Lapping nylon
 

Folks,

I'm attempting to fit a 1.5 inch nylon cylinder into a 1 inch diameter
metal sleeve. The nylon will act as a piston for my spring piston
pellet gun. I've heard for years that auto engine pistons are lap
fitted for each cylinder and would like to give it a try. Could someone
give me the basic idea of how this might be done for nylon?

Thanks

Tut

Lapping nylon
 

wrote:

I'm attempting to fit a 1.5 inch nylon cylinder into a 1 inch diameter
metal sleeve. [snipp] I've heard for years that auto engine pistons are lap
fitted for each cylinder and would like to give it a try.

Auto engine pistons lapped? No!
And getting the 0.5" off with lapping is also not the right way. :-))

I'd take some PA with MoS2 in it (the black one) and simply turn it on a
lathe. Just make it a nice gliding fit. I have done that with pistons
(diam 12mm) for a compressed air engine to my satisfaction.


Nick
--
Motor Modelle // Engine Models
http://www.motor-manufaktur.de
DIY-DRO // Eigenbau-Digitalanzeige
http://www.yadro.de

Lapping nylon
 

Lapping nylon
 

Polymer Man wrote:
A principal of lapping is the softer material (the lap) will pick up
the abrasive and lap the other part. Nylon will pick up the lapping
abrasive, and in turn wear the bore in your air gun.

Things are lapped to get a very precise fit and a fine finish. Due to
the extreme difference in coefficient of thermal expansion between
nylon and your metal bore and the fact that nylon changes dimension
with exposure to water (kinda like wood), you would not want an
extremely close fit between your nylon piston and metal sleeve or you
risk binding. You will need some clearance, probably around .005" plus
or minus .001". It does not take lapping to get that kind of fit. A
fine lapped finish would probably not be highly beneficial to the
function. And I don't think nylon will lap well. My opinion is your
best bet is to just turn your piston on a lathe.

If you want a high performance plastic that does not absorb water, has
good lubricity and high wear resistance and a low coefficient of
thermal expansion and friction, look at polyimide (Nylon is polyamide
and is very different) such as Dupont Vespel.

Though some plastics polish well (Delrin for example) if you want an
actualy lapped part, you should probably use metal.

Polymer Man--Thanks!

Calculations indicate that I'm looking at 2000-2500psi and thought
lapping might result in less gas leakage. I'll give Vespel a look see.

Tut

Lapping nylon
 

Nick

Thanks for your comments--I'm not sure what PA is but assume its
polishing compound. I have four different grades--from rouge to black.

Tut

Lapping nylon
 

Trevor Jones--

Thanks for your comments--my last auto engine rebuild was over 30 years
ago--guess thing change.;-))

Tut

Lapping nylon
 

wrote:

I'm not sure what PA is but assume its polishing compound.

PA is Polyamid. I would use it for the piston.


Nick
--
Motor Modelle // Engine Models
http://www.motor-manufaktur.de
DIY-DRO // Eigenbau-Digitalanzeige
http://www.yadro.de

Lapping nylon
 

Lapping nylon
 

Thanks again for your comments. The air rifle is a $135 Chinese one
imported by Compasseco--it shoots extremely well but needs some
refinement. This is the only model the importer doesn't sell extra
piston sets and springs for. The original machining on the piston
assembly wasn't quite right and gouged the piston sleeve when fired.
The sleeve then roughed up the plastic piston creating a sealing
problem--(a plastic piston sits on a metal assembly.) It still shoots
fine (dime sized groups at 35-40 yds) but the cronograph says the exit
pellet velocity has fallen to around 815 fps--down from 1100 fps. My
plan was to hone out the sleeve and then replace the plastic piston
with a tight fitting nylon one.

Tut

Lapping nylon
 

Lapping nylon
 

Jeff

Your car wheel piston cup may work. Thanks

Tut

Lapping nylon
 

Lapping nylon
 

Nick--Thanks for the PA info....

Trevor--some good ideas in your post. Jim Maccuri actually made the
main spring for the rifle. What would your recommend for button
material?

Thanks again everyone

Tut

Lapping nylon
 

Lapping nylon
 

Trevor--

Thanks I'll check him out.

Best Wishes

Tut