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Strange effect when turning

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larry phelan 124/05/2021 17:16:44
1050 forum posts
14 photos

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.

ega24/05/2021 17:20:03
2186 forum posts
179 photos

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?

mechman4824/05/2021 17:27:55
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2923 forum posts
456 photos

+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 124/05/2021 17:37:15
1050 forum posts
14 photos

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 Connelly24/05/2021 17:37:19
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1789 forum posts
190 photos

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

Martin C

Oily Rag24/05/2021 17:54:17
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434 forum posts
147 photos

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 mart24/05/2021 20:08:26
3185 forum posts
201 photos

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

Pete Rimmer24/05/2021 20:25:46
1004 forum posts
57 photos

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

Andy Stopford24/05/2021 21:13:43
88 forum posts
9 photos

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.

Robin24/05/2021 21:15:03
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468 forum posts

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" smiley

Oily Rag24/05/2021 21:29:03
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434 forum posts
147 photos

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

Robin24/05/2021 22:39:25
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468 forum posts
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 wink

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 125/05/2021 09:00:03
1050 forum posts
14 photos

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.

DiogenesII25/05/2021 09:10:14
255 forum posts
137 photos
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 125/05/2021 09:40:03
1050 forum posts
14 photos

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

Hopper25/05/2021 10:08:32
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5505 forum posts
137 photos

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.

Hopper25/05/2021 10:12:04
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5505 forum posts
137 photos

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 Connelly25/05/2021 10:12:25
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1789 forum posts
190 photos

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 125/05/2021 11:02:15
1295 forum posts
50 photos

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 Connelly25/05/2021 13:14:04
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1789 forum posts
190 photos

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

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