Turning a rectangular plate into a circular one

[Sam Longley 1Participant @samlongley1]

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?

[Sam Longley 1Participant @samlongley1]

[norman valentineParticipant @normanvalentine78682]

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

[JasonBModerator @jasonb]

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

[Ian PParticipant @ianp]

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

[Speedy Builder5Participant @speedybuilder5]

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

[Anonymous]

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

[SillyOldDufferModerator @sillyoldduffer]

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

[Neil WyattModerator @neilwyatt]

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<

[Brian Sweeting 2Participant @briansweeting2]

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

[not done it yetParticipant @notdoneityet]

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

[peak4Participant @peak4]

Posted by Brian Sweeting on 07/09/2017 21:10:17:

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

[HopperParticipant @hopper]

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.

[HopperParticipant @hopper]

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

[JasonBModerator @jasonb]

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

[SillyOldDufferModerator @sillyoldduffer]

Hopper deserves a prize for 'Cuttus Interruptus'!

[KWILParticipant @kwil]

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

[HopperParticipant @hopper]

Posted by JasonB on 08/09/2017 09:23:21:

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

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..

[HopperParticipant @hopper]

Posted by SillyOldDuffer on 08/09/2017 09:42:54:

Hopper deserves a prize for 'Cuttus Interruptus'!

[Sam Longley 1Participant @samlongley1]

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.

[Neil WyattModerator @neilwyatt]

Posted by SillyOldDuffer on 08/09/2017 09:42:54:

Hopper deserves a prize for 'Cuttus Interruptus'!

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

[JonParticipant @jon]

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.

[Sam Longley 1Participant @samlongley1]

Well thanks you everyone for the replies.

i have done it & 3 points came up

1) Brian sweating reminded me that sitting on my shelf was an 80mm hole saw so I have cheated for now.

2) I did experiment on one piece turning from the face but grinding an HSS cutter with the right cutting profile was not successful & produced a lot of chatter, although I did get the radius clearance on the side OK. It did take a while & not really worth the effort. A bit like using a cut off blade.

3) I did like Jon's suggestion of tapping the centre & turning to size & that worked great. It pulled the piece square to the jaws & i was able to face the quite rusty piece as well & get tight to the stud with no slipping. However, his comment about turning off the corners is still a problem that I am contemplating. I take the advice about Kennametal tips. They must be better than my cheapo chinese tips but I do baulk at the price of the holders at £ 60+ each from MSC

Has anyone any comments as to how I can reduce the cost of holder ( 12mm) to enable me to use the better quality tip with the correct screw? I read on other forums that replacement screws for Kennametal tips can be an issue as they vary from tip batch to batch

Also which tip. Does it need to be thicker for an interrupted cut. Is CCMT OK as it can be used for other work

 

Edited By Sam Longley 1 on 09/09/2017 11:49:32

[Anonymous]

Posted by Sam Longley 1 on 09/09/2017 11:49:15:

Is CCMT OK as it can be used for other work

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

Andrew

[Simon Williams 3Participant @simonwilliams3]

Jenny at J B Tools does a good range of suitable holders at sensible prices. Usual disclaimer

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