Trouble with fixed center

[David George 1Participant @davidgeorge1]

Recently I have been making a replacement leadscrew for the cross slide on my Myford M Type lathe.

The steel I am using is EN24T and first turning was using a live center as it is only 1/2″ diamiter with parts 3/8″ and 5/16″ parts. when it comes to screw cutting I am limited to miss the body of a live center so I change to a fixed half center which gives me enough room to start the thread. I noticed the leadscrew was not running true so I had a look at the center and it had worn where the contact was on the center of the leadscrew. Inspecting the center it was not very hard and could be filed with a hand file. I decided that I could repair the center and drilled a 6mm hole into the end of the center about 12mm deep and fitted a piece of tungsten carbide into the hole and brazed it in place.

 

The point was roughed out using a off hand bench grinder and a diamond wheel on my cutter grinder. It was finnished by mounting the center into the morse taper in the spindle  to ensure it runns true. I then used my toolpost grinder with diamond wheel set at 30 degrees to finnish the point to runn within 0.0003″ dial reading.

 

David

 

[Michael GilliganParticipant @michaelgilligan61133]

That’ll do very nicely !

MichaelG.

[David George 1Participant @davidgeorge1]

in place and working well.

 

[Anonymous]

I had a dead centre like that. After it wore badly on first use, and proved to be unhardened, I binned it and have never used that (ME) supplier again.

Andrew

[Clive Brown 1Participant @clivebrown1]

I thought that unhardened centres were live centres, intended to be lightly machined “in-situ” in the headstock for good concentricity. My Boxford was supplied with both hard and soft MT2 centres. Carbide tipped centres are commercially available, eg ArcEuroTrade.

[JasonBModerator @jasonb]

If it were intended to be a soft “live” ctr then why shape it as a half ctr which is intended to be “dead” in the tailstock to clear the tool

[Mike HurleyParticipant @mikehurley60381]

My understanding has always been that the hardened centres were for tailstock use, as  in normal use they are a lubricated bearing surface, and will wear (as the OP found out). The unhardened ones designed for the head end, as they are effectively locating only and not a running bearing surface – also that way so you could routinely re-shape that one with normal tools to ensure it was preciseley on centre.

So, if you’ve got a bit of an assortment of centres from different sources and mixed MT sizes, you may simply be using the ‘wrong’ type in one end or the other. Can’t think why you would have an unhardened half-center though, unless ,as suggested, it was just ‘faulty’

Mike

 

[Michael GilliganParticipant @michaelgilligan61133]

David has successfully upgraded it from Sow’s Ear to Silk Purse

All credit to him !

MichaelG.

[BazyleParticipant @bazyle]

I think George is one of the more advanced engineers on this site and knows the difference between hard and soft centres, but often suppliers don’t and provide anything at random. He was demonstrating how the situation could be recovered without resort to ebay and another pot luck item. Commercial carbide dead centres (not ahem far eastern maybes) are not cheap.

[Michael GilliganParticipant @michaelgilligan61133]

… That would be Mr George to you, Bazyle !

🙂

MichaelG.

[JasonB]

On JasonB Said:

If it were intended to be a soft “live” ctr then why shape it as a half ctr which is intended to be “dead” in the tailstock to clear the tool

True. At risk of thread drift however,I’m not sure that I’d call the centre in the OP a “proper” half-centre.; more a centre with some tool clearance. A half-centre, to my thinking, allows facing cuts across to the drilled centre-hole.

[Michael Gilligan]

On Michael Gilligan Said:

… That would be Mr George to you, Bazyle !

🙂

MichaelG.

Whoops, sorry David.

[David George 1]

 

On David George 1 Said:

in place and working well.

 

Looking at David’s picture I’m not even sure a half centre was needed to make this part.

There looks to be plenty of axial distance when the unthreaded shaft length and the ‘triangular’ gap are combined.

I agree though that it is odd (for what appears to be) a manufactured half centre to be supplied unhardened. Nice work fitting and grinding the carbide end.

Ian P

 

 

 

[JasonB]

On JasonB Said:

If it were intended to be a soft “live” ctr then why shape it as a half ctr which is intended to be “dead” in the tailstock to clear the tool

Maybe a second hand centre, home modified by someone who didn’t understand, or overheated and lost its temper.
Alternatively a soft one deliberately modified to work with rotary table’s tailstock to clear some cutter.

Bill

[David George 1Participant @davidgeorge1]

The center was brand new, first use, but it only took a couple of hours to drill, braze a piece of 6mm carbide rod which was spare, from some inpection jigs I made for RR for inspecting turbine blades, a while ago, rather than send it back and wait for a replacement which could be also soft. Job done.

David Also known as George by many friends with no animosity 🤣

[Author]

Posts