Easing out the bore of a 3-jaw, s/c chuck

[GreensandsParticipant @greensands]

I have a very nice 3.25” O/D Prat 3-jaw self centering chuck which annoyingly will just not pass a 3/4” diameter rod stock through the bore. For example, a recently purchased length of aluminium rod measuring a dead 0.750” in diameter had to be reduced to 0.746” dia in order to enter the bore which seems a tad ridiculous. The answer I feel is to put the chuck up into the 5ins chuck and take say 0.005” off the bore but before taking such drastic action to a perfectly good chuck I am just wondering what others may think of the operation or have been faced with a similar problem.

[Michael GilliganParticipant @michaelgilligan61133]

I bet it’s bore is nineteen of those nasty furrin millimetre things !

MichaelG.

[David George 1Participant @davidgeorge1]

Hi I had the same problem with a Pratt chuck which was new and was 25mm bore I bored it to a few thou over 1″ bore with no problems now I can get 1″ black bar through to the spindle face which will only take 3/8″ diameter.
David

[Paul LousickParticipant @paullousick59116]

A Pratt chuck is fairly robust and you do not locate on the bore, so taking a few thou off the diameter should not be a problem.

Pratt Burnerd chucks were made in Halifax, so I doubt if is was made using those nasty furrin millimetre things !

[Michael GilliganParticipant @michaelgilligan61133]

For info.

https://www.colchester.co.uk/Data/Product_Downloads/Pratt_Burnerd_International_Workholding_Brochure_2022_Web.pdf

MichaelG.

[Clive Brown 1Participant @clivebrown1]

As said, no foreseeable problem with machining, but as the OP already has a 5″ chuck, then why bother?

[Clive Brown 1]

On Clive Brown 1 Said:

As said, no foreseeable problem with machining, but as the OP already has a 5″ chuck, then why bother?

I’m guessing the Pratt chuck might run truer.

[JasonBModerator @jasonb]

Why Bother? Could have better run out than the 5″‘, less wear and as members get older the smaller chucks are easier to lift.

Michael I thought by the description it was an old imperial chuck not a newer 80mm one.

[JasonB]

On JasonB Said:

[…]

Michael I thought by the description it was an old imperial chuck not a newer 80mm one.

Possibly  ‘transitional’
They obviously ended-up going for 19mm

MichaelG.

[old martParticipant @oldmart]

Going to 0.754″ would be fine, but you should strip out the chuck so that the minimum of parts have to be cleaned after the boring. Cast iron dust gets everywhere. Going larger would take careful evaluating of the design of the chuck. Make sure the OD of the body is running true before boring.

[JasonBModerator @jasonb]

How do you get on machining cast iron parts if you don’t want the swarf on the chuck? Anyway I think the bodies are SG iron which machines more like steel than grey iron

As for dust I aim for chips and a minimum amount of dust.

[John PurdyParticipant @johnpurdy78347]

I had the same problem with my 4″ PB Griptru chuck.  It would not take a piece of 1″ stock in past the jaws, the bore was just shy of 1″. I removed the jaws, centred the body, and skimmed a few thou out of the body with no problems. worked well as stated by others. After doing it I dismantled the chuck and gave it a good clean and re-lube.

John

 

[old martParticipant @oldmart]

If the body parts are SG iron all the better, but there is also the depth inside that has to be bored. I’m never afraid of dismantling and cleaning a chuck. That said, I did fully dismantle a 6″ Toolmex 4 jaw independent for machining the body, and it was probably more trouble than it was worth,especially getting things lined up so precisely so that the grub screws went in easily.

Somebody has a P B 80mm three jaw for sale on ebay and the label just says “iron body”.

[KEITH BEAUMONTParticipant @keithbeaumont45476]

I have often wondered why the Mini-Lathe specification boasts a 20mm clearance spindle,yet is fitted with an 80mm chuck that only passes 16mm. With the thousands that have been made of this lathe I would have expected at least one of the  suppliers to have a chuck that matched the lathe.  Perhaps they have and I have missed the fact, but when I visit shows, I have checked 80mm chucks and they are always 16mm.

Keith

[JasonBModerator @jasonb]

You can always stick the 4-jaw on as they have a larger bore but would indeed be nicer with both passing at least 20mm and quite possibly one of the reasons people like to put a 100mm 3-jaw on them.

[KEITH BEAUMONTParticipant @keithbeaumont45476]

I do use a 100mm for most work, plus E 25 Collet set that I have the full range to 20mm. So I have got arround the problem.

Keith.

[BazyleParticipant @bazyle]

Hum. It’s a bit like the people who think that if their toolholder has a 3/4 in gap they have to use enormous tooling. Probably not the design intention but the larger bore allows for a draw tube for collets taking maybe 15mm. The small chuck supplied will just be the accountant’s choice.

[Author]

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