Mandel sizes

Hi everyone, sorry if this is a simple question but I'm looking at turning a mandrel to fit a workpiece on but I'm worried the mandrel will be too small and won't be rigid enough.

The workpiece is mild steel round stock dia 2.2" and 0.8" long. The drawing calls for a 0.250" hole in the middle, my plan was to drill a hole undersize, fit it on a mandrel with locktite to turn the outside then drill and ream the centre hole to final dia. I've drilled an 11/64 hole in the middle but I feel a mandrel this narrow wouldn't be rigid enough. Any advice?

I'm fond of superglue for this sort of job. Face a rod, say 1" diameter, which is held in the chuck, and glue the 2.2" round stock to it. For extra security, fit a live centre to the tailstock, and use it to push the round stock against the chuck. Boil item to break the glue joint later.

So much pressure can be applied by a tailstock live centre that the glue may not be necessary - light cuts only though.

On it's own the 11/16" mandrel is likely to flex, but adding tailstock pressure as described above would provide the necessary support.

Alternatively, thread the end of the 11/16" mandrel and make it long enough to be used as a drawbar through the spindle. Tightening the drawbar would pull the 2.2" disc hard against the jaws, making the thing rigid even before the jaws clamped the mandrel.

Dave

I never thought to use my centre! I don't have a live centre but I have a dead centre I could use with some grease. Would that be okay? Or would the part get too hot? I have some inch dia aluminium I could use to make a "clickspring style" arbour (?) They put grooves in the end I assume to increase the surface area for the glue to contact.

Try it and see. A dead centre will definitely get hot, especially when used as a clamp, but it might last long enough without overheating if only a little metal needs to be skimmed off. Rather than grease, maybe set up a drip and run oil over the centre continually.

Don't forget heat breaks superglue and that the job might get too hot to touch. How about investing in a live centre - I use mine a lot.

Dave

A live centre is definitely on my shopping list, perhaps it needs to move a little further up. I think glueing and clamping with a live centre is the best option, I'll sort myself out with one of those! Thanks for your advice!

Yes, I wouldn't start from there.

Use a piece of stock long enough to be held in a chuck so that the face, OD and hole can be machined at one setting. Then turn round and face to thickness.

When making mandrels i normally drill the spigot and partially tap whatever metric thread is convenient. Probably M4 in the above case. The spigot is then hacksawed axially so that when a screw is inserted, and reaches the partially tapped section, the spigot opens up. This will hold the work quite tightly, although in the above case very light cuts will be the order of the day.

Andrew

Please report back! Although gluing and/or pushing with the tailstock are recognised methods, the fixing they provide isn't as firm as gripping in a chuck. I often glue to a mandrel for cleaning up discs and have never had one come off, light-cuts only though.

Supergluing two clean metal surfaces together results in a remarkably strong bond, as you may be upset to find when trying to get them apart, but the bond is weaker sideways, so hard sideways cutting like parting-off could go pear-shaped! Same with creating a friction grip by pushing with the tailstock; side, heavy or interrupted cuts are likely to cause trouble. Take it easy.

Never tried glue and tailstock pressure together. I guess it will be significantly stronger.

The tailstock method may not be possible if the tailstock, saddle, or tool-post foul each other. I guess a 'live centre extender' could be made to overcome that, but I've never had the need.

Dave

If you are going to use a dead centre for this sort of push-location, use a small rolling bearing over the end of the tailstock centre, with a thick washer resting against the work. Much less heat will be generated, and much less wear on the centre etc. A thrust bearing would be best, but a conventional ball-bearing will take end loads for a reasonable time.

Then put the bearing and washer in a matchbox and keep it with your other turning odds and ends, ready for next time.

Cheers, Tim

Edited By Tim Stevens on 16/07/2023 10:55:20

I like that a lot, thanks Tim. Not just for the job in question but I can see lots of other times your idea would be helpful.

David

Andrew Johnson's recommendation of clamping with a setscrew, either exial clamping, or radial on a split mandrel, to turn the OD.

Then held on the OD (Centre in the 4 jaw ideally, ) to ream the central hole.

HTH

Howard

I recently made a mandrel for some aluminium parts. The centre was Ø32 so I didn't have the size issues you are worrying about. I put a hole through the mandrel than hacksawed a split along the axis so the resulting tubular spigot could be spread and used a MT2 to MT1 sleeve to spread it when the part was over it. Combined with a live centre this made the mounting solid but easy to release. If you can turn a suitable tapered pin on a piece of stock with a centre drilled hole in the end you could do something similar on a smaller scale if you get a live centre or use Tim's bearing method.

Martin C

Thanks all, I decided to shake my piggy bank upside down and buy a live centre. I think that's the most sensible option since I'll see myself using it a lot. I decided on method similar to what I had in mind originally. First I turned some mild steel down to the ID of the hole in the stock, it's a locating fit I would say..

The portion of the round stock turned down is slightly shorter than the depth of the hole it's going through

Then I took a scrap piece of aluminium, used my centre drill to put a (notch? Divot?) in it to locate the centre. I figure if I locktite all this up, the mandrel and the aluminium disk, and use the live centre to apply pressure to the part against the shoulder of the mandrel I should be okay?

In this setup I can't see the mandrel actually resisting any forces, so it's acting more to locate the work on the chuck and not in my face should the clamping action of the tailstock fail for any reason.

I haven't locktite anything yet, I'd be interested to hear if you think this setup will be rigid (and safe) enough

Edited By Matt T on 20/07/2023 17:20:19

Would help to know what other operations you intend to do on the workpiece.

All you have said is that it is 2.2" dia and it needs a 0.250" hole.

That could have been done with it in the chuck (reverse jaws) and no need for an arbor

Without knowing if anything else needs to be done to it it would be pure guess work as to whether the arbor is upto the task.

But if you need to turn the OD then I would have gone for a longer spigot turned to fit the hole cut onto a larger diameter piece of material. Thread the end of the spigot for a nut and drill the end of the spigot with a small ctr drill for tailstock support. The shoulder created by using larger stock provides enough friction to stop the job slipping so no need for loctite etc

This sort of thing

Edited By JasonB on 20/07/2023 19:00:51

Edited By JasonB on 20/07/2023 19:07:00

Sorry, I didn't include these details. I just need to turn down the OD, I don't think the reverse jaws will fit in a small hole?

In which case try what you have using light cuts with sharp HSS or **GT inserts or change to the arbor shape I showed.

If there is/was spare material on the length then I would have opted for soft jaws and done the OD, hole and one face all in one setting. Then reverse to face to length. You probably don't have soft jaws but they are worth having.

In hindsight I shouldn't have cut the stock so short as that method would be a better option. I'm planning on getting some copper sheet to make some soft jaws with my next project. Light cuts will be the order of the day and HSS tools are all I've used so far.

My dad swears by carbide insert tooling but from what I've read they require higher speeds and heavier cuts. Since my little hobby lathe tops out at 2500 and is less powerful than say a myford I'm not sold on the idea of carbide inserts.

The method in your picture looks like a good one, if this doesn't work Ill see if I can recreate that one. I won't get chance now until Sunday afternoon as I'm going down to visit my dad for this birthday but I'll have a go when I'm back and let you know how I get on

lathe soft jaws are unhardened steel and can be machined to suit the job, on a part like that a recess say 1mm deep is cut into the face of the jaws to the required diameter and you then just hold the job by a very short length.

The **GT polished inserts meant for non ferrous metals will work fine on a small lathe and don't need a high spindle speed. You could do the OD of that at 500rpm with Carbide.

Quick update as I tried it out yesterday, the setup did work, and it was solid enough to turn down the extra OD that I needed. I had to do the setup twice as the glue bond on the mandrel broke the first time but I don't think I gave it enough time to really cure. This definitely wasn't the easiest way to go about doing the job and next time I'll plan out my order of operations better so I can turn down the OD from the larger piece of stock first. On the upside, I now own a live centre and I've learned a bit along the way so all in all its been worth it! Thanks again for the ideas and advice!