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A question of Mandrels..

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MW09/02/2016 11:38:31
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Hi,

I was wondering if anyone had any good ideas on how to successfully clamp a round part on a bar in order to turn the whole length without rechucking it and therefore keeping all the diameters concentric to one another.

I, at the moment, have a method of simply threading a bar with a larger diameter at the back as a kind of "back stop" and am really asking if the form of the clamping nut makes a big difference to how well the bar holds it, i've seen someone do this for gear cutting in MEW before.

I do find that despite this i get a kind of "judder" effect when roughing steel where it skips round either in the chuck holding the bar, or the part itself. (it just ruins the finish)

 

Edited By Michael Walters on 09/02/2016 11:40:35

roy entwistle09/02/2016 11:41:47
1436 forum posts

Michael A lot depends on the diameter A taper on the mandrel usually works

Roy

MW09/02/2016 11:53:55
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were talking 38mm mild steel, i was trying to take 2mm cuts with a large roughing tool. I guess that makes sense when you think that a morse taper works a bit like that.

JasonB09/02/2016 12:09:26
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21632 forum posts
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If you drill a ctr hole and add tailstock support to the threaded end of your Mandrel that will help stiffen things up.

MW09/02/2016 12:26:45
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Yeah i didnt use a centre but i guess its rather alot to take off in one go, i shouldve considered that.

Michael W

colin hawes09/02/2016 14:06:25
548 forum posts
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It is best to make a press fit mandrel and turn the job between centres. Ready made mandrels for this type of work have a very small taper 0.002 on the diameter over 10" would be satisfactory. The taper allows easy fitting which gets tighter as it is pressed in.

Howard Lewis15/02/2016 01:14:18
5545 forum posts
13 photos

If you have the work on a mandrel, remember that heavy cuts may deflect the mandrel , or cause slippage, as you have already found.

2mm sounds like a roughing cut to me. Better to spend time taking more smaller cuts, than to risk damaging work by trying to rush.

This isn't piecework is it?

Howard

Hopper15/02/2016 06:19:49
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5505 forum posts
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Or you can make a mandrel that is a neat fit in the hole in the job, then loctite the job on to the mandrel.

Heat it with a propane torch to get the job off the mandrel.

IanT15/02/2016 10:11:11
1918 forum posts
185 photos

I gather the part is 38mm in diameter Michael but it would help to know the length of the part and the size of the hole through it. Also what material did you start out with (was it already cored/drilled or solid for instance?

If it has a small (relative to the diameter) through hole then the approach you originally mentioned might well be the best one. The need to provide tailstock support would depend on how far it extends from the chuck, although if in any doubt then of course support it. Small cuts are the order of the day (with a sharp tool).

However, it may also be possible that thinking through the machining operations before starting might have helped you avoid the problem. You may well have done this of course but the reason I mention this is that I have often found myself in a (self-imposed) bind because I didn't think the whole process through to begin with.

For instance, starting from the solid, could you have turned the work between centres before drilling/boring it? Maybe not - but this might have been easier than making a mandrel or other fixture. Alternatively - with a pre-cored piece, could you have filled the holes at each end and then centre drilled them (for between centres turning). I don't know the answers in this case but you may want to think about it.

I should mention that I do have some commercial mandrels (acquired at a boot-sale) and have only used one of them (once). They would be very useful if I was machining volume parts in some circumstances but most of my work is 'one-off' from stock material. Having dug myself into holes in the past, I now try to think through the work holding required before jumping in. For instance, I try to keep the work part of the parent material for as long as possible and allow extra length if I'm turning between centres (parting/turning off surplus later) .

You may well have done all this of course (I'm not suggesting you didn't) but the advice is good but easily forgotten and I still make silly mistakes in this area even now - which is why I mention it. smiley

Regards,

IanT

.

MW15/02/2016 12:03:45
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2051 forum posts
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Thanks Ian, the length of the bar i had was quite short and stubby (50mm?) and i needed to turn the entire outside of the stock and then turn a smaller step which then kind of turns the part into a "flanged bush", it needed a centre bore of 5/16 (7.93mm)and a counter bore so i did have the ability to use a mandrel and thought that this was the best option, although i couldn't machine a taper to make the fit better.

In the end, the part was scrapped and i bought a much longer piece of mild steel that allowed me to machine from the head stock held in a 3 jaw and gave me a successful piece of steel. (its worth remembering that i dont like switching the bar around to machine from the other side because this almost always changes the roundness of the bar spinning in the lathe even if you use collets)

My lathe is only 1HP 750W inverter variable speed at the moment so it cant handle the slower speeds needed for steels and iron very well. I intend to make a different pulley setup that makes the top speed 500-600rpm(a kind of "slow mode", i can test this, but thats for another day.

I thank you all for your recommendations, its good to know i'm not missing out on much with regards to the commercial mandrels and as you say, for one offs, its better to make up a special bar to hold it.

 

Michael W

 

Edited By Michael Walters on 15/02/2016 12:08:57

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