Pratt 4 jaw tight jaws

[grim_d]

I have two Pratt 4 jaw chucks, one is 9" and the other is 10", they have both done hardly any work (the 10" is verging on "as new").

All of the jaws are extremely tight in their grooves.

The 9" is my "big chuck" for my Harrison and is usable.

The 10" was getting cleaned up so I can sell it but the jaws are so tight that I couldn't.

What's the script? There are no burrs, no rust, no chips or swarf and the 10" particularly is as clean as a whistle..

Thought it may have been the screws but removed them and I physically can't push the jaws in their grooves by hand.

 

[brightspark]

put some micrometer blue on them and see where there tight then rub them down on some emery cloth on a bit of glass plate. try swapping them round may help

 

[grim_d]

put some micrometer blue on them and see where there tight then rub them down on some emery cloth on a bit of glass plate. try swapping them round may help

Yeah but why are they so tight?

Surely without external influences such as burrs etc they should fit from the factory?

These ones are even numbered so one presumes they fit at some point.

 

[mike os]

Bit of lube?

 

[grim_d]

Bit of lube?

Tried every slippy stuff in the workshop upto and including moly.

 

[Kram]

Dont know the answer but mine are extremely tight to start on the screw.
Are they ok once there fully in the groove?

 

[Agroshield]

They are a mass-produced item (but in rather lesser quantity than a toaster). Both the jaws and the chuck are made to a tolerance range.

With the jaws, I am not sure if they grind before or after heat treatment but that can introduce a little variation.

To get them smooth as silk, you need to hand fit them using a stone on the high spots.

 

[grim_d]

They are a mass-produced item (but in rather lesser quantity than a toaster). Both the jaws and the chuck are made to a tolerance range.

With the jaws, I am not sure if they grind before or after heat treatment but that can introduce a little variation.

To get them smooth as silk, you need to hand fit them using a stone on the high spots.

So it's par for the course that the jaws don't fit from new?

To be clear, getting then to fit is not an issue, but I find it bizarre that I have to? These are not modern cheap imported goods.

 

[MetalMonkey]

Egg sucking, but have you confirmed they are the right jaws for the chuck? - they are usually stamped with the same serial number as the chuck body.

 

[Agroshield]

So it's par for the course that the jaws don't fit from new?

Maybe your idea of 'fit' and the manufacturer's idea of 'fit' differ. If they sold then 'just right' from the word go, are they going to wear tighter or looser in everyday use? So if, as a manufacturer, you have to err, which is better for your customer - send them out tight and they will loosen with age and give a long product life or send them out loose with a correspondingly limited product life?

When the production racing boys buy the latest whizz-bang Fire Ninja from Yamuki, the first thing they do is strip it to the last nut and bolt and blueprint it.

 

[Screwdriver]

Do you have any more on the history of this part? Did you acquire it several years (decades?) after its manufacture? There may be a reason it is "as new" and virtually unused if it is virtually unusable. I doubt if each jaw is produced to fit a specific body, there will be a tolerance range and jaws (one assumes) will be selected from a batch which match the finished part most closely. Chances are someone has simply paired up a random set of jaws with the nominally correct body, either (or both) of which may be at opposite ends of their tolerance ranges.

If you'd bought it new then you should expect a quality item to work perfectly on day one of course. Secondhand - meh, make it fit.

 

[grim_d]

Maybe your idea of 'fit' and the manufacturer's idea of 'fit' differ. If they sold then 'just right' from the word go, are they going to wear tighter or looser in everyday use? So if, as a manufacturer, you have to err, which is better for your customer - send them out tight and they will loosen with age and give a long product life or send them out loose with a correspondingly limited product life?

When the production racing boys buy the latest whizz-bang Fire Ninja from Yamuki, the first thing they do is strip it to the last nut and bolt and blueprint it.

They are "break a sweat" tight. Definitely not right.

I'm not saying that stripping things down and improving them from new is a bad thing, but I would consider the 10" to be unserviceable as it is.

Do you have any more on the history of this part? Did you acquire it several years (decades?) after its manufacture? There may be a reason it is "as new" and virtually unused if it is virtually unusable. I doubt if each jaw is produced to fit a specific body, there will be a tolerance range and jaws (one assumes) will be selected from a batch which match the finished part most closely. Chances are someone has simply paired up a random set of jaws with the nominally correct body, either (or both) of which may be at opposite ends of their tolerance ranges.

If you'd bought it new then you should expect a quality item to work perfectly on day one of course. Secondhand - meh, make it fit.

To be honest I think I bought it from Clive a couple of years ago, but no I don't know it's history, it's entirely possible that they are the "wrong" jaws.

Had I bought it actually new, It would be rejected, rectification isn't an issue I just wondered if this was normal with Pratt chucks because I have two now with the same "issue".

 

[brightspark]

its not the end of the world just get them measured and rub them up job done . i had one that was tight just on the one jaw and it ****** me off so took a midges off it using a disk sander on the sides till a smooth fit

 

[mike os]

Just tap and turn, then fettle them

 

[grim_d]

its not the end of the world just get them measured and rub them up job done . i had one that was tight just on the one jaw and it ****** me off so took a midges off it using a disk sander on the sides till a smooth fit

You are so dramatic.

 

[brightspark]

You are so dramatic.

say it how it is

 

[grim_d]

The issue was never the solution but rather the reason for the issue, sometimes the why is as important as the how.

 

[Munkul]

Howevere good Pratt stuff is, I wouldn't be surprised if there were a few duds out there - a chuck with tight ways, paired with jaws that were on the large side - and therefore interference fit.

I've got a 250mm Pratt chuck which came with knackered soft jaws, I got the right hard jaws to replace them I thought, but they're TOO slack!

I wonder actually, now i think about it, if the difference is metric vs imperial? If the 10" chucks have almost the same dimensions as the 250mm chucks... Thing is, they measure the same with very-nears!