[11bravo]

Wasn't sure where to post this.
I build and fly high power amateur rockets and am in the process of building an all/mostly carbon fiber rocket.
I need to make a male plug from which to make a female mould from which to make the nosecone.
Attached is a zipped folder with about all the information I can think of in regards as to what I need.
It would be one half of the nose cone split lengthwise, similar to what is shown in the Alibre file; the shape of the actual nose cone in that file is NOT correct however.
Nor is the shape in the AutoCad file.
They do show the need for a rear shoulder portion.
I do not know how to import a series of points or an equation into Alibre so there are a couple of files from which the information can be found.

Points found in:
Nosecone.lst X y values.xls
Equation found in:
The Von Kármán Nose Cone.pdf

The surface would have to be really quite smooth.
In the long run I would actually need two identical halves.
Not sure what to have them made out of though.
I have some maple glued up but thought of the HDPE(UHMV) today.
Have considered aluminum.
These would be mounted to a flat piece of something and be covered with a surface coat epoxy and some fiberglass cloth to make the mould.
When cured, the mould will be pulled off and these pieces will be set aside.
They will not be exposed to elevated temps other than the heat of the curing epoxy.

Anyone think they could make one out of a 2”x4” to see how well they turn out before committing to the fancier materials?
If so, shoot me a quote for your time and material and we’ll see if we can do business.

Thanks,

Greg

[SORCHEROR]

why not just make the mold out of aluminum?instead of epoxy and glass

[11bravo]

I don't have a way to make an aluminum mold, although someone here might.
Also, I want to learn/practice that method of mould making.

[mrphyyber]

Wasn't sure where to post this.
I build and fly high power amateur rockets and am in the process of building an all/mostly carbon fiber rocket.
I need to make a male plug from which to make a female mould from which to make the nosecone.
Attached is a zipped folder with about all the information I can think of in regards as to what I need.
It would be one half of the nose cone split lengthwise, similar to what is shown in the Alibre file; the shape of the actual nose cone in that file is NOT correct however.
Nor is the shape in the AutoCad file.
They do show the need for a rear shoulder portion.
I do not know how to import a series of points or an equation into Alibre so there are a couple of files from which the information can be found.

Points found in:
Nosecone.lst X y values.xls
Equation found in:
The Von Kármán Nose Cone.pdf

The surface would have to be really quite smooth.
In the long run I would actually need two identical halves.
Not sure what to have them made out of though.
I have some maple glued up but thought of the HDPE(UHMV) today.
Have considered aluminum.
These would be mounted to a flat piece of something and be covered with a surface coat epoxy and some fiberglass cloth to make the mould.
When cured, the mould will be pulled off and these pieces will be set aside.
They will not be exposed to elevated temps other than the heat of the curing epoxy.

Anyone think they could make one out of a 2”x4” to see how well they turn out before committing to the fancier materials?
If so, shoot me a quote for your time and material and we’ll see if we can do business.

Thanks,

Greg

The easiest/cheapest method would be to find someone local that has a wood or metal lathe. Turning the part would insure it is symmetrical. When you dimension the plug make sure you take into consideration the thickness of the finished product. The plug will have to be under cut to get the finished part the correct size. Once you have a plug, I recommend using casting epoxy to make the mold. It is inexpensive and relatively easy to use. The mold should be 3 piece, 2 outer halves and a plug to insure uniform thickness.

[shibi]

I don't have a way to make an aluminum mold, although someone here might.
Also, I want to learn/practice that method of mould making.

1. What type of carbon fiber/grpahiite are you using?
2. What is the target cure temperature of your epoxy?
3. Are you using an outer mold in addition to the plug? or are you vacuum bagging it.

You have to be really careful with a solid aluminum plug. The thermal expansion of aluminum is a whole lot more than carbon fiber and typically in the cure cycle the aluminum plug may expand more than the carbon fiber, leading to excessive residual stress.

Off and on you'll get a part that looks nice on the outside but the graphite under the surface graphite fabric layers are all broken in bits due to the expansion of the plug during cure.

-S

[11bravo]

mrphyyber, shibi, and anyone else-
Thing is, I want the shape to be pretty close to exact and a wood lathe run by hand just isn't going to be that close.
Second, to turn it I would not only need to use the "split turn" techinque, just by the nature of the nose cone it would need to be turned at the far end without support (cut free of the tailstock).
The guys I know that use a CNC turning center are a little hesitant to try that.

For the method I am using, the plug will need to be exactly the size and shape of the finished nose cone.
And the plug will only be used to make the mould, which cures at room temp, so the plug will not be used during elevated temp curing so it's thermal expansion is not an issue.
See-

http://www.jcrocket.com/nike-asp.shtml

and scroll almost all the way down.
This is essentially the method I will be using but I plan on modifying it a bit and use two halves instead of making a cone, messing around trying to get a plate cut to fit the cone and then making two female moulds off of that.
Instead, mount the two halves to a flat plate and make a double mould in one shot off of that.
Using the Epoxy Surface Coat about 2/3 the way down this page-

http://www.fibreglast.com/showproduc...0Resins-5.html

to make it.
Them lay up the cone halves;
Outer surface and third layer will be some twill that I have; do not remember the weight right off the top of my head.
Middle layer will be unidirectional CF oriented along the length of the cone.

I haven't used this method myself so will probably make a fiberglass version or two before trying the CF; give myself some less expensive practice first.

Thanks guys.

Greg

[elgamiller]

just by the nature of the nose cone it would need to be turned at the far end without support (cut free of the tailstock).
The guys I know that use a CNC turning center are a little hesitant to try that.

why not have a false centre on the end for the roughing out op, then hold on the od to machine the ogive.

Paddy

[sharrestom]

That would be a lot easier, if I understand the problem. If that could be done, I might be able to knock them out of 6061-t6 bar for you on my Haas.

I would have a problem machining that sharp transition from the cone to the plug, and I'm assuming that you would vacuum bag the carbon fiber.

Let me know.

You might consider a female split mold for the external shape, and a silicone plug, which has a fairly high rate of thermal expansion, to force the wet layup into the nosecone shape. You would have to create the wet layup on the silicone plug with an absorbent layer and peel ply to make this work, similar to a vacuum bag procedure.

You would need a second mold to make the silicone plug.

tom

[11bravo]

why not have a false centre on the end for the roughing out op, then hold on the od to machine the ogive.

Paddy

Sorry, I have to plead ignorance here; I don't know what you are saying.

[11bravo]

That would be a lot easier, if I understand the problem. If that could be done, I might be able to knock them out of 6061-t6 bar for you on my Haas.

I would have a problem machining that sharp transition from the cone to the plug, and I'm assuming that you would vacuum bag the carbon fiber.

Let me know.

You might consider a female split mold for the external shape, and a silicone plug, which has a fairly high rate of thermal expansion, to force the wet layup into the nosecone shape. You would have to create the wet layup on the silicone plug with an absorbent layer and peel ply to make this work, similar to a vacuum bag procedure.

You would need a second mold to make the silicone plug.

tom

Tom-
If I understand correctly, you are suggesting taking a block of aluminum and machining a nose cone shaped hole into it and using that as the female mould.
I guess that would work.
Off the top of my head I don't see any reason it wouldn't anyway.

I'm not sure where it is that you are concerned about the sharp transition.

[garfieldsimons]

You have equation which means for every x you have a y. All you need is a tight metal working lathe and just dial it in. Move out y then move in x. Add step to y and repeat with new x and y. The smaller the step the finer the finish. It will take you a while, but is no big deal. Light sand to clear off the fine steps and you are done.
If you were to cut the profile on a template, any competent wood turner could make it faster than any cnc. This is not a high tolerance part. It is a just a part to minimize air resistance. I would wager that with a couple of tries, you could make it with a belt sander that would have no functional difference from one made down to the tenths.
That said, you could also just use a solid model (say solidworks) and any CNC mill could make it.
Regards,
Richard

[Rico55]

What you were originally asking for is relatively straight-forward; a 3d carving of half of the nose cone cut out of wood. This can be done on a 3-axis router, with a little clean up of tool marks afterward. There may be a very slight radius at the transition step, but that could be cleaned up by hand.
Contact me if this is what you want. I will cut this out of clear Mahogany for stability. Once I have the model in the computer, I can reverse the cut to make a mold pocket that could be coated with penetrating epoxy for release purposes.

[sharrestom]

I'm not sure where it is that you are concerned about the sharp transition.

I would be using ballend mills, .125 being about the smallest, to machine the part, so I can't get that sharp transition in where the nosecone mates with the rocket body: it would be a .063 fillet.

It could be added in as a secondary operation (clamping the mold in a vise and interpolating a keyseat cutter to create the sharp edge.

The main issue is whether is would be practical to build your laminate around the silicone plug. It might be quite tedious, and difficult, but it would give a completely seamless nosecone.

[11bravo]

You have equation which means for every x you have a y. All you need is a tight metal working lathe and just dial it in. Move out y then move in x. Add step to y and repeat with new x and y. The smaller the step the finer the finish. It will take you a while, but is no big deal. Light sand to clear off the fine steps and you are done.
If you were to cut the profile on a template, any competent wood turner could make it faster than any cnc. This is not a high tolerance part. It is a just a part to minimize air resistance. I would wager that with a couple of tries, you could make it with a belt sander that would have no functional difference from one made down to the tenths.
That said, you could also just use a solid model (say solidworks) and any CNC mill could make it.
Regards,
Richard

Actually, this is a high tolerance part.
OK, in reality, I probably will not notice the difference between a conical cone, an ogive, a parabolic or an elliptical, but this is what is known as a Von Kármán Ogive and gives the least drag for diameter and length of just about any nose cone shape.
Might seem a little obsessive to worry about getting it as perfect as possible, but rocket guys (most don't call ourselves rocketeers) get just as obsessive about our rockets as tool guys get about their tools.
To do it by hand on a metal lathe would be 11,936 X's and 1,194 different Y's.

I am working on modeling it in Alibre Design Express, but I need Excel 2000 or later and I have '97 (looking hard at upgrading)

I did manage to get the profile into Esprit but that is at the community college that has the CNC turning center and guys that are reluctant to do this part.

Thanks for putting the time in to think about it though,
Not intending to sound snotty,

Greg

[11bravo]

Rico and Tom-
OK, I see where you both talking about now.
Not a problem; I can deal with that.
If either one of you are up to giving it a try, let me know how much you'd need and we can go from there.

THANKS!!!

Greg