Rotating Centre Problems

I am making some axles, appx. 6" long by 3/4" diameter, from EN8. After facing and centre drilling each one I am mounting them between centres on the lathe and turning the journals at the tailstock end.

Now, I am measuring the finished and polished journal diameters with a Mitutoyo digital micrometer and so know what is happening to 0.0001". I was going to use a solid centre in the tailstock but decided to try an older MT3 rotating one. The journal ended up exactly where I wanted it but I then noticed the rotating centre had about 0.003" free sideways play so replaced it with a MT2 modern but unbranded item bought recently.

The turning seemed to proceed well but I had a few problems measuring the diameter as I got to the finish. When I took the work out to see what was going on I was amazed to find that the journal was OVAL with the peaks some 0.0014" larger in diameter than the waist. This had happened on two separate journals with the same result.

To test each centre, I put one of my satisfactorily round journals between centres and measured the total run out at the tail stock end with a dial gauge at the top journal point.

RESULTS:

Solid centre = <0.0001"

MT3 worn centre = 0.0002 / 0.0003"

MT2 modern = 0.0008"

Since the MT2 modern one has replaceable centre tips I figure that the tip I am using is not in line with the bearing axis. But what I cannot understand is when I cut with it how does 0.0008" TIR out of true result in an oval piece of work that is effectively 0.0014" out of true.( i.e. twice as much assuming my measurements are all +/- 0.0001" ) . Or has the tip gone further into its socket and aligned better as I have swapped things around?

Any engineers with experience of rotating centres please? Am I expecting too much and should I stick with a solid centre? I will try and get the MT3 one apart to see if I can replace the bearings perhaps.

Norm

Edited By norm norton on 29/12/2015 19:56:55

IMO, if you want a really accurate job, use a fixed centre. Use a live centre to rough down.

There ARE good live centres out there, but they cost a lot because they are high quality precision kit

http://www.model-engineer.co.uk/forums/postings.asp?th=82419

Edited By Ady1 on 30/12/2015 00:53:07

The difference in runout may be due to the extra sideways forces on the revolving centre when actually taking a cut.

In my experience, Chinese bearings are too often very poorly made. Quite often replacing them with good quality brandname bearings such as FAG, Nachi, SKF etc will cure the problem.

.

I see that the Gepy link that I posted in that thread is dead.

Try here.

MichaelG.

If you are calling your axles 'scrap' , turn a test section down to the same diameter at the headstock end and measure that. This would check to see that there wasn't some sort of cyclic vibration coming from the headstock.
BobH

Can the pre load on the bearings be adjusted in the rotating centre? I have a Skoda rotating centre, it's my second one, wore the first one out, and although it's possible to repair them , it's cheaper to buy new, last one cost about $NZ100. These centres are adjustable, but have never needed it. Best use, a solid centre with some Moly grease.

Ian S C

The solid centre should give better results, but you do not say what machine you are using, its condition or indeed what tolerances you are trying to achieve.

If you are expecting to get accuracy to a tenths of a thou then you have to establish whether your machine is capable of such tolerances. Indeed you may, if wanting very close tolerances, have just found out why commercial products are often ground to achieve the final size and tolerance. You state that your "worn" centre is showing a .0002/.0003 compared with .0001 for the fixed centre. This would be truly impressive if using a model makers lathe IMHO.

Backlash alone in your slides would normally prevent you achieving such accuracy.

There is a lot of good advice here, but first you must check what you are trying to achieve, and determine whether the lathe you are using is capable of doing this, and then look at the centres. HTH.

Many thanks for the comments. Nice link back to 2013, thank you Andy.

Agree with what is said and today have done it properly with a carbide tipped fixed centre. You do have to try to leave 'a quarter thou' clearance between tip and socket or the grease is pushed out and it starts getting warm. I decided to run it with the 'quarter thou' slack for all but the final cuts when I removed all play. I do wonder if a high performance grease will be better, moly grease as you suggest Ian.

Last night when posting I was pretty miffed at finding the £50 multi-tip live centre was out and wondered what you thought. Checking today, swapping tips, ensuring all is clean, it clearly shows a run out of some 0.0008". I have had it for more than a few years but never used it for a precision task. I suppose one might be lucky and buy another £50 one and find it reasonable, but I now realise this is the too-cheap end of the market. I see that Bison and Rohm items are around £150 and even budget Vertex are £80. I have come to the point where I have stopped using the hobby suppliers for tooling. Not sure that I will afford a Swiss Gepy with its 1 micron precision though…..

My Czech MT3 item looks like it comes apart so hopefully I will find a taper bearing in there and can shim or adjust it. The other cheepie one is going in a bin!!

Mike, I have got a reground Super 7, but more precise is my Harrison 140. I am machining the axle journals down to 0.0008 - 0.0010" over size and then finishing with 240/400/1200 grit paper. Knowing where you are for sizes all becomes quite straightforward with a good digital micrometer - the Mitutoyo has been one of my best second hand purchases. You are right about the difficulty of trying to get tenths of a thou along a job. Bed alignment with the head, and/or tailstock alignment with the axis, means that I am well aware that we are always cutting tapers!. Several tenths of a thou in a couple of inches is the best I can manage between centres. The Harrison is reasonably good from its headstock though. When I set it up a couple of years ago an alignment test cut of a hefty bar in the chuck gave 1.1094" at one end and 1.1092" eight inches away.

Norm

I am somewhat confused Norm. Exactly what sizes are you trying to produce by turning and what are your finished sizes. You see you have said that on your Harrison 140 you can achieve 1.1094 and 1,1092 over an 8 inch length with a chuck held piece of bar. Now you are trying to turn final diameters of 0.7500 inch over 6 inches! So why don't you simply use the Harrison?

I am somewhat sceptical that your lathe is that accurate.

A further confusion is caused by your comment of producing journal size 0.0008/0.0010 oversize. Eight tenths to one thou......and then using grit paper to finish to size???????????? Seriously??? What size and tolerance is the finished product, and what surface finish are you looking for?

Can I ask you check these sizes and tolerances, because they simply do not make sense to an old school chap like me. Sorry if that sounds rude, it is not meant to be but your descriptions of sizing capability on your lathe would suggest that you can turn to the finished size.

As someone who spent 50 years in toolmaking and 30 of those in a very high precision environment, a lathe is not for the kind of tolerances being bandied about here. To achieve the 2 or 3 tenths sizes we had a temperature controlled shop and ground to those sizes. Our Jones and Shipman 1400 machines where never turned off so that they stayed stable. Inspection was at up to 100 to one shadograph and digital probing. A new toolroom lathe needs a very smokey operator to hold better than 1/2 a thou.

Edited By jason udall on 30/12/2015 19:18:42

Turned to 0.7510" - 0.7508" and then taken down to the finished size of 0.750 with abrasive, simples

Quite commonly used to take the crankshafts of model engines down to final size and at the same time get a better finish than with the turning tool, almost a ground finish.

A tenth of a thou is a heck of a tolerance on an axle, and indeed a heck of a tolerance on a lathe. In all my time in engineering I only had one job where any tolerance was less than 0.001". I would think the level of accuracy you are trying to acheive will be difficult to get.

Mike, sorry if my descriptions do not add up, it's difficult describing everything. Turning EN8 leaves me a fine but still rough surface. The final 0.0008" (or so!) I removed CAREFULLY with 240 and 400 grit paper, measuring as I do so, then 1200 to polish to a shine. I agree, it is not a precision ground surface, I don't have that sort of equipment. But I now have two of my three axles finished and each journal is 0.7880" +/- 0.0002" and within 0.0002" along the 0.625" journal length. Good enough for a plain bearing surface sitting inside 0.7888" bronze axle boxes. Goodness knows how precise the axle boxes are after I lapped them but the plug gauge tells me that is what size they seem to be. 0.0008" is what I want for the oil film clearance (0.001" per inch)

But never mind precision discussions. Happily my MT3 live centre readily came apart and it has a nice expanding taper sleeve around the rollers with an adjustable front screw. So after cleaning I can adjust it to make a good useable centre.

Norm

Edited By Tony Pratt 1 on 30/12/2015 20:32:55

Norm: You don't say what sort of tooling you are using, but with insert carbide tooling I've found that you need to run at high speed to get a good finish on EN8; probably over 2000rpm for 3/4" stock. On a larger diameter, running at a surface speed over 500fpm, I got a finish around 1.6µm Ra.

Andrew

Andrew - no nothing like that speed, I dare not with the lathe dog and a fixed centre! It is a 0.2mm radius finishing insert. The literature says that EN8 tears on machining but polishes up well and that is just what I find. But thanks for the info - I might try a piece in the chuck and run it up to 2,000 to see any effect.

Jason - thanks for your support

Norm

(p.s. previous post typo, bearing journals are 1.156" long not 0.625"