+1 for ega reply. check your lathe alignment across the shears you may have a slight twist in your lathe bed. Lathes can be out of level lengthwise without being detrimental ( think of ships machine shop ) but if not level from front to rear shear at headstock / tailstock then there will be a liability to turn tapered shaft.

George

Do you have a travelling steady you can use? This would be a good time to make use of one.

Martin C

I have to ask, but are you using the compound slide?

This can happen if the chuck is worn so that it's gripping more tightly at the base of the jaws, allowing the workpiece to flex away from what's effectively line contact at the gripping point.

Robin,

Best of luck trying to straighten a lathe bed out via the sheet metal cabinet mountings. Good for a levelling of the whole assembly but then you need to 'tweek' the bed by its mounting on the cabinet. A well engineered lathe will have a 3 point fixing, two under the headstock and a single central mounting bolt at the tailstock end, then adjusters either side of the tailstock end mount (rear and front) in the cabinet coolant tray (given that it is strengthened in this area ? The reason why a cabinet stand needs to weigh at least half the weight of the lathe itself) , will allow the twist to be taken out. Shimming is the alternate route.

Martin

PS, here is my 0.02c worth from the above mentioned thread:

Adjusting the headstock is an absolute last resort and very rarely needed. First try this:

There are two taper turning causes, with two tests and two solutions:

1. Bed Alignment. Stick a piece of 1" diameter bar in the chuck with about 4 to 6" sticking out. NO tailstock centre in place for this test. Take a fine finishing cut along the length of it and measure the job for taper. It should be within a thou or less (0.025mm). If it's not, the adjustment is made by shimming ONE of the mounting feet where the lathe attaches to the bench, at the tailstock end. This "twists" the bed to get it aligned to your headstock spindle axis.

2. Tailstock Alignment. Stick a short piece of bar in the chuck and turn a 60 degree point on it as close to the chuck as possible. Then put a known good centre in the tailstock and slide it up so the two points almost meet. Pinch a thin steel ruler between the two points, with the quill extended about the amount you normally use it at. The steel ruler should stand up vertical and also should lay square to the main lathe axis when viewed from above. If it does not, adjustment is done by adjustment bolts or screws in the tailstock base that move it from side to side. If the tailstock centre is lower than the headstock centre, you will have to put shim between the base and main body of the tailstock to bring it up to headstock spindle level. Further fine adustment is made by turning a piece of 1" diameter bar 6 to 12" long between centres and measuring the resulting taper after a fine cut. Adjust tailstock offset until less than a thou of taper.

The best and simplest written/pictorial description of how how set up your lathe's bed alignment and tailstock alignment I have seen is in the front of the Myford ML7 Owners Manual. PDF copies are available free all over the net. It applies to all lathes, not just Myfords. You should read it carefully before adjusting anything. It gives two ways of doing the bed alignment, using either an expensive precision level, or by the simple turning test outlined in 1. above, which is all I ever use in the home workshop. Then they describe the tailstock alignment by turning test as in 2. above.

This is all assuming that your lathe's bed is not worn out beyond reprieve.

Edited By Hopper on 25/05/2021 10:13:17

Did you use the toolslide to turn it?

If so, very quick check:

Turn it again, using the toolslide, check result.

Then again turn it in the same way, but lock the toolslide, and use the saddle to advance the cut.

Check both results and compare.

As it is happening over such a short length, my guess would be the toolslide gibs.

..ah, yes – wondered when someone would spot that – look on it as kind of Quality Assurance check..

I must say that was quite a kind response..

First check should be for twist in the bed of the lathe.

Check as per Ian Bradley "The Amateur's Lathe", or his "Myford Series 7 Manual"

This advocates what has already been suggested, or turning a "dumbell" and adjusting or shimming the Tailstock end of the lathe unrtil both ends of the dumbell are the same diameter.

Read through the thread posted by Robin, the method will probably be described there. The same problem is present.

Either that or use an Alignment bar, or even try making one.

Use Silver steel, of the largest diameter that will pass into the mandrel, (ideally 1" or 25 mm so that it is stiff ),

Ideally, mount in a 4 jaw chuck and clock to centre as accurately as possible.

If only a 3 jaw available, take a very light cut over a slightly longer length.

DO NOT CHANGE THE CROSS SLIDE SETTING. Preferably, lock it.

Mount with only a short length protruding from the chuck and centre drill. Repeat the light cut

Mark in line with a chuck jaw.

Reverse the Silver steel, end for end, with the mark in line with the chuck jaw, and centre drill.

Turn a longer length to the diameter of the first end.

Grip this longer end in the chuck,again with the mark in line with the chosen chuck jaw.

Set a clock on centre height and set to Zero on the turned section at the Headstock end.

Move it to the Tailstock end, on the turned section.

Shim, or adjust the Tailstock end of the bed until both readings are the same.

You have now taken the twist out of the bed, and can check the Tailstock.

Mount the bar between centres, and set the clock on centre height and Zero at the Headstock end. Adjust the Tailstock (there should be a screw, low down and facing you, and another at the far side of the Tailstock.

Adjust these screws until the clock reads Zero at both ends.

The Tailstock should now be aligned with the Headstock, in the horizontal plane.

Hopefully, it should be aligned in the vertical plane. To check, bring the Tailstock and Headstock centres so close that they grip a thick feeler gauge, or a steel rule. It should be held vertical by the centres..

Hopefully, the lathe will now turn parallel between centres or without.

But remember that long slender work will deflect under cutting forces, unless supported by a travelling steady.

HTH

Howard.