# modifying a lathe chuck.



## kd0afk (Mar 30, 2013)

I wonder if this is a good idea; I have a 6" chuck and I would like to make it kind of a run-true chuck by turning down the base plate slightly and adding four set screws to the chuck so it can be trued up then tighten up the back bolts.


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## GWRdriver (Mar 30, 2013)

kd,
Many years ago there was an article (or two) in Model Engineer magazine which described the the conversion of a conventional 3-jaw chuck to "adjust-tru" capability.  IIRC it was done as you suggest, so it is certainly possible but would of course depend upon the specific design of the chuck and backplate.


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## kd0afk (Mar 30, 2013)

I have about a 1/4 register on the chuck so I'm thinking 1/8" or #6-32 set screws maybe? This thread has been cloaked. I actually turned the base plate on my new 4 jaw chuck a bit under, no wiggle but not a press fit either, and I didn't want everyone to think I was a moron.
Thanks for the help


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## kd0afk (Mar 30, 2013)

Also, would it be advantageous to use lock washers on the back bolts or will loctite work good enough? Once I have it trued I should need to loosten everything except for a one in a while cleaning, right?


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## kd0afk (Mar 30, 2013)

I guess, now that I think about it, I wouldn't really need to do this to true it up to be able to have the stock centered, mainly just to balance it.


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## GWRdriver (Mar 30, 2013)

#5 or #6 screws seems awfully small and light and if it was me I'd want the adjusting screws to be larger.  More information would be helpful.
How deep is the recess in the chuck body?
How wide is the back plate spigot (aka, shoulder, register, nose)?

The adjusting screws don't necessarily have to be centered on the width of the recess in the chuck body, or the back plate spigot, they only have to contact the spigot somewhere on its width with enough metal underneath it to make flat surface contact and not break the metal.  Sometimes, unintentionally, when you are trying to snatch that last .0005" of adjustment, a lot of force ends up on one or more of the screws and if there isn't enough metal under them they could break a chip off.

In this case, if the spigot is 1/4" wide the contact point (center of the screw) could be moved out to say 3/16" from the back plate face.  If you allowed say 1/16" clearance (from the nearest thread crest) from the back plate face that would easily allow a 1/4" adjusting screw.  I would then work that back to the chuck to see if a 1/4" screw would fit the metal available in the body.  There should be a balance to be struck between largest screw size and the minimum amount of metal surrounding the screw threads.  

As for lock washers or Loctite, I have two such chucks both of which bolt on from the front (with Allen head alloy bolts) and neither of them came with or call for lock washers.  I think you overestimate the potential for those screws to loosen or back out.  I'm not saying it can't happen, but I've never had one loosen up (in 40 years.)


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## bigo (Apr 1, 2013)

Several years ago i made my cushman 8' ajustable like a buck chuck. I used 1/4" 28 set screws and ground off the end flat on a surface grinder. I turned the back plate deeper to work with the 1/4" 28 set screws, and .015 under for adjustment. I also put harden collars on the bolts that hold the chuck to the back plate, not lock washers. When you go to ajust the chuck you loosen the bolts  a little bit, adjust it , tighten the bolts and re check it. I can get it down to .0002 using a plain gage for checking.


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## kd0afk (Apr 1, 2013)

bigo said:


> Several years ago i made my cushman 8' ajustable like a buck chuck. I used 1/4" 28 set screws and ground off the end flat on a surface grinder. I turned the back plate deeper to work with the 1/4" 28 set screws, and .015 under for adjustment. I also put harden collars on the bolts that hold the chuck to the back plate, not lock washers. When you go to ajust the chuck you loosen the bolts  a little bit, adjust it , tighten the bolts and re check it. I can get it down to .0002 using a plain gage for checking.



Why the hardened collars? What is the advantage over lock washers?


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## bigo (Apr 1, 2013)

they don't dig in when you lock them down and through off your setting


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## kd0afk (Apr 1, 2013)

Are hardened collars like washers? I'm having a brain fart. I know what a nut with a hardened collar looks like but I'm just not getting it. Sorry.


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## bigo (Apr 1, 2013)

yes harden collars are like washers, they are thicker some are ground on both flat surfaces


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