#MT Collets, tolerances

[Bill PudneyParticipant @billpudney37759]

I want to bore the front bearing recess on a small model aeroplane compression ignition engine. The normal process is to do everything possible from one end, in this case the back end, remove the workpiece and place in a previously made fixture, which registers/locates on the previously turned rear bearing recess, which ensures concentricity of the first bearing recess with the second.

The "previously made fixture" is not possible to locate accurately in the ER32 collet, there is an out of true of about 0.1mm, when I was aiming for about 0.01mm max. I have checked the collet chuck and its less than 0.01mm out in the taper. Without the collet chuck, the MT3 spindle taper produces as near as I can see no deviation, certainly less than 0.005mm, which I reckon is pretty good.

So I was thinking about getting an MT3 collet to put in the spindle, then putting my little fixture in that. The question is, the fixture has a spigot for the collet of 18.85mm diameter. So would a 19mm MT3 collet or a 3/4" collet (19.05mm diameter) hold and locate accurately my 18.85mm diameter spigot??

I would prefer to get a 19mm collet than a 3/4" one as its liable to have a metric drawbar thread. Any ideas, as they seem to be pretty rare??

Thanks in advance

cheers

Bill

Edited By Bill Pudney on 12/01/2016 10:21:39

[Bill PudneyParticipant @billpudney37759]

[Michael GilliganParticipant @michaelgilligan61133]

Bill,

In a word … No

[unless someone has come-up with a clever new design]

Please see my response to OldMetaller, on this thread.

MichaelG.

.

P.S. … You would probably do better to buy a larger [20mm+] collet, and make a split-sleeve slave collet to accept the pin on the fixture.

Edited By Michael Gilligan on 12/01/2016 10:40:09

[Martin ConnellyParticipant @martinconnelly55370]

This is the sort of job where mounting a piece of metal in the lathe and then machining it to create a mounting fixture would be good practice. As long as you do not dismount it it will have theoretically zero run out. The actual amount of run out may be tiny or immeasurable but there is always some give in the spindle bearings that varies with the pressure on them when something is cut.

Alternatively have you got soft jaws for a chuck that can be bored. Once again this will give a low tir bore to mount things in.

A third option is to mount your existing fixture in a 4 jaw independent chuck. Do you have one or is there some other reason why this is not an option?

Martin

[Bill PudneyParticipant @billpudney37759]

Yes I have a 4 jaw chuck. That was actually plan C. Plan A was the 3MT collet, which doesn't seem to be a goer. Plan B was to get a blank end arbor and make a new fixture. Thinking about it, the 4 jaw would be a pretty good way. I just don't like 4 jaw chucks!! However, the common sense, practical approach would be that it's good practice. So I will give it a go when the temperature drops, today was 39 degrees C, which means about 45 to 50 in the shed. Tomorrow is forecast to be the same, but Thursday is looking good. Right now it's 10.30p.m. and it's 32 degrees. That's too hot for this little black duck!

Thanks chaps, I'll let you know how we go wrestling with the 4 jaw.

cheers

Bill

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Bill Pudney on 12/01/2016 12:08:26:

Plan A was the 3MT collet, which doesn't seem to be a goer.

.

Bill,

Did you see my PostScript ?

I may have missed some important feature of your fixture, but [from here] it seems a viable solution …common practice, on a smaller scale, amongst clock repairers.

MichaelG.

[JasonBModerator @jasonb]

Don't remove the mandrel/fixture once it is machined

When I have done aero engines that need bearing recesses from both ends I turn up a parallel mandrel and then add a slight taper as it nears the chuck. The parallel part is a good fit in the engien part and then a quick tap locks it onto the taper. An expanding mandrel also works.

First side bored through and bearing cecess done

Expanding mandrel skimmed to fit and then without removing from lathe the bearing housing on the other side done.

Edited By JasonB on 12/01/2016 13:25:08

[Michael GilliganParticipant @michaelgilligan61133]

Bill,

1. Searching for an illustration of a 'slave collet' … Google seemed determined to offer me 'slave collar'. The concept is simple enough though [a split bush, or reducer sleeve].
2. My suggested 20mm bore is non-preferred for MT3 … But it is available as an oversize.

That said: The four jaw is fine, unless it's for repetition work.

MichaelG.

[Bill PudneyParticipant @billpudney37759]

MichaelG, yes I certainly understand the concept you described of using a slave collet. I've just had a look at the link you gave and have inquired whether or not they ship to Australia. If they do ship here I will order a collet and have a go with your concept.

I have just set up the 4JC and it was quite good, I got the runout down to under 0.01mm, which was pretty pleasing. Then the temperature got to me (before 10 a.m., over 35 degrees!)

One of the reasons that the 4JC option was down the list was because I have several to do, two immediately and then quite a few after that at some distant point. Hence my desire for a more reproducible process.

At the moment I intend to progress across a broad front, getting an o/s collet and using a slave collet as MichaelG suggested, (assuming I can get an o/s collet), making a new fixture based on a blank end arbor, and as a backup, in case everything goes to poo, the 4 jaw chuck.

cheers

Bill

[Bill PudneyParticipant @billpudney37759]

For what it's worth, I just had a quote from the company MichaelG suggested for an MT3 x 20mm collet. The collet was 10 Euro, which was fine, however the postage to Australia was 120 Euro, which may be what their post office quotes, but it's a bit O.T.T. for me.

It's a pity as they had some interesting stuff on their website, but there you go.

So it's back to the 4JC for now!!

cheers

Bill

[Danny M2ZParticipant @dannym2z]

Posted by Bill Pudney on 14/01/2016 03:09:17:

For what it's worth, I just had a quote from the company MichaelG suggested for an MT3 x 20mm collet. The collet was 10 Euro, which was fine, however the postage to Australia was 120 Euro, which may be what their post office quotes, but it's a bit O.T.T. for me.

Oh dear Bill, the dreaded postal bandits strike again! Maybe they were going to deliver it in person.

Would it be more convenient to use a 3/4" ER collet and purchase a suitable long bolt for the drawbar?

(That's what I did when H&F sent me an MT3 boring kit for my mill, the kit had an imperial drawbar thread).

* Danny M *

[Bill PudneyParticipant @billpudney37759]

Hi Danny, Hows things??

Did you mean a 3/4" MT3 collet? That wouldn't leave room for the "slave collet" suggested by MichaelG.

I initially tried an ER32 collet in the collet chuck, and the collet (not the chuck) was giving unacceptable runout, so that's why I started on this saga.

Today I did the first bearing housing in the 4JC, it took a lot of fiddling about to get the T.I.R. to an acceptable number, less than 0.01mm. It was avoiding this fiddling about that made me want to go down some alternative route.

I keep telling myself, "I'm doing it because I enjoy it"….but I do!!

cheers

Bill

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Bill Pudney on 14/01/2016 03:09:17:

For what it's worth, I just had a quote from the company MichaelG suggested for an MT3 x 20mm collet. The collet was 10 Euro, which was fine, however the postage to Australia was 120 Euro …

.

Sorry, Bill … I had no idea that the postal charges would be so high.

I think the problem is that they use a flat rate for anything up to 30Kg.

21 Euros to the UK is bad emough … but 120 Euros to ship 0.20Kg is ludicrous.

MichaelG.

 

[it seems that only the Chinese are capable of shipping Worldwide at low cost]

Edited By Michael Gilligan on 14/01/2016 08:37:02

[Danny M2ZParticipant @dannym2z]

Posted by Bill Pudney on 14/01/2016 08:30:08:

Hi Danny, Hows things??

Did you mean a 3/4" MT3 collet? That wouldn't leave room for the "slave collet" suggested by MichaelG.

Bill, I did mean an MT3 collet with a 3/4" hole. Should be no requirement for a slave as 3/4" works out to be 19.05mm. Surely a 3/4" collet is capable to collapse 0.05mm when tightened. Otherwise try wrapping the shank with cigarette paper.

* Danny M *

[Michael GilliganParticipant @michaelgilligan61133]

Danny,

Bill's original question was: "The question is, the fixture has a spigot for the collet of 18.85mm diameter. So would a 19mm MT3 collet or a 3/4" collet (19.05mm diameter) hold and locate accurately my 18.85mm diameter spigot??"

… To which, I answered "No" [and this remains my opinion]

Hope that clarifies.

MichaelG.

[Tim StevensParticipant @timstevens64731]

It seems that the ER collet you tried may be eccentric, but I recall reading somewhere (else) that there can be problems with ER collets if they are pulled up using the conventional C or claw spanner. The solution suggested was to tighten gently, then to tap all round the retaining ring – this seemed to settle the collet more accurately in its bore.

Perhaps the offset load from the tightening would be reduced if you used a spanner which did not rely on applying a side load on the spindle – a T spanner with a load equally on both arms, rather than a single sided job. How you link a T-bar to an ER slotted spanner is a problem, I admit. But it may be worth a try before you invest in sending stuff around the world.

Regards, Tim

[Bill PudneyParticipant @billpudney37759]

The original ER32 collet was eccentric, but within tolerance with a T.I.R. of about 0.1mm. I have some "high precision ones on the way from CTC. It will be interesting to see how they perform!

I remember seeing an article where the writer suggests giving the collet nut a (light) tap to reduce the T.I.R., was it in M.E.W.? Anyway, wherever it was, I did try that, I also tried swapping the ballrace nut for a standard nut, with no obvious change.

cheers

Bill

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