Chucking a Piston in 3 jaw lathe chuck

[drf255]

Sorry for the dumb question.

I picked up an old 618 Craftsman/Atlas lathe. The CAD continues.....

Haven't fully cleaned and set it up, but got it for $350.

It's got a 4" 3 jaw chuck. Just for shizz and giggles, I tried to chuck up a ms250 piston I welded a dome on for machining. The skirt on both sides encompasses less that a quarter of the circumference. No matter how I rotate it, I can't find a point where all the jaws are contacting the round of the piston.

Any advice here? I'm sure I'm just missing something. I tried the search function, here and on google, and came up with nothing.

Thank you.

 

[Echoboy86]

Hey there, seeing as no body has replied i'll go first. The best way i found was to find a PVC plumbing socket (think wall) with an inside diameter smaller than the O.D. of the piston. Take this socket and chuck it up in the 3 jaw and using a boring bar, open it up till you have a snug fit on the piston skit.

This little jig will allow you to hold the piston securely and allow you to true it up without marring the skit.

Here is a pick of a Meteor piston i did for a Jonsered 2040. Hope this helps

 

[drf255]

Did you put a slit in the pipe, or is it full round?

I was thinking a sleeve as well.

 

[Echoboy86]

Nope, still fully round. Just bore it so it's a really snug fit. It will lock up nicely as you tighten the chuck. Don't over tighten the chuck!!!! Just enough that you can take light cuts without it slipping, otherwise you'll warp the skirts.

 

[drf255]

Hmmm.

Looks like a simple option.

Next question, if you don't mind. I plan on making a mandrel for the jug I'm working on next. Has anyone ever tig welded a piece of round to the top of a jug to chuck it up for base and squish band cuts? Seems like it would be easier than buying a square chuck and making up plates. I'm thinking I make mandrel, insert and find centerline of the jug, tack pipe or solid bar to top of head, use to spin jug, cut off when done.

Sound feasible?

 

[Echoboy86]

Its doable but i wouldn't recommend it personally. very hard to get it true enough for cutting the squish and base as accurately as is needed and the heat could lift the plating near the top of the jug. better to turn up a mandrel for cutting the base and one to lock in the vice for cutting squish. tree monkey did a thread on this method just last week. i use the same method and it works a treat, its also just as accurate as setting it up in the 4 jaw but you can take your time and i feel like i have more control over the process. hope that makes sense

 

[drf255]

It does.

I'm thinking of using TM's method. Can one use the same mandrel? One side with the cutter and flip in lathe to other side for insertion for base cut?

 

[Echoboy86]

Yep, I have several like that. For some saws I have seperate ones but, that's just because I only had enough aluminium or Delrin off cuts to make it in 2 pieces. What saw are you working on???

 

[cuttinties]

Make a draw bar. And if you're building an ms 250 I doubt that you're going to need to cut squish, unless you are planning to cut the base, and bearing pockets . Anyways good luck

 

[drf255]

Sorry. New to the game.

What's a "draw bar"?

My lathe came without a boring bar holder. I either have to find or make one.

MS250 project done, except for possible custom piston domes.

I have enough 4047 to tig piston domes forever.

 

[old-cat]

Hey there, seeing as no body has replied i'll go first. The best way i found was to find a PVC plumbing socket (think wall) with an inside diameter smaller than the O.D. of the piston. Take this socket and chuck it up in the 3 jaw and using a boring bar, open it up till you have a snug fit on the piston skit.

This little jig will allow you to hold the piston securely and allow you to true it up without marring the skit.

Here is a pick of a Meteor piston i did for a Jonsered 2040. Hope this helps
View attachment 405322

This looks really good, except I would get the chuck jaws all the way up on the ring groove for a more solid grip. If you ever had a piston come loose while you're cutting you would understand my concern!

 

[blsnelling]

You need a 4-jaw chuck.

 

[drf255]

I think I may

 

[old-cat]

I think I may

You WILL be glad if you do!

 

[Agr516]

For those who produce pop-up pistons with a 4-jaw chuck: Do you hold the piston directly with the jaws of the chuck or do you use some sort of holding device? I have wondered if the chuck jaws would leave indentions in the piston skirt if contacted directly.

 

[jar944]

by the ring lands with a set of custom soft jaws is the safest method.

http://www.abs-products.com/balancing-supplies/piston-fixtures.shtml

 

[old-cat]

For those who produce pop-up pistons with a 4-jaw chuck: Do you hold the piston directly with the jaws of the chuck or do you use some sort of holding device? I have wondered if the chuck jaws would leave indentions in the piston skirt if contacted directly.

YES the jaws will leave marks. Thick paper or gasket material between the jaws and the piston works.

 

[blsnelling]

I use a center to help hold it in place as well.

 

[Agr516]

Brad, do you drill an indentation in the top of the piston head for the center to "ride" in? I am thinking about trying a popup piston in a cylinder that is close on squish with base gasket removed and have not made one before.

 

[blsnelling]

No I don't. I'm not using it to find center, rather to apply a little pressure to make sure it doesn't come loose.