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Three jaw indipendent chuck

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BOB BLACKSHAW26/07/2019 18:16:27
290 forum posts
63 photos

I've made a 6 inch flywheel in three parts and its came out well, no wobble. The axel was made from hexagon steel so I had to use the three jaw chuck for this.

My question is that when its on the axel and rotated there is the slight out of true caused by the centring on the three jaw chuck when the axel hole was drilled.

Why are three jaw independent chucks made commonly available, as setting up for true on a four jaw is easy to do.

I made a mistake so I had to take the hexagon out of the chuck which caused the slight out of true on the finished flywheel, but with a independent chuck I could of got it spot on.


Zan26/07/2019 19:22:27
161 forum posts
12 photos

3 jaws are quick to set up and almost on centre. Never remove work from it until it’s finished or as you found out it won’t grip in the same position

a 4 jaw would have been much worse as gripping hexogon material in it is not easy to set true, even worse if it has to be removed and later re machined.

If your flywheel was correctly secured in the chuck, drilling. AND. Reaming the bore should leave it without wobble, but probably a faceplate job to get it really true

i can’t exactly understand why you use hexogan fir the axel , how and when does it wobble, with the hex in the lathe chuck or when the flywheel is mounted in the engine?

the axel would probably have been better turned between centres or as a minimum with a large overhang and tailstock support

do a test. Grip a bit of round stock in the 3 jaw, and use a dti to determine the runout. If it is more than sat 5 thou, remove the chuck from the backplate skim a 1/16” off the register. Re mount the chuck and tap it with a soft hammer to get the bar to run true. Mine will grip to 2 thou on all diameters after this treatment, and it’s a cheap polish chuck

BOB BLACKSHAW26/07/2019 19:37:14
290 forum posts
63 photos

The hexagon is the housing of the axle,

But my question why are independent three jaw chucks not available


Clive Brown 126/07/2019 19:46:19
403 forum posts
11 photos

But my question why are independent three jaw chucks not available


Because there's probably not much demand for them, (some-one will now post a link!).

They would be tricky to adjust as moving one jaw would release the work and also require both of the other jaws to be adjusted, a different amount.

V8Eng26/07/2019 20:22:07
1408 forum posts
27 photos

There are the Grip Tru type in 3 jaw, not independent jaws but can be set up accurately, they do seem to need setting up for the size of work piece. I have one for my ML7.

A bit of online searching will produce plenty of info.


Sorry the linky does not seem to work properly: it does go to the Rotagrip site and you will be able to search on there.

Edited By V8Eng on 26/07/2019 20:30:04

Edited By V8Eng on 26/07/2019 20:36:32

SillyOldDuffer26/07/2019 20:32:00
5630 forum posts
1157 photos
Posted by BOB BLACKSHAW on 26/07/2019 19:37:14:


... why are independent three jaw chucks not available


They are available Bob, just not particularly popular. Here's an example made by Bison. The type is more complicated to make and therefore pricey. I guess as most people only need them rarely they prefer to spend their cash on something else. Never seen one but I suspect they're less robust than ordinary 3-jaw types.

The old-guys had a type of chuck consisting of a tapped cylinder with three bolts that could be tightened independently to grip and centre the work inside: I imagine they would be a right pain to use. I've an idea they were called 'Bell Chucks' but can't find a reference. Easy enough to make.

Ordinary 3-jaws are usually OK provided the work isn't moved. It's difficult to reset them. I've had mild success marking the job and one jaw to recover the approximate position and turning the replaced work inside the semi-tightened jaws for minimum run-out on a DTI. Not as reliable or as easy to do as using a 4-jaw in the first place.

Recently I did some work needing a job to be moved too and fro between lathe and milling machine a few times. An ER32 collet chuck and Stephenson's Block provided good resetability, and is much faster than a 4-jaw.


not done it yet26/07/2019 20:43:24
4501 forum posts
16 photos
Posted by BOB BLACKSHAW on 26/07/2019 18:16:27:

Why are three jaw independent chucks made commonly available, as setting up for true on a four jaw is easy to do.


Probably for a similar reason as to why milking stools were nearly always (well, all the sensible ones) were 3-legged!

A bit of unevenness is easily accommodated. Self-centring 4 jaw chucks are useless if the stock is out of round, and square stock must be perfectly square as well.

A lot of newbies might have a lot of trouble with a 4 jaw independent.

Setting two perpendicular axes can be problematic (accounting for high spots between the 90 degrees of separation) for some, but setting thee separate axes is very much harder - even for seasoned machinists. Getting results any better than a simple self-centring would eat up time!

Michael Gilligan26/07/2019 21:37:10
15478 forum posts
668 photos
Posted by SillyOldDuffer on 26/07/2019 20:32:00:
The old-guys had a type of chuck consisting of a tapped cylinder with three bolts that could be tightened independently to grip and centre the work inside: I imagine they would be a right pain to use. I've an idea they were called 'Bell Chucks' but can't find a reference. Easy enough to make.


This one has eight screws [for fairly obvious reasons], but it illustrates the style nicely: **LINK**


peak426/07/2019 22:14:30
1075 forum posts
124 photos

I have a rather interesting Skinner 3 jaw geared chuck.

Unlike a conventional scroll 3 jaw self centering chuck, where the scroll moves the jaws, these have an internal, removable, ring gear with teeth on the face of the ring.
The ring gear operates all three screws at the same time, which in turn operate the jaws in the conventional independent manner, similar to a common independent 4 jaw. The screws though are fully engaged in each jaw.

They did both 3 and 4 jaw types, but unfortunately mine is missing the removable ring gear, so can only be used in independent, rather than self centring mode.


vintage engineer26/07/2019 22:18:55
247 forum posts
1 photos

When I did my apprenticeship we were only allowed to use 4 jaw chucks. You can set up a 4 jaw just as quick as a 3 jaw.

John Reese26/07/2019 22:46:02
836 forum posts

One argument against an independent 3 jaw is the difficulty getting the work to run true. I have Japanese 3 jaw with the adjust true feature. There are 3 screws for adjusting runout. It take me far longer to get that chuck dialed in than with a 4 jaw.

larry phelan 127/07/2019 11:27:25
667 forum posts
24 photos

This is a most interesting post. I like that hint about skimming the backplate to improve run out. Never had to do it, but who knows, some day, perhaps?

I did buy a 4 jaw self centering chuck many moons ago thinking it would be better than my 3 jaw, but it is still in the box, never got around to making a backplate for it. Tend to use the 4 jaw independent more and more these days, quick enough to set true. I can get it near enough by eye then use my dial gauge to get it dead on.

Ian S C27/07/2019 11:42:56
7468 forum posts
230 photos

Recently a bit came up in one of the threads about gripping hex bar in a four jaw chuck, it works. This is one place for collet chucks, put it in take it out often as you like. A scroll type three jaw chuck will have varying accuracy through out it's range, and as the chuck ages some areas will wear more than others. If things get too bad a shim under one jaw can help.

Ian S C

Plasma27/07/2019 12:00:39
391 forum posts
45 photos

Yes ian, it was the "what to do what to do" thread. But it was shown using a 4 jaw self centring chuck, not sure how easy to do it in and independent.

The adept came with a three jaw chuck that took a bit of setting up but could be made accurate.

As regards skimming the back plate I believe the poster was advocating take 1/16th of the register, not just facing the back plate. Then the chuck could be tapped about on its mounting holes to achieve accuracy. Not sure I'd want to ruin the register though.


old mart27/07/2019 19:23:39
1532 forum posts
136 photos

You can certainly hold a hexagon in a four jaw independent if you want to.

Those three jaw scroll chucks with independently adjustable jaws are intended for use with odd shapes which are all the same. Once the chuck is adjusted for the first one, then simply by using the scroll, you can rechuck successive components quickly. Otherwise they are a PITA.

 Most of my three jaw scroll chucks have reduced registers and enough play in the backplate screws to allow a few thousandths of an inch fine adjustment. It is easy to increase the number of bolts/screws holding the chuck to the backplate from three to six for added security. The exception is my Pratt with the serrated jaws, which is normally used with soft jaws, so it always runs true.

Edited By old mart on 27/07/2019 19:31:58

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