Checking lathe alignment?

[Andrew TinsleyParticipant @andrewtinsley63637]

I am slowly rebuilding my Myford ML7. Next problem is how do ensure that the mandrel axis is aligned to the tailstock centres and also the latter two adjustments parallel with the lathe bed shears?

I ask because I have refitted the lathe with new white metal bearings. I did ask about refitting new bearings but got no advice from forum members. So I am not sure that I have done the correct fitting method.

Andrew.

[Andrew TinsleyParticipant @andrewtinsley63637]

[HopperParticipant @hopper]

Alignment of the mandrel axis has nothing to do with the tailstock. Tailstock is the very last thing to be set up when rebuilding a lathe, simply because is independent of everything else and can be adjusted to suit on its own.

Headstock mandrel needs to be aligned so it is parallel to the bed ways in both the horizontal and vertical planes. Very tricky to do if you have a worn bed with a .005" hollow worn in it at the chuck end where the carriage is most used. But if you reckon your bed is in useable condition, and has been set dead level at both ends,  and if your carriage ways and gib are in OK condition, the quick check is to place a 2MT test bar in the headstock spindle with the 6" or 12" long parallel ground section protruding out in the normal area where a turning job would sit. No tailstock is used for this test. A dial indicator mounted on the carriage with the plunger resting on the test bar and the carriage racked back and forth while observing the dial indicator. First done with the dial indicator on the side of the bar, then the top. If the dial indicator reads 0 or within one thou of it all the way along, job's good. If you get a bigger reading, the headstock may need adjusting to suit. (Be sure to check the test bar is running concentrically when rotated before starting the alignment test.

Another way to test headstock mandrel alignment, and a test that should be done after the above anyhow, is to hold a piece of one inch diameter bar in the three jaw chuck, sticking out about six inches, and take a cut along it. Again, no tailstock in place. If the mandrel is aligned, the turned bar will have less than one thou taper over the length of the cut. If the bar turns down tapered, headstock may need adjusting to suit.

Headstock is adjusted by either shimming or scraping in the vertical plane and in the horizontal by using the two clamping screws that are in the front of the headstock at its base, along with shims or scraping the clamped surface of the headstock where it bears against the bed, which necessitates the loosening and tightening of the four headstock hold-down cap screws inside the headstock below the backgear assembly.

Once you have headstock mandrel aligned to the bed axis in this way, THEN you can go on and set the tailstock. Quick and rough way is to put a centre in the headstock mandrel and one in the tailstock and then pinch a thin steel rule between the two. It should stand up vertically and also square across in the horizontal plane if all is aligned well. It is adjusted horizontally by the offsetting screws in the tailstock. If the tailstock is too low, shims can be added between the tailstock body and its base. You can also chekc the alignment of the tailstock quill's number two morse taper with the lathe bed by inserting the MT2 ground test bar into the tailstock taper and running the carraige mounted dial indicator up and down it as was done for the headstock alignment.

There are of course other ways of doing this but this will get you going.

Edited By Hopper on 17/04/2017 11:47:18

Edited By Hopper on 17/04/2017 11:48:35

Edited By Hopper on 17/04/2017 11:51:02

[Andrew TinsleyParticipant @andrewtinsley63637]

Thank you Hopper,

Those tests have been done with the exception of checking the vertical height of the bar. All done during the levelling process for the bed. I shall go back and check the vertical alignment. Both the lathe bed and saddle have been reground, so these should be fine.

Thanks for an excellent and very clear explanation of how to do the job. I am a trifle annoyed with myself for not working it out myself. I suppose there is always the thought that the job involves some wizardry, but it is straightforward logical thought. Not a strong point with me at the moment due to some pretty strong medication!

Thanks once again,

Andrew.

[David George 1Participant @davidgeorge1]

I also check the centre of the tailstock after aligning the headstock by putting a shaft into the chuck fastening a dial test indicator on it and spin the chuck while the DTI is in contact withe the morse Taper and adjust as necessary then repete with a test shaft in the tailpost at different lengths. Make sure tailstock is locked when testing.

David

[mechman48Participant @mechman48]

Posted by David George 1 on 17/04/2017 23:09:23:

I also check the centre of the tailstock after aligning the headstock by putting a shaft into the chuck fastening a dial test indicator on it and spin the chuck while the DTI is in contact withe the morse Taper and adjust as necessary then repete with a test shaft in the tailpost at different lengths. Make sure tailstock is locked when testing.

David

Reminds me of reverse alignment procedures I used to do on gas turbine/ gearbox/ pumps etc back in the days of yore in the desert environs of the middle east, only a wee bit bigger !

​George.

[Swarf, Mostly!Participant @swarfmostly]

I'd like to put in a word about use of the DTI. They usually come with a ball tip that works best if it is accurately set to the same height as the test bar.

My personal preference for this situation is to change out the ball tip and fit an 'elephant's foot' type of tip. That makes the set-up more tolerant of minor height differences.

You just need to be a bit careful when buying the 'elephant's foot' tip – there are at least two different stem threads depending on the DTI maker and whether the instrument is Imperial or Metric.

Obviously, if you support the DTI by a clamp around its stem, you have to fit the 'elephant's foot' after you've passed the DTI stem through the clamp.

Best regards,

Swarf, Mostly!

[HopperParticipant @hopper]

Posted by mechman48 on 20/04/2017 10:06:18:

Posted by David George 1 on 17/04/2017 23:09:23:

I also check the centre of the tailstock after aligning the headstock by putting a shaft into the chuck fastening a dial test indicator on it and spin the chuck while the DTI is in contact withe the morse Taper and adjust as necessary then repete with a test shaft in the tailpost at different lengths. Make sure tailstock is locked when testing.

David

Reminds me of reverse alignment procedures I used to do on gas turbine/ gearbox/ pumps etc back in the days of yore in the desert environs of the middle east, only a wee bit bigger !

​George.

I used to align steam turbines in power stations, to about the same tolerances as a hobby lathe – but the shaft and rotor unit weighed in at 80 tons. All the same principles though. In fact I think it was easier to scrape the bearings for an 18" diamter shaft than the 1" jobs on small lathes!

[HopperParticipant @hopper]

Couple more things to look out for: if you use the dial gauge mounted on the lathe spindle, make the mounting bracket or arm as short and stout as possible or you can get a thou or more of sag due to the position of the guage changing as you run it round the 360 degrees.

And be aware that many of the old time Brit lathe manufacturers used to make the tailstock a few thou higher than the headstock spindle to allow for wear over the years. A bit of vertical misalignment is not critical and can be compensated for by horizontal alignment with test cuts on a test bar between centres used for final setting. This actual cutting test is more important than getting your dial gauge reading dead on. It is the "real world" test under working conditions, which can cause deflections etc that make a perfectly dial-gauged lathe not turn perfectly true.

[Martin ConnellyParticipant @martinconnelly55370]

I have had to work out the sag on very large arbours for reverse alignment years ago. I also wrote spreadsheets for calculating the machining of spacer shims to go between turbine and generator underbases from the resulting figures. Lasers alignment has made alignment so much easier. I gave a small machine optical alignment kit to a local astronomical society for use as spotter scopes rather than scrap it off when we started using lasers.

The empirical method for finding the sag on a support arm or arbour was as follows.

With the arbour resting on the floor use a spring balance to find the force needed to lift the free end of the arbour off the floor. Clamp the arbour securely to something solid and vertical (we used building stanchions) then put a suitable indicator underneath the free end and set to zero. Apply the force needed to lift the end found earlier and note the deflection of the indicator. stamp this value on the arbour for use when doing reverse alignment.

These arbours were often well over 1 metre in length.

Martin C

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