I have a question about lathe alignment

Mark_f

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I'm writing this from my sick bed. It seems I have a lot of time to think right now.
I have always had about .004" to .006" taper over a 6" long cut. I have leveled my lathe but I have a theory why it may not stay level. This is a South Bend 9" lathe. It is mounted on a nice shop built steel cabinet bench with a top made of .125" steel. Where I think the problem may be is, this lathe is in a shed beside my home. The floor is 3/4" particle board sub-flooring on 2 X 8 joists 8 ft long. With the heavy machines, this floor has some give or "bounce". If I lift one corner of the lathe bench even .050" it throws the level out. I don't believe it is possible to keep things level on this floor. Now to my question. The lathe only need to be level to itself ( not the world. They put them on ships in a shop and the ship is not always level to the earth). Short of rebuilding the shop, I am wondering if placing a 40" X 20" steel plate 1/2" or thicker on the bench top and then mounting the lathe on the plate if that would help solve the problem of constant changing of the leveling. The lathe could be aligned to the very stiff plate and remain constant. Am I way out in left field in this thinking?
 
Would that make the top 1.750 thick steel,+ lathe, that's a lot of weight for a particle board wood floor
I would think the 1.250" steel top would be good enough. Could somthing else be causing the taper?.
Hope you get to feeling better soon

Edit: sorry misread the thickness of the top , .125+.500 steel plate mo better but still heavy
 
That is a lot of taper! I’m not there looking at your lathe of course. But it seems to me you have some other problem going on? Unless you meant 0.0004” to 0.0006” in 6”. I don’t think the lathe could cut that kind of taper/error with a twisted bed. Head stock miss-alignment yes, twisted bed, don’t think so. I think the bed would have to have a crack or something to throw it out that much. Or it is worn very badly. Please get well…Dave.
 
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Hi Mark- seems to me a metal frame with a particle board top would make a stable platform and be a whole lot lighter; supported on wooden legs. Maybe a tripod of wood. ?
Mark S.
 
That is a lot of taper! I’m not there looking at your lathe of course. But it seems to me you have some other problem going on? Unless you meant 0.0004” to 0.0006” in 6”. I don’t think the lathe could cut that kind of taper/error with a twisted bed. Head stock miss-alignment yes, twisted bed, don’t think so. I think the bed would have to have a crack or something to throw it out that much. Or it is worn very badly. Please get well…Dave.

It is .004" to .006". The bed is not worn that much. I know if I align the tail stock up at the spindle and then slide it back 12 inches, it is a several thousandths off. I am sure the bed is not worn that much. Could the head stock being out of alignment cause the problem? I have been chasing this for a few years. I have made a lot of things on this lathe, but it would be nice not to have to work around it all the time. A while back I needed to turn a 12" long piece to an accurate diameter. I had to move the tail stock several thousandths to do it. When I moved the tail stock up about an inch from the spindle to hold a short piece, the tail stock was far enough out it broke the point on my carbide tipped dead center. Does that indicate the headstock may be off?
 
My dad's 9" SBL, now mine, has never been leveled in it's life! The headstock has always been secured to the bench top with two 3/8 bolts tighten down somewhat. The two out board bolts at the tailstock end are always been left loose. (Dad's instructions to me) Last time I checked, turning the OD of a part, I was getting near zero taper in two inches. Don't really do anything long in the lathe anymore. Have a bigger lathe for that.
 
If your turning between centres, headstock alignment would have no effect Mark. Holding the work in a chuck and centre on the tailstock will cause the part to flex and possibly walk out of the chuck if the headstock isn't aligned. Same effect as not getting a steady rest aligned.
Could you make a stiff sub frame for the lathe to sit on. Support the sub frame on 3 points, so the building can't cause it to twist. Then level (or take the twist out of the bed) using the sub frame.
My Hardinge lathe uses a similar arrangement, the headstock is fastened at two points and the end of the bed at one point. Those points aren't really fastened, they use coil springs so the bed can't twist if the cabinet does,

Greg
 
If I want to check the headstock alignment with the ways, would the best way be to use a 3MT test bar in the spindle, NOT in the tail stock at the outer end and indicate it? Using the tail stock would pull it if the head stock was out , correct?
 
One thing you might want to try when you're feeling better is to level the lathe and take a test cut and see if the lathe cuts straight. Yes, the lathe might move some with that foundation, but it doesn't seem to me that it would move immediately... so, level, take a test cut, check the piece and then you can even re-check level and see what results you get. Then, take action based on the results you see.

Hope you feel better and good luck,
Ted
 
The headstock on a Colchester I had was out of alignment. I used a 2 or 3 inch dia piece of aluminum 6 or 8 inches long in the three jaw. Take a light cut, check for taper. On the colchester there were 4 bolts I think holding the headstock to the bed. Loosen 3 and tap the headstock and try another cut, measure and repeat till the taper is gone.

Greg
 
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