Accurate blind hole in brass disc.

I fouled up (yet again) today. I wanted to make a 5.5 mm deep 6mm diameter blind hole in the centre of a 17mm diameter by 7.5mm thick brass disc to accept a 6mm axle. Plan was to glue the axle in place with retainer.

I put the disc in the lathe and had at it with a 6mm slot drill held in the tailstock chuck. In retrospect a disaster in the making. The hole turned out true to centre, flat bottomed, but about 6.7mm in diameter. I'm going to have to try again - any advice would be welcome.

Robin.

Do bear in mind I've never received much formal training on this sort of thing.
I suspect the slot drill wasn't held either rigidly enough or central enough.

Last time I did something similar, I held an undersized slot drill in a boring bar holder in the toolpost.
Make sure it is central to the axis of rotation and also with the front cutting edge at centre height, just like a normal cutting tool.
Wind the slot drill into the workpiece, just like you would from the tailstock, and then outwards towards you until the hole is the correct diameter.
A carriage stop might be useful to get the depth spot on when re-cutting after measuring the diameter.

Bill

If you have to do it with a slot drill, I'd at least sneak up on it with several smaller slot (or regular) drills first.

I'd be inclined to turn and bore the whole thing on the end of a piece of brass bar then part or hacksaw off and face the other side (could be held by the axle).

As Peter says going straight in with the final size cutter can see it cutting oversize, I would drill a pilot hole say 5mm, then use a 5mm milling cutter followed by the 6mm but would not expect it to be as accurate as say a reamed hole. For cutters I find the 3flute ones plunge better than 2 flute.

The other option is to plunge most of it with a milling cutter of 5mm or 7/32" after pilot drilling and then use a small boring bar to finish, as shown in the recent 14.5mm hole thread a milling cutter can be use das a small boring bar as shown here doing a tapered hole in a pulley (note setup for reverse spindle direction)

Knurl the axle, to increase its diameter, and loctite in place? Go all the way through and fill the hole later?

I would likely do it on the mill, rather than in the lathe.

Having failed, maybe sleeve the axle with a brass cap and then insert this, however you choose.

I would bore it with a 5mm slot cutter, used as a boring bar and held in the toolpost as above. 5mm pilot drill and then bore to size / length with the slot cutter. Small V block under the cutter if needed.

Can you approach it the other way round? Make the fractionally undersize hole in the disc then machine the end of the axle (just the seating part of it) to suit?

I'd start by checking the alignment of the tailstock.

The cheap, nasty, but successful way to do this is to first cut an (oversize) 5mm hole with a slot mill. Then mount the  cutter in the toolpost, one cutting edge aligned just like a boring bar. Now size the hole using it as a boring bar. When you think about it, this is the same as it being offset due to tailstock misalignment, except you can vary the offset accurately.

Edited By Macolm on 19/07/2023 10:18:16

Thanks for replies. I'm confident about tailstock alignment. I think the method is wrong - the tailstock / chuck / slot drill assembly just isn't rigid enough to prevent the tail wagging the dog.

NDIY - thanks for the reminder about the knurling trick. I always forget about that! Unfortunately it wont work here because the 6mm shaft has to pass through a bearing during assembly.

I don't have a boring bar small enough, but the 'cheap and nasty' method Malcolm recommends sounds good - I think JasonB suggested essentially the same method as well. I'll give that a go.

Thanks to all for suggestions, Robin.