Not so live centre

[Runner]

Hi,

I made myself a quick and dirty live centre. The design utilised a MT2 Morse Taper with a machineable straight shank that was reduced in size to fit the shaft diameter of a single roller bearing. The housing was made and incorporated the centre, which was similar in form to the dead centre. The MT2 Morse taper was loctited to the shaft diameter of the bearing and similarly the housing was loctited to the OD of the bearing. This design was opposite to general practice in so much that the body rotated instead of the shaft. I assume that this is OK. However, although the roller bearing was new and Japanese manufacture, the Morse Taper and housing could be flexed with regard to each other indicating that there was some play in the bearing. Undaunted I tried it out. The live centre didn't rotate because, I assume, that contact surface and or contact force to the work piece was inadequate. The contact force was increased and the live centre rotated but the play in the bearing manifest itself by a slight wobble of the centre and because of the increased contact force the bearing quickly got worse. Thinking about the latter point the bearing is not designed for a sideways force, the live centre required a thrust bearing of some sort, but how is this feature realised in professionally made live centres? I would be grateful for any comments on where I went wrong in my design.

I have decided that I should purchase a live centre, being so cheap. However, maybe the cheap ones are no better than mine. So I would like any recommendations and or points to look for when purchasing one.

Regards,

Brian

 

[bezalel2000]

Hi Runner

Roller bearings and common 62xx type which have a shallow groove are intended for forces exerted at right angles to the shaft (the force applied by the cutting tool across the job) but these bearings are not suitable for forces exerted Parallel to the shaft (along the bed)
As you mentioned, what you need is a thrust bearing but one that handles significant side load.

If you are intent on making your own live center, a cone and Chanel ball bearing, like those in the front fork of a bicycle, should serve well

Bez

edited

 

[Runner]

Thanks Bez,

the bearing is a NSK 6202Z, which is, as you indicated, a shallow groove not suitable for where a sideways force is applied without the use of a thrust bearing. The bicycle cone bearing is a good idea, which I shall give some thought to how I can incorporate it into a live centre.

Brian

 

[MachineTom]

You stated that a Roller bearring was used, the # is a ball bearing? I'll assume that the # is correct the description is incorrect. Look for a Deep groove BB, or just buy one, Concentric Centers brand name makes a MT-2 low profile center, it uses needle bearings and a preload spring, great when working small diameter stuff, and Royal brand for standard stuff. unless you have a heavy duty lathe stay with the single bearing stuff, The replacable point chinese stuff has too much runout for my taste,( tested 2 different sets) YMMV

 

[Runner]

Yes you are right it is a ball bearing, I was unthinking when I wrote the description. Thanks for the info. I "googled" both Concentric Centers and Royal Centers and both provided information that answered my concerns. Royal particularly helpful in providing a pdf file that shows the contruction of their live centres, showing a double ball bearing and thrust bearing construction. Comparing my design to theirs indicates that there is no such thing as a quick and dirty live centre.