Lathe Chuck Wrench question

[AKPrybar]

I just bought a Craftsman Commercial/ Atlas 12x36 and it came with 1 abused check key/ wrench. I want to purchase 2 more so I can have 2 good ones to use with the 4 jaw chuck. Here is the question: What size key do I need to buy? They are square and seem to be in between 1/4 and 5/16. The actual measurement of the square end is .275 and it fits the check well. Thanks for the help, I don't want to order the wrong thing.
Jay

 

[mmcmdl]

Easy to make them ! Welcome aboard .

 

[AKPrybar]

Thanks! I currently only have the lathe, no mill to mill the flats. I would have to grind them by hand which is why I was hoping to figure out exactly what size I need and just purchase a couple.

 

[mmcmdl]

I most likely have a few for the 3996 although your chucks may be different . You're going to have to measure the square in your chucks .

 

[AKPrybar]

I don't have a super accurate way to measure the inside square on the chucks, but the outside square on the key is .275 and its a perfect fit in both the 3 jaw and the smaller of my 4 jaw chucks. That is the key I'm looking for. I assumed it would be a standard 1/4 or 5/16 key but the size seems to be in between.

 

[Tozguy]

I recommend that you start by measuring the square socket on the chuck for yourself. An adequate caliber to do this can be bought for a very reasonable price. It is a basic tool and you will need it forever after.
It is the only reliable way to learn what size key you need.

 

[NortonDommi]

0.275" is 6.985 mm. 7 mm is a standard European key size plus you can buy 7 mm square tool steel. 7 - 6.985 = 0.015 mm = 0.00059" so some 7 mm stock could be a start or you could buy an end mill and mill the ends of some bar stock on your lathe. Easier than hacksaw and filing.

 

[RobertB]

It's a chuck key, a super precise fit is not required. If you can't find a 7 mm one start with a 5/16" one and grind down to fit.

 

[jwmay]

I made one with only a lathe. Purchased a piece of key stock of the correct fit, turned it to Slip fit in a bored hole and secured with loctite. Cross drilled the other end, and drove some 1/4” round through that. Done.