My $0.02 worth – bear in mind I have limited experience in these matters

Would it make sense to start with the 20mm hole and then finish the rest of the piece using the hole circumference for reference?

I'll get my coat…

I would make a stepped mandrel to fit the the hole and the recess. Clamp the mandrel in the tailstock chuck and use this to hold your inplace whilst you set the four jaws to the job. Hope this makes sense.

Either clamp it to the lathe faceplate and bore it, or make a "sub-faceplate" out of a piece of flat bar or plate. I use 10mm thick aluminium alloy flat bar about 50mm wide and 150mm long for this. Then drill and tap a couple of pairs of 6mm holes in the plate, strategically located so that a pair of clamping bars, — cut from 20mm x 5mm or so steel flat bar with two holes drilled to match the tapped holes in the base plate — can be held held down with 6mm screws.

It looks like you will need to put a bit of packing under your job so it sits on the square face. I use makeshift "parallels" cut from a length of 6mm square keysteel or bar for this.

Attach the job to the plate, then bolt or clamp the plate to the faceplate. I have a hole drilled each end of the plate so a bolt goes straight through and thtrough the faceplate. Then I use a clamp on the other end to allow movement to adjust it to get the job centred.

A bit of mucking about, but I use that sub-faceplate over and over again for drilling and boring small parts in teh lathe where I want more accuracy than my cheap Chinese drill press will give me (I found its table is half a millimetre out of square with the spindle!) so its well worth the effort to make up. You could use a piece of steel flat bar if no ally in the scrap box.

ER collet block to hold by the round spigot and then fix that to a face plate and bore out to 20mm if you only have the tooling to cut on the lathe. No ER block then just bore a hole in a bit of square stock and drill & tap a cross hole for a grub screw to lock the part in position.

If on the mill hold by spigot and use a boring head or home made cutter.

Edited By JasonB on 17/02/2018 07:41:57

if you want it absolutely central in the lathe, why not take a piece of bar, mount it in the 3 jaw, and bore it out to be a really tight fit on the diameter of the stem. Mark the position of No 1 jaw, remove and slit along the axis, with a hacksaw.

Deburr, replace in the 3 jaw with the mark aligned on No 1 jaw.

Fit stem into this newly made collet, clamp up hard.

Bore out to 20mm.

HTH

Howard

As you say its would be difficult to get a flat bottom using a boring head in the mill, unless it was the boring and facing type, and it would be an OTT method. For speediness and expediency my money would be on using a four jaw in the lathe.

Once you can hold it (4 jaw or faceplate) then holding on a rotary table in the mill is another possibility. By using an end mill less than half the recess diameter, getting a flat bottom is easy. I have a simple manually rotated table that I use frequently to create large counterbores. Sitting an already drilled hole on a spigot easily ensures the counterbore is concentric. The same technique works without a rotary table, just by hand rotating the part as long as its size and proportions make it amenable to hand rotation.

Ian P

Hi Robin, you may find it useful to drill and tap some holes in the faceplate to enable parts to be fixed for turning, especially useful for holding smaller parts for turning.

Emgee

It's a classic, another "useful" and "informative" post from one of our leading armchair users.

Andrew

Good to hear it Robin.

Hopper – enjoy the fame and fortune for the full 15 minutes!

Hopper!

We're not worthy, were not… hang on, his 15 minutes has run out!

Neil