I may be stupid but

Hi guys. As the description & lots of members on here know i am a bit stupid, Some don't.

So the question is when i watch youtube video's, lots of tubers when using a 3 jaw chuck. They put in a piece of stock & then tighten the work with the key. Then rotate & tighten again , then rotate another 3rd & again. Why ????. I never do this. I know the chuck is a scroll device so why would i want to rotate & TIGHTEN ALL 3 KEY HOLES.

So please answer honestly am i stupid or maybe i am missing something. Please correct me if i am wrong.

Steve.

No you are not stupid. I can't imagine why they do that, either.

I rarely watch You-Tube videos these days, driven away by the advertising clutter, but I can't say I have noticed that habit. However, it does seem odd.

Perhaps many of them use a lot of rough old bar or knackered chucks (knackered by rough old bar?) and find, or think they have found, they need use all three pinions to cope.

Or they have picked up bad habits from each other.

Allegedly this practice reduces run out by helping balance the forces on the scroll so it moves closer to true centre.

It is said that any wear in the chuck allows the scroll to float slightly off centre causing run out.

I've never seen any figures of calculated run out associated with any degree of wear so am mor ethan a little skeptical. Maybe if a chuck is well worn.

A few brief tests showed no sensibly repeatable effects on the three jaws I use. But mine are in decent to very good condition with known history. I suspect supporting the work rather than relying on the lower jaw(s) to lift it onto centre helps control any imbalace. As does nip, rest, final tighten technique rather than simply pulling up in one go.

Life is too short to do proper, statistically significant tests backed up by wear measurements.

Any three jaw needing that level of playing to get decently acceptable results is, in my view, only fit for the bin. By its very nature a three jaw is of limited precision. Trying to do better than the concept allows is guilding the lilly in on uncertain terms. I understand the logic of a GripTrue and the various home brew equivalents but the faffing involved in setting up seems a vast waste of effort. Although certain limited circumstances can exploit such it doesn't alter the fact that you are trying to make an unsuitable tool work for the job in question.

Clive

The scroll in a 3 jaw chuck has to have a small amount of lateral clearance within the chuck body or it would seize up. this creates a small and unavoidable radial error that grows as the chuck gets older.

Try this; put a piece of ground stock in the chuck using one of the tightening points. Mark it with an A using a felt pen. Then check the concentricity of the ground stock with a dial indicator. Note the max high and low amounts and points for A using the felt pen on the chuck body. Now repeat this for the rest of the tightening points.

You will find that the noted points are not all in the same position. most likely one of the drive points will give the least run out. this is because no scroll chuck is perfect, when you drive the scroll from one point on its side it is pushed laterally by the angled drive gear inside the chuck. this may increase or decrease the total error in the system.

Some machinists take advantage of this phenomenon and mark the drive point on their chuck that gives the least run-out error.

Unfortunately there are other errors in the chuck mechanism the scroll itself and the teeth on the back of the jaws also are not perfect; different diameter workpieces may give differing results.

For normal 3 jaw chuck work I do not worry about small errors, however if doing a long run of the same part that requires high precision it may help to find the best drive point to use to reduce error.

All this can be avoided by using a 4 jaw chuck and a dial indicator however the three jaw is a great time saver.
 

Edited By John McNamara on 24/06/2022 00:51:06

Well that is food for thought guys. I do have the Griptru & have never had any issues by just tightening 1 keyhole.

Steve.

Being of the "Hammer and chisel" fraternity, I never , or very rarely bother.

Theoretically, tightening all three jaws should be better since it loads the scroll in three places, (But tightening one jaw, will load the scroll via the other two anyway. )

If you want better concentricity, use a four jaw and clock the work. In that way you should be able to get better than 0.012 mm run out. Even less if you are prepared to invest the time and effort to do better than that.

Howard

An old mentor of mine in the now distant past "always tighten all four holes in a three jaw chuck". If one to tighten suits you that's fine. But why is there movement at each hole tightened and very hard to undo.

Same applies to a Drill chuck it settles the load evenly on the jaws.

Hi Steve,

I agree with Nigel, you’re not stupid. Anyway I thought you had an Albrecht chuck, the Rolls Royce of keyless chucks. 😀

If I am going to use bar stock and part off the turned piece, I tighten all 3 jaws and there is usually some movement on each position - so something moves!

Bob

Good quality chucks such as the old Crowns and Burnerds etc often had a "0" or other reference mark stamped on the chuck body next to the chuck key hole that was used in the factory to tighten the jaws for final grinding or testing. So they seemed to think at the factory that which keyhole you used could influence runout. The "tighten all four holes" regime seems to have come about more to ensure a tight grip with worn chucks (which begins the first time you use it) and does seem to work, and to help minimise runout in my experience. Ditto drill chucks. Or you could do like all the machinists in those YouTube videos from Pakistan Truckers etc whose preference is a good three-foot length of pipe over the chuck key handle. Seems to be universal over there. And it seems to work for them. I would hesitate to recommend it though.

My grandad, who was a professional engineer working from the 1920s onwards, used to drum into me the need to use all three key holes on a chuck. Why, I never knew, but presumably it was established practice at one time.

Regards,

James.

I though we were talking 3 jaw lathe chucks here or am I stupid ?

regards

If one hole is adequate, why would the manufacturer add others? There must be an advantage other than convenience.

If you think about the forces on the scroll, it is better to tighten all three, ideally, you would turn all of them at the same time.

When you turn one you are applying a tangential force to the scroll, This will rotate and move the scroll laterally, hence the off-centre tightening. Also, there is friction between the scroll, body and jaws which will resist the rotation of the scroll so it can get wedged in a non-ideal position.

Tightening all three wiggles the scroll around so improves the tightening.

Yes, you can get a chuck tight on one point, but I find you can always get a little more if you use all three points.

Mine said the same. It does seem to make the chuck grip a drill better.

So I tighten all three jaws on the lathe chuck when I think the part would benefit from more security - like roughing out a fresh piece of stock. I don't do it on fragile, already machined parts.

My thoughts exactly!

I have always used all three holes on a three jaw. and on a drill chuck. It was what we were taught at Tech. in the 40s

Roy

As the bishop said to the actress.

Exactly James, I have been doing this from 1960 -- same reason, my mentors instruction -- several different ones and it does work ! you get a less runout by progressivily tightening the chuck working around the holes, I also always start at the same hole every time.

John

I bet if chucks had a single hole then there would be a thread about what a great idea it would be to have 3 holes so there was always one on top for immediate use. On a larger geared head lathe it could be something of an effort to rotate the chuck, I seem to remember setting one of the gearbox levers to a halfway position to effectively disengage the gearbox when setting up a 4 jaw job.

Mike

Edited By Mike Poole on 24/06/2022 09:44:30