Working with large diameter stock

A pesky little problem for me....

1. I need to reduce 3" diameter mild steel lump down to 64mm diameter for a short distance.

2. I then need to bore a 30mm hole in the centre

3. And then part off an 8mm "slice"

I have mounted it in the outside jaws of a chuck, and drilled a centre into it. Only 8mm of the piece is held in the outside jaws, on the second "step", and I cant use the outer jaws as they don't close down onto the 3" diameter stock. (Neither will a set of inside jaws open up wide enough to hold it).

Problem 1:
I feel that the piece would benefit from the support of a centre in the tailstock. But the tailstock quill won't allow the centre to reach the work due to the cross slide getting in the way.

Will I be safe facing the piece and turning it down to 64mm diameter?

Problem 2:
After boring out the centre hole to 30mm diameter I need to part off a "slice" ideally 8mm thick, but 5mm will do. I have 2.5 and 3.5mm carbide insert parting tools, but the larger of the two won't leave enough clearance over the jaw faces to part off even 5mm. The 2.5mm parting tool would do it.... just. The problem is that the piece has to be mounted on the second step of the jaws, so the outer step gets on the way.
In addition, the thin parting tool will need to be set at least 18mm out of the tool holder in order to reach the central bore to part off the "ring" of finished material. That's a pretty large tool overhang. Dodgy?

Ideas and suggestions please!

Thanks
Rod

Hi Rod,

Can you move the work piece out more in the chuck, i.e. to the next step in the jaws?

I've seen a number of videos on youtube where the machinist would do the bore first, then add a temporary mandrel/shaft with superglue supported by a center as you have suggested, maybe do the bore at 25mm first. Do the work on the piece and then heat the superglue joint to remove it from the mandrel. Then finish the bore out to 30mm?

Ed.

YOu can use the outer steps of the jaws by putting a piece of packing between the jaw and the work piece. Three short pieces of mild steel flat bar or square bar should do the job for you.

There is no need to have a tailstock centre in place on a short job like this one.

Hi Rod,

I recently had to do something similar and had to hold a piece of 220mm diameter x 5mm thick plate in a lathe chuck to remove 2mm from the OD.

What I did was to face the end of a piece of pipe on the lathe, remove it from the chuck and tack weld it to the back of the round plate to make a temporary mounting boss.

Locating the pipe on center with the round plate was not critical for welding because I then used a 4-jaw chuck to hold the round plate by the piece of pipe and adjusted the jaws to find center. The plate was perpendicular to the lathe axis because I had already faced the end of the pipe and clamped it to the plate while it was welded. Then ground off the piece of pipe.

With your job, after welding, you could hold your piece of round bar in the chuck and machine the OD and end of a piece of pipe (or round bar) to make it square and centered, then hold hold it in the chuck by the temporary boss. (The OD of my job was too large to fit my chuck.)

Paul.

Use a left hand tool to reduce the diameter rather than the right hand one you show in your photo as it will fit without the risk of catching the overhanging jaws

First deduce about a 10mm length to 40mm then hold by that in the outside jaws while you turn the 64mm dia, bore and part off.

Alternatively get a set of soft jaws for your chuck

Edited By JasonB on 18/04/2016 07:10:38

Thanks folks for wrapping your collective brains around this. Some good ideas there.

I see now that its all a matter of "visualising" the finished piece (a disc 64mm diameter, 8mm thick, bored out 30mm) "inside" the piece of steel 3" diameter and 1" thick.

Following Jason's idea I can first turn a spigot down on one side of the 3" disc (on the left on the drawing below) so that it will fit into the normal 3 jaw chuck. Then the finished piece can be machined by turning it down to size, boring and then parting it off.

The "blank" should be pretty secure in the 3 jaw chuck (set right in on the spigot so that the face sits hard against the face of the jaws too).

I am still a little concerned about parting it off (down to the bore) with an 18mm overhang of the thin parting tool. Perhaps someone can reassure me that this will be OK?

Thanks
Rod

Edited By Rod Neep on 18/04/2016 09:18:15

A pesky little problem for me....

1. I need to reduce 3" diameter mild steel lump down to 64mm diameter for a short distance.

2. I then need to bore a 30mm hole in the centre

3. And then part off an 8mm "slice"

----------------

A nice big hole in the middle so you could bore the 30mm hole, then "plug" it onto the normal 3 jaw jaws like a pipe and support it wif the tailstock

Make sure 30mm will fit onto the jaws first, if you are going to take this route

gl

As an extra snippet if you do a lot of short length work it may be worthwhile taking a small unimat style 3 jaw and adapting it to fit your spindle nose

This can then be used to "mount" awkward short pieces via the hole and the tailstock

Edited By Ady1 on 18/04/2016 10:20:59

This may not apply to other setups, but with the chucks I have the four jaw is much larger in diameter than the three jaw. This means that sometimes I can grab a larger job with the jaws the normal way around on the four jaw when the three jaw would not do it. With these particular chucks, the four jaw also has less overhang which is also a bonus. The downside of course is that it is not self centering so you have use a dial gauge or wobbler to set things up. But that is no big deal.

Of course that won't help if you don't have a four jaw, or if it is no bigger.

John

That works nicely. I have used the idea to get a small faceplate mounted (and maybe I should get a Unimat chuck too for tiny work)...

RDG Tools supply an M14 x 1mm size 3 MT so that a Unimat faceplate or chuck can fit direct without any adaptation.

There is nothing to stop you from grooving as deep as you are comfortable with using your parting tool and finishing off with a hacksaw. Flip the part and face the sawn side to give the required thickness. Just leave an allowance for the final facing operation.

Martin

Have you got a keats angle plate if so you can use the face plate.

Frank

Almost there... just needs a chamfer on the central hole, and then to be parted off.

It all worked out fine thanks to the ideas and suggestions from you folks.

Then turned round and in the regular jaws of the 3 jaw chuck for the main event machining....

Cheers
Rod

Well.... I got the disk that I wanted to make separated from the "blank"....

.... and then kicked myself !

The piece that I had made was fine, but the "scrap" piece that was left over (shown here on the right) could have been something else useful. A small 3" faceplate! Problem is, when doing the boring out for the blank, I had first drilled through with a 14mm drill, the largest I had, ready for the boring operation. If only I had seen the result, I would have drilled it to a size suitable for an M12 internal thread to mount on an M12 (Emco thread) 3MT arbour that I have.

Hindsight is a wonderful thing.

Hmm..... I suppose that I might be able to make an adaptor.

Rod