Lathe alignment

[jgb7573]

Yesterday evening I did my first bit of turning between centres since installing the Super 7 at my new home. I was turning a five inch (approx 125mm) length of bar to size before setting over the tailstock to turn a taper (I'm making the columns for a vertical engine). Having taken the first cut I miked the workpiece to determine how much was to come off. I checked it at both ends of the cut and was surprised to see a difference of 6 thou from one end to the other. Now I was using self-act here so there should have been no difference. I cleaned out the centres, checked the setup and then took another cut, still 6 thou difference.

So I have an alignment problem with the lathe. I shouldn't have been surprised of course because I hadn't checked this since the lathe on its wooden bench was moved from the leaky garden shed with the wobbly floor at the old house to the concrete-floored, dry garage at the new one.

So I got out my between centres test bar, 15 inches long and with collars at either end. This was mounted in the lathe and then a cut taken over each collar without disturbing the cross slide setting. I checked the size of the two collars and there was only 1 thou difference between them. So it would seem that the twist or other misalignment is bad near the headstock and then gets better. That was a bit of a surprise.

So in went my 5 inch soon-to-be column again and I took another cut. Still 6 thou difference from one end of the cut to the other. Well at least I have a consistent problem to solve.

As it was getting late I stopped for the evening. I've slackened off the lathe holding down bolts and the first thing I'll do when I get an hour or two in the workshop is to check and adjust alignment from scratch, just as I should have done before I used the lathe for real.

At least there is still enough meat on the column to finish to size!

[Noddy]

is it wider or narrower at the headstock end?

6 thou = about 4 hairbredths you wouldn't easily see it.

can you mount a dti on the spindle and run it around the tailstock barrel to see if the two are actually aligned or where any mis alignment is?

failing that, my first thought is bed wear, as I'd have thought any wind would give consisitently increasing mis alignment. easy test is to snug up the saddle gib mear the headstock end and see if it gets tight as you rack the slide towards the tail end

Those tests are easy to do and easy to put back

if snugging up the gibs shows only marginal wear, then try messing with loosening and tightening the hold down bolts. an engineers level would be a big help in determining which bolts to tighten or which feet to put shims under.

keith

[alanstepney]

I wont comment on the alignment problem. You have and will get plenty of advice on that subject. Indeed, it has been mentioned here before.

However, I REALLY dont like off-setting the tailstock. It can be tricky to get right again afterwards, so once it is correct, I like it to stay that way.

Fortunately, there is a simple alternative.
Make a small attachment that has a morse taper (to fit the tailstock) to which is fixed a flat plate (which you arrange to be horizontal). That plate has a slot, along which one can fix a short centre, set as required by its fixing bolt, that runs in the aforementioned slot.
(I am sure someone will quote the ME or wherever I first saw it.)

Simple and quick to use, and it doesnt mean altering the tailstock.

[jgb7573]

Thanks for reminding me of that tool Alan. Like you I'm nervous of shifting the tailstock over so I'm going to see what I can do about it before turning the columns. There is a design by Ian Bradley for which Hemingway sells a kit.

But first I should get the alignment sorted. I'm going to follow the instructions in the Myford handbook. That method has worked well for me in the past. Hopefully that will fix the problem as the lathe was turning parallel in the old garden shed before we moved house (and workshop).

[alanstepney]

Ahh yes, it could well have been the Ian Bradley one I am thinking of.
Certainly worth making.

As for the alignment, Myford should know what they ae talking about so that is a good place to start.

[ravensworth2674]

Alan's memory is holding up because Ian Bradley DID cover it in his Myford book.
However, Tom Walshaw as Tubal Cain covered it as his Super 7 was on a slippery slope in the Southern End of the Lake District. More recently, Thomas has it in MEWM.
He goes into the use of a good spirit level to eradicate the possible twist in the bed.
I suspect that if the Myford was moved and re-installed that if it was OK earlier, the tailstock would be OK again.

No one has mentioned the razor blade 'technique' of two decent centres and holding a blade horizontal or skewed. It is not 100% accurate but it gives an idea. It also works for setting lathe tools at centre height- but using a softer metal insert to avoid chipping a tool.

( I was musing over the first coffee and wondering if the lathe blocks are Myford or if not, correctly shimmed)

Cheers

Norm

[stan64]

hi i am looking at the model engineer 2nd feb 1996 in foyles portfolio there is plans for a taper turning attachment that bolts on to bed of lathe leaving the tailstock in parallel position