Lathe chuck size problems!

Hello again gentlemen, me again!

I need to use a 19mm ball end cutter to cut a half sphere into a piece of 25mm brass. However, now looking at the drill chuck on dad's lathe, it only takes a 13mm shank - the shank on the ball cutter is 20mm.

So, do I buy another chuck for a one off use? Or is there another way? Is there an adapter (I doubt it but might be worth asking) that would fit into the 13mm chuck and accept a 20mm shank?

Another option, could I turn part of the piece of 25mm brass bar stock I have down to 13mm and mount that in the drill chuck, then mount the ball cutter in the lathe chuck - would that work or could you see disaster looming on the horizon?

If I need to buy another chuck - how can I tell what taper - 2MT or 3MT etc the lathe takes? All I know is, it is a (Chinese I think?) lathe from Harry UK about 12 years ago.

Thanks in advance.... again!

Margaret

Holding the tool in the lathe chuck and the reduced dia of the work in the tailstock would probably be the simple option. Another would be holding the work in the tool post or clamped to the lathe cross slide but harder to set up. Lastly like Ramon showed in the other thread mount to tool in your toolpost if it will accept that size but probably wont.

Whatever way a pilot hole about 5mm dia and just less than 9.5mm deep will make life easier

Regarding MT sizes measure the thick part of the taper just beyond where it sticks out of the tailstock, MT2 will be about 17.8mm which is most likely what you have, MT3 23.8mm approx

Edited By JasonB on 19/07/2023 08:53:10

Thank you Jason, great advice as always. I really do appreciate your help.

Since ER32 collets go up to Ø20 it is possible to hold cutters up to this diameter in the tailstock if you have a suitable ER32 collet holder for the tailstock. I use an MT2 to ER32 collet holder in my tailstock for Ø19 shanks on annular (Rotabroach for example) cutters. Probably not worth getting unless you are going to have a regular need for this type of size.

Another option is to mount the cutter in the tool post. How easy this depends on a number of factors we don't know regarding the machinery and tooling you have.

The holding of tools with hard polished shanks in a lathe chuck can be problematic due to the low friction of the shank in the lathe. This will probably not be as big an issue with brass as it would be with working on steel but you are doing quite a large diameter with the ball end milling cutter. It will have a long cutting edge in contact with the brass when you take all cutting edges into account. If you have a suitable ER32 collet and a collet block then using the collet block in the lathe chuck would help overcome this problem.

The size of a morse taper is easy to figure out from the diameter of the open end. Just look up diameters and dimensions of morse taper sockets. As an example the open end of a MT2 socket is 0.700" . There is also the possibility that you have some morse taper shanks that have a number stamped on the flat of the tang at the end. Doesn't take long to find out if this is the case.

Martin C

Thanks Martin. I don't have an ER32 collett, and looking at prices of them, they are as expensive as buying a new chuck.

I'll try turning down the workpiece and mounting that in the chuck I have and see how it goes.

Thanks for your advice though, it is much appreciated.

Hopefully the brass will not slip too much in the drill chuck. If you do find the brass is slipping (since it is a relatively slippery metal) put some paper between the brass and the drill chuck jaws.

Martin C

If you learned how to use a bench grinder you could make a form tool out of an old file and save yourself a lot of money.

Martin - thanks for the tip re the paper - I will remember that.

Duncan - I will have to learn how to make tools out of a bench grinder for sure. 👍

Or you could save the money and not buy the grinder and use that old file to file a 1/4 round on some gauge plate and use that as your form tool.

Or rough turn some silver steel to shape then finish with the old file to a ball nose, saw and then file off half the depth then turn a reduced shank on the other end and you have a D bit that will cut the hemisphere and can be held in the tailstock