Chucking a Piston in 3 jaw lathe chuck

[blsnelling]

I guess I'm not sure why a 3 jaw isn't good enough.

You can't get ahold of many pistons with a 3-jaw chuck. The skirts aren't wide enough.

 

[Chris-PA]

You can't get ahold of many pistons with a 3-jaw chuck. The skirts aren't wide enough.

Ahh, of course.

 

[super3]

You can't get ahold of many pistons with a 3-jaw chuck. The skirts aren't wide enough.

That depends on how deep the jaws are.

 

[Marshy]

Need some soft jaws to machine to size. Of course the problem becomes they fit only one size piston.

 

[jar944]

Need some soft jaws to machine to size. Of course the problem becomes they fit only one size piston.

make your own soft jaws from hex stock, and you have 6 different sizes all for cheap

or round if you like

 

[Rx7man]

Jar, you have a nice setup.. I have a 3 and 4 jaw chuck for my lathe, but I have no way of attaching different faces to them, which is really lacking for this kind of work.

I've had to make all sorts of holders, etc for various stuff.. For boring carbs I have a plate I mount the carb to thats bigger than the center bore of the chuck, then I get it at least square, from there I can fiddle with the center quite easily.

 

[LegDeLimber]

Rx7man: just sorta spitballin an off the cuff idea here.
Do you have the option of drilling and tapping the jaw faces?
But you'll need to be able to mill a set of steps into a set of blocks,
to match the jaw faces.
Then drill the appropriate holes to through bolt them to the jaws.

 

[LegDeLimber]

Through the center of your chuck you push a bar and tighten up. You'll use a wrist pin slid through a hole drilled in the bar. Place the piston on the end of the bar, push the bar through pull snug and tighten the chuck. The flat part of the skirt will be on the chuck with the draw bar being held, not the piston. I would give a better description but I don't really have time until this evening.

Here is my setup, all done on a lathe except for the milled slot in the arbor.

View attachment 405391 View attachment 405392

You know.... i'm looking Caffeine1fgs jig and wondering IF you could get away with
a hybrid of using the drawbar idea and a pin , to pull the piston against the base.
Use a wrist pin that's only just small enough to fit in all piston holes.

Now add a set of concentric steps bored about 2.2~3mm into the face of the jig.
these would be in the diameters of your most likely/common piston skirt diameters.

Thinking of the light cuts such as these saw pistons here, nothing heavy.
you'd want to be sure you have a good sharp point on the tool and the angles
would be pretty steep/agressive
So as to give a really free cutting - lowest pressure tool bit.

 

[Rx7man]

I've thought of it, but it would probably take quite a bit to drill and tap those blocks, they're hardened steel.

I think your idea for the concentricly cut rings would work nicely, I'd probably go with the correct wrist pin for each piston just to make sure you don't mar or oval the hole in any way, but you could have a couple of the most common sizes.. 10 and 12 mm, and you'd just use a different clamping block for each.. or you could use the right wrist pin, and have a big enough hole in your clamp block to fit as big a wrist pin as you'd like.. which is probably the easiest way to go

 

[LegDeLimber]

Funny how you can have a "what am I forgetting here..." sensation.
yep, the undersized pin could quite, quite easily dimple the piston hole.
I definitely like the idea of interchangeable drawbars and pin sizes.

Edit: try to clarify my ramblings a bit.
In this case when I say drawbar, I'm referring to the center part of
Caffeine1fg jig.
I would only build the jig with a drawbar all the way through the headstock
if i was running a LOT of same sized pistons.
Otherwise, I'd would try to build it all into the jig and just load the piston and clamp it before putting the jig in the chuck.

 

[Rx7man]

I think if you oversized the drawbar hole to fit the biggest piston pin, you can muck that up a bit and it won't matter too much. I'm with you on the drawbar going all the way through the headstock.. though perhaps cheating with a piece of 1/2" Readyrod (allthread) with a pair of doubled nuts on each end (or welded) would give you the possibility of loosening and tightening it easily.... guess you could put a long extension on a ratchet and get in there too.

 

[drf255]

I don't see how a drawbar would effectively center a piston.

If you chuck up the drawbar and the piston is off, looks like adjustment would be quite difficult.

I cannot wait to get the parts to fix this lathe. It appears that I won't have by this weekend. I'm bummed.

 

[LegDeLimber]

That's why we wanted the concentric rings/steps bored into the face of the jug.
erhum, least that's what was going through my silly brain.

edit: make 'em to fit the common diameters of pistons.

 

[cuttinties]

You know.... i'm looking Caffeine1fgs jig and wondering IF you could get away with
a hybrid of using the drawbar idea and a pin , to pull the piston against the base.
Use a wrist pin that's only just small enough to fit in all piston holes.

Now add a set of concentric steps bored about 2.2~3mm into the face of the jig.
these would be in the diameters of your most likely/common piston skirt diameters.

Thinking of the light cuts such as these saw pistons here, nothing heavy.
you'd want to be sure you have a good sharp point on the tool and the angles
would be pretty steep/agressive
So as to give a really free cutting - lowest pressure tool bit.

His jig is a modified drawbar. And you can't use a drawbar without a wrist pin. But all in all the drawbar or sleeve is the best option for 3 jaw chuck.

 

[cuttinties]

I think if you oversized the drawbar hole to fit the biggest piston pin, you can muck that up a bit and it won't matter too much. I'm with you on the drawbar going all the way through the headstock.. though perhaps cheating with a piece of 1/2" Readyrod (allthread) with a pair of doubled nuts on each end (or welded) would give you the possibility of loosening and tightening it easily.... guess you could put a long extension on a ratchet and get in there too.

That's what I've found to be the most affective. I wouldn't worry about interchangeability as it's pretty cheap to make drawbars.