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Carbon Teflon Tapping

Rogue_Machinist

Hot Rolled
Joined
Jul 26, 2011
Location
Oregon, USA
So this is isnt for work. This is for a personal project im working on. But I made a huge mistake and assembled something incorrectly. besides that. I need to make a ring for a high power rocket im building. The ring CANT be aluminum or Stainless. So I decided to use Carbon filled Teflon. But I was wondering if anyone has tapped carbon teflon and if it will accept the tap without too much deformation Or am I better using something else. I just happened to have this piece handy. It was either this or Bakelite. But I know this carbon teflon will handle the heat the ejection charge. Ive never worked with Bakelite too much.
 
Are you sure you should be building "high powered rockets"? It was on hand is not a good way to select materials for such construction.

Perhaps you should get some experience with something less ambitious under your belt first.
 
As Muckalee points out, Teflon(R) and other PTFE materials have some design limits. It sounds like you used phenolic resin in similar applications. Phenolic resin is about 4 times stronger than PTFE. There's a wrinkle in that the tensile strength of carbon filled PTFE is less than that of pure TFE (which sounds weird, but that's what I found for data). But the key point is that filled bakelite is MUCH stiffer than C filled PTFE.

One other thing. Plastics have what is called a glass transition temperature. They turn from stiff solid material to flexible, rubbery material at that temperature. For PTFE (and some other high-performance plastics like PEEK and PEI) the GTT is in the range of 130°C. So none of these materials can see exhaust gas at all. Well, they CAN see higher tempertures for the brief period before failure.

I personally would not use PTFE* in any form for a structural member in a rocket. If its just a seal, and the seal never sees exhaust gas, perhaps. Otherwise I'd use a stronger engineering plastic like PEEK or PEI, or Ti.

*I'm not an expert in the application of engineering plastics, but I did work for DuPont at the Washington Works in Ohio, where they make Teflon(R) and other Fluoroproducts. So my bias is towards Teflon(R), but (depending upon your application) I'm recommending against it.
 
The key bit of information which is not in your post is ...where is this on the rocket? As stated, if it is just a general purpose ring for holding stages together or something then you have a lot more latitude for material. If it is in the nozzle assembly then you have to go high tech. When I worked at the flight dynamics lab in Dayton we used a lot of titanium for nozzle assemblies. Pretty much anything less exotic just melted.
 
The key bit of information which is not in your post is ...where is this on the rocket? As stated, if it is just a general purpose ring for holding stages together or something then you have a lot more latitude for material. If it is in the nozzle assembly then you have to go high tech. When I worked at the flight dynamics lab in Dayton we used a lot of titanium for nozzle assemblies. Pretty much anything less exotic just melted.

No this is literally just a reinforcement ring for a rail button to be screwed into. And adds rigidity to the tube. Its not going to be anywhere near a prolonged heat source.
 
As Muckalee points out, Teflon(R) and other PTFE materials have some design limits. It sounds like you used phenolic resin in similar applications. Phenolic resin is about 4 times stronger than PTFE. There's a wrinkle in that the tensile strength of carbon filled PTFE is less than that of pure TFE (which sounds weird, but that's what I found for data). But the key point is that filled bakelite is MUCH stiffer than C filled PTFE.

One other thing. Plastics have what is called a glass transition temperature. They turn from stiff solid material to flexible, rubbery material at that temperature. For PTFE (and some other high-performance plastics like PEEK and PEI) the GTT is in the range of 130°C. So none of these materials can see exhaust gas at all. Well, they CAN see higher tempertures for the brief period before failure.

I personally would not use PTFE* in any form for a structural member in a rocket. If its just a seal, and the seal never sees exhaust gas, perhaps. Otherwise I'd use a stronger engineering plastic like PEEK or PEI, or Ti.

*I'm not an expert in the application of engineering plastics, but I did work for DuPont at the Washington Works in Ohio, where they make Teflon(R) and other Fluoroproducts. So my bias is towards Teflon(R), but (depending upon your application) I'm recommending against it.

As I posted to a few other responses. This is just a mounting point for where the launch rail button will be tapped into. It will experience the ejection charge of the motor. For about a second.
 
Are you sure you should be building "high powered rockets"? It was on hand is not a good way to select materials for such construction.

Perhaps you should get some experience with something less ambitious under your belt first.

I have built over 200 rockets. This is yes my first high power rocket. But I have made many custom rockets all worked fine. I was asking because I cant use a metal. But dont have access to phenolic. I do have access to Bakelite. But never worked with it.
 
If it were me, I'd still use something other than PTFE. Reason is, even a small amount of thermal degradation in PTFE produces some pretty toxic stuff. If everything works fine, you're probably ok, But if you have the same luck I had as a kid, sometimes things stick and the chute begins burning inside the tube and, well..

If you note that there are no baking sheets (often used for broiling) that are PTFE coated, you'll understand. Broil some tater tots on a non-stick sheet and your parakeet will die. Literally. Decomposition products (carbonyl flouride and perfluoroisobutylene and others) are lethal to budgies at very low levels. And to humans at levels a little bit higher. Seriously, the products are nerve-gas level toxic. I take dead budgies as a little warning to us non-budgies.

One other reason not to use PTFE is that drllled and tapped holes close in after drilling or tapping. Carbon filled may be better (and I think you have more experience here than I do) but getting the hole and threads just right might be tough. Although, one might regard it as a self-locking hole... Last, the strength and stiffness of PTFE compared to other plastics is like Manchego cheese... Last (really, this time) glueing the ring in may present difficulties and I could see the ring-body joint failing. Maybe I'm being too engineringish, though.

Let us know if you do this and it works. What class engine are you using?
 
If it were me, I'd still use something other than PTFE. Reason is, even a small amount of thermal degradation in PTFE produces some pretty toxic stuff. If everything works fine, you're probably ok, But if you have the same luck I had as a kid, sometimes things stick and the chute begins burning inside the tube and, well..

If you note that there are no baking sheets (often used for broiling) that are PTFE coated, you'll understand. Broil some tater tots on a non-stick sheet and your parakeet will die. Literally. Decomposition products (carbonyl flouride and perfluoroisobutylene and others) are lethal to budgies at very low levels. And to humans at levels a little bit higher. Seriously, the products are nerve-gas level toxic. I take dead budgies as a little warning to us non-budgies.

One other reason not to use PTFE is that drllled and tapped holes close in after drilling or tapping. Carbon filled may be better (and I think you have more experience here than I do) but getting the hole and threads just right might be tough. Although, one might regard it as a self-locking hole... Last, the strength and stiffness of PTFE compared to other plastics is like Manchego cheese... Last (really, this time) glueing the ring in may present difficulties and I could see the ring-body joint failing. Maybe I'm being too engineringish, though.

Let us know if you do this and it works. What class engine are you using?

Thanks for listing out all the reasons. I was just gonna say "don't do that, PTFE is a stupid choice for that application"

Our friend bosleyjr has listed out a nice smorgasbord of downsides, here are the reasons why you would want to use PTFE for you application:

1. It's chemically inert, doesn't react with hardly anything.
2. It's quite slippery
3. It has a nice wide service temperature range (I know everyone was talking about the toxic chemicals released at high temp, but also as others said, keep it below the danger point and it will serve you well). Also keeps its properties down into cryo-temps.

If any of those three reasons makes you want to use it, then by all means. But it won't tap properly, it exhibits much less stiffness than a wide variety of common plastics, and it's $$.
 
If it’s inside the booster section opposite a rail button I wouldn’t worry about the eject charge. It comes and goes quickly. I take it the upper rail button ins above the top motor mount centering ring?
Good luck w/ your presumably pending L1 cert. launch.
 








 
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