Myford bed twist

Hi,

Im just wondering about the bed twist on a Myford lathe. I do regular diagnostic checks on my S7. I always follow the recommendation of setting the bed twist to avoid issues with tapering especially on long materials being turned. I did the check on a 1 inch stock over 8 inch long from the chuck without any tail stock support and used a micrometer to check both the distal and proximal cuts. This came back as equal measurements. All is aligned and cutting as expected.

The problem I noted is when a turned down an aluminium round stock from a 12mm down to 10mm over 8 inch long. This was supported by a tail stock with proper live center (center drilled) and properly set. Im surprise to see a very slight taper. 10mm distally (away from the chuck) and 10.2mm proximally (near the chuck). Cuts are light around 1-2 thou with a brand new CCGT insert. All are tight on the set up. No loose gibs cross slide are locked. The tailstock was not adjusted and I can see the witness marked on the tailstock dead on. Is this just deflection? is this due to the length of the aluminium? I don't have a round steel that size to test at the moment. If anybody can help with this it would be much appreciated.

Thanks,

Raphael

Edited By RAPHAEL VAL GOLEZ 1 on 23/12/2019 11:06:07

Could be a number of causes.

Is the tailstock centre in good condition?

Is it cleanly mounted in taper?

Last and by no means least, how did you drill the centre for in the aluminium material?

What you have effectively done is the test for confirming that your tailstock is aligned except that you might have been holding the work in the chuck and not on a freshly cut centre. If the error on diameter is 0.2 then the offset is 0.1 or 4 thou in real money. You had tested on a 1 in bar but lots of chucks vary at different diameters, especially around 1/2 in where it is most used and depending on how it was tightened. Then getting the tailstock aligned and snugged up and extended and locked in the same way as when previously tested all add possible problems.
I don't think I have ever done such a long length trying for truly parallel as I find things only need to be accurate at each end and if I need a long (5in) accurate piece for a sliding fit I use silver steel. Perhaps that's cheating.

I would suggest that the point of the tailstock centre isn't quite central.

Several possible reasons, tailstock itself not central, barrel wear allowing clamp to offset barrel in tailstock casting, a bit of debris in the morse taper bore, or between tailstock and bed tilting it over, slightly bent running centre spindle; I'm sure there are others.

How to find out which is harder;
Maybe the following. Do you have a dial gauge? if so set it up in the toolholder.

Edit, Sorry just realised you are using a chuck at the headstock end;
You already have a long bar which appears to be tapered, so zero the clock on that bar, at the headstock end on a portion you have just turned to size (any size).

Try turning a bar between centres and see if the error repeats. If so then move carriage to tailstock end observing the clock; since you've already turned the bar between centres, using those settings, the needle should barely move.
Leaving the same settings on the toolpost, reverse your workpiece; I presume the clock will now give a different reading due to the different diameter. Keep the same settings and investigate how each of the above causes a change in that reading.
i.e. loosen and tighten tailstock clamp, barrel clamp, slightly rotate running centre body, and then full 180° etc.
Replace running centre with a plain one, clean taper etc. This should hopefully point to where the tailstock centre offset is being generated.

Further to that, if nothing immediately shows, then the tailstock casting centre line might be off, which is adjustable as you know. If there is a need to make a new bar perfectly parallel, and the same length, then you already have the correct setting on your clock gauge. Using the tailstock offset screws adjust it so the clock reads zero.
This should be repeatable for your new workpiece, but perhaps only for this exact length.
 

Alternatively if you need various pieces, you could always use an MT2 boring head with a centre in it, so you can dial in an offset, much like turning a taper; in this case it's just a parallel taper. 

In my case I made a dead centre for the boring head using a broken carbide centre drill.

Bill

Edited By peak4 on 23/12/2019 12:02:55

Edited By peak4 on 23/12/2019 12:07:24

Also need to check the tailstock centre is inline with the headstock centre in the vertical plane. Not so easy to correct though.

John

Thanks for the input gents.

Peak, thanks for the advice. I have more or less check the bed twist and it seems ok on my test cut unsupported. I will get a tailstock test bar and indicate it. I don't have a large drive dog to accommodate a steel round stock sturdy enough to make the in-between center test. A ground test bar will be faster and I can see how much to adjust the tail alignment. The bed is not that bad but I was just thinking of what Journeyman have said. How do you verify the vertical plane? height gauge?

You will find an alignment bar to be of great usefulness. I have two of these with a MT2 one end and centres each end, one about 7" and the other 11" long. To check the tailstock, it can be used in two stages.

1 put a piece of 3/8" stock in the chuck projecting about 1/2" and turn a 60 degree taper on the end. Do not remove it from the chuck. Then put the test bar between the centre you have just produced and a DEAD centre in the tailstock. Mount a dti on the saddle and run it along the side of the bar to check the tailstock alignment. Adjust as required. Then run the dti along the top of the bar to check the height of the tailstock relative to the headstock.

2 fit the MT end of the bar in the tailstock, then you can check if the quill is in line. Try this over the full stroke of the quill.

As you have proved that the error is not with the headstock, most of the error will be in the tailstock, and if the bed was twisted, it would show up when you turned the 1" X 8" bar. 

https://www.ebay.co.uk/itm/2MT-Lathe-Alignment-Test-Bar-MT2-Alloy-Steel-Over-All-Length-283mm-11-1-8-Inch/174007649507?epid=19028343798&_trkparms=ispr%3D1&hash=item2883aa04e3:g:5vUAAOSwxBNcypvK&enc=AQAEAAACUBPxNw%2BVj6nta7CKEs3N0qULvMK1kOsJidtOIv6tvoFynHVpYAhNT3AO6m1jdy3yT6ga38eVq60l7SkJV3oAAhGC4p6bxn5N13yisHN7T%2FmhH08Ka4nZOK7nkZSqP9u1%2BPQ5EDPMjW4i1Lw0gLkB8s7oltqacLqMWcjuTqswwi2fceN3%2BuwiVXKGHyTPMYOZFZpn3%2BsIGRYVlwGzj8jgT%2FD%2Fhk4cD0wqthCN5PQ%2FhPPrM%2B0NSlCwhYZSdCbgTS62tgeq7srEeBukvIFsc73AfVBqhLtB%2BTWLTE1Exz9ogURD6K0dslLDct6qUf8FbPxyqanh1FGAoTL%2BJbK8kcMIWqdxVNRkPgqtBtjHbItmmCInuXJWTj%2Bjvg5L%2FHrxyzUJ35%2Fnq%2FOFeLF2QqTn%2F7g6Nwnbp1RQlUZiImzAc0PsMI13PQE%2F70xj%2F3urN9QcMu%2FSoHPH%2FLpFCpMa%2FmvOfnybmF9cyUpck1XGtlcNmIh6ik%2F7BFoyYHWOfO%2BMY62ZDL0%2FLSTWMTQiB0tKCb9bHzPa5q0AfpkkjZCjb%2BTEyltIpsbPItkv0xKASwasuwiJkWD1niHk3sia7p5A%2Bks3Z8omYcysh89rlDPHrhWRMEhZu2mevKMqKh2v7%2FFt1duVh6D9hcwu%2FRxMR%2ByX%2F2KekEBYA5xc0X54q2wGPJ%2BsxmiSlDrs4D17WNIBUyxnfGtqPKFdS956qg3p9KaGGo845%2FaPxxm3lIV7t0x0dD08Z09MS1Q3y%2BJyCNN%2B9uoORuWasywoQ4Yzuk%2FYwlxvx4QSvzNn%2Fkc%3D&checksum=17400764950763df7ac517714bffa2fafc72247dacd5&enc=AQAEAAACUBPxNw%2BVj6nta7CKEs3N0qULvMK1kOsJidtOIv6tvoFynHVpYAhNT3AO6m1jdy3yT6ga38eVq60l7SkJV3oAAhGC4p6bxn5N13yisHN7T%2FmhH08Ka4nZOK7nkZSqP9u1%2BPQ5EDPMjW4i1Lw0gLkB8s7oltqacLqMWcjuTqswwi2fceN3%2BuwiVXKGHyTPMYOZFZpn3%2BsIGRYVlwGzj8jgT%2FD%2Fhk4cD0wqthCN5PQ%2FhPPrM%2B0NSlCwhYZSdCbgTS62tgeq7srEeBukvIFsc73AfVBqhLtB%2BTWLTE1Exz9ogURD6K0dslLDct6qUf8FbPxyqanh1FGAoTL%2BJbK8kcMIWqdxVNRkPgqtBtjHbItmmCInuXJWTj%2Bjvg5L%2FHrxyzUJ35%2Fnq%2FOFeLF2QqTn%2F7g6Nwnbp1RQlUZiImzAc0PsMI13PQE%2F70xj%2F3urN9QcMu%2FSoHPH%2FLpFCpMa%2FmvOfnybmF9cyUpck1XGtlcNmIh6ik%2F7BFoyYHWOfO%2BMY62ZDL0%2FLSTWMTQiB0tKCb9bHzPa5q0AfpkkjZCjb%2BTEyltIpsbPItkv0xKASwasuwiJkWD1niHk3sia7p5A%2Bks3Z8omYcysh89rlDPHrhWRMEhZu2mevKMqKh2v7%2FFt1duVh6D9hcwu%2FRxMR%2ByX%2F2KekEBYA5xc0X54q2wGPJ%2BsxmiSlDrs4D17WNIBUyxnfGtqPKFdS956qg3p9KaGGo845%2FaPxxm3lIV7t0x0dD08Z09MS1Q3y%2BJyCNN%2B9uoORuWasywoQ4Yzuk%2FYwlxvx4QSvzNn%2Fkc%3D&checksum=17400764950763df7ac517714bffa2fafc72247dacd5

Edited By old mart on 23/12/2019 17:20:31

I never expected such a long link, still, it works.

I have found that the tailstock quill sometimes needs clamping very tight to get the centres to align properly.

.

I’ve mentioned this before, but ebay still keeps catching us out

Everything from the question mark onwards is superfluous to our needs

... it’s all ebay tracking how you reached the page.

MichaelG.

.

Demonstration:

This should link to exactly the same page

https://www.ebay.co.uk/itm/2MT-Lathe-Alignment-Test-Bar-MT2-Alloy-Steel-Over-All-Length-283mm-11-1-8-Inch/174007649507

Edited By Michael Gilligan on 23/12/2019 17:50:13

Old Mart, Thanks.

I am planning to use a MT2 dead center on the headstock attached straight within the spindle bore to eliminate the chuck. I can see that you mentioned to use the chuck and turn a 60 degree round stock to align the chuck itself. I can do both but which would be more accurate, chuck or straight onto the spindle bore?

Personally, I'd use a bit of bar held in the chuck and turned to a 60° taper.

That way it's guaranteed to be concentric with the spindle rotation. Keep it, but re-turn the taper each time you re-mount it.
It does leave more overhang from the headstock, but this might be an advantage as none of the saddle will be overlapping the gap part of the gap-bed. Also it provides a drive dog for your right angled driver; that is you use one of the jaws to drive the workpiece when turning between centres.
Also depending on the length and shape of the workpiece, swapping between 3 and 4 jaw chucks allows you to move the actual point of the home made centre along the lathe bed a bit. (Myford independent 4 jaws are often shorter than 3 jaw self centring chucks.)

Bill

Edited By peak4 on 23/12/2019 18:25:13

A cheaper way to make a test bar is to take a length of, say, 25mm bar, centre it both ends then turn away the middle and ends so it has a slightly raised collar near each end.

Then take a light cut over the collar at the tailstock end and, without changing any settings, reverse the bar and reduce the other collar. Ideally do this between centres, but you can hold the headstock end in a chuck.

Because the collars are narrow and machined in the same position they should be very close to each other in size.

The bar can now be used to align the tailstock by putting it between centres and taking measurements to the collars using a gauge in the toolholder.

Neil.

You would be best to turn the soft centre to get the alignment done, but putting a dead centre in the spindle taper and repeating the tests would be an excellent check of consistency. The freshly turned soft taper absolutely guarantees alignment with the spindle axis, until you remove of course. I keep the shank of a bolt for exactly this purpose, and reskim the taper before testing. When I bought the test bar, I made the mistake of using a live centre in the tailstock, and what I got was the runout of the centre. changing to a dead centre, and turning the test bar in it proved that the centre in the bar was good, I got zero detectable runout. The test bar in the advert is cheap enough and has the added advantage of having the Morse taper built in. It can also be used in the mill spindle and in the appropriate size collet for testing and alignment.

With the myford tailstock I found I had to push it against the back shear before locking it. Even with the tailstock gib adjusted, bed wear meant mine was ever so slightly slack as you got towards the chuck, but the back shear front edge (if you get my meaning) doesn't get much wear.

You don't need to buy a test bar, either make one, as Neil suggests, or centre a bit of bar, hold it in the chuck, relieve the middle, then support with tailstock, light cut each end without moving the cross slide (lock it if you can), measure the difference, halve it. If it was big at the tailstock end move the tailstock towards you by this value, or vice versa. Measure the movement with a dti. You need to slacken the tailstock clamp to do this. It's a while since I got rid of my ML7, so reading the manual would be a good idea as I might have forgotten something.

Edited By duncan webster on 23/12/2019 22:24:22

Thanks everyone. Problem solved. I managed to zero in and eliminated the taper issue from the tailstock. Its now cutting perfectly fine again, no more taper cuts.

Cheers,

Raphael