Looking for advice before I CNC a mold to lay-up a fiberglass nosecone

jlabrasca

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I've watched John Coker's tutorial on fabricating a FG nosecone -- in which he first made a FG negative over a turned form. It occurs to me that it should be possible to mill a mold from machinable wax, tooling board, or something similar.

I have a Shapeoko XL, with nominal 3" of z-travel. The added clearance required by the endmill means I'd have just a little more than 2" of cutting depth -- if I am clever about how I program the cuts. So I should be able cut molds for cones to fit 3 inch -- maybe 4 inch -- body tubes.

Fiberglass is a new material for me. I am reading up, but I am sure I am going to miss something critical. Anybody have any experience doing something like this? Any advice on how I should proceed and/or where I should be looking for information?

L2nosconemold.png
 

Reinhard

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Looks like an interesting project. Seeing the 4 holes on the corners, I'd suggest adding some extra ones. You will need some for alignment pins and some for bolts to keep it closed.

Another thing is to consider how you will open the mold after the nose cone has cured. Depending on some factors, like surface finish and mold release treatment, the halves might not come apart easily. Keeping this in mind while designing the mold will help a lot when working with it later. For example, you can add some steps or a chamfer on the circumference of the mating surfaces. Then you can drive a wedge or a similar tool in between the halves.

Reinhard
 

rharshberger

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Finish smoothness of the form is super important, it will ease removal of the mold and will require less release agents, and less work on the finished casting. Any imperfection in the mold will transfer to the part. I have spent many hours buffing molds in my fathers fiberglass shop where he made bathtubs and cultured marble sinks and slabs. Part of the reason for starting with a turned form is to eliminate as many imperfections as possible via sanding, priming, painting and waxing, a negative such as what you are desiring to make is much harder to surface prep in part because its two separate pieces so any material removed from the common edges needs to be identical or the parting line will be larger and harder to remove on the finished part. Other than those reasons and some that others might list, the negative will work, its just how hard is it going to be to make work easily.
 

jlabrasca

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Finish smoothness of the form is super important, it will ease removal of the mold and will require less release agents, and less work on the finished casting. Any imperfection in the mold will transfer to the part. I have spent many hours buffing molds in my fathers fiberglass shop where he made bathtubs and cultured marble sinks and slabs. Part of the reason for starting with a turned form is to eliminate as many imperfections as possible via sanding, priming, painting and waxing, a negative such as what you are desiring to make is much harder to surface prep in part because its two separate pieces so any material removed from the common edges needs to be identical or the parting line will be larger and harder to remove on the finished part. Other than those reasons and some that others might list, the negative will work, its just how hard is it going to be to make work easily.

I got a couple blocks of wax from Carbide3D; I can get a pretty smooth surface by setting a very small step-over. A fast pass with a propane torch removes the tool marks completely -- but it is too easy to melt the edges. I am thinking I could machine the mold with some excess above the radius of the cone. Smooth the interior with the torch, then use a facing mill to take it down to finish height to create a crisp edge at the mating surfaces for the mold halves. This reduces the z-margin for the mold, though and I'll have to think of a way to preserve the sharp edge at the shoulder against melting.

L2nosconemold2.png

Large precast wax blocks are expensive (although not nearly as expensive as Renshape tooling board). I found an Instructable on making machinable wax. I'll pick up some paraffin this weekend and give it a try. This it adds a layer of complexity to the project though -- so I've also ordered some bulk wax from these guys

I also need to think about how I will cut the excess FG at the edge of the mold before mating the halves -- when the FG gets to the "leather stage". The wax I have won't stand up to an Xacto. I have flush cutting shears that might work...I'll have to make up some FG pieces and test them.
 
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