"Tim Wescott" wrote in message
...
Bob La Londe wrote:
"Tim Wescott" wrote in message
...
Bob La Londe wrote:
Well, I tried to assemble the hole thing and the push rod threaded into
my piston crooked... How is that possible I wondered. I did
everything on the lathe. Oh, ****. No I didn't. My mini lathe was
too small so I only center drilled the piston on the lathe. I drilled
it on the drill press. I really need to throw this damn thing away.
The table was tilted side to side. I straightened that today, but
found the table is also slightly tilted front to back. GACK!!! Its
not horrible for punching holes in sheet, but I guess I need to add a
mill drill to my wish list for anything else.

Next time, rough drill it undersized in the press, then bore the hole to
size, with a boring bar in the lathe.

I'm not sure about "too small", though -- unless the tailstock chuck
won't take the drill, you should be able to keep loads light by feeding
slowly.

Actually it has an amazing amount of power for a cheap tiny lathe. HF
calls it a 7 X 10, but I think its really a 7 x 8. Several people
on-line have commented that when they swap out to the 14" bed from The
Little Machine Shop they gain about 6 inches of working length rather
than the 4 you would expect. As soon as they are back in stock I plan to
order the longer bed for it, and turn the old bed into a tail stock
parking rest. I still want a bigger lathe but this little one is handy
sometimes.

Now to save that piston... I am thinking I might bore it out further,
hammer in a plug, and re drill it concentrically. Then either drive in a
couple wedge pins or just slap a couple weld tacks on it.

(a) Press in a plug -- even doing this in a vise is going to be more
accurate than hammering it in.

Well, the little 12 ton air over press would have probably been my tool of
choice too, but I might not have thought of it until too late

(b) How much force on the piston, and which way?

Not all that much. (maybe 20 or 30 if there is a plug up) If the guide rod
is straight it's will have a few pounds of vacuum in one direction as it
draws in material and a few more of pressure as it pushed out material.
Right now its hand operated, but the dimensions are planned for it to be
able to drop into an electric caulking gun. (different piston entirely when
I do that conversion)

If the piston isn't going to be pulling hard on the rod, a press fit or
shrink fit may be plenty strong enough. Or a light press fit with one of
the more insanely strong Loctites, or epoxy. Or just fill the hole with
epoxy & call it a plug (hmm).


I'll have to thank about that. Epoxy would work for strength, but this is
an aluminum hand injector that may be handling liquid media upto about 400
degrees... Ideally the media should never run above about 350, but my
thermal remote tells me its hitting 400 occasionally. You know I bought
that thermal remote for checking the preheat on welding thicker aluminum
plate, and since then I have used it for all kinds of things.



(c) Press, weld (not if it's epoxy, though), _then_ bore out the hole --
and check the outside for concentricity and warps, too.

A few thousandths wouldn't hurt a thing. There is a lot of clearance on
this piston. The o-rings fill the gap. The degree of accuracy for the push
rod to be perpendicular to the piston o-rings is important though. I
already have a rod guide, but can't use it the way it is. If it doesn't go
in and out straight the piston will lose suction or pressure out the back.


--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com