Drilling holes for tapping through the back plate of a chuck.

Hi there.
I am new to this community, so pardon me if this question is not fit here.

I got a back plate with only the thread for fitting it on the lathe shaft.
It was about 5mm to big for the chuck, so I cut the diameter smaller as well as cutting the recess into the front for fitting on the chuck.
I have no idea of what the the back plate is made of except that it is very hard. The metal chips coming off the plate was not the normal small curly type but kind of needle like shards or splinters. I have burnt a tool steel cutter and haven't been able to cut much at first. I changed to a carbide insert cutter which then prove to be a great improvement. The shards was still the needle like splinters but came off in a constant "stream".

I now have to drill and tap 4 holes to attached it onto the chuck. For that I have bought a 7mm cobolt drill (tapped hole should be 8mm to accept the bolts. I believe, reading it from this community and others that the cobolt will do the job provided I feed it a good amount of cutting oil.

My problem/question is the following. Should I drill and tap the holes of the back plate and the into the chuck, or could I drill a 8mm hole through the back plate and in the chuck I drill the 7mm holes and tap that as the chuck is a softer kind of metal and would be easy to do.
I have a small milling machine. The back plate is for an old Southbend lathe.

Or should I rather take it in and let an engineering company do that part of the job?

Does the chuck not already have tapped holes?

But I would drill the back plate 8mm and drill & tap the chuck so the chuck is secured from back. If the chuck is already drilled for bolts, then drill & tap the back plate.

When drilling hard materials, use slow speed with a lot of pressure on the drill bit. I would most likely drill with no oil.

Jim, it seems that someone at some stage was trying to attach the chuck to some other back plate because there is 2 skewed and worked out holes. It is however shallow holes. The new holes have to be drilled.
So just to confirm you suggest drill 8mm holes through the back plate and 7mm into the chuck and tap only the chuck.

thank you for your input.

Quote:

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Originally Posted by Fcsmetal

So just to confirm you suggest drill 8mm holes through the back plate and 7mm into the chuck and tap only the chuck.

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Yes, I think that is what I would do, unless the chuck is factory through drilled for bolts from the front.

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