Turning a rectangular plate into a circular one

I have a Warco 250 MV lathe & a WM 16 mill

I have a large piece of 8mm steel plate & i want to cut some pieces from it to make 4 No 75 diam * 6mm th pieces each with a 30mm hole in the centre.

I have a 225mm angle grinder so can cut 4 pieces approx 100* 100 then cut the corners to make octagons. I can drill 12mm holes to mount on a mandrel.

The problem is that when I start turning there is going to be a lot of vibration as the corners hit the cutter.

So how would the team tackle this?

Would they use HSS or indexable cutters?

Would they machine from the edge & slowly cut the corners off ( thus ignoring the chatter & the vibration on the lathe) or would they leave the plates square & cut from the face so that they cut away the outer ring of the corners plus a bit of metal thus giving a smooth cut? (Rather like boring a hole)

If so what sort of cutter? would it bind on the groove as it went in or could a HSS bit be ground in any way, or is that the wrong approach?

Or should I put it on a rotary table & mill the corners off then stick it in the lathe?

Once you have cut them into octagons have another go at then with the angle grinder to turn them into sixteenagons.

Cut your bits to 80x80, 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

I generally machine from the face by creating the central hole first so it can be gripped internally with the 3 jaw, . I use whatever cutter is in the toolpost (slightly oversize to allow for cleaning up) until the corners fall off.

Depending on what it is required for I might then grip the part on its OD and bore the ID to the final size.

Ian P

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

If you've got the speed and feed correct there shouldn't be chatter or vibration; thump, thump, thump yes, but that's not the same thing. Personally I'd use carbide inserts. The idea that they don't like interrupted cuts is outdated. The top bar was reduced to the bottom bar on the lathe, with a round carbide insert at 800rpm and 4 thou/rev feed:

Like this:

Andrew

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

Interrupted cuts with carbide?

I lopped a steel gear in two with a parting tool a few days ago.

It went BRRRRRRRRRRRRRRRRRR..... but not >PING<

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

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

Why not use the lathe as the drill? If I had several to do, that is what I would likely use. I've got several 2" discs to cut from aluminium offcuts, so I just bought the next size up hole cutter. Steel is that much harder, but you can go steadily and evenly with the cutter in the tail stock.

A cutter in the mill would do the same job.

Edited By not done it yet on 07/09/2017 22:15:24

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

Bill

An old dodge when taking interrupted cuts is to take a piece of wood about 1" x2" x 2 foot long and wedge one end of it into the lathe bed, with the side of the wood bearing on the OD of the chuck. You then hold the end of the wood, keeping firm pressure on the chuck, or take another peice of wood about one foot long and wedge it between the first piece and the drip tray of the lathe. The wood bearing on the OD of the chuck acts as a shock absorber, taking up the backlash in any drive gears etc, and quietens the whole operation down considerably.

It works very well when taking interrupted cuts on larger diameters where the lathe is running in back gear and there is that extra backlash in the back gears etc. It stops the rotating job and spindle jumping forwards in the non-cutting sector of the rotation so the backlash is already taken up when the tool bit engages with the job. Much less of an impact on the gears that way.

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

Sam should not need the wood trick on the 250 as it is belt drive so no gears to keep in mesh

Hopper deserves a prize for 'Cuttus Interruptus'!

New technique WAE, wood assisted engineering (of a sort).

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..

Still quite a handy tip for some though !! & the picture of the Drumond M type brings back memories.

If I did that with my Warco the gutless motor would just die. It is weak enough as it is.

As long as the wood doesn't fly out of the lathe - projectile dysfunction.