Griptru chucks

[GavlarParticipant @gavlar]

Just a quick question on Pratt Burnerd Griptru chucks, or the equivalent

Are they worth the expence if you already have decent three jaw, four jaw and collet chucks?

Having three ajustment screws, are they any more difficult to accurately centre than a four jaw with it's two pairs of opposing ajustment screws?

Thanks in advance

G

[GavlarParticipant @gavlar]

[old martParticipant @oldmart]

Griptru chucks are intended for use in a production enviroment. If there are large quantities of identical workpieces, a three jaw scroll saves time and the fine adjustability at the start of the run is a great advantage. Each batch may be a different size, but can be zeroed in perfectly. A four jaw independent can be dialed in just as accurately, but even if two adjacent jaws are loosened each time, the repeatability is not guaranteed unless it is checked. The cost of this type is double that of their standard chucks. PB is the only type using three adjustment screws, the other makes use four, and there have been comments made on other forums that trimming four is much easier.

Edited By old mart on 26/03/2021 21:47:02

[not done it yetParticipant @notdoneityet]

I’m just guessing here, that concentricity is likely only perfect when accurately set for any one diameter?

I have a smaller example but will only use it as a three jaw chuck, if it ever gets a backplate for the lathe.

[Martin KyteParticipant @martinkyte99762]

Posted by not done it yet on 26/03/2021 22:24:05:

I’m just guessing here, that concentricity is likely only perfect when accurately set for any one diameter?

I have a smaller example but will only use it as a three jaw chuck, if it ever gets a backplate for the lathe.

Yes, they are intended to be set up to run true for a single diameter. Not sure if I would go out and buy one in a hurry, however I have ended up with two through various means. The first came with my first Myford and I inhereted the second which is in much better condition. I use the best one 'in anger' when I have the need to set something truely concentric in the GHT dividing head on the mill or the lathe where the 4 jaw will not fit being of a larger diameter. Also for repetition work (on the odd occasion that it matters) on the lathe and finally for jobs on the lathe needing that degree of precision better than I can get with collets. The 4 jaw could be used but I always feel there is less chance of marking the work with the griptru. So as I have one I would not like to be without it. That said soft jaws get round many of the jobs the griptru would be used for and there is something to be said for homemade split collets.

regards Martin

[SteviegtrParticipant @steviegtr]

I have the 4" version on the myford. I had it for over a year before i found out it could be adjusted. Since i have done that it is now very good to use. I have not had any problems with different diameter holding. I did a youtube video of this some time ago, showing a strip down to see how it worked.

griptru chuck adjustment

Steve.

[Chris CrewParticipant @chriscrew66644]

Personally I wouldn't bother with the expense of a Grip-tru. It is standard practise to do as much as possible on a work-piece in the lathe at one setting, and if possible finish it completely before removing it from the chuck or before parting off. I have removed chucks from the Myford with the work-piece still in situ and remounted the chuck on a Vertex dividing head on the mill for milling flats or key-ways etc. before putting the chuck back on the lathe for a final operation or parting off. It's been accurate enough for what I have ever needed to do in a back-shed workshop but it's only been possible because the Myford nose is replicated on the dividing head. This may not be the case with other combinations of lathe and attachments. There's a way round everything if you just stop and think for while. Just because you are a beginner doesn't make you stupid and your common sense and logic is just as good as anyone else's.

[Joseph Noci 1Participant @josephnoci1]

Posted by old mart on 26/03/2021 21:44:46:

…………… PB is the only type using three adjustment screws, the other makes use four, and there have been comments made on other forums that trimming four is much easier.

Edited By old mart on 26/03/2021 21:47:02

Not quite – Most 3 Jaw chucks of that type have 3 adjusting screws – Even Rohm ( The ZS series), a prominent German make, have 3 adjusters. I have a 120 and a 200mm Rohm for my EMCO lathe and the 120mm chuck is on the lathe all the time – I use it for most jobs and is a doddle to set up.

Expensive yes, but having become used to this way of working, worth it.

Joe

[Bernard WrightParticipant @bernardwright25932]

I've modified two 7-1/4" PB 3 jaw chucks, these were OEM on my Colchester Master Roundhead, that had integral L0 backplates fitted, to GripTru capability.

The first one I copied PB's design, ie; with 3 tapered screws.

The second, I used an old method of 3 grub screws, where the tapered screws would be.

I found this later design far quicker to setup.

Quite honestly I wouldn't be without either.

I only ever use a 4 jaw if the work is odd shaped.

Having said that, I certainly wouldn't buy a new one at the prices quoted, if a reasonably priced SH one came along I'd snap it up.

P.S. Just looked in my photo gallery and see I'd posted a pic of both types there

Edited By Bernard Wright on 27/03/2021 07:17:07

Edited By Bernard Wright on 27/03/2021 07:17:39

Edited By Bernard Wright on 27/03/2021 07:22:11

[Neil LickfoldParticipant @neillickfold44316]

I make mandrels for trimming pistons quite accurately. One advantage of having a grit true chuck , is that I can hold the mandrel and check that it runs true in the swash plane. Then I can put the piston onto the mandrel and then proceed to dial in the piston while adjusting the 3 tapered screws. With patience I can get it dialled in to 3 microns quite easily. I can not achieve this if I used a 4 jaw chuck and get the swash within the acceptable level needed for high performance model engines. In the grip true chuck, the part is held once and stays in that held position. The chuck just adjusts the radial runout. In a 4jaw, when the part is adjusted, the chuck jaw is released a little then another jaw is adjusted and the part moves, but does not stay in the same exact swash plane.

[jacques maurelParticipant @jacquesmaurel42310]

Have a look at my article: "Achieving accurate coaxial chucking", model engineer workshop issues 292 & 293 april and may 2020.

JM

[old martParticipant @oldmart]

Soft jaws are the best way to hold work perfectly and without risking marking the work.

[GavlarParticipant @gavlar]

Thank you all.

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