Thread: Another question

Originally Posted by morrisman

I cannot seem to get my head around this . What is this setup supposed to check ? The cross slide is 90 degrees to the spindle ? Mike

you have got it mike. Ive never seen it befor thou

One of the most foolproof ways I have seen is to face off your faceplate then clock the back Half of the faceplate with a dial on th cross slide. The amount measured will be twice the error. No clamp to move on you this way, and no special jig to make.
Remember that a lathe should turn slightly concave, but never convex. I don't know what the allowable figure is but it is in Schlesingers.

Ew

Originally Posted by morrisman

I cannot seem to get my head around this . What is this setup supposed to check ? The cross slide is 90 degrees to the spindle ? Mike

Mike, the cross slide is NOT 90 degrees to the spindle axis. It is on purpose slightly off, such as to ensure a surface faced in the lathe will be slightly concave, but never convex. Think about it, if you put a convex faced workpiece onto a flat surface, it will rock. Whereas a concave faced workpiece will be stable. Same if you assemble faced components together, if the surfaces are convex the assembly will not be stable.

To ensure that a new lathe faces slightly convex, the cross slide dovetails are slightly angled in respect to the bedways as seen from above. All inspecton protocols for lathes demand a facing cut to be either perfectly flat or something like 0.05mm concave over a 100mm facing diameter. No convexity is tolerated. An old and worn out lathe OTOH is likely to face convex. Of course, a very expensive precision toolroom lathe will only cut a very tiny bit concave, whereas a cheaper lathe will usually cut quite a bit concave.

The picture that you show is just another way to measure the same thing. It eliminates errors that mat be due to using a blunt tool or taking a too deep facing cut or failing to take surface speed into consideration when choosing the spindle rpm. If you are able to take a facing cut over a 100mm diameter workpiece without letting the tool rub at the periphery where surface speed is highest, or deflect the tool towards the center where surface speed is slowest, taking a facing cut is the fastest method to assess concavity. Do not cut past the center though, as this will falsify the result. If you do not yet have the experience to do a perfect facing cut, it may be more reproducible to use the method shown in your book.

But, as I understand it, the cross-slide isn't checked against the spindle axis. Spindle axis and cross-slide are checked against the bed ways.
If you used the above test to set your cross-slide arent you stacking errors?

MTR says 0.0005" per 6"(if I'm reading it correctly lol)

Stuart

Originally Posted by Stustoys

But, as I understand it, the cross-slide isn't checked against the spindle axis. Spindle axis and cross-slide are checked against the bed ways.
If you used the above test to set your cross-slide arent you stacking errors?

MTR says 0.0005" per 6"(if I'm reading it correctly lol)

Stuart

In reality it is..

When I did the 10EE..

bed was ground..

headstock scraped into the bed, aligned parallel with spindle axis (I think the spec was the error had to be towards the tool pressure)

The cross slide had been scraped in previously... So after applying turcite to the saddle ways, the saddle was scraped so the cross slide ways were aligned with the headstock....

The test to test this was a parallel in the four jaw chuck, adjusted so upon a 180 degree revolution the DTI read 0-0.. Then slide the cross slide along the parallel and check the error and correct... Getting the parallel to read 0-0 took a lot more effort then you may think...

Remember all these DTI tests rely on the bed being straight parallel and unworn..

If your bed is worn, then what part of the bed do you align the headstock to?

A lot of it comes back to what sort of work you expect from the machine...

Not sure how Phil gets on when he has to relocate a worn machine, and how he manages to relevel/realign it.. Experience I guess.

Originally Posted by Stustoys

But, as I understand it, the cross-slide isn't checked against the spindle axis. Spindle axis and cross-slide are checked against the bed ways.
If you used the above test to set your cross-slide arent you stacking errors?

MTR says 0.0005" per 6"(if I'm reading it correctly lol)

Stuart

Of course you are right. When manufacturing the cross slide ways into the saddle, you reference them against the bedway Vee's at the bottom of the saddle at 90 degrees. But like all machining operations, you have to allow for a tolerance. And this particular tolerance happens to be an offset tolerance. Meaning it is not 90 degrees +/- 0.xx degrees. It is 90 degrees +0.0000/-0.xx degrees.

Later, when the lathe is assembled and adjusted, there will be absolutely NO means to correct this anymore by adjustments alone. For this reason, the facing test cut is usually the very last item at the bottom of the test protocol. If the facing cut is convex, the saddle is a reject and must be removed and replaced, or the cross slide ways re-worked (scraped) in situ until the lathe faces concave.

See also the bible of lathe alignment: "Testing Machine Tools" by Georg Schlesinger. At the very end of his test chart for "finish turning lathes up to 400mm swing" you can read:
- Lathe faces (hollow or concave only) 0 to 0.02mm per 300mm in diameter

Originally Posted by .RC.

The test to test this was a parallel in the four jaw chuck, adjusted so upon a 180 degree revolution the DTI read 0-0.. Then slide the cross slide along the parallel and check the error and correct... Getting the parallel to read 0-0 took a lot more effort then you may think...

(I think) This is why Nick Muller suggested using some dowel pins, square and a DTI to check the squareness of the cross-silde ways by moving the carriage.

Originally Posted by cba_melbourne

Of course you are right. When manufacturing the cross slide ways into the saddle, you reference them against the bedway Vee's at the bottom of the saddle at 90 degrees. But like all machining operations, you have to allow for a tolerance. And this particular tolerance happens to be an offset tolerance. Meaning it is not 90 degrees +/- 0.xx degrees. It is 90 degrees +0.0000/-0.xx degrees.

Yeap I had it ass about.
As long as the "error" of the cross-slide is less than the "error" of the spindle its all good*. No stacking of errors.

I think

Stuart

*as long as things aren't so bad you have problems else where

Originally Posted by Stustoys

(I think) This is why Nick Muller suggested using some dowel pins, square and a DTI to check the squareness of the cross-silde ways by moving the carriage.

Cannot say I am real fussed on that idea.... Error stacking comes to mind when I think of that..

Originally Posted by Stustoys

(I think) ......
As long as the "error" of the cross-slide is less than the "error" of the spindle its all good*. No stacking of errors.

I think

Stuart

*as long as things aren't so bad you have problems else where

???
Stuart, this is not simply a cross slide "error". The "error" is intentional, it is an integral part of lathe design. It is a paramount requirement for the lathe to face hollow. A lathe that faces convex, no matter how little convex, is a reject and cannot be sold with an inspection certificate.

And this built-in cross slide offset tolerance is usually twice as large than the horizontal spindle alignment tolerance or "error" as you call it. It is up to 0.02mm over 300mm facing diameter (measured over 150mm from edge to center of face), eg it would be the equivalent of turning a taper of 0.02mm over 150mm if the headstock alignment was off by the same amount. There is no stacking of errors, as it is two completely different things that you measure here.

Originally Posted by .RC.

Cannot say I am real fussed on that idea.... Error stacking comes to mind when I think of that..

Almost all stacking errors can be removed by taking two measurements(In fairness to Nike I think is was more intended for keeping an eye on things and at least moving in the right direction. than a final alignment)

Originally Posted by cba_melbourne

???
Stuart, this is not simply a cross slide "error". The "error" is intentional, it is an integral part of lathe design. It is a paramount requirement for the lathe to face hollow. A lathe that faces convex, no matter how little convex, is a reject and cannot be sold with an inspection certificate.

Yes. By "error" I meant from perfect(be that in spec or not). Even if the spindle was out by 5 degrees... as along as the cross-slide is out by less than that the lathe will cut concave. Yes as I said I had it ass about...... so no stacking.

Stuart