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Myford ML7 Turning Between Centers

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JOHN BRIDGE 109/02/2012 11:11:43
104 forum posts
11 photos
I am new to turning between centers and have just tunrned a piece of ally between centers and adjusted the tailstock until this test piece was uniform in width along it's length, spot on in fact. I turned the test piece through 180 degrees to turn the bit of the test piece that was held in the clamp to the same diameter as the rest of the piece but test piece does not now seem to have excactly the same center line as before. Is this normal or is more adjustment to the ML7 required.
Ady109/02/2012 11:17:24
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If you turn it around you won't get the same readings.
 
Try and imagine the cylinder as a parallelogram, where each end isn't exactly 90degrees to the cylinder, but the cylinder is 100% equal along its length.
 
If you turn it around it will be slightly out.
 
Annoying innit
JOHN BRIDGE 109/02/2012 11:22:14
104 forum posts
11 photos
Thanks Ady1, Yes it is Annoying when I cut the test piece I thought turning between centers was the answer to a Maidens prayer but not to be, still another lesson learned.
John
Les Jones 109/02/2012 12:20:32
2162 forum posts
149 photos
Hi John,
I think the most likely cause is the centre at the headstock end not running perfectly true. (Or possibly some slight end float between the centres allowing the drive dog to move the headstock end of the with the driving force.) If the tailstock end is a dead centre then tailstock end must be concentric with the centre hole. If it is a live (Rotating centre ) then if that does not run perfectly true there will be a lack of concentricity at the tailstock end.
 
Les.
JOHN BRIDGE 109/02/2012 13:12:38
104 forum posts
11 photos
Hi Les, This is causing confusion now Les are you saying it should be exactly the same either way, I am using live centers both ends, there is no play, the test piece cuts spot on throughout it's length, could the tailstock still be out of adjustment under these circumstances. There is one thing I dont understand even though even though the test piece cuts accurately if I run a DTI along it on the crosslide there is a 4 thou. difference from tailstock to mandrel end.
John
Dave C09/02/2012 14:03:33
94 forum posts
8 photos
It sounds to me that you possibly have a slight twist in the bed and you have set the rear tailpost to turn correctly with the twist in place already. Thats possibly why when you run the saddle along the bed you are showing the 4 thou difference ? I'm only guessing as I have had similar problems with my lathe untill I spent a few hours setting it up as accurately as possible. An exercise well worth doing before attempting to turn accurate work. As I say this is purely a guess as I am still learning the hard way myself.
Might be worth a check though. Hope this is of some help.
Dave
Donald Wittmann09/02/2012 14:07:24
40 forum posts
John,
When you run the DTI along the workpiece unless your bed is perfect then you will notice it in your reading [bed wear] you would also have a difference in your original measurement. How did you measure the part originally? did you use a micrometer? or calipers[not good for precise measurements]. is it slightly larger at the spindle end?
you can have your centers spot on, but if you have bed wear [or a slight twist] then you will notice a difference, the more pronounced the wear the bigger the difference.
Donald.
Nobby09/02/2012 15:04:10
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587 forum posts
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Hi
Try turning a dead centre in the chuck end . Then using a dead centre in the tailstock.
See how that goes .
Nobby
 
Ady109/02/2012 15:07:35
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3870 forum posts
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There are also "built in" errors.
 
There was a recent post about flatness when facing a workpiece.
When you face a workpiece a properly made lathe will create a slightly concave facing, something about the headstock alignment with the lathe bed having a slight tweak.
 
Because a lathe has so many different things it can do I suppose that there will always be a wee bit of compromise.

Edited By Ady1 on 09/02/2012 15:10:15

Les Jones 109/02/2012 15:51:34
2162 forum posts
149 photos
Hi John,
I agree with Nobby's advice. You achieved creating a parallel bar so the axis of rotation must be parallel with bed. The tailstock being out adjustment would not cause lack of concentricity. It would cause the bar to be tapered. A live centre at the tailstock end can cause lack of concentricity if centre is not true. you can check this by rotating it with a dial gauge against it. As a dead centre does not rotate it cannot cause lack of concentricity at the tailstock end. It could cause it at the headstock end as it is rotating. Nobby's advice guarantees the centre at the headstock end is running true as you have just turned it. Even if the taper in the headstock was not true the centre you created would be. (PROVIDED YOU DO NOT REMOVE IT AND REPLACE IT.) I have never heard heard of a live centre being used in the headstock.
 
Les.
KWIL09/02/2012 16:58:15
3309 forum posts
63 photos
John,
 
Have you followed Dave C's advice and checked that the bed is flat and free from a twist. Is the lathe bed properly bolted down to a stable (and strong) surface ? If it is correct, then the other comments can apply.
Les Jones 109/02/2012 17:13:49
2162 forum posts
149 photos
Hi John,
Can I confirm that my understanding of your phrase " does not now seem to have excactly the same center line" mean that the end that was the chuck end (Now the tailstock end) is running eccentrically. If this is not the case then ignore all of my comments.
 
Les
Nobby09/02/2012 17:14:00
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587 forum posts
113 photos
Hi Les & Guys

Les is right having turned the 30 degree angle dead true if you remove it from the chuck on replacing you must trim it true again . I ground a test bar between 2 dead centres about 8" long for checking set up. as you know on grinders the head does not rotate just the dog & workpeice/ Sorry drifting away from original thread.

Nobby

JOHN BRIDGE 109/02/2012 19:02:29
104 forum posts
11 photos
Posted by Les Jones 1 on 09/02/2012 17:13:49:
Hi John,
Can I confirm that my understanding of your phrase " does not now seem to have excactly the same center line" mean that the end that was the chuck end (Now the tailstock end) is running eccentrically. If this is not the case then ignore all of my comments.
 
Les
 
Hi Les, Confirmed.
John
 
Les, I am new to this it seems obvious now that a live center is not required.
John
 
Kwil, The lathe is mounted on a Myford stand on a concrete floor it has adjusters on the stand feet to level it, it is level I used one of the digial levels to achieve this, the Lathe was placed on it and this was levelled using the same digital level. I then cut the test piece as describeed in the lathe instructions, this resulted in the tailstock end cut being larger than the mandrel end so the tailstock end front adjuster was raised until the two cuts were yhe same. This did result in the bed at the tailstock end now not being level front to back at the tailstock end, You would not beleive how much time I have spent trying to adjust this lathe before I do any important turning.
The saddle now travels correctly across the bed after applying Nigel's mod. Now this problem, will it ever end.
John

JOHN BRIDGE 109/02/2012 19:04:12
104 forum posts
11 photos
Michael Thanks for your tip I will check this.
John
KWIL09/02/2012 19:31:41
3309 forum posts
63 photos
John,
 
Interesting that your lathe was level. By the way it is not conventional to level from the ground with a Myford, with the cabinet/stand set, you must then adjust under the bed feet using the height adjusters set in the raising blocks, merely leveling via the stand will not take out any bed twist. Are you sure about your digital level, they do not seem to be accurate to me and certainly not repeatable once you move them, their resolution seems to me to be coarse compared with the bubble in an engineer's level.
JOHN BRIDGE 109/02/2012 20:40:00
104 forum posts
11 photos
Kwil, I think I may have misled you here I levelied the stand using it's levelling feet I then levelled the lathe using the levellers on the top of the stand which just level the lathe and not the stand the stand remains as it was levelled using it's own levelleling feet. The digital leveller was bought to confirm that the normal bubble type spirit levellers were reading correctly two of them. These digital levellers also have a bubble built into them.
John
 
I have now readjusted it all levelled the bed done the test pieces not now using live center in mandrel, no change DTIstill reads 4thou difference the mandrel end being 4thou closer to the front of the lathe than the tailstock end, the test piece is 12" long.
John  

Edited By JOHN BRIDGE 1 on 09/02/2012 20:46:02

Donald Wittmann10/02/2012 16:14:25
40 forum posts
John,
IF, as you say the bed has no twist, and I think we can safely assume that your headstock in not "pointing up" nor your tailstock "pointing down" then it points to bed wear
at the headstock end. If you run your DTI along the length of the workpiece then you will notice the reading changing. For every 0.01mm the reading changes then the part Ø will change by 0.02mm so if your part is 4thou larger in Ø at the headstock end that would be round about 0.08mm which means your DTI reading would change by 0.04mm.
If you are SURE that there is no twist, then stick in a piece of steel in the chuck and turn a 60° center and use a fixed center in the tailstock. then take new measurements and see.
if you still see a difference in Ø then run the DTI along the bed and that will confirm your bed wear.
EDIT. 
John,
            Depending on the amount of wear the part Ø will not necessarily be double the error. 

Edited By Donald Wittmann on 10/02/2012 16:19:13

Edited By Donald Wittmann on 10/02/2012 16:20:01

Nobby10/02/2012 18:57:10
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587 forum posts
113 photos
Hi John & Guys
I may be missing the point here .If the lathe is level etc. And it the job measures 4 thou bigger at the tailstock end .Cant you just adjust the tailstock over towards you / tool 2 thou
as been said by Don & Myself turn a true centre 60 degree inclusive angle first and dead
centre in tailstock. Mark the center so you can put it back the same way ie line at the top
Ps I am waiting for the flack !!!!!
Nobby
JOHN BRIDGE 110/02/2012 19:03:48
104 forum posts
11 photos
Hi Nobby, Yes I think you are missing the point, the bar measures excactly the same at both ends it is only when you run a DTI down the bar using the carriage that there is a 4 thou difference. The bar is nearer to the front of the lathe at the mandrel end .
John

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