Originally Posted by
longbow
I usually cut my push out moulds slightly small then lap to get the final size I want. If I use a full nose form rather than an ejector pin, I lap the whole cavity spending slightly more time "out" than "in" to produce a very slight taper of 0.0005" or so for easy ejection.
Just cast a slug, drill a hole in the base for a wood screw, file a flat on the slug, put some valve lapping compound on the flat, insert into the mould and lap away. I use an electric drill rotating slowly.
Another small problem I ran into was that sometimes the D bits warp when I heat treat them after milling the flat on them. I don't have a jig made up to grind to final form after heat treating which is what is recommended so I make the final version in soft form then heat treat but the D section tends to bow slightly making it cut about 0.003" large.
I took to making the D bit about 0.003" small then doing a final light lapping to finish the cavity. That workks well enough.
Longbow