Are we talking about a shaft or a bore

I'm thinking combunation of spindle bearings not set up correctly plus bed not level.

So acombination of work deflection which get smaller towards the chuck and a taper because you have adusted the bed to compensate for the deflection.

What does everyone else think?

regards Martin

All sorts of weird thing happen when you induce twist. What he has going on is very unusual if a tailstock centre is not being used.

This is a classic symptom of compensating for an out-of line headstock by inducing twist in the bed.

Can you get your hands on a machinist's level to get the bed un-twisted?

Mike what sort of tool are you using?

I use only a well honed HSS tool for machining test pieces after checking or resetting alignments. The type of tool and depth of cut are going to greatly influence the results. I limit the depth of cut to 0.05 mm or 0.002", this standardises the machining conditions.

I do think you are asking a lot of the Myford as regards the unsupported length, which I take to be 100 mm or 4". As you say you have 1" in the collet. On my Emco Maximat Super 11 which has a taper roller bearing headstock I only ever use a 75 mm protrusion of the test piece beyond the jaws of the chuck.

Personally I would start with the test bar in a spotless mandrel socket and run a clock along where a tool would normally pass and over the top. The latter test will tell you if the spindle is dipping or pointing skywards. The first test tells you whether the spindle is pointing down the bed centreline.

Provided you have not disturbed the headstock casting during your overhaul?

Then if the clock in the tool position is reading higher or lower towards the tailstock end then the bed is out of alignment.

To set this I either use an Engineers Level or a piece of 25 mm diameter silver steel kept specifically for the purpose. Grip one end in the 4 jawed chuck using protection pieces and clock to run true. Check the outer end and any high spot should be put at the top.

Return the clock to the tool position on the Silver Steel and clock from headstock to the tailstock end and adjust the bed feet until you get the same reading.

Remove this set-up and return to your test piece. It should not be that far out and minor adjustments to the bed feet at the tailstock end should put it right. Two collars on the test piece save a lot of time machining and preserve a HSS cutting edge.

Spot on concentric centres in the piece of Silver steel make setting the tailstock a doddle.

Regards

Gray,

Pete

Myfords have thrust screws to abutt the headstock to the rear shear before the 4 hold down bolts are tightened.

Personally I would ditch the collet in the original set up and hold a piece of free machining steel in the four jaw for rigidity and turn a collar test with a very sharp tool taking very little off at a time to set the bed up.

regards Martin

If the basics are ok such as no twist in the bed, reasonable bearings and no loose items, then I'd say it could well be attributal to wear of the vertical shears. You would only need ~ 0.0008" of movement to give a 0.0015" change in diameter, which would relate somewhat with your measurement of 0.0025" horizontal wear across the bed.

Get an accurate measurement of the full width of the bed in the worn area and also the width of the front way. Compare these readings with similar unworn areas of the bed to determine how much deviation exists at the inner front shear, which is the one likely to be most worn.

There's a conversion to 'wide gauge' guide that can be done to these lathes if it is wear that's causing your problem.

Martin.

I think, at the stage of testing umpteen times I would test with cutting between centres. At least that would avoid most of the possibilities?

If it is the same diameter at both ends and different in the middle, you have a wear problem with the bed/carriage,I would suggest. Both may well need attention.

Or just take a continuous cut over the 4" length of the 1" diameter bar. It does not take that long. And gives a full reading of what is going on.

Yes, using a level on a worn bed is fraught with danger as 5 thou wear on one shear can throw a sensitive level out by a lot. Over the 4" width of the Myford bed that could throw the level out by 15 thou per foot! By the time you twist the bed to compensate for that, it could be all over the place.

NDIY's between centres turning test might be worth a shot. If you adjust the tailstock to turn the same diameter on the test bar at both ends but the middle is still barrel shaped, it would tend to confirm suspicions that the bed wear is the culprit.

Very hard to check headstock spindle alignment with a bed that worn. A ground test bar etc may sit dead true to the spindle axis but then when you wind the carriage along the worn bed, it is going to follow the worn section, so the bed overall may be in line with the spindle, but the worn section is not.  Maybe an extra-long test bar so you can run the carriage on the far end of the bed where wear is least? Problems then with long bar flexing etc.

Best shot may be to try the wide guide conversion if yours is a pre-1972 narrow guide machine. Your carriage then at least will follow the unworn rear shear vertical surface instead of the worn front shear vertical surface. But if yours is a later factory wideguide machine, you can't do the reverse because the tailstock will have worn the inner vertical faces of both shears, including the narrow guide surface.

Edited By Hopper on 24/05/2020 09:39:57

I was refering to Jason's suggestion of a three-collar test bar in the previous post.

But the saddle runs on the full width of the top horizontal surface of both ways. So saddle wear is full width. Problem is the front way way takes most of the load and wears more than the rear way. Unless the Super 7 carriage is different from the ML7? (It doesn't appear to be but I dont have one to hand to check.)

I suppose you could measure the actual wear on the bed and shim the level accordingly??? Or build a "sled" that referenced off the relatively unworn underside of the shears where the lift plates run and sit your level on that??? Tricky though.

Edited By Hopper on 24/05/2020 09:57:47