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Posted by on May 6, 2006, 8:42 pm
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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
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> 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.