|1992 forum posts|
Yes, I'm with Nick-G here. All of my chucks are old - mostly Pratt I think (without checking) and well worn. But I always work on the assumption that the 3-jaws are not accurate and for many things this doesn't matter in practice.
If I need to do a second operation or hold something like silver steel true, then I either clock it true (in a 4 Jaw) or use a collet instead.
|Martin Kyte||20/02/2017 14:56:53|
2751 forum posts
Are we forgetting the primary purpose of a chuck is to grip the material firmly so it does not either slip or wobble about (by that I mean move rather than stay fixed albeit off centre). Worn jaws do have a tendency to do both which defeats your ability to turn parallel or round, produce a good finish or tap from the tailstock without the work turning.
Wear on the scroll will matter much less than wear in the jaws.
That's not saying you cannot get a bargain now and again.
1808 forum posts
Chuck jaws with age tend to go 'bell mouthed' with wear. Especially with us model engineers who often have to grip short parts.
Things like parting off with a bell mouthed chuck can be a bit of an 'adventure'. been there. ........ Tried that.
|Martin Kyte||20/02/2017 15:43:12|
2751 forum posts
So my conclusion is if you want a cheap chuck go for an economy standard precision new one rather than a high end old one. You are unlikely to get the low run out that the high end one once had and the jaws will have seen some use to boot. With then new one at least the jaws should close parallel. I know you can always regrind but that will not put the wear back on the tenons and new jaws are expensive.
That said I pensioned off my old griptru by buying it a set of soft jaws. In addition I have a 2 independent four jaws, a self centering four jaw an a Super Precision Pratt Burnerd for Sundays.
2051 forum posts
Unless you know for certainty that it's good then yes, I can say I have experience of prestigious large chucks that well and truly had it at 0.2mm runout, and gave me a difference in 0.05mm in diameter every 5mm or so. Try making gauges out of that!
Luckily it wasn't my machine but it was an extremely good make of industrial lathe that had seen a thrashing over it's lifetime.
My own home lathe can certainly do better than that, but anyways, standard 3 jaw chuck for my £800 machine gives 0.03 runout on nominal bar stock.
May have not been all the old chucks fault but I can't imagine it helped matters.
Edited By Michael-w on 20/02/2017 16:49:25
|Nick Taylor 2||21/02/2017 09:20:35|
|102 forum posts|
Out of interest, everyone is always talking about old Pratt chucks or new import chucks - is no one interested in new Pratt chucks?
I have an old 125mm 3 jaw super accuracy which is very worn, the outside jaws are not so bad but the bar gripping on the inside jaws is terrible and very badly bell mouthed.
I'm temped to buy a 125mm standard accuracy Pratt chuck, seeing as they can be had brand new for less than £200 online. Any reason not to? Other than price compared to the other offerings.
|Martin Kyte||21/02/2017 09:30:43|
2751 forum posts
Sound like a plan to me Nick although new Pratt chucks would be outside the topic of the tread.
|John Stevenson||21/02/2017 10:07:12|
5068 forum posts
Pratt chucks EXCEPT the grip-true are now made by TOS in Poland and have been for quite a few years.
Made on the same line they make their own chucks.
|Nick Taylor 2||21/02/2017 10:43:59|
|102 forum posts||
Ahh ha, thanks John - didn't know that!
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