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Lathe turns convex

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JOHN BRIDGE 125/01/2012 19:01:37
104 forum posts
11 photos
Hi, Hello to all,can anyone help with this problem or suggest some areas to examine, I recently bought a Myford ML7 Tri- Leva, it would be nice to be able to use this to machine a Stuart Twin Launce engine. I have had no end of problems with this Lathe and am now a little sorry I bought it. The latest problem is when facing off the resultant surface is not flat it is dome shaped, it is convex not pefectly flat. I have had a dti on the nose of the Mandrel and there is no deviation there, it seems to me with my limited knowledge that the cross slide is not traversing true across the bed, it seems to be taking more off at the begining of the cut thah it is when it nears the center, is this a likely scenario if not has anyone any suggestions as to were else to look. I have set up this Lathe on it's Myford stand according to the handbook using a digital spirit level and turning the test piece for final adjustment. After these final adjustments the tailpost end had to be adjusted to accuratly cut the test piece and the front of the bed at the tailpost had to be slighly raised.
TAA John
Steve Withnell25/01/2012 19:14:07
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848 forum posts
215 photos
My lathe can do this if I don't lock the saddle. I seem to recall someone mentioning in a different thread that lathes don't face perfectly flat, so I guess it depends on how "flat" you need to be.
magpie25/01/2012 19:28:56
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498 forum posts
98 photos
Never had anything to do with Myfords, but my old box ford did the same. The cure was as Steve said above. Lock the saddle,then take up any play by trying to wind it back toward the tailstock. Adjust the top slide,then try again.
 
Cheers Derek      P.S. This is realy a case of the blind leading the blind, but it worked for me

Edited By magpie on 25/01/2012 19:32:27

AndyB25/01/2012 19:45:32
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167 forum posts
7 photos
Hi John,
 
I have to ask, sorry if it offends;
 
Are you using the topslide or cross slide to traverse your face?
 
If it is your topslide then it is not set square to the work. You want to use the cross slide ie. the handle on the front of the apron.
 
If you are using the cross slide, is it secure on the saddle? Can you get sideways movement on the cross slide? If you can, you need to adjust your gib strip which is done by slacking off the nuts and adjusting the screws to get an even, smooth feed across the bed. You will have to move it all the way across and back to get an even adjustment.
 
Does the saddle move easily but firmly along the bed? You may have to adjust the gib there too.
 
How far from the chuck are you cutting? A lot of overhang can move your workpiece with the cutting pressure. Try using a fixed steady or a half centre to support the job, especially for thinner pieces where deflection will definitely take place and give the shape you are describing.
 
Does your cutting tool have a lot of overhang? That will deflect too (but you would get a horrible bumpy finish to the cut)
 
Hope this helps.
 
Andy
mgnbuk25/01/2012 19:47:34
1205 forum posts
72 photos
I had a similar problem with a Super 7 when I first got it. My machine is mid-60s at a guess & has the "narrow guide" saddle arrangement. With this, the saddle is guided longitudinally on the front shear only, with the "fast side" on the inside of the front shear (shared with the tailstock) & the gib strip bearing on the outside.
 
The "fast side" under the saddle is much shorter than the gib strip - what appeared to have happened on my machine was the gib had been adjusted too tightly at the tailstock end of the saddle, which is actually outboard of the end of the fast side & had caused excessive wear. This caused the saddle to be skewed on the bed - easily checked with a set of feeler gauges in the gap between the saddle and the rear face of the rear shear. The gap should be equal (and around 1/16" IIRC). You may find that it is not equal (smaller towards the tailstock) & this will cause convex facing. (I guess that the person who overtightened the gib strip was the same person who greased all the saddle oil points !).
 
I "fixed" my machine by effectively doing what Myford eventually did with their "wide guide" arrangement - I Loctited a piece of ground flat stock (1/16" x 1/2"x full length of casting IIRC) onto the outer vertical face on the saddle, so that the "fast side" is now the ground flat stock bearing on the previously untouched rear face of the rear shear . The saddle now bears on the outer edges the bed & does not share a guide face with the tailstock . This also has the advantage of making the fast side equal in length to the gib strip and so gives a greater bearing area. After doing this, the lathe faced correctly (very slightly concave) & no further problems have arisen.
 
HTH,
 
Nigel B.
David Clark 125/01/2012 19:57:27
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3357 forum posts
112 photos
10 articles
Hi There
Sounds like the carriage is worn.
A job for Darren Boden, ex Myford fitter I think.
Regards David
 
 
Tony Pratt 125/01/2012 20:03:22
2023 forum posts
12 photos
Hi John, as "new " lathes will cut slightly concave ie hollow in the middle, so your lathe does seem to have some wear.
Tony
Bazyle25/01/2012 20:06:02
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6381 forum posts
222 photos
As the tri-leva is not a V bed I would first suspect that the saddle is loose and twisted, or a bit of swarf has got in to missalign it. Next the same for the cross slide. Perhaps during the moving process it got jiggled around enough for this, or perhaps it happened to the last owner ages ago and they didn't do the sort of work to notice.
 

Edited By David Clark 1 on 25/01/2012 20:27:40

JOHN BRIDGE 125/01/2012 20:30:26
104 forum posts
11 photos
Thanks for all your replies.
Steve, Yes the saddle is locked, it did not lock correctly when I got it but I have sorted that one.
 
Andy, Yes the crosslide, I have ajusted all the various gib strips there is absolutely no movement at when I am facing , I have been facing close to the chuck, I have also faces an old 6 inch cast iron face plate, this has the same problem now.
 
Nigel, This sounds like a solution if I can get my head round it, many grey cells have now gone.
 
David, Do you have any contact details for Darren.
 
Tony, Yes with mine when you put the piece on a flat surface you can rock it, you can see it with a straight edge across it.
 
Bazyle, Point taken, the lathe is now very clean I have stripped the whole thing down and cleaned it to try and rectify this fault.
 
John
 
David Clark 125/01/2012 20:35:59
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3357 forum posts
112 photos
10 articles
Hi John
Email me direct
 
JOHN BRIDGE 125/01/2012 20:47:13
104 forum posts
11 photos
Hi David, Email sent but it is still in my outbox something about server problems, should send eventually.
jason udall25/01/2012 21:09:08
2031 forum posts
41 photos
sorry if off topic.
BUT this thing about the cross slide "out of true"
Heard it else where. never seen it at work (CNC'S)
. Is it a manual thing?
Tony Pratt 125/01/2012 21:31:01
2023 forum posts
12 photos
All machines will wear and go out of tolerance, so I would say it is a lathe thing. I would imagine the Myford type of flat bed etc would be especially prone to this type of wear?
Tony
Alan Worland25/01/2012 21:33:50
247 forum posts
21 photos
I remember reading in some Myford paperwork that the saddle was always set up to turn a very gradual 'dish' when facing off (dont think we are talking much, just so that when 2 faced off parts are put together they wont 'rock')
Perhaps with a bit of tweaking of the saddle gib screws things might be pulled into line?
Is the headstock true to the bed and not on the wonk? - test bar and DTI
My Myford (early 50's) always faces off flat
 
Alan
blowlamp25/01/2012 21:37:11
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1641 forum posts
105 photos
It's a common fault with Myford lathes.
 
I think Nigel's got it sussed when he mentions wear on the saddle guide that bears on the inner front bed shear.
 
These lathes are prone to wearing like this because the guide is quite short and the normal turning forces tend to skew the saddle in a clockwise direction when viewed from above, which concentrates pressure and thus wear on the section closest to the tailstock side of things.
 
You could probably sort this out yourself with a bit of light and careful milling and/or filing, although if the wear is more general in nature it may pay to get in touch with Darren..
 
 
Martin.
JOHN BRIDGE 125/01/2012 21:41:25
104 forum posts
11 photos
Nigel, I think you have hit the nail on the head with this, I have just done the check you described, the gap between the saddle and the rear face of the rear shear is 31 thou at the chuck end and about 2 thou at the tailstock end, this would certainly cause my problem. Does this piece of ground 1/16" x 1/2" not intrfere with the saddle lock and is the space for this strip created by backing off the gib strip adjusting screws, I have never used Loctite in this way does it get a good grip, is this ground flat stock easily available, what do you mean by "fast side". Sorry if these questions are simple but as you can see I am very new to this and am rareing to go.Thank you once again for your help.
John
JOHN BRIDGE 125/01/2012 22:54:34
104 forum posts
11 photos
Blowlamp, Yes Nigel is correct in my case, I ahave just packed out the saddle and rear face with a 30 thou feeler gauge faced a 2" ally bar and for the first time with this lathe the ally does not rock on a flat surface and no daylight under the straight edge. Your suggestion is interesting to do some light milling I will look into this, Thanks.
mgnbuk26/01/2012 07:58:53
1205 forum posts
72 photos
what do you mean by "fast side"
 
Sorry to have used jargon. This term seems to be a term pretty well universally used by machine tool fitters from the (now mainly disappeared) machine tool builders from the Halifax area.
 
The "fast side" on a slideway is the fixed or reference surface that guides the slide on the shear (face on bed). It is the face opposite the gib strip, and is usually arranged to take the forces applied to the slide in normal operation.
 
So, the "fast side" on the cross slide is the face nearest the spindle, the "fast side" on the compound is nearest the centreline & the "fast side" on the saddle is (in this case) a section underneath the saddle the bears on the inside edge of the front shear on the bed,
 
The addition of a strip to use the rear face of the rear shear has caused no problems with my saddle lock & I cannot see how it would. Yes, you back off the gib screws to allow the strip to be fitted. The gap appears to have been originally designed to be 1/16", so there are no adjustment issues with the gib & the apron lies where it was intended also - something to bear in mind if you decide to re-machine the existing "fast side" - the saddle will move forward on the bed & there may not be sufficient clearence in the apron attachment screw to align the leadscrew correctly. You way also have to make a thicker gib strip. This face was most probably gang-milled at the same time as the intended "fast side", so should get you pretty close to original alignment by packing it with a parallel spacer (which is what I found in practice).
 
I used ground flat stock because I had a suitable piece. A flat, parallel brass strip would have done equally well. If I were doing this at work, I would use a piece of a PTFE-based slideway liner like Turcite, Guidecoat or Rulon. When checked with marking blue, my "modification" showed as being flat - normal practice if the face were being scraped would be to slightly relieve the central area (just a couple of scrapes) to ensure that the face bore on two pads at the ends & so would not rock.
 
I suppose ideally a Loctite High Strength Retaining compound should be used to attach the metal strips - I didn't have any to hand and used Studlock. Araldite (slow setting) may also work - that is what I would have used with a PTFE liner. It has been giving satisfactory service for a number of years now. To attach the strip, I slightly roughened both the saddle face & one side of the strip with coarse emery cloth & degreased (brake cleaner), then lightly coated both surfaces with Loctite. A piece of writing paper was folded over both shears of the bed & the saddle (with strip held in place) was placed on the bed over the paper & the gib inserted & tightened to press the strip flat against the bed. This was left 24 hours. The paper prevents any squeezed-out Loctite from glueing the saddle to the bed. After the cure period, the saddle was removed & any excess Loctite and stuck-on paper were removed with a scraper & the flatness and bearing of the new strip checked with marking blue on the bed. Adjust if required with a scraper, clean down, lubricate and re-assemble. Then a final test cut to make sure all is OK.
 
Regards,
 
Nigel B.
 
JOHN BRIDGE 126/01/2012 09:09:32
104 forum posts
11 photos
Nigel, Thank you very much for your very detailed explanation, this will be the route I take to try to resolve my problem, as is very obvious I am very new to this but I am determined to get there, with all the problems I have had with this lathe I have regretted buying it instead of a new Chinese Clarke but I do like the messy thing, Thank You.
Best Regards John
JOHN BRIDGE 126/01/2012 09:19:48
104 forum posts
11 photos
David, Thank you for the number.
Best Regards John

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