tivar 1000 or delrin

[yel01z06]

was going to make a spring guide I know most of you make the guides out of delrin I have seen some people using tivar 1000. has anyone used Tivar? is it better or worse?any info would be appreciated

[Machinist]

Hey Trek, I've been thinking along the same lines. The spec's. I've seen leave me thinking it may be superior to Delrin, especially in lubricity. If a place like McMaster-Carr sells it, "little people" like us could get some.
Mike

[moe1942]

At Mcmaster carr it is $5.44 for a piece of Tivar 1000 is 1 1/2" X 1'.

[Bhawanna]

why don't people make spring guides out of metal like brass, aluminum, stainless, etc...?....i am just learning this stuff and was curious.

thanx.

[tjk]

Hey Roger, I didn't think that was a stupid question. Like you,  I'm picking up tips everytime I get on the GTA. If we don't ask questions, then either we'll never know, or learn the hard (and often expensive) way. I'd think it might be to keep the weigh down and/or keep the rifle balanced better, but that's just a guess. Then again the friction of the spring might cause a steel/metal spring guide to expand after continous shooting, ad interfere with the spring?!?!? I'd like to know the answer to your ? and hopefully someone can explain why. I do have a question you you, How do you crown the breech of the barrel. What kind of abrasive bit should one use? like a conical shaped stone or deburring bit? I've got lots to learn too!!! The Sierra'a needin' a chop!!!!!!tjk

[Bhawanna]

check your private mail box....

[3n00n]

http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=9420&mid=69539#M69539

[gunsup0331]

I used a conical stone with my dremel then the tapered polishing felt to crown my breech with great results.
  IMHO the breech crown can be less precise than the muzzle end.
HTH

[mjb108]

That's kind of what I was thinking, too.  It seems a close-fitting  plastic guide will function as follows:

(1) when spring is fully compressed, the spring OD (and ID) will increase slightly, allowing the spring to move without friction (againste the guide) as it expands (lengthwise) when the gun is fired.
(2) upon full extension, the piston will hit the end of the cylinder (not wanting to discuss how hard it hits, but it does hit, right?)
(3) a vibration results, which would like to immediatey "ring" through the length of the spring (TWANGGGGGGG!)
(4) Except that the spring has just now returned to its free-state (or nearly free-state) diameter, clamping upon the spring guide
(5) which, being made of relatively dense plastic, dampens the vibration to almost nothing.
(5A)  Alternatively, a metal spring guide, being much more massive than the spring, does not have a similar harmonic, and fairly well dampens the spring also, but still may have some tendency toward a lower frequency reverberation (TWUNG?)

Seems like the plastic one is almost better, given the service life you can expect of a decent one, and the comparative costs.....


This is just a thought experiment that has been rolling around in my head as I drive to and from work - does it sound right???

Mike

[ccjr]

I noticed that the top hat & spring guide I made of UHMWPE in my QB88 gradually began to bulge and shorten after a few weeks.  
The top hat locked into the piston. My surplus teflon supplier warned me that teflon would behave the same way.

I found this interesting:
http://www.eng-tips.com/viewthread.cfm?qid=205376&page=3

[mjb108]

ccjr,

Good observation!  I followed up to your search by going to http://www.matweb.com, as well as a couple of commercial websites - Tensile yield strength of teflon (PTFEl) is nominally 3,000-5,000 psi, depending on the specific type and the data source.  This compares similarly to about 3,000 psi for Tivar 1000 (Ultra High Molecular Weight PolyEthylene or UHMWPE).

Note that many people use Delrin (acetal) for these types of parts.  The tensile yield strength of delrin is nominally 10,000 psi!  (The very weakest steel, by comparison, will easily exceed 25,000 psi.)

The observation of bulging would be related to the compressive (rather than tensile) yield strength - I suspect the differences between these three polymers is approximately the same as the differences in their tensile yield strengths (i.e I expect UHMWPE & Teflon would be roughly 3x less able to resist a compressive load without permanent deformation as compared to delrin).

For what it's worth,

Mike

[yel01z06]

the impact strength of Tivar is about 5x higher than delrin from what I have read and the oil filled tivar says that the impact strength is unbreakable is that the same as the compresive yield stength?