Why Delrin/Acetal and not oiled nylon 66?

[Skunky]

I've been using Delrin/Acetal like most others, for spring guides and top hats.

But why do most people use it?

Why not Moly impregnated nylon or oiled nylon. (Nylatron etc.)
I would think nylon maybe a little less brittle?

Has anyone had a delrin top hat self crack/destruct due to high impact, small diameters sharp angles etc?

Just contemplating as I'm due to get some more delrin and wondering if its worth changing?

[tinbum]

Delrin is a trade name like Hoover or Sellotape it covers a whole host of engineering plastics. Some are brittle, some are not. Some are hardwearing, some are like cheese!
I have tried most over the years, included ones impregnated with various substances. The one's I've settled on machine well and are durable, but for one offs the fancier ones may work well

[jonnyone]

More years ago than I care to recall John Bowkett supplied me with a guide made of the above material for my HW80.
Excellent product and it's still going strong inside the gun.
Dave

[Black Beard]

I've used oilon (green oiled nylon) abd it works fine. It should be good for long spring guides where there is a lot of vibration.

But Delrin/Acetal is a lot easier to machine- particularly drilling deep holes.

[WebleyWombler]

Why do people use that type of material in a steel cylinder anyway? is it to do with friction or resonance. I don't work on anything made after about 1960 unless it's a Webley pistol but I would be very wary of using any hygroscopic material inside an air rifle . Nylons can be particularly bad for absorbing water. Wouldn't a material such as PVC or PTFE be a better choice?

No axe to grind, just interested.

[Born Again]

Delrin is acetal homopolymer, generic acetal is usually acetal copolymer.

Delrin is usefully stronger, especially in thin sections, although not quite as good for chemical resistance.

VS Nylon, both types are much nicer to machine and more dimensionally stable. Both types are low friction.

[Skunky]

I think if Delrin/slightly hygroscopic materials are in side the air rifle and lightly lubricated, the chance of moisture being absorbed are very minimal?
I'm toying with the idea of Mos2 impregnated nylon.
May get some to try.

[Born Again]

I think if Delrin/slightly hygroscopic materials are in side the air rifle and lightly lubricated, the chance of moisture being absorbed are very minimal?
I'm toying with the idea of Mos2 impregnated nylon.
May get some to try.

I have some oil filled nylon, and it is useful for bearing use, but delrin / acetal machines far better. Delrin on delrin is very low friction, almost as slippery at PTFE but much harder, it would be my choice.

[TonyL]

Why do people use that type of material in a steel cylinder anyway? is it to do with friction or resonance. I don't work on anything made after about 1960 unless it's a Webley pistol but I would be very wary of using any hygroscopic material inside an air rifle . Nylons can be particularly bad for absorbing water. Wouldn't a material such as PVC or PTFE be a better choice?

No axe to grind, just interested.

Good questions.

I don't have a lathe so can't make my own guides etc., but really wish I had.

The advantages of synthetic over steel would be, as you mention, reduced friction and also, in many cases, resonance. Much will depend on the fit and nature of the guide in both respects, though. Ease of machining and lighter weight (sometimes desirable for top hats) are other considerations. I think PTFE, although having very low friction properties, is usually avoided as it doesn't have memory and may take on an undesirable and less efficient form.

All interesting stuff and there's lots of useful information all over the 'net on the usefulness and properties of various engineering plastics, as well as on various suppliers' sites.

[hmangphilly]

Nylon is funny old stuff to machine .

You deffo need to practise

[Antoni]

I have zero experience with tuning airguns but I do have some knowledge from other hobbies.

Nylon 66 is very hygroscopic. From memory it will absorb 6% of either its weight or its volume of water.

We were taught that this particularly important part of our machines should never be oiled even though there is relative movement between it and an anodised aluminium surface. Does anyone know why we were told that?

[pajj]

Hi,

I haven't machined that many plastics but the only one that I have tried that I would say machines well is delrin/acetal. So on the positive side we are lucky that we have a very usable (top hats, guides, washers, etc.) plastic that also machines very well.

Cheers, Pajj.

[Skunky]

Hi,

I haven't machined that many plastics but the only one that I have tried that I would say machines well is delrin/acetal. So on the positive side we are lucky that we have a very usable (top hats, guides, washers, etc.) plastic that also machines very well.

Cheers, Pajj.

Sure is easy/nice to machine on my little benchtop lathe.

[TopDog]

Having used "Delrin" for custom guides for ages, it is entirely suitable. Easy to machine and good wear resistance particularly when lubricate properly.

To achieve a good lasting fit, I machine the guide to fit the chosen spring at the pre-load required (have a test rig for that!) to achieve a good interference fit! This ensures that the guide does not wear quickly and need a re-setting on ME.

Since the spring expands with compression, the "slop" is taken up with a good molypaste on the guide and a wipe-on/wipe off application of a high temp/high pressure grease on the spring O/D (MS WS2 is my preferred one atm) takes care of O/D friction in the piston and dampens spring vibrations without running everywhere when the rifle is stored!

A piston sleeve is mandated where the spring fit may provide excessive clearance and tend to buckle as the rotation is curtailed by end-load friction (30 - 40 degrees being typical - have measured it!).

Note that each guide is matched to the particular spring used to eliminate tolerance errors - not a really commercial proposition!!!

Can't do ALL the wrinkles in this post but it really does work! Rifles I have "fettled" some years ago still produce the same ME with the same pellets as they did when modified!

Delrin good and easy steel good but not very easy!

Happy fettling

#atvb
david

[Skunky]

Thanks everyone for the input
Topdog -

I turn my guides to suit the spring "snugly" to cut down on twang.
As far as moly grease - I have a tin of Molyslip open gear lube. Tacky and heavy on the moly side.
A quick search on the web and it looks like its no longer available. very good at quietening springs as well as lubing though.