a strap to go round the outside

smfr:
There's a block at the end held in place with a pin (my model is wrong here; the pin just has a rounded head on one end, and I presume fits into a reamed hole through the conrod and the block, tightened down with a nut):



Jason, I thought about broaching one side, then turning the broach around. I'd be a little worried that a square broach will eat away at the sides though.

Simon

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