Rotating (live) centre disassembly.

I purchased an inexpensive rotating centre from Chronos Ltd., which has replaceable bits. It's only for light duty and odd jobs not accomodated by my other centres (bit of tube work, thin parts needing a fine point etc.,). Anyway, the one that was delivered had rough bearings and Chronos quickly replaced it with no questions (cudos to them!). That left me with a faulty centre representing a challenge as to whether or not I could effect a repair of sorts. Disassembly seemed difficult, there were no visible screw threads holding it together, and a cautious prod under the arbour press suggested that, if it were a pressed together assembly, then a heck of a lot of pressure was needed to get it apart.

Instead, I tried flushing the thing through (after removing the end cap) first with engine oil, then running the thing for an hour, then refilling with moly grease using the same method. I taped a 5ml syringe on the end to force the lubricants through. The result seems to be a success, at least no more crunchy bearing - whether the bearing was damaged in the process I can't say but it seems to be OK.

I'm still curious how the thing was put together originally...

Edited By Chris Heapy on 21/04/2013 12:28:21

Edited By Chris Heapy on 21/04/2013 12:30:05

Hi Chris

I too purchased one of these - due mainly to the variety of replaceable "bits" as you put it. I've not used it a lot but so far I've not encountered any problem with the bearings. I'll bear in mind your remedial trick in case of trouble in the future!

By the way - are you the Chris Heapy of the wonderful web site? - if so many thanks for all the useful and entertaining info. I've been able to access over the years.

Regards

Norman

I believe they simply forgot to clean the swarf out before assembly so if yours is not rough to begin with then it's unlikely to develop the problem (albeit there don't appear to be any dust seals to protect it).

Yes, thanks for the memories

Rgds, Chris.

Hi Chris,

A quick google tells me your website was the stuff of legend. Is it gone forever?

Neil

Hi Neil,
After seeing Norman'scomment about Chris's website I too Googled it and came up with this. I got the impression that it contains the information that was on Chris's site.

Les.

I see you've constructed a new light sabre! Your skills are complete.

In the centers Ive taken apart

end cap

small circlip or E clip

small bb race..

center spindle now pulls out from other end..needle rollers and trust race...

clean out

replace bearings with insttument grade and silicon grease(doesnt wash out)..donot over grease

stiff but true and comes into own at 2000 to 8000 rpm
..will need warming up for best performance..

Skoda, Rohm ..all last about about 1000 hrs outoff the box ..

then 3000 plus with the above..upgrade..and less vibration/runout.

.btw new bearing cost typically twice what the center did

I'm on my second Skoda 2MT size, the bearings collapsed on the first one, and parts to repair it were about the same, or maybe a little more than a new one, so I'v got one that one day may get something done to it one day, ie., i might redesign the insides, and rebuild it my self. I think it was a bit light for the work it was getting at the time. Ian S C

Ian,  all mine are mt3..so just that bit easier

I have quite a few in need of rebuild ( from the years when a new one was bought before I came on the seen and rebuilt them.. ..) I count at least 12... unfortuantly many have lurked straight off the machine in boxes for years.. and might not be worth the effort ( if needed anyway) but all that I have done fail in the same way..

Anothe pet peeve is operators grinding down the noses to get that last bit extra clearance...I have about 5-10 not fit to be seen or rebuilt..

..Gees thats a lot of dead live centers....

Edited By jason udall on 23/04/2013 15:53:39

If there is a circlip in the rear it's hiding itself very well. I''ve even poked about with a probe and can't feel one. In the absence of any definite evidence otherwise I have to conclude it is just pressed together.

Most of those centres that fail do so because the operator fails to re-tension the tailstock during a heavy turning operation and heat expansion of the work crushes the bearings. If you can remember to keep checking, or don't clamp the chuck so tightly (let the workpiece slide into the chuck as it expands, or let the tailstock itself slide back a little) then the center will last a bit longer . You have to know your own lathe to get those settings right! However, the accuracy of even these cheap Oriental centres is better than most lathes' ability to turn parallel, a wayward 0.0005" at the tail end is not normally critical. It's a different story on a cylindrical grinder of course.

Edited By Chris Heapy on 23/04/2013 16:24:04

Chris

Another method that might have found its way into high precision but relatively low cost tooling, is assembling with Loctite and its ilk.

The extreme precision demanded of hard disk platter and head mounting arms is acheived by making the shafts and housings a clearance fit in the bearings and then assembling using precise jigs to get the optimum preload and location. Its a good way of mass producing near silent motors that run at 10,000 RPM for thousands of hours

I do have one bit of workshop kit that is assembled with Loctite. It is a non genuine 'Foredom' handpiece for a flexible shaft. It uses two angular contact bearing in a plain tubular bore, no circlips on the shaft or housing. As long as it does not get above 200c (I will have let go long before then!) it will do the job.

Ian P

Very true Ian, and if it's Chinese Loctite I'd best not drop it... However, it would have been a tricky job filling the bearings with grease and then assembling with Locitite in the hope of it sticking anything - or perhaps they assembled it and forced grease through afterwards (much like I had to) which is not a good way of packing bearings.

Chris Heapy .."Most of those centres that fail do so because the operator fails to re-tension the tailstock during a heavy turning operation and heat expansion of the work crushes the bearings. If you can remember to keep checking, or don't clamp the chuck so tightly (let the workpiece slide into the chuck as it expands, or let the tailstock itself slide back a little) then the center will last a bit longer"

bit hard to adjust the collet pressure "in flight" with the rest of the stock bar ( 10foot) still attached.. tailstock pressure is set hydarlically and chosen to suit each job ( or should).. and is typically 10% of the rated load for the centers in question.. ( only checked this once when I was looking into why they fail so oftern ... machine max tailstock pressure was 1/5th of the Rohm rating.. and we seldom operate at that pressure ( tends to bow the parts ).... as to rpm and lubrication again Rohm don't advise lubrication below 2000 rpm and only recomend oil from 2500 to 8000 .... any way 1000 hrs would be buckets for home use but is only months for production...we just treated them as expendable like tooling..that said put £200 of bearings in them and a better than new result was obtained. even tried solid carbide bushes ( now they cost! ).. well thyey out last the spindle and housing ....( I kid you not reused them in about 3 more rebuilds)..but carbide rebuild did not last as long as instrument grade bearings..

glad you have a solution

mmm maybe spindle is interference fit in small end? or heat fitted?

I never tried having 10ft of stock hanging out the back of my S7B, must try that sometime (will have to drill an appropriate hole in the garage door though...)

well not quite10 foot the headstock/collet closer is about a metre..and the rest is in the barfeed....

but it all needs driving which relies on the collets grip....a fair mass at 42 mm

Hi Jason & Chris

I remember ( who could forget!) at a night school class - remember those? - someone leaving a long length of bar hanging out of the back of the headstock spindle on a Colchester. On power up it started an increasingly violent oscillation and then wrapped itself around like piece of liquorice before someone thankfully hit the master emergency stop! I've never forgotten it - and it all happened so fast. It would have killed anyone standing in the way.

Norman