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  1. #1
    Join Date
    Aug 2011
    Location
    Melbourne
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    4,779

    Default Lathe chuck sizing

    Hi all,

    I have a universal divding head with various pieces missing/damaged. I'm keen to bring it up to speed (to some extent) since it came with my mill. I has a Pratt (England) 3 jaw chuck with a set of outward facing jaws. I'm keen to find a corresponding set of inward (standard) facing jaws. The question is, how do I know what to look for? Maybe a stupid question but how are chucks measured? My lathe has a 150mm 3 jaw and a 200mm 4 jaw which I have always assumed is measured across it's diameter however this Pratt chuck measures 190mm across which seems a strange size. It's definately imperial.

    Thete are numerous S/H chuck jaws on offer on eBay but how would you know what fits? Is there an easy way to determine what size chuck jaws that I am yet to learn?

    Cheers,

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  2. #2
    Join Date
    Oct 2008
    Location
    Cairns, Q
    Posts
    666

    Default

    Quote Originally Posted by simonl View Post
    I has a Pratt (England) 3 jaw chuck with a set of outward facing jaws. I'm keen to find a corresponding set of inward (standard) facing jaws. The question is, how do I know what to look for? Maybe a stupid question but how are chucks measured?
    Hi Simon,

    To the best of my knowledge conventional three and four jaw lathe chuck sizes are usually quoted as the O/D of the chuck body in either inches or millimetres.

    I just checked an early 1960s IBC catalogue, and they quote the diameter of some of their chucks in half inch increments, so I assume you have a 7 1/2" chuck. I have a Pratt chuck marked "Pratt EP760 Made in England" which we bought about 1948 whose body is 4 1/2 inches diameter. I don't recall seeing any recently manufactured chuck specifications mentioning sizes in other than whole inches or ten millimetre steps. This being the case I think it would be pure luck to find new jaws for an odd sized Pratt chuck, since Burnerd and Pratt amalgamated as far back as 1953. If you do find a source, I would probably be interested in getting a new set of inside jaws for my well worn 3 jaw.

    Frank.

  3. #3
    Join Date
    Aug 2011
    Location
    Melbourne
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    4,779

    Default

    Thanks Frank,

    Since my chuck says "The Pratt Chuck" with no mention of Burnerd, do your suspect it's older than 1953? I think your are correct at 7-1/2" as a size. I could be wrong but I think any set of jaws from a 7-1/2" chuck with the same sliding fit dimensions should fit. Whether they fit and produce good concentricity when holding a workpiece is another question though. I assume if you were using a 2 jaw self centering chuck that concentricity is important on a dividing head because unlike a lathe, you would not be turning a part down and making it concentric.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  4. #4
    Join Date
    Jul 2014
    Location
    Alphen aan den Rijn, Netherlands
    Posts
    208

    Default

    It is not impossible to make your own jaws. I think Stefan Gotteswinter has a video on YouTube where he does just that. Something to consider if the chuck is worth the trouble.
    Peter

  5. #5
    Join Date
    Oct 2008
    Location
    Cairns, Q
    Posts
    666

    Default

    Simon,

    I would think any chuck bearing just the Pratt name would be a fair age, though they might not have changed the name on individual chuck models immediately on amalgamation. There was a short discussion on Pratt chucks on PM a while ago. In post 13 here:
    https://www.practicalmachinist.com/v...-chuck-331908/
    the poster mentions that "some of the older 'Pratt only' chucks had different sized jaws to the later Pratt Burnerd, and are nigh impossible to find" which seems to indicate that the pitch of the scroll and/or the jaw slide sizes for a given chuck size are not necessarily standard. It is surprisingly hard to find information on many common products that were made in pre-Google days!

    Frank.

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