How much in the chuck

[JimM]

I know that as a rule of thumb you should not have more than 3 times the diameter of the work sticking out of the chuck without support but are there any guidlines on how much you need to be inside the chuck.

More precisely I need to make a top hat washer - the 'brim' is 1" diamater and 1/8" thick the remainder is 1/2" diameter and 3/4" long. Assuming I'm using 1" bar what would be the minimum length of stock I need if I was to do this in the 3 jaw chuck

Thanks

Jim

 

[Lew_Merrick_PE]

Jim,

As is so often the case, the answer is, "It depends..."

The most general answer is: as much as you can stuff in there. If you have a collet and appropriate clearances, a foot or so is not going to hurt. If you are holding it in a chuck, how tall are the jaws of your chuck? I understand that it hurts your Scotch soul to throw material away, but you are going to be taking off 1/4 inch from the diameter to end up with the ø.500 final size (.750 long) and the better your "grip," the more you can remove with each pass.

I am currently paying about $1.20/lb for CFS merchant shafting. That is .22 lb/in or $0.27/in. I don't care how little you value your time, but spending another $0.30 to save yourself from ducking flying steel is cheap!

The jaws of my 3-jaw chuck on my "little lathe" are 1.18 (30 mm) tall. I would use a piece of stock cut to (1.18 + .125 + .100 + 1.050 =) 2.45 nominal length. This would allow me: 1.18 for the chuck jaw, .125 clearance, .100 parting tool (mine is .094 wide), and .050 excess to trim back on the bushing when everything is said and done.

My "big lathe" has C5 collets with an outboard bar guide. I would shove a 6 foot nominal length of ø1 bar through the collet to stand out (about) 1.5 inches from the face of the collet. I could turn the ø.500 diameter, drill/bore the bushing hole, part it off, swap it off to a ø.500 collet, and face the end.

As I said, It depends...

I hope this helps.

 

[websterz]

I wouldn't personally do it without at least a half inch of material to hold. The more the better actually as you can easily deform the scroll if you apply too much pressure on just the ends of the jaws by grabbing very short lengths of material.

 

[mklotz]

Yes, as stated, it depends on a number of factors. That said, for a job like you're describing, I start getting uncomfortable when the chuck jaws are only gripping over about half their height. Of course, that's not to say that I haven't violated that rule on numerous occasions with no resulting disasters.

 

[Jasonb]

A lot depends on the size of your lateh, personally I would turn the 1/2" dia on the end of a length of bar then part off to leave a little over the 1/8 wide of the flange, then grip by the 1/2" dia and face off the flange. That way you don't have any issues with trying to hold a short bit of metal.

If you have softjaws for your chuck then that would allow you to grip a shorter length, particularly if you bore first and bring up the tailstock ctr for support then reducinng to the 1/2" dia

If you only have a small lathe and short lengths of 1" stock why not put all the length in the chuck and bore it then mount it on an arbour to turn the 1/2" and face the ends, that way you can safely make it all out of a bit just over 7/8" long.

Jason

 

[JimM]

Thanks for the responses guys

The jaws on the chuck are 30mm so I'll always treat that as the minimum in future but Jason's idea of boring and then using a mandrel is great and is what I'll be using this time. I'm sure it's the sort of solution that comes naturally to experienced machinists but it didn't even cross my mind ! Another great tip to add to the many I've gotten from this site

Cheers

Jim

 

[Tin Falcon]

the way I would make a hat bushing like that is chuck up a piece at least 2 -1/2 inches long and make the bushing maybe leave a little extra for a finish cut on the brim.
Chuck up material leave a 1 1/2 stick out.
first face off and clean up the end.
Aply layout dye thith a fat marker
use a jenny or hermaphrodite caliper to mark both edges of the brim.
then rough cut the top of the hat eyeball to the first line.
setup a stop or indicator for final shoulder lenght.
feed for finish cut on diameter and then take final should cut at the same time feeding from the inside out.
cut a relief groove at the edge of the brim about .010 deep.
spot drill drill and ream the bore past the end of the bushing de bur before reaming
locate your parting tool ans take a cut an 1/8 -1/4 deep
deburr all corners with a file finish parting off then debur the brim side of the bore
if needed flip the bushing and clean up the brim.
Tin