Strange effect when turning

[larry phelan 1Participant @larryphelan1]

I am getting a strange effect when turning a piece of round bar.

What,s happening is that

I am getting a strange effect when turning a piece of round bar onto which I wish to fit two bearings. What is happening is that while the dia at the point away from the chuck is correct, the dia nearest to the chuck is undersize, even when I use support from the tailstock.

Any ideas what I am doing wrong ? The machine in question is a Craftsman lathe, which seems to work well otherwise.

Have tried locking the cross slide, but made no difference.

Do I have a problem with the carriage or the headstock ?

All advice welcome.

[larry phelan 1Participant @larryphelan1]

[egaParticipant @ega]

A quick fix might be to turn the piece between centres, adjusting the tailstock to eliminate taper.

Could you turn the bearing journals to correct size separately eg with a relief between them?

[mechman48Participant @mechman48]

+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

[larry phelan 1Participant @larryphelan1]

I am concerned because the same thing would happen if I tried to turn a simple shaft parallel, even over a very short length . In effect, this is what is happening.

[Martin ConnellyParticipant @martinconnelly55370]

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

Martin C

[Oily RagParticipant @oilyrag]

Sounds like either:-

1. Twist in the bed

2 Tailstock out of centreline with headstock, or

3. A combination of both!

Remedies:-

1. Turn a scrap length of aluminium bar with a relieved centre section (dumbell like) make the ends at least 6" (150mm apart) then without the tailstock supporting the far end take a VERY light cut of around 0.002" depth (0.05mm ) off the far end and continue to the chuck end (fast traverse between obviously! ) then measure the diameters with a micrometer. If the tailstock end is bigger then the bed is twisted 'away' – correct by shimming under the rear foot at the tailstock end (unless the lathe has some adjustment provision). If t'other way of twist shim under the front foot.

2. Turn a soft centre in the mandrel to a sharp point. Fit a hard centre in the tailstock barrel and bring the tailstock up to the mandrel centre. Put a razor blade between the two and see whether its up/down, canted left/right. Adjust tailstock accordingly.

3. Do both again after testing.

Martin

[old martParticipant @oldmart]

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

[Pete RimmerParticipant @peterimmer30576]

Depending on the model of Craftsman it could be the headstock misaligned on the bed, or the bed is twisted somewhat.

[Andy StopfordParticipant @andystopford50521]

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.

[RobinParticipant @robin]

I feel your pain, my lathe is waiting on some bolts and an M14 stub turnbuckle to straighten the bed out.

OTOH, it is worth it just for the delight of using expressions like, "M14 stub turnbuckle"

[Oily RagParticipant @oilyrag]

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

[RobinParticipant @robin]

Posted by Oily Rag on 24/05/2021 21:29:03:

Best of luck trying to straighten a lathe bed out via the sheet metal cabinet mountings.

Martin

Guess we will find out once that M14 stub turnbuckle arrives

I hear what you are saying, but in this instance, messing with the floor mount has already reversed the unwanted taper. I am now merely adding a fine adjustment.

Robin

[larry phelan 1Participant @larryphelan1]

Oily Rag,

Just been to check my lathe and I dont see any screws at the tailstock end, except the ones bolting the machine to the cabinet. I dont see any at the headstock end either, thought there might have been some means to adjust it, but dont see any. Looks like I will have to do some shimming.

Andy, I doubt if the chuck is to blame. It,s my 4 jaw chuck, from new and seldom used, but I,m open to all ideas !

The error I am getting is 1 1/2" thou over two inches, which sounds a bit much.

Never noticed it until this job came along.

[DiogenesIIParticipant @diogenesii]

Posted by larry phelan 1 on 25/05/2021 09:00:03:

…I doubt if the chuck is to blame. It,s my 4 jaw chuck, from new and seldom used, but I,m open to all ideas !

The error I am getting is 1 1/2" thou over two inches, which sounds a bit much.

Never noticed it until this job came along.

Did you mount the chuck to do this job? ..it's not summat simple like a piece of debris that's got trapped behind the chuck…?

[larry phelan 1Participant @larryphelan1]

Just thinking ,since the tailstock was not involved in the job, would that not point to a headstock problem ?

[HopperParticipant @hopper]

See this recent thread on setting up your lathe to turn parallel: **LINK**

It contains all the information you need to set your lathe up to turn parallel, first without the tailstock centre and then with it. In your case, with no centre, it is quite likely that all you need is some shimming under one of the mounting feet on the bottom of the lathe bed.

[HopperParticipant @hopper]

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

[Martin ConnellyParticipant @martinconnelly55370]

The original post made me think of the workpiece springing away from the tool at the end of the chuck which is why I asked about the travelling steady. Even the sharpest of tools will cause this to some degree since there is no perfectly rigid material with easy cutting characteristics. The travelling steady will stop the part moving away from the cutting point. This type of taper is worse with relatively blunt carbide compared to finely honed and small radiused HSS.

Size the shaft close to the chuck, fit the travelling steady on the turned section near the chuck and machine the remainder by travelling away from the chuck. This is the sort of job travelling steadies are suited to.

If it is possible then turning this part between centres may give a more consistent diameter to each end.

Martin C

[David Standing 1Participant @davidstanding1]

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.

[Martin ConnellyParticipant @martinconnelly55370]

DiogenesII, an offset chuck would not cause this problem. The workpiece will still rotate on the spindle centreline regardless of the chuck being canted or not.

Martin C

[DiogenesIIParticipant @diogenesii]

..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..

[larry phelan 1Participant @larryphelan1]

I am happy to be able to report that due to careful shimming, I have reduced the error to almost nothing, something like 0,002mm as far as I can see, over 150mm. At that point, I decided to leave well enough alone [like a Mother-in-law ]

My thanks to everyone who helped out. I shall now go away and play [until next time ! ]

PS Stick with it Robin, you will get there in the end !

[Howard LewisParticipant @howardlewis46836]

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.

[RobinParticipant @robin]

Posted by larry phelan 1 on 26/05/2021 13:15:09:

PS Stick with it Robin, you will get there in the end !

Well done that man. If my M14 stub turnbuckle arrives on time I should have it all back together over the Bank Holiday.

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