Screw-cutting – Is There Something We’re Not Told?

[Nigel Graham 2Participant @nigelgraham2]

Thank you, Hopper, Howard!

Yes, I have used what I call “temporary centres” on quite a number of occasions.

Hemingway Kits even specifies it for using their Boring Bars, my first of their projects.  I’m not sure it gives much advantage there over using the conventional spindle centre because the tool will still rotate in a circle at a fixed distance along the machine. It does though work for turning long surfaces.

I made a driver to use with a chuck, consisting of a flanged ring with three grip screws, and two lugs spaced for a jaw to go between them.

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My first test for warping using that two collars method was with the better of my self-centring chucks, a 4-jaw unit I bought a few years ago and not used heavily since, and never on rough material. (Most of the basic turning I do is fine even with the old 3-jaw chuck, because it does not demand very closely parallel surfaces over long lengths.)

I will though try it again perhaps with smaller diameter bar I can hold in an ER collet.

 

I still need test the tailstock setting, and that will need making an indicator mounting for the face-plate.

The saddle prevents bringing the tailstock close to the chuck so whatever I set the probe to, probably a centre, needs the barrel projecting by much of its travel. I thought first this may lose test accuracy by magnifying any angular error, then I realised that can be an advantage.

What stops running the probe over the barrel itself? A sizeable keyway, that’s what, along the barrel’s horizontal-plane centre-line.

[HopperParticipant @hopper]

The outside of the tailstock barrel is not necessarily perfectly concentric with the tapered hole up the middle, so better to read off the internal taper. If it is a bit rough around the mouth as some old ones are, you can put a nice new centre in there and read off the morse taper on the centre right next to the end of the barrel.

Also using the dial indicator method, you can get a thou or more of “droop” in the set up holding the heavy dial indicator between the 12 o-clock position and 6 o’clock. Such a rig really should be tested by setting it up on a rigid shaft, zeroing the dial and then turning the shaft and rig upside down and observing the needle movement. Hence, my preference for a real-world turning test. But it makes an interesting exercise.

Just an aside, if you did ever have to align two shafts with a groove down one like that, using a dial indicator, you only need to get a reading on three points around the circle. At 12, 9 and 6 oc’clock in the case of the Myford with a groove at 3 o’clock. So you only have to swing the dial indicator by 180 degrees, not 360 or even 270.

What your are doing is aligning two circles when aligning two shafts. One is the circle the dial indicator body travels, the other is circle the dial indicator POINT travels. So if you are aligning two (theoretically) perfect circles, if three of the four points spaced at 0, 90 and 180 degrees apart are giving a 0-0-0 runout readings (or within tolerance of say 1 thou TIR) then the fourth at 270 degrees point MUST be also 0 (or within tolerance as the others). It can not be otherwise.

 

[Nigel Graham 2Participant @nigelgraham2]

To be honest if the tailstock barrel itself is not fully concentric there’s precious little I can do about it. The most likely cause is probably wear, particularly from tools like drills slipping.

Anyway I set up an indicator on the tool-post, despite sparse equipment for doing so. This showed a run-out of 0.002″ on a piece of ground bar in an ER collet, and the same error on the collet chuck body itself, giving some limit of accuracy for measuring the tailstock.

I cannot measure the tailstock barrel bore, only the outside of the barrel or some point on the protruding part of a centre. However, the outermost part of the barrel is least likely to be worn because it rarely goes inside the casting.

Next I mounted the DTI via two bits of aluminium-angle on the faceplate, projecting enough to just reach the barrel extended over the saddle. With the rear tool-post and top-slide removed for access I could swing the rather ungainly assembly perhaps 150º, giving something like that three-point test.

This proved first that despite careful adjustment of the tailstock’s slide there is some lateral swing due to giving it enough play to set the unit at the end of the bed: the vertical surfaces of the bed are worn in their central section. The tailstock starts to grip at its furthest forwards position. The lateral play is probably only about 0.001″ but produces enough yaw to affect significantly the centring at further extension.

Moving onto the lateral adjustment itself, I succeeded eventually in bringing the measured error down to about 0.002″, on opposite sides of the barrel (being careful not to catch the DTI probe in that keyway).

I tried it on two or three centres of varying lengths and possible wear-and-tear, but mainly measured on the barrel itself slightly back from its muzzle.

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I left it for the evening at that point, but the next stage will be more turning tests. I will probably repeat the test for bed twist but the result I obtained yesterday suggested I have set the bed level (by lathe-levelling definition).

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I have a Rabone-Chesterman 6″ engineers’ level, which is part of the “book” method, but whether it is sufficiently sensitive is another matter. The bubble is somewhat shorter than the space between the 2 marks, but I can test its relative sensitivity by feeler-gauges and surface-plate – possibly the milling-machine table. The sensitivity specification is 0.003″ or better per foot but I doubt this level is readable to such limits.

I’ve also a Wixey digital level but it’s not the most inspiring looking thing, from its small spirit-level that appears to have given up the ghost to three discs in the underside that might function better as surface contacts (if they are that) if they were not slightly recessed into the instrument case. Still, it may work as a comparator. And whilst the instructions are not quite the King’s English, I must admit I cannot read Mandarin.

The Myford manual though does give two further tests for level, one using a DTI and a bar in the chuck, the other by turning two collars on the bar.

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It would appear trying to fettle an old lathe that may have had a rather unhappy past is a matter of trying to play several unexpected sources of inaccuracy against each other towards a rather unsatisfactory compromise. While though feeling somewhat better myself than I did last week, realising the desperately, ridiculously slow progress and rough quality of my magnum opus steam-wagon – itself now over than 20 years old and its entire project over 40 – was sharpened by realising my own progress as a septuagenarian having lost no less than three long-standing friends within the last two or three months – and a fourth last Thursday.

Very likely, this poor old Myford ML7 lathe is like its owner…. well past its best!

[Graham MeekParticipant @grahammeek88282]

Non-concentricity of the tailstock barrel is more likely to be cause by someone using a Morse taper reamer mounted in the Headstock and trying to ream the Tailstock Barrel. That is without first ensuring the tailstock barrel is on centre-line with the headstock. This has been found from experience with other lathe owners when questioned.

Alignment or Levelling of the bed is carried out using a known ground “straight” piece of bar, like Silver Steel. Checking for straightness is done using two matched 90 degree Vee blocks on a surface plate and a DTI. Rotating the bar in the Vee blocks, with the DTI between the two Vee blocks. Should give a zero reading through out this test if the bar is straight, (please not we are not checking roundness as this requires 60 degree Vee blocks).

The two collar method will work but it is very time consuming when compared to the DTI method.

I have the same level. It is perfectly adequate for getting the lathe somewhere near. Turning the level through 180 degrees will improve the accuracy as you reach level.

Fine tuning comes from turning using the two collar method. With a well honed HSS tool, cuts do not want to be deeper than 0.001″ or 0.025mm. Carbide inserts usually rub with such small cuts and tell you nothing.

Regards

Gray,

[Howard LewisParticipant @howardlewis46836]

As you are finding, sorting out an elderly, and probably abused, machine can become a lifestyle in itself.

But the satisfaction of finding that every correction makes an noticeable improvement, as the sow ear increases the silk content.

What was a big door stop begine to be a useable machine. Sometimes, it can then be used to make the bits thatb will improve bit even more!

Keep up the good work.  Look how much better it is already.

Howard

[Nigel Graham 2Participant @nigelgraham2]

Well, supported by the workshop radio though an entire concert of Bruckner, the (church) service of Complin and half of “Music Matters” with Sir Mark Elder as guest finally saw me go as far as I can with this lathe.

I re-visited a few points, statically tested the variation on two points on an old gear-shaft mounted between centres, and reached a stage where the likely error will be only about 0.004″ diameter over 6, maybe even 8 inches, length.

Replacing the rear tool-post, squaring it against the faceplate for holding a parting-tool, finally revealed that the two nut sizes are indeed metric (13 and 17mm A/F). Grrr… That means the lathe needs yet another spanner, one not compatible with the rest of the machine and its attachments.

Cleaning the cross-slide to refit the top-slide showed a collection of pimply scars from trapped swarf. A few minutes of careful work with a very small, smooth file cleared them.

(I’d removed the top hamper for access and to give a mounting-point for the DTI stand)

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Only actually using it will tell.

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Thankyou all for your help and encouragement.

[Nigel Graham 2Participant @nigelgraham2]

SEQUEL:

After a few days leaving the lathe in peace rather than pieces, and me before I went to pieces, I set out yesterday to make the Feed-screw for the ‘Stent’.

Its main threaded portion is just over four inches long.

I held the stock between an ER collet and tailstock half-centre.

I found I have a chaser for the chosen 1/2″ X 20tpi UNF rather than the designed 3/8″ X 20tpi BSF, so removed the bulk of the metal with a single-point carbide tool and used the chaser to ease the thread to fit an ‘Autolock’ collet test “nut”.

It worked!  WOW!

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The fly in the ointment was the rough, sharp finish. I thought an aluminium or brass lap might sort that out, but inspection by lens showed no, the Vee-form thread was a W-form.

I might have inadvertently closed the half-nuts at an intermediate point on an early pass. They are not easy to engage smoothly, and it’s sometimes necessary to snap them smartly back fully open when they don’t close just when the thread-dial says. So I might have “jumped” over the divisions.

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So today I made a new screw, and obtained a very satisfactory result.

I nearly messed it up after all that by forgetting to put the gearbox itself back from the fine feed to screw-cutting setting for threading the hand-wheel spigot, though!

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One other little task on the lathe paid off.

The primary feed / screw-cutting choice is by removing a gear-pair from the banjo and replacing it the other way round. The retainer is a simple, rectangular steel strip hook-plate which originally had to be removed completely, risking losing a small screw, to let the gear by.

A few minutes of filing, with a washer as template, rounded the retainer’s end so it needs now only swinging clear to use. Much quicker, easier range-changing, no lost screw!

[bernard towersParticipant @bernardtowers37738]

Why change the 3/8″ thread for a 1/2″?. My Stent has been in use for close on 35 years and there is still no appreciable play between nut and screw. My son has just refurbed a Stent and one of the jobs was to replace the feed screw which to say the least was appalling, but I found a piece of quality rolled thread 3/8″ BSF which was inserted into the original feed shaft and now its like new. If you want a piece let me know.

[Nigel Graham 2Participant @nigelgraham2]

Good question! 🙂

It seemed a rather small thread for moving a lot of heavy metal. Not by the strength of the screw but by the bearing area of the thread – the larger being less loaded so of potentially longer life. All other things being equal, like my workmanship!

I do have a length of 3/8″BSF Studding bought for the purpose, but thankyou for the offer.

I was going to increase the diameter of the vertical feed-screw as well but that is probably be even less necessary. I may though fit the vertical slide’s central gib screw as a small thumb-wheel for clamping.

[old martParticipant @oldmart]

For aligning a tailstock, assuming that the Morse taper is ok is a test bar with the Morse taper end. They are invaluble for all sorts of alignments. And they have centres in the ends.

The Atlas at the museum has never had the “levelling” done even though it has been raised about 40mm on a block of aluminium at the headstock end to enable special adjustable screws to fit at the tailstock end for levelling. I did notice the other day that my 12″ Moore & Wright spirit level moved noticably when the belts were tightened even though the stand is made from 2″ box section welded steel. It is spot on with test cuts up to 4″ long are made just held in the chuck. And the test bar between centres is ok to 0.001″ in 12″ length.

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