Mounting then re-mounting material in a lathe

[jps1982Participant @jps1982]

I’m starting to learn to use a lathe (Myford ML7) and have a very basic question.

Is it necessary to do all the turning work in one sitting.

Or is it acceptable to put material into a lathe, machine it, then remove it and bring it back later for more machining.

Im planning my first piece of work, which will be to make a pair of bushes from Delrin. I’ll find it easiest to carry out a sequence of cuts which mean I turn the work around in the chuck partway through machining it, but not sure if I can reliably make two matching pieces that way,  or if there will be too much variance in the exact mounting that I’ll be able to achieve?

[Pete RimmerParticipant @peterimmer30576]

Bushes suggest ID, OD and facing off ends for a simple operation. You should drill and bore the ID first then you have a choice, you can turn the OD and face one end, chamfer the inside and outside corners and part it off then put it back in the chuck to face off the other end and chamfer the edges again.

Much depends on the dimensions of the part but that would be my sequence of operation. If you can, drill and bore to double-depth plus parting allowance so that you can cut both in one setup.

[Chris CrewParticipant @chriscrew66644]

No 3-jaw chuck will repeat the concentricity of the work if it is removed and then replaced. You can possibly improve the concentricity by marking on the work and one jaw of the chuck with a marker pen so that the work can be returned to the same position in the chuck and it would pay to make a note of which key hole from which you released the work and use this same hole to re-tighten the chuck. The work, in all probability, will not be perfectly concentric but it it may be acceptable, depending on the level of precision you require.

If you want to achieve concentricity you will need to ‘clock’ the work with a DTI using either a grip-tru chuck (expensive!) or, more commonly, a 4-jaw independent chuck. I recently had to do this to accurately centre-drill both ends of a mandrel that was going to be re-sized on a cylindrical grinder between dead-centres and achieving an accuracy, at least as indicated by the DTI, to 0.0005″ was relatively easy. It takes a little practice and ‘feel’ to know which jaw to slacken slightly and which jaw to tighten but it’s not a difficult skill to acquire.

[SillyOldDufferModerator @sillyoldduffer]

Generally, it’s best to plan cutting sequences to minimise moving the job.   Particularly important when using an ordinary 3-jaw chuck because these don’t reset accurately.   Thus, when a rod has to be drilled and turned so the hole and outside are concentric, do both operations without dismounting the work.  This guarantees the hole and the turning share exactly the same axis.  If job is moved between the two cutting operations, the common axis is lost.

Of course, most complex jobs can’t be done without moving them between machines, or into new positions on the same machine.  On a lathe, 4-jaw chucks are often used for this.   As the jaws can all be adjusted independently, the operator can accurately reposition the job  with a Dial Indicator.   With practice this can be done quite quickly so some machinists use a 4-jaw for everything.   Otherwise, three jaw chucks are fast, but not accurate when work is moved.   Four jaws take time to set up, but can reset work accurately.

If money is no object, accurately made and adjustable 3-jaw chucks are available.  They don’t perform better than a 4-jaw, but save time,

For speed and accuracy, collets are the best answer.  They’re repeatable without requiring the operator to measure anything.  Unfortunately collets are expensive and only grip over a narrow range, so not general purpose.  Brilliant for work like clock-making though, and occasionally useful in my workshop.  I mostly use ER32 collets to hold milling tools, but they’re also good for work-holding.  So I own an ER lathe collet chuck, and some Stevenson Collet Blocks.   Blocks can be held by a 3 or 4 jaw chuck, or a bench vice, or clamped to a milling table.  Again not general purpose, but they allow work to be reset accurately:

 

I’m sure other members know other ways of repositioning work accurately after it’s been moved.

Dave

[BazyleParticipant @bazyle]

Because you are using Delrin which is soft that anyway will introduce a lot variation. One of the traps is to assume that because the part close to the chuck is accurately concentric that the far end of a bit sticking out 3 inches is also in line. chuck it 3 times and every time the far end will be ‘off’ by a different amount so pay some attention to this as you tighten the jaws.

[Howard LewisParticipant @howardlewis46836]

As already said, the chances of finding a 3 jaw chuck that holds work concentric, at almost any diameter, are very small.

If you are making plain bushes, with just one internal and one external diameter, I would suggest that your procedure to ensure concentricity betweem Od and bore, is

1 Face end of material.

2 Centre drill

3 Drill almost to size, in stages. (slightly longer than you want the bush to be – allow for parting off, and final facing op.)

4 Bore to finished size  You might want to chamfer the bore before the boring operation begins.

5 Turn OD to size, (slightly over length).

By machining the bore and the OD without disturbing the work in the chuck, they should be concentric.

6 Part off slightly over length.

7 Reverse part finished job in chuck and face to length.

8 Chamfer finished bore.

Since you are making more than one bush, I would suggest leaving Ops 7 and 8 until Ops 1 – 6 have been carried oiut on all the bushes.

HTH

Howard

[Chris GunnParticipant @chrisgunn36534]

Pete has got it in one as far as making bushes are concerned, if you want more accuracy a low cost option is to get a set of soft jaws for your 3 jaw chuck, these can be bored to suit the diameter of the work, and provide a square face to locate the back end of the work, and will give as good concentricity as you will probably need. I use mine a lot, and when they gradually get machined down, I weld more material on the front end and start again.

Chris Gunn

[jps1982Participant @jps1982]

Thanks all for the guidance, much appreciated!

[JAParticipant @ja]

As experience increases and the work you do becomes more complex the only answer is a four jaw chuck and dial indicator or good quality collett chucks (ruling out face plates because of the skill required). Things will become more complex and you have to start thinking about and using datums.

Many will castigate me for making machining sounding hard. However, as Chris says, centering a job in a four jaw chuck is a skill easily mastered (as an mechanical engineering apprentice, the first day on lathes and using chalk). One day that three jaw self-centering chuck will of more use as a doorstop than on the lathe. I did not use mine for years, until recently to hold a fly cutter.

JA

[Nigel Graham 2Participant @nigelgraham2]

I would, and do, use Howard’s approach.

Some soft plastics are awkward to machine thanks to that softness. For the last operation, facing the flange end of something like a thin-walled bush, it can be better to hold it with the bore plugged with a short piece of metal or rigid plastic for support.

They can be awkward to machine in another way. I don’t know about ‘Delrin’ (it is a brand), but when drilling some thermoplastics the drill can bind, either by expanding from the heat more than the plastic does, or by the soft swarf jamming. So needs careful feeding and frequent swarf clearing.

[HuubParticipant @huub]

Delrin is POM.

Machining plastics to some accuracy, requires some finesse when tightening the chuck. I prefer my ER32 collet chuck (range 3..20 mm) but that is not always possible.

I try to do the most critical dimensions first in one fixation. If you have to remove a lot of material, the inner tensions will deform the part. Do a rough outer and inner turning and than finish the part to the required dimensions.

Plastics are elastic so drilling using a 10 mm drill, results in a hole slightly under 10 mm. That can be a benefit if you want a tight fit.

If you feed to slow, the tool will rub and the plastic will melt. So keep the feed “high” and the RPM “low”.

Delrin is “easy” to machine on the lathe with a good finish. Use a sharp tool, an insert for aluminum or a fresh ground tool.

 

[Martin KyteParticipant @martinkyte99762]

Your options are:-

GripTru chuck 3 jaw

Soft Jaws

Between centres turning

Collets

4 Jaw independent chuck

Mandrels and Faceplate fixtures

this is a general list for you to explore and not all methods will apply to your bush job.

regards Martin

[Ramon WilsonParticipant @ramonwilson3]

On ops like these described by the OP, providing there is a through bore, then a simple and quickly made mandrel to fit the bore will allow repetition and perfect concentricity to be achieved until the mandrel is removed. On any soft material the use of any type of chuck is fraught with distortion issues.

If no through bore then a very slightly tapered mandrel that is a push fit into the bore can suffice providing light cuts are taken

 

BTW Dave – surely that collet set up means the collet is only as good as that three jaw chuck 😉

Tug

[Ramon Wilson]

On Ramon Wilson Said:

…

 

BTW Dave – surely that collet set up means the collet is only as good as that three jaw chuck 😉

Tug

Good point.  The set-up Tug questions is this one, which I copied off the web:

Obviously considerably inferior to a Collet Chuck mounted directly on the spindle, but I think it addresses the 3-jaw reset problem.  The Collet isn’t moved in the chuck, so the collet remains aligned on the same axis.   Therefore work can be moved in and out of the collet, and other collets fitted.    Accuracy must suffer though:  even if it’s perfect, the collet block overhang amplifies the 3 jaw’s run out.  But I think the arrangement is better than just using a plain 3-jaw.

I say “I think” because I haven’t tested it with a DTI, and I’ve no chance of trying today!  I’m already late for an appointment…

Dave

 

[Ramon WilsonParticipant @ramonwilson3]

The thing is Dave the whole set up defeats the use of a collet – if the accuracy of which is that what is required.

As a means in which to use a collet for better work holding of small diameters however then yes, okay apart from, as you point out, the overhang.

I may not have made myself clear in my last post to JPS, the original poster.

Once your material is faced to length and the bore drilled/reamed/bored (as you wish) then all external work is carried out on the mandrel allowing removal and reversal if required ensuring good concentricity (providing the fit to the mandrel is a good one)

Hope that helps

Tug

[bernard towersParticipant @bernardtowers37738]

The basic 3 jaw is what it is, its for completing 95% of intended work just leaving parting facing and deburring.

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