Major flaw in the world of engineering

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Rotagrip will show you: **LINK**

http://rotagriponline.com/Photos/4%20Jaw%20SC%20chuck%20Hex.jpg

MichaelG.

I've got a 4 jaw self centring chuck, only every used on square bar.

My regular 4 jaw only gets used for "odd" shaped parts.

99% of the time I use a 3 jaw chuck, with either hard or soft jaws.

I also have an ER40 collet chuck that gets used on production work.

As a skilled engineer with 40 years experience on both manual and CNC lathes, I have never really understood most of the arguments for 4 jaw chucks...........

Jim

Well I don't have a SC4-jaw and have not really felt the need to buy one.

SC3-jaw gets used for a lot of round(ish) things and the soft jaws get regular use more as a backstop for shallow work than for concentricity. Mine is better than half a thou runout, I notice the cheap SC4 jaws on the ebay have accuracy of 0.003" so you may not even get all jaws in contact even if a "round" item were gripped.

Independent 4-jaw gets used for odd shapes as well as square, rectangular and offset. As it it larger than my 3-jaw I also hold larger round items or smaller ones I want a tighter grip of and 4 points of contact will distort large tube less than 3 points anyway.

The hobby engineer is more likely to use the lathe to turn odd shaped items (Ind 4-jaw & Faceplate) than in industry as they don't have the choice of tooling available that a machine shop would have, particularly so in the past before many had mills in the home workshop. Same goes for size as the 4-jaw is often larger than the 3-jaw eg typical Myford would have 6" Ind 4-jaw and 4" 3-jaw so the 4-jaw will get used at home whereas a machine shop will just stick a large round item into a large 3-jaw.

Small detail work I like to use the 5C collet chuck as it grips short lengths and is better if you need to get your hand close for using a graver or file. I do have some imperial hex and square collets but these days tend to start off with round and machine the metric profile I want as it gives a crisper profile.

The famous myfordboy seems well-pleased with his:

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MichaelG.

I always thought that a SC four jaw chuck was intended for woodturning

Roy

It’s amazing what we ‘always thought’ and how effectively the internet can disseminate alternative ideas.

There is of course a good reason why wood is perhaps more appropriate: it’s soft enough to let the jaws settle into it.

… the harder the material, the more severe the risk of only getting three point contact.

MichaelG.

Doh!

Thanks - yes, I should've thunk of that.

Nevertheless, as a turner who started in 1974, I don't believe I've ever actually seen a 4 jaw S/C, never mind used one, though I've known of them for I don't know how long.

I would expect one of those with a matching level of concentricity to be more expensive than a 3 law.

Since reading all the above comments some very informative (some judgemental for lack of my knowledge!) I have also bought the much talked about 100mm three jaw, Which seems to be very accessible, with a backplate available (pre-machined too!) Strange that. But of course it's my own fault for not knowing these things... Learning all the time though.

It's just a shame that not everybody knows the things that make engineers roll their eyes up at eh?

Thank you for all the helpful answers by people that understand where I'm coming from. It's good to know there are some that like to share their knowledge not just scorn...

When I started out with my lathe , bought new from Chester 20 odd years ago, I knew as much about the machine as it knew about me, So many things to learn about, but now I know twice as much nothing as when I started.

It,s called " The Learning Curve". My machine came with two chucks and a faceplate [Do they still supply faceplates now ?, never see them listed } However, later on I bought another 3 jaw chuck, from another supplier which came with no backplate. This was not really a problem because surprise, surprise, I found it easy enough to buy a piece of round bar from a steel supplier, just a slice off a length of stock bar to make my own backplate. A smaller piece of round was used to bore and thread this to suit the spindle. Both welded together [ dont cringe ! ] and the whole thing mounted and turned to suit the chuck. Still using it, so it can be done.

As others have pointed out, there are too many different chucks, lathe spindles ect not all interchangeable, so it is quite normal to sell chucks without mounting plates. Also, a 4 jaw sc chuck is not a great idea to start with, I used to think it was, now I know better.

Someone mentioned Sparey ? I think you could do worse than buy and read that book, carefully, since it will answer many of your questions in plain fashion. I seldom refer to it now, never more than every two days or so. It covers almost everything you need to know at hobby level and way above ! and it deals with the type and size of machine you are dealing with.

When all else fails, do not be afraid to ask stupid questions on this Forum, I do it all the time, to fill in the many gaps in my knowledge, most of them quite large,. Remember, no question is stupid if you dont know the answer, that,s why you have to ask them.

Do not be disheartened, it will come to you OK. Yes you will make mistakes, cock ups ect but that,s all part of the game, and the start of a scrapbox which will get bigger and BIGGER as yo go along. Dont ask how I know !

Guys take years to learn this stuff, so dont expect to know it all overnight.

I thought there was quite a bit of adverse judgement in your initial post. It's OK to rant, but don't be surprised if others rant back.

The SC 4 jaw chucks for woodturning have different jaws than those designed for metal turning as the jaws have a much wider jaw face which form a complete circle at a set distance out of the fully closed position this is the ideal size of a spigot to give maximum grip.

Many also have interchangable jaws so allowing for larger spigots to be made to fit this is so larger pieces can be turned.

Metal working sc chucks have a very small contact area making it unsafe to use on wood as fibres compress and can lead to wood departing the chuck.

Look at this chuck as an example. There are a few exceptions but they are more specialist jaw types

MickB1

Thye one and only 3 jaw that I have ever seen held work, AT THAT DIAMETRER, without eccentricity. Tyhe worst was worn and the work (Solver steel) was 0.036" eccentric.

As you say a GOOD 3 jaw, in reality a rare beast!

But handled on the correct way, even a bad 'un can be used to produce concentric work.

Howard

The only chuck that has no runout is the 4 jaw independent and this is only if you have the patience or desire to adjust it to that level. Any self centering chuck will have a tolerance and this can vary with the work diameter, the scroll mechanism can eat a swarf chip at any time and lose the accuracy you thought it had. If you need the surface you are holding in the jaws to be concentric with the surface you are machining then you will need to check with an indicator it is within the tolerance you require. I have no idea of the runout of my three jaw chucks because it does not matter to me. If the job needs to be concentric and cannot be completed without repositioning then it will have to be remounted in an independent jaw chuck or a collet. The accuracy or a three jaw should be of little concern for most work and some thought given to the order of operations can often make the runout irrelevant.
Mike

Mike, 3 jaw self centering with freshly bored soft jaws at the required diameter or a Griptru adjustable chuck.

Martin C

Just a comment: Before you do any cutting make sure the work is absolutely firm in the chuck (or whatever is holding it) and the cutting tool likewise. Doing so may still mean you could mess up the job but not doing so will mess up big time.

I use a face plate on the lathe far more than a 3 jaw self centering chuck. A Keats plate is also a really useful work holder. In addition to these a good set of collet chucks is invaluable.

JA

Mike, if you want an accuracy of say 0.001" and know your 3-jaw can do 0.0005" or better then what is the point of spending time clocking it in a 4-jaw when the 3-jaw will do?

I just looked up my 3-jaw maker says repeatability better than 0.02mm that is 0.0008" and I have clocked it at less than 0.0005". They also make better ones as mine is just "standard precision" but I wonder if as Howard says they are not seen much as mine costs about £500 and the better ones £1500 for a 125mm 3-jaw so not likely to be seen much in the home workshop, luckily mine was NOS and cost £100.

Industry these days seems happy to CNC a part on one end of a bar and then transfer to the second spindle opposite the first to machine the second side using 3-jaw chucks and get the concentricity they need. Though they may be using machined in-situe top jaws or just high quality chucks

I accept the points Martin and Jason make but if concentricity is critical you will need a quick confirmation check with a clock as you never know when a swarf chip is undermining your expected accuracy. I was taught that a 3 jaw is an everyday general purpose work holder and the accuracy and repeatability should always be questioned. Of course for many jobs concentricity is not critical and a decent chuck in good condition will be entirely satisfactory. For the few moments required to just position an indicator and spin the job to check that all is well could save a job from a random burr, dirt or swarf particle upsetting things. Changing chucks and collets or machining soft jaws takes time so only doing it when the job demands it will save time and effort. The case was made by the late Commander Barker for machining the backplate register for the chuck body undersize to allow the chuck to be adjusted to run true and the bolts tightened, a poor man’s Griptru. When you consider the construction of a scroll chuck it is miraculous that they are as accurate as they are.

Mike

Edited By Mike Poole on 23/04/2023 20:42:29

I think it was Prof. Chaddock who wrote in his Quorn series that when this was first suggested, the editor of ME refused to print it.

Paul, you might like to watch this video for general information about chucks:

https://www.youtube.com/watch?v=_eTSmQGEvwo&t=1s

and definitely watch this one:

https://www.youtube.com/watch?v=8t5yLHqbRBU

Thanks mate I will.

It makes it a lot easier to understand seeing the work being done as opposed to reading about it. which with some authors can make it sound quite daunting...