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If it ain't broke don't fix it - or should I?

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Robin Graham19/07/2019 01:05:30
651 forum posts
148 photos

I've made an ER40 chuck for my lathe. It works well enough - I'm getting a TIR of better than 0.008mm (if my dial indicator is to believed) using 'standard precision' collets, which is more than OK for what I do. However, a blush rises to my cheeks when I look at the finish on the the internal taper:


Not a great pic, but the finish is somewhat less than superb. I'm not sure what went wrong - maybe swarf rattling round as happens when boring - I cut it dry.

Should I just say that it works for now and leave it alone or attempt to shine it up? If the latter how would folk go about it?






Edited By Robin Graham on 19/07/2019 01:18:42

Mark Rand19/07/2019 01:18:43
850 forum posts

Don't worry too much what it looks like. Is it actually rough when you drag a fingernail along it or it is just an optical effect?

Either way, unless it has been hardened, it'll eventually wear smooth.

Robin Graham19/07/2019 01:35:55
651 forum posts
148 photos

Thanks Mark, it's 'slightly rough' to the fingernail but hard to quantify. Not hardened so probably best to leave as is and let it bed in. Just annoyed that I didn't get a better finish and wondering why.


Kiwi Bloke19/07/2019 05:54:02
301 forum posts
1 photos

Probably would have been better with lubricant, assuming tool geometry and all other variables were OK. It's often helpful to bore with the tool 'upside down', working on the 'back' of the bore - the surface furthest from you (perhaps you did...). That reduces the risk of chips getting dragged between tool and work and spoiling the finish. Also, the feed is applied by conventional and familiar use of the dials, rather than having to subtract. But if it works OK, why bother?

Michael Gilligan19/07/2019 07:30:43
14786 forum posts
635 photos
Posted by Robin Graham on 19/07/2019 01:05:30:

... It works well enough - I'm getting a TIR of better than 0.008mm (if my dial indicator is to believed) using 'standard precision' collets, which is more than OK for what I do.

[ ... ]

Should I just say that it works for now and leave it alone or attempt to shine it up?


I think you answered your own question, Robin

Unless and until the time comes that it is not "more than O.K." ... Just use it.


not done it yet19/07/2019 07:45:09
3952 forum posts
15 photos

Question might be: What difference would it make if you simply improved the finish with a fine abrasive? Likelihood is that there would be a good deal more collet contact with very little change in diameter or taper.

A test with engineer’s blue might be enlightening, just to check the contact between collet and holder.

Mick B119/07/2019 08:57:24
1366 forum posts
75 photos

You're not worrying about the dried-up oil stains on yer headstock.

It works well. Why fuss about unseen cosmetics - this is engineering, not fashion?

If you hadn't put it on this forum, only you, and perhaps He Who Sees All Things would know.




Edited By Mick B1 on 19/07/2019 09:25:25

pgk pgk19/07/2019 09:24:02
1520 forum posts
285 photos

I'm sure I'm not the only one that has trouble getting the best finish when hand turning the top slide on a blind taper. I made an ER32 holder and used emery wrapped round a collet to improve things.


Baz19/07/2019 10:06:37
323 forum posts

Leave it! If you re machine it you may make it worse, angle wise or finish wise, use it until it needs re machining, which will most probably be ten years time with the amount of use model engineers stuff has.

SillyOldDuffer19/07/2019 10:08:41
5148 forum posts
1074 photos
Posted by not done it yet on 19/07/2019 07:45:09:

Question might be: What difference would it make if you simply improved the finish with a fine abrasive? Likelihood is that there would be a good deal more collet contact with very little change in diameter or taper.


The smoother the mating surfaces the better the grip, which - in theory - is a 'good thing'. Whether Robin's taper need improving is a much harder question. Depends how rough it is. I've often found feel to be better than a visual inspection. Running a finger or finger nail lightly over an object can detect invisible defects.

So I'd compare the feel of the new taper to that of another. If the new taper feels markedly rough in comparison, I'd polish it gently with a fine paper. (With care - overdoing metal removal could spoil the taper.)

There's a difference in my mind between the miniature screw-thread type of rough finish with pronounced peaks and troughs due to severe tearing, or wrong feed-rate, and the spoilt look you get when swarf is caught by the cutter and smeared over an otherwise flat surface. Ideally tapers should be polished, but I suspect smears matter less than ridges.

If the taper slips due to poor contact there's a high risk of damaging both male and female tapers, making them even more likely to slip in future. I suspect it would take a lot of time for such damage to become bad enough to matter in a home workshop. But it depends on how hard the taper is worked.

Some much for theory. In practice, I wonder if dirt getting into the tapers might be more serious than a mildly poor finish? Despite wiping off before each insertion, I've still found swarf crushed on the male. As well as reducing the grip, presumably dirt must also tilt the taper slightly. But I've not noticed dirt in the taper causing a real-world problem with a cut.

On balance, if the taper works and doesn't feel awful, I'd leave it alone.


Howard Lewis19/07/2019 10:28:52
2752 forum posts
2 photos

Like pgk pgk, I used fine emery wrapped around a collet to polish the internal taper.

BUT, do not press too hard, or it will grab and snatch things out of your hand, if you are lucky.

Putting some oil onto the emery tape, will soften the cut slightly and improve the finish.

This technique has worked for ,e, for the five or so times that it has been used.

Gently does it!


larry phelan 119/07/2019 12:01:00
579 forum posts
11 photos

Let sleeping dogs lie !!

mechman4819/07/2019 12:15:23
2551 forum posts
385 photos

An old engineering adage … 'if it aint broke don't fix it'. So if it runs accurately enough for you … 0.008mm is pretty damnm good so...


Clive Foster19/07/2019 13:56:53
1993 forum posts
73 photos

Time to blue up and twist collet in the taper to see how good the fit really is. The look at and finger it test is remarkably sensitive and often something that doesn't seem to be as wonderful as you wanted is actually pretty good.

Which seems to be the case here.

However there is considerable potential friction involved in finally snugging down a collet so things generally go better with smooth polished contact surfaces. The contact area on ER systems is relatively large leading to potentially more friction. Which is why the published tightening torque figures are so high. My guess is that a less than smooth but still accurately shaped, surface will limit the maximum grip of the collet at sensible tightening torques. We Home Shop and ME types generally don't tighten the things up to full rated torque anyway and, usually, do just fine even tho' the grip in practice is less than book.

So long as the blue test shows 50% or more coverage and no shape errors, as in sorta like crappy thread, just surface finish I'd say you are good to go. If blue coverage is bit low then best to make a matching plug and lap these surfaces to improve the contact area.

Sounds like you re doing just fine but if you do do a blue test you will know.


old mart19/07/2019 14:16:08
1107 forum posts
113 photos

Have you tried all of your collets for consistency using undamaged drill shanks, or milling cutters? If the average is only 8 microns, then its best to leave well alone. If you have a longer test bar, and that is ok with several successive fitting and refitting, turning the collet in the nut each time, then what you have is as good as it gets.

Robin Graham20/07/2019 01:07:54
651 forum posts
148 photos

Thanks for replies. It seems that the consensus is to leave well alone and not risk compromising the accuracy I have achieved (probably more by luck than skill). Because I don't have any training in machining I just bash on, make mistakes and try to learn from them. In this case I seem to have achieved one goal (making an accurate chuck) but but failed on another (using the topslide to bore a taper with a decent finish). Hence the tension!

I tried bluing (with the right stuff, not layout!):


This was with a minimal wipe of blue on the internal taper, then rotating a 28mm collet supported by a nominal 28 mm bar (it was actually 27.85mm, which gave a light push fit) with hand pressure. I don't know how to interpret that though, other than it shows that the surfaces are in contact along the length of the taper.

I'll probably try a light skim with suds - my boring tools have through coolant holes, but I hate the stuff, wet, smelly, goes everywhere and leaves horrid dried oil stains on the headstock. But it clears the swarf.

Regarding torque, I first tested the chuck with what seemed normal force for tightening something like this and the TIR was a depressing 0.04mm. It was only when I really leaned on it that it went down to 8 microns.


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