Paxolin rod/bar anything similar?

[raynerd]

Hi Guys, I`m about to start a project which requires a 1/2" paxolin bar however quite a bit of googling has not brought anything up at all! -there a few places selling paxolin thin sheet but that is it. I need about 12", probably less.

Does anyone know anything with similar properties. Without giving away my intended project at this point, I believe the intended use of paxolin is for low expansion with small changes in room temperature.

Could anyone suggest anything else suitable (or a paxolin bar supplier if possible). During my googling I`ve found Tufnol mentioned often in the same sentence as paxolin. I also know someone who has used a fiberglass rod in place of the paxolin which apparently worked quite well - I`ve googled this instead but I think I need to be more specific than simply "fibreglass".  I expect some of these other plastics would work as well but I was hoping someone with more understanding of the different types could give me some advice.

 These guys seem to stock a good range:  http://www.directplasticsonline.co.uk/

Any help appreciated.

Chris

[andyf]

Tufnol is posh paxolin, Chris. ShopforPlastic say they do it in rod form:  http://www.shopforplastic.co.uk/plastic-engineering-materials-1-c.asp . I've used them for Tufnol offcuts and found them prompt and cheap.

Also, a search on Ebay for Tufnol Rod turns up two results. One is 50mm bar and the other 12mm.

Andy

[Bluechip]

Chris

Tufnol is a trade name ie, a company. Like Biro / Hoover.

Two general groups: SRBF synthetic resin bonded fabric. SRBP sythetic resin bonded paper.

Many different grades / uses.

Carp, Whale, Kite etc. etc come to mind, others exist.

http://www.tufnol.com/tufnol/default.asp?id=92

And ..

http://www.tufnol.com/tufnol/default.asp?id=21


Dave BC

[Spurry]

You might find what you need at MacGregors, I have bought paxolin  and grp sheet offcuts from them .

http://www.macgregor.co.uk/plasticmain.htm

Pete

[Boz]

I'll second http://www.shopforplastic.co.uk (Davis Industrial Plastics) as a supplier , I've recently bought Nylon66 from them recently and got good service.

A couple of reminders - best to wear a dust mask while machining or sanding it. The dust can do nasty stuff to your lungs. The other thing is that the thermal expansion and the strength is different in different planes. Think plywood. Tufnol and its' relatives are layered products so they expand more between the layers than they do along the layers. They also delaminate very easily. So they are strong along the layers (especially the glass based products like 10G40), but will split like plywood between the layers.

I'm rustier on the rod forms - I think they are spiral wound, so strength wise they are similar to the grain in a tree. So they expand more radially than along the rod. They can also split radially if a hole is too near the curved surface.

Regards
Boz

[raynerd]

Thanks for all the replies, I've just ordered from daves industrial plastics as some of you recommended as it was a good price and obviously you have had good service. I've just got 30cm of 12mm dia. Hopefully should work ok - I don't think it is too critical, just anything instead of metal to reduce changes due to temperature change. Thanks again!! 

[Bluechip]

Thanks for all the replies, I've just ordered from daves industrial plastics as some of you recommended as it was a good price and obviously you have had good service. I've just got 30cm of 12mm dia. Hopefully should work ok - I don't think it is too critical, just anything instead of metal to reduce changes due to temperature change. Thanks again!!  

Chris

Are you sure you're on the right track ??

A lot of 'plastics' expand considerably ...

Like PVC for instance ...

You might think aluminium is bad, PVC is > 2x aluminium.

Dunno about tufnol, can't find the numbers ...  

Table at the bottom of this. That Invar stuff is used for clock pendulums. As you probably know  ..

http://en.wikipedia.org/wiki/Expansion_rate

Dave BC

[lordedmond]

Good  for no metal bits usual disclaimer other that a satisfied customer  they are very quick with del.

http://www.directplasticsonline.co.uk/


Stuart

[Lew_Merrick_PE]

Chris,

Paxolin is a tradename for a phenolic resin/linen composite material.  It is probably also available as a phenolic resin/paper material, but that did not leap out and bite me on a quick Google search.  The question for you is what are the temperature, humidity, and operating stresses in which this part is required to function?  Typical epoxy-glass (e-glass type fiberglass) materials are relatively limited in temperature and humidity regimes (i.e. < 100°C and <65% RH) if there is any significant load (stress).

[raynerd]

The working conditions are room temperature ... for the pendulum rod of a clock     The plans call for a 1/2" paxolin rod of 6" length. I believe paxolin is used to reduce expansion and lower it from that which would occur if a metal pendulum rod was used. Obviously if the pendulum rod length changed, the period changes and your clock goes out of time.

I can only presume paxolin is well suited to the job since it is called for in the plans in preference to any other material. I`ll see how I get on with the Tufnol now I have ordered it - to be honest, if I pull this off and it keeps to a few minutes a day I`d be chuffed

[75Plus]

Chris,

See if you can find a suitable piece of Invar. It is a nickle/steel alloy that is known for it's resistance to thermal expansion. Invar is used on a number of precision regulators most of which have a one second period. i.e 60 beats a minute. At that length (36" +/-) thermal expansion can become a problem.

If your pendulum length is only 6 inches you will not see a great variance even if you had the clock located outside so it was subjected to much wider temperature swings than indoor environments.

[SPiN Racing]

Another place to check, that has lots of information about the products, is http://www.onlinemetals.com/ They have a nice selection.. granted they are on the US side of the pond.. its worth looking at for info at least.

Also I have found in the past Fleabay has tons of stuff in the plastic lines, as well as metal, lots of Drops. I have purchased lots of drops that are ULTRA cheap, and are perfect for making suspension bushings, as well as a wide variety of plastic, delrin, plastic and other parts.


Scott

[raynerd]

Well chaps, thanks for all the info. I`ve spent many hours looking and I now have ordered a 400mm length of Invar36 as well as the tufnol (it`ll come in useful at some point). I hear that Invar is sun of bitch to cut and I think I`m going to have to thread it.

I ordered the invar because I was looking at the thermal expansion co-efficients and Tufnol was 1.9 x 10e-5 which seemed quite high. I believe the expansion on Tufnol is primarly along the laminate and I think the tufnol rod is bound in rings to make up the bar and so I`m guessing expansion along its length (and therefore causing the period to change) could be tiny? - which is why it could be OK to use.

Anyway, for the sake of £3 I have got a length of invar as well so I guess I can choose which I use.

Thanks again for all the detailed info
Chris 

[Lew_Merrick_PE]

I ordered the invar because I was looking at the thermal expansion co-efficients and Tufnol was 1.9 x 10e-5 which seemed quite high.

Chris,

What units are you using?  1.9 X 10^-5 as in/in/°F is different from m/m/°C.  Most steels run about 6 X 10^-6 in/in/°F (1.08 X 10^-5 m/m/°F).  Most aluminums run about 12 X 10^-6 in/in/°F (2.2 X 10^-5 m/m/°C).  The "length/length" part is always the same regardless of units, it's the degrees of temperature that make the difference.

[raynerd]

That is a good question Lew, I am pretty sure it was m/m/oC.

So tufnol at 1.9 x10e-5 m/m/oC which would make it a little less than aluminium!

[Lew_Merrick_PE]

That is a good question Lew, I am pretty sure it was m/m/oC.

So tufnol at 1.9 x10e-5 m/m/oC which would make it a little less than aluminium!

Chris,

The basic design of a pendulum is to concentrate the mass at the bob and setting your period as t = 2*pi*(l/g)^0.5 where 'l' is the length of the pendulum and 'g' is the (appropriately unit-based) acceleration due to gravity.  If we assume that "indoors" is the environment, then somewhere in the 20°F is going to be your maximum delta-temperature.  At 6 inches nominal length and 20°F your nominal delta T, then aluminum (12 X 10^-6 in/in/°F) would have a delta-l of .00144 inches.  Steel would give you a value half that.  Steel will be (nearly) 3X as stiff (27 X 10^6 psi tensile modulus as opposed to 20 X 10^6 psi for aluminum), so the only question is which you could turn to a thinner wall tube to minimize mass?  The answer is almost certainly aluminum -- though you can get very thin-wall steel tubing that might be a better choice.

I don't know the numbers in metric, but most steels are about .284 lb/in³ while most aluminum are about .099 lb/in³ and most plastics run in the .05 lb/in³ range.  The general factor of stiffness for a symmetric column or beam is EI where E is the tensile (or compression) modulus and I is the area moment of inertia.  I would assume that you would want to maximize the stiffness while minimizing the mass of the pendulum's connector.  Right?

[raynerd]

Lew,

Would stiffness really be such a consideration with any of the materials we are discussing in the situation that it will be in? Don`t get me wrong, it needs to be stiff but would the period really vary due to this factor whether aluminium, steel or plastic was used?

I do understand most of what you have written but I`m curious why such a prolific clock builder, John Wilding, would have suggested Paxolin as the most suitable material if it was not the case - aluminium, brass and steel are used elsewhere in the clock build so he clearly could have choosen this during the build - yet he specifies Paxolin.  
Regards
Chris

[andyf]

FWIW, most Foucault's pendulums you see in museums etc (one is shown here http://en.wikipedia.org/wiki/Foucault_pendulum ) have their bobs suspended on flexible wire.

Andy

[Boz]

I`m curious why such a prolific clock builder, John Wilding, would have suggested Paxolin as the most suitable material

One suggestion is that Paxolin is lower density than any metals, so you'll end with a larger pendulum, same mass. Does larger look better for the design? ie is it a purely aesthetic choice? Paxolin won't corrode either, although I suspect that won't help the clock's accuracy significantly .

Boz