How to turn an article partly covered by the chuck jaws

Hi all,

I have a piece of work which I need to face both ends, then externally turn to various diameters and finally bore. Obviously all needs to be concentric etc.

So far, using the 4-jaw chuck, I have faced one end and then turned the adjacent diameter at that end. I have then turned the work end for end and set it up using the middle step on the reversed chuck jaws, and then faced the other end. I can't hold it with the outer step because the jaws meet in the middle before they contact the work.

Now, I need to turn down a length of 26mm from the right hand face without moving the work in the chuck, but the left hand edge of the 26mm section is about 3mm under the outer step of the chuck jaws. Later I will need to turn down a section about 5mm wide which will be entirely under the outer step. It seems to me that I need a bent tool capable of getting at the work under the jaws. Alternatively, I have wondered about using something like a SCLCL tool parallel to the lathe axis rather than at 90 deg to the axis and cutting towards the headstock rather like facing at 90 degrees to a normal face.

Thoughts? Suggestions? Comments?

The work diameter is about 50mm diameter and about 45 mm long. It is being held in a 160mm diameter 4-jaw chuck. I do have available a 80mm diameter 4-jaw chuck and I am also wondering if I should simply transfer the work to that chuck and accept the possibility of any slight loss of concentricity.

MTIA

Peter G. Shaw

How about producing the bore first and then using centres or making a mandrell to locate it for turning the ODs.

Run lathe backwards and use a boring bar on the rear edge.

Do you have soft jaws? if so machine to hold by the first turned side ond only deep enough that they don't get in the way of the second cut

Or just use a L/H facing tool like I did to turn the OD of this part, tool shank along axis of lathe

SCLCL as you suggest

Edited By JasonB on 08/02/2016 15:36:42

To ensure concentricity I would do the bore first then put it on tight mandrel between centres to machine the ends and diameters. So the mandrel should be made first and can be used as a plug gauge. It's an advantage if the mandrel has a 0.002" reduced diameter for a short distance to give an indication that the bore is "nearly there" and to aid its press fit alignment. Colin

Plus one for the left hand tool.

Thanks all for the suggestions.

Mandrel. Why on earth I didn't think of that myself I do not know. Perhaps my brain goes open-circuit at times! What's more, as Colin says, it would achieve that concentricity between both ends which, given what I have already done, I can't now guarantee, not without scrapping the work and starting again.

Boring tool. Thought of this one, but scrapped it because I hadn't thought about reversing the lathe. In any case, without major re-working of my cross-slide and tool posts, I can't get the cross-slide far enough over to be able to get the tool down the far side of the work.

Left hand tool. I tried this with a high carbon steel home-made tool. It did work after a fashion, but the resultant surface was not very good which suggests that probably my angles & cutting point were wrong, yet it would seem that I was indeed on the right lines. As it happens, I don't have a SCLCL tool, or indeed any other suitable left hand knife type tool so couldn't try it, but it does seem from Jason & Chris to be a goer so I will shortly be looking for a suitable tool for future use.

But in the meantime, here's what I've decided to do. First, I must point out that although working for the best precision I can achieve is always desirable, the actual end result is nothing more than a handle with an adjustable graduation collar hence a slight loss of concentricity probably doesn't matter. Therefore I have decided to transfer to the 80mm 4-jaw chuck since I can hold the work on the outer step of the reversed jaws and hence do the necessary two-stage turning and then the boring.

Now, why didn't I use the 80mm chuck to start with? Well, I was hung up on holding the work with as much jaw as I could get onto the work, and since the 80mm with the jaws in standard position wouldn't hold the work, I decided to use the 160mm chuck, and hence the problem as outlined above. Had I thought to reverse the 80mm jaws and accepted the slight loss of grip, then the problem would not have arisen. A case, I think, of not thinking the job through thoroughly coupled with a lack of experience.

Anyway, thanks to all,

Peter G. Shaw

At this size I would be thinking I need a length of material about 1 1/4 longer than the work or even longer if the bore of the spindle is big enough.

Assuming the bore is too small grip approx 1" long in the larger chuck, jaws the normal way round. Face end, centre drill, drill and bore hole, fit a tube centre and then do the od's. Then part off. All one setting so as good as the lathe can be.

If parting off might leave a messy face I would part off a touch longer. Put some tape on the work where the jaws will be, set it in the chuck still using the tube centre but not gripping the work tight enough to puncture the work, remove centre and take a light facing cut. You could use the same sort of technique to allow a shorter bar to be turned up fully but concentricity wont be so good. A decent 3 jaw and some care plus use of a tube centre should keep the error down to a few thou max. I'm assuming both ends of the bar are square on before you start. They will be close if you face an end and then press that end hard against the face of the chuck and then face the other end.

Centres can often be useful for getting better alignment when there is a hole in a part and the work has to be reversed. 3 jaw chucks can be pretty good alignment wise if the work is put in correctly. On that score I noticed a real turner doing that and hand facing correctly on youtube, an add for the recent Ceriani lathes. Short sharp and sweet. There is a nack to centring the work in the chuck but it comes easily with a bit of practice.

John

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Spot on John was thinking the same until read your post.
Not only is it easier its also accurate, all but last op done without removing from chuck.