Right pressure for tailstock

[jani]

If I turn heavy shaft between centers, for example 500kg, what is suitable pressure for tailstock pinol to press workpiece? 250kg, 500kg, 1000kg or what??

[RICHARD ZASTROW]

jani, If this is a live center, there is a thrust / axial limit to the bearings. The center manufacturer should be able to give those specifications to you.
If a dead center, you must consider the weight of the piece and the lubricant you are using in the center hole.

Note: Here is an example.
Neidlein type RN3MK3 3MT 10000 RPM 1400 LB. THRUST
RN3MK4 4MT 10000 RPM 1400 LB. THRUST
RN3MK5 5MT 10000 RPM 1400 LB. THRUST
RN4MK4 4MT 8000 RPM 2500 LB. THRUST
RN4MK5 5MT 8000 RPM 2500 LB. THRUST
RN5MK5 5MT 5500 RPM 4400 LB. THRUST
RN5MK6 6MT 5500 RPM 4400 LB. THRUST
RN6MK6 6MT 4000 RPM 6600 LB. THRUST

[jani]

Its lathe's original live center, Daewoo puma 700lm. Maximum pressure for tailstock is about 40kg/cm2. Now i'm used 20kg/cm2 for 500kg workpiece, which gives about 1500kg thrust for tailstock pinol.

[RICHARD ZASTROW]

jani, Do not forget you must not exceed the thrust capacity of the machines headstock bearings. Just a reminder.

[jani]

Thank you for answering Richard.

I think that after 40kg/cm2 headstock bearings start to suffer. Daewoo says that 40kg/cm2 is lathes maximum allowable pressure for tailstock, so headstock bearings should be ok with 40kg.

[RICHARD ZASTROW]

jani, For some reason I have a limit in my head from some old lathe I programmed of 4000 lbs thrust limit. I think that equals 1800 Kg. So if you are at 1500 Kg. you should be ok. Don't forget the limit on your tailstock center. For example, if you have a need to use a small center you might be down to 32 lb. thrust for a #2MT Concetric (brand name) live center.

[jani]

Parts which we run now are ball valve's balls with shaft, made of stainless castings. Ball diameter is 25in with 16in hole and with shaft the lenght is 50in.

Do you have any idea Richard how down I dare to go with pressure or is it safer to keep it where it is now?

[RICHARD ZASTROW]

jani, Is the shaft at the tailstock end, both ends? I assume the hole is thru the ball. If so, does the tailstock pressure deform the ball while machining?
If centralized, the weight is aprox. even on both ends. Half the weight on each end. The weight will exert pressure on the tailstock. I use 50% of that weight as the pressure from the part towards the tailstock. That would only be enough to neutralize. You still have to add cutting force and eccentric centrifugal force plus a safety factor. I would have pressure as high as I could without deforming the workpiece or exceeding either headstock or live center limits. Since I do not know all the details of either the machine and equipment available or specifics of the work piece,I can not give you good advice. Just remember, advice is only worth what you paid for it including free advice. Good luck!

[jani]

Richard, thank you for many advices, I really appreciate that.

Shaft is at both ends, short and long shaft. And hole is thru the ball.
Today I tested with dial indicator how much tailstock deform ball. At tailstock side, dial indicator showed 0.01mm change, which is ok. I used screw jack inside the ball giving firmness.

[RICHARD ZASTROW]

jani, Is this part a repeat type job, serial production or one time or seldom repeating? If a production type job, you might consider a "pass thru" hydraulic steady rest. Then you could support the workpiece in the steady rest and use lighter pressure on the tailstock.

[jani]

This is first time on puma when we run those balls, but maybe in the future we run more.

There's manual "pass thru" steady rest on puma, but we took it off, because those balls are so "small" and they seems to go well with current set up.

[RICHARD ZASTROW]

jani, It seems then you have no problem. I hope your tailstock center pressure question is answered. God Luck.