Cut your bits to 80×80, no point in having them bigger and then lop the corners off so you have an octagon.

Hold in 4-jaw and drill but a better way if your chuck jaws will fit inside a 30mm hole is to bore the hole first.Just a standard boring bar, no point in trepaning like the second photo

If not just drill your hole

You can then turn the OD using a standard cutter moving along the bed not facing.

Finally face to thickness

Knocking is not really an issue, no different to facing off a big square or rectangular block in the 4-jaw

Edited By JasonB on 07/09/2017 19:38:32

Edited By JasonB on 07/09/2017 19:43:50

Hi there Sam. Don't turn them from the outside once mounted on your faceplate, chuck etc, but turn them on their face. You may have to grind up a tool from HSS similar to a short parting off tool, but allowing a side rake similar to the radius you are cutting. – As you say, a bit like boring. Slip a piece of hardboard between the faceplate and the work to protect the faceplate.
BobH

I'll be interested to see what the experts say Sam. Quite often I find I'm doing it wrong!

I found trepanning difficult beyond a few mm because, as you say, the tool binds. It's worse than parting because the groove is curved as well as deep.

So I prefer rounding the square to a hexagon or octagon and then turning across the edge as normal. To reduce shock on the tool tip I take tiny cuts at first, gradually increasing depth as the square gets rounder. If the vibration is alarming, back-off a bit. As the vibration reduces, go in harder.

I've read that carbide is more likely to crack than HSS when used to make interrupted cuts. In practice I've not had a problem with carbide – just don't be too brutal at first.

Finally, I'd super-glue all 4 plates together and turn them all in one go. Boil the plates to break the bond. Glue may not be necessary if you have a big fat Stub Mandrel. (Neil is far too skinny.)

Dave

Edit: Gosh, EVERYBODY can type faster than me!

Edited By SillyOldDuffer on 07/09/2017 19:45:35

Or, you could use a hole saw in a drill to form the large diameter and then transfer to the lathe for completion.

That's what I normally do, it also has the advantage of leaving you with some spare embryo rings for the next project.

Bill

Here's a couple of pics better showing what I was waffling on about in the previous post.

1. Base model, single piece of wood jammed gently by hand against the rotating chuck to dampen impact when performing Cuttus Interruptus on a square block of steel.

2. Deluxe two piece model featuring the second board jamming the first into position, allowing the operator free use of both hands while gear cutting with a single point fly cutter type tool. Tension on the first board, and therefore the damping effect on the chuck, can be finely adjusted by tapping gently on the second board with a clenched fist. It doesn't take much pressure to effectively do the job.

Note that positioning of the first board is such that it rubs on the chuck body clear of the area where the chuck key barrels are located.

Edited By Hopper on 08/09/2017 09:14:03

Hopper deserves a prize for 'Cuttus Interruptus'!

Depends. Belts can stretch while under load, then catapult the job forwards like a spring when the cutting pressure is taken off. Probably not the loud clanging noises that back gear gives, (and the associated drive pin in loose fitting hole) but certainly some variation of rotational speed..

Opposite way to above piccies.

Cut the steel out and leave square ish it don't matter.

Drill and tap through largest high tensile long enough bolt you have, ie Hex head long enough to drop behind jaws and pull in to them when steel threaded on.
If drill straight through and bolt, most likely it will slip and or rotate no matter how tight.

Just simply turn o/d with a suitable carbide and wear a face helmet, arm and neck protection. Best carbide found to date for eccentric interrupted cuts are the Kennametal Top Notch bull nose or parting, virtually unbreakable.
At this point now have an external working edge.

Now if jobs on tight, dead simple remove the bolt with job and unscrew by hand.
Swap jaws over to external and slap job n and drill and bore the rest out, easy used to have to do it that way on a small lathe.

Holding off the bore will impregnate/deform.mark up more in to the job and have substantially reduced holding force than off outside. Done hundreds.

That's what I use, interrupted cut or not.

Andrew