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Work holding?

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Vic21/09/2019 18:52:39
2256 forum posts
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They say is an art in itself.

I have the following quandary. I have some 32mm stainless bar that I want to cut a 18mm slice off then bore 19mm right through. I want the bore to be as concentric with the OD as possible with as little loss to the OD as possible. I hope that’s clear. I have an ER32 collet chuck so too small. I also have a SC three jaw and a couple of four jaw ind chucks. The only way I can think of doing it is to waste part of the bar by cutting off an oversize slice to enable me to grip it in a four jaw. Or am I missing a trick? If I have to waste say 10mm of “gripping” material then so be it but I’d like to avoid it if I can. I rarely use a four jaw as I find indicating work a chore but I don’t have much choice as my three jaw SC is not very accurate.

JasonB21/09/2019 19:20:55
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Can you not simply clock a 20mm slice true in the 4-jaw then face one end and bore after which flip it over and face the otherside back to 18mm long. Will keep OD at it's max and be as true as the effort you put into getting it right in the 4-jaw.

If you have soft jaws for your 3-jaw chuck that would be the other option as you would only need 1mm length to grip so could skin the OD at same setting as boring and facing. This would be totally concentric with a slight loss of OD

Edited By JasonB on 21/09/2019 19:23:01

HOWARDT21/09/2019 19:30:52
452 forum posts
14 photos

Where concentricity is important.

Place slightly longer piece in 3 jaw chuck.

Face end, centre drill, drill and ream.

Place a mandrel in a collet chuck to hold the part to face the unfinished end and skim the outside. My mandrel are usually a bit of bar turned while held in the collet with a screw or nut to clamp the part in place. I realise you have to use material to make a mandrel but mine get reused for different parts, or you could buy a mandrel. In day gone by you could use a standard mandrel which has a slight taper to hold the part and centres in each end to hold between two centres.

i am sure you will get many suggestions, but that is the way I work on my mini lathe.

Clive Foster21/09/2019 19:57:26
1844 forum posts
59 photos

Details of how you do the job depends on personal preferences and equipment available. Jasons method works just fine. I've done just that.

But these days I'd do it a little differently by facing both sides of the slice true before verifying that the OD runs true and boring the hole and flipping again to bring to length and clean up any tool exit burs on the backside. The extra flips over Jasons method make the job theoretically slower but I prefer to have a good surface pushed back against the jaws for the money cut and to de-bur the ends of the hole and slice under power with a proper chamfering tool. Overall its pretty much a wash.

As my Rapidor power hacksaw is decently accurate I'd expect to waste maybe 2 mm on facing and bringing to length. 1.5 or even 1 mm waste is feasible with care on such a small diameter but generally I'll leave more as making a quicker, easier job.

If the stock outside surface is poor enough to need machining and best concentricity is needed I usually make a stub arbor to hold the stock via the bore with a nice hefty nut to clamp the part on.

Although I have soft jaws I don't, for some reason, use them. All the jobs I seem to encounter that soft jaws could be appropriate for have other satisfactory ways of doing them. I use one time arbors a lot.

Clive

Daniel21/09/2019 20:11:56
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249 forum posts
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Probably, a lot depends on whether the op needs to skim the od of the original bar.

Will the parent bar pass through the spindle bore, etc, etc ?

It's unclear what level of precision is required for this job.

ATB,

Daniel

Michael Gilligan21/09/2019 20:32:03
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14023 forum posts
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Posted by Vic on 21/09/2019 18:52:39:

They say is an art in itself.

I have the following quandary. I have some 32mm stainless bar that I want to cut a 18mm slice off then bore 19mm right through. I want the bore to be as concentric with the OD as possible with as little loss to the OD as possible. I hope that’s clear. [ ... ]

.

Hacksaw your slice at [say] 20mm long

Drill it on the pillar drill, and tap [say] M16

Mount it on a screwed mandrel in the collet chuck and turn all the external dimensions.

Re-mount it in the four-jaw and bore to 19mm diameter.

MichaelG.

Vic21/09/2019 21:20:50
2256 forum posts
11 photos
Posted by Daniel on 21/09/2019 20:11:56:

Probably, a lot depends on whether the op needs to skim the od of the original bar.

Will the parent bar pass through the spindle bore, etc, etc ?

It's unclear what level of precision is required for this job.

ATB,

Daniel

That’s part of the problem. Spindle bore is only 20mm. I think my original idea of an over size workpiece will probably be easiest. It means wasting some bar but I’ll try and keep it to a minimum. Thanks for the ideas though all.

Clive Foster21/09/2019 22:55:25
1844 forum posts
59 photos

Its worth building up a stash of free / throwaway material to practice or verify methods on if you aren't sure of the best way to tackle a job. No sense in wasting paid for stock if you can help it.

Something like that I'd be thinking in terms of screwing together a couple or three layers of ex kitchen cabinet sides or a chunk of worktop to get a reasonable thickness to play with. Doesn't have to machine well as a rubbish finish doesn't matter. Idea is merely to get a feel for the steps involved in holding the job and what will be tricky to get at / machine with your equipment.

Probably not stricktly needful on a simple job like that but sometimes its pretty much essential. I've done more than a few jobs a hard way 'cos I was too sure I knew the best way to do it without practice.

Clive

peak422/09/2019 00:27:02
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A couple of thoughts, Is the lump of bar stock shorter than between centres, and do you have a 3 point steady.

If so drill, bore, face, and turn the outer, in whatever order suits. Cut off a slice hold in the chuck of choice and face off the final side. I keep a stock of old ball/roller bearing races to act as round parallels for setting up stuff in the chucks.

Alternatively, depending on accuracy/concentricity required, slice off a lump first, set up in a 4 jaw, maybe using the clock gauge off the saddle, so you can move back and forth between the jaws whilst adjusting. Drill and bore, maybe even taking one facing cut.

Knock up a sacrificial mandrel to bore diameter with a mating face to ensure that the previously machined face is in exactly the right plane. Loctite together and take the finest of cuts on the OD, same again on the outer face.
If you don't want to use loctite, slide work onto mandrel and hold in place with one of those spare bearing races, a large washer and a running centre in the tailstock; again a very fine cut on the OD, then machine the final face on a 3/4 jaw. Since you want to remove minimal material from the OD, there should be enough friction to drive the workpiece, providing the location shoulder is of sufficient diameter.

Bill

Hopper22/09/2019 09:00:18
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3714 forum posts
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Posted by Vic on 21/09/2019 18:52:39:...
...I rarely use a four jaw as I find indicating work a chore but I don’t have much choice ...

Time to get a bit of practice then.

It only takes a few minutes if you carefully note/mark the high spots as measured with a dial indicator then move the job appropriately by half of the indicated run-out. At the end stages, don't loosen off the jaw in the direction you want to move the job, just tighten up the jaw on the other side and job will still move by a thou or fraction there-of.

If you want better than quarter-thou concentricity, best and simplest way is to sacrifice the extra 20mm of material, hold it in the chuck and turn inner and outer diameters in one setting. This has the added advantage of ensuring (theoretically) that the OD is circular and not some odd shape as the bar stock was rolled into.

Mandrels and steadies and between centres really make a dead simple job harder than needs be.

Apply Occam's Razor and learn to set up in the four-jaw. It's an invaluable skill that is worth learning and practicing.

Vic22/09/2019 09:46:40
2256 forum posts
11 photos
Posted by Hopper on 22/09/2019 09:00:18:

If you want better than quarter-thou concentricity, best and simplest way is to sacrifice the extra 20mm of material, hold it in the chuck and turn inner and outer diameters in one setting. This has the added advantage of ensuring (theoretically) that the OD is circular and not some odd shape as the bar stock was rolled into.

Mandrels and steadies and between centres really make a dead simple job harder than needs be.

Agreed Hopper. It doesn’t actually need to be super accurate I’d just like it within a couple of thou. I’m going to go with my original idea, the same as you have said. Before getting out the four jaw I will try using my bump tool on the three jaw and see how far out it is before I start. I don’t mind losing a bit off the diameter.

Michael Gilligan22/09/2019 10:33:09
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Posted by Hopper on 22/09/2019 09:00:18:

[ ... ]best and simplest way is to sacrifice the extra 20mm of material [ ... ]

Mandrels and steadies and between centres really make a dead simple job harder than needs be.

[ ... ]

.

Absolutey, Hopper yes

But those of us proposing the use of such devices were trying to address Vic's stated preference to not 'waste' material.

MichaelG.

Hopper22/09/2019 11:33:57
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3714 forum posts
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Posted by Vic on 22/09/2019 09:46:40:
Posted by Hopper on 22/09/2019 09:00:18:

If you want better than quarter-thou concentricity, best and simplest way is to sacrifice the extra 20mm of material, hold it in the chuck and turn inner and outer diameters in one setting. This has the added advantage of ensuring (theoretically) that the OD is circular and not some odd shape as the bar stock was rolled into.

Mandrels and steadies and between centres really make a dead simple job harder than needs be.

Agreed Hopper. It doesn’t actually need to be super accurate I’d just like it within a couple of thou.

Ah well in that case your three jaw chuck should do the job. If it is more than a couple of thou out it needs replaced or jaws grinding as per article in MEW in the past year or so. It's worth spending a bit of money to get a good quality three jaw chuck as it is the mainstay of lathe workholding. And there are good ones coming out of the Far East at reasonable cost. No need for astronomically priced "connoisseur" kit.

MadMike24/09/2019 14:43:09
196 forum posts

I have just seen this thread so my apologies for a late entry, as it were.

As usual there is a lot of well meant, and good advice. However perhaps we need a tad more information.

How long is your piece of 32mm stainless? What OD do you want to achieve? How accurate do you want the concentricity of the OD and the Bore? How accurate do you want the final thickness?

The nominal difference between the OD and the bore is a mere13mm, that is a 6.5 mm thickness of metal.

Why has nobody suggested the obvious. Face off the bar, turn your required OD, drill and bore, if it needs to be that accurate to require boring, the through hole and then simply part of the required thickness. If the finish is critical, turn it in the jaws. Set it accurately in place and just face off the second side.

JasonB24/09/2019 14:48:07
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Posted by MadMike on 24/09/2019 14:43:09:

Why has nobody suggested the obvious. Face off the bar, turn your required OD, drill and bore, if it needs to be that accurate to require boring, the through hole and then simply part of the required thickness. If the finish is critical, turn it in the jaws. Set it accurately in place and just face off the second side.

Because Vic wanted to avoid that method

"If I have to waste say 10mm of “gripping” material then so be it but I’d like to avoid it if I can"

not done it yet24/09/2019 17:03:26
3372 forum posts
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MadMike has made the point of how long the bar might be. If I where to have this ”problem”, I would consider cutting off a useful length (after machining one end and parting off) finishing with two useful pieces for future use, or by using my fixed steady, machine and part off (or cut with a hack/band saw) from the original before facing the back side. Easy enough to decide in most cases.

Generally it is not a real issue - well not enough to need umpteen different possibilities. Like I doubt I would go to the lengths of welding on an extra piece, or anything like that, to save a few mm of bar.smiley The problem only usually arises when I want two pieces from a bar that is barely long enough!

Vic24/09/2019 18:52:31
2256 forum posts
11 photos

The jobs done now. It seemed quicker and easier to waste the 10mm or so for holding. The three jaw chuck was a surprise though as clocking it revealed it was only 2 thou out, less than expected. Onto the next project.

Howard Lewis25/09/2019 12:55:32
2341 forum posts
2 photos

You don't say what OD you want, so are we to assume that you want a 19 mm bore that is concentric with the 32 mm OD?

If so, why not, as already suggested, by Jason.

Cut off a piece 20 mm long

Mount in 4 Jaw chuck and adjust to run true within your tolerance for eccentricity.

Face and chamfer

Assuming that the faced end is outermost, centre drill, and gradually open up to say, 12 mm part way through (To minimise chance of drill wander )

Reverse in chuck, and realign for minimum eccentricity.

Face to length, and chamfer.

Centre drill and gradually open up to 12 mm. Hopefully the drill will be able to pass right through without problems.

Enlarge hole to 19mm. Bore through for the last 1mm, taking several spring cuts.

Hopefully, you will now have the end product that desire.

Howard

Vic25/09/2019 14:22:02
2256 forum posts
11 photos

See above, the jobs already finished.

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