Carbon Fibre banjo

[Dave Sayers]

Hello everybody, I am new to this forum and I hope you all had a good Christmas and have a Happy New Year.

My question is really related to the fact that I got a very nice banjo in a charity shop. I'm enjoying learning to play it now that it is in tune. I like to have the occasional challenge, and having looked my banjo over I'm thinking of building one with a carbon fibre pot, if that's what it's called. It's a simple enough shape.

Does anyone here have any experience using carbon fibre? I know there can be some serious health issues, so any advice will be gratefully received. I am anticipating making the pot mould and laying the fibre mat and doing all the glueing. I will be looking to put a high shine on the outside. The first one will be difficult, but I expect to be able to use the mould again and again. I'm interested in the tools and techniques so I can properly plan before starting or abandon the idea if there are too many difficulties in the way.

I'll almost certainly build a more traditional banjo some time. Incidentally, as far as I could make out the banjo which I got very cheap was in the charity shop because the bridge was way out of place, so it was never in tune. I suspect the original owner thought it would come with the bridge in exactly the right place.

Thanking you in advance for any advice.

[Mario Proulx]

I'm enjoying learning to play it now that it is in tune.

Wait right there!!!! You got a banjo in tune?!?

[Bob Menzel]

It's real easy if you use wire cutters.

Dave: have you done any composite work before?

[Dave Sayers]

Firstly I shall answer Mario. Yes I believe I have my banjo in tune, and playing the harmonic and then the note at the twelfth fret the tuner seems to agree. It was out of tune when I got it because the bridge was in the middle of the skin and nowhere near the correct position. So I tightened up the skin and moved the bridge around according to some online instructions. I also replaced the friction fifth tuner with a geared one. It now sounds pretty good, and installation of pickups is to follow when they arrive. If I have encountered any difficulties it is that being a guitar finger picking player I have to have a break between using either instrument at the moment. I also believe that whoever invented banjo picks had never heard of amplification, as they are clearly the design of an evil demon.

Bob, it's best if you consider me to be a cross between a hippopotamus and a know nothing when it comes to composite work. Assume I am a Hipnoramus and then nothing will be missed out. I will say that when it comes to dangerous or skin irritating chemicals I do not cut corners or compromise on safety. I want to make an honest appraisal of the difficulties of working with carbon fibre, including cutting, shaping and joining. Luckily I do not have to use my workshop for carbon fibre work. I have a separate brick built hovel garage which I can adapt if necessary, keeping contamination to a minimum. I thank you in advance for any information.

[Michael Lewis]

Is there a reason for using CF other than the novelty of the material? CF is worked in the same manner as fiberglass, with hardening resins (epoxy or polyester) and a guaranteed mess to clean up. Practice (experience) really helps.

I think your strength will come more from your architecture rather than the material.

[Dave Sayers]

There is a very good reason for using CF. If you have any knowledge of cutting the map or preparing moulds information would be gratefully received. Thank you for your interest, Michael.

[Michael Lewis]

Dave, do you have vacuum equipment? Or do you plan to just hand lay the CF? I don't have any direct experience with vacuum forming but it is a respected method of getting stuff into shape and minimizing thickness. Carbon fiber fabric can be sort of stiff, so you have to plan how to hold it in place if there is an angle or other interruption of the surface, and vacuum is good for this.

To make your mold you need to make a "plug" the same shape you want the rim of the pot to be. Make it as perfect as possible and then shine it up. You can use wood or urethane foam as a core for the shape and add Bondo body filler over that. When it is as perfect as you can make it then you wax it with paste wax. When that is dry rub it down and wax it again. This will be the surface you get when you pull the part from the mold down the line, so make it as shiny as you can. Spray PVA mold release on the wax and let it dry. Avoid making any runs or drips as this will show up in the finished mold. Then spray it again. This will help get your part out of the mold as it dissolves in water.

Now you are going to make your mold. Start by applying resin to the plug and laying the fabric on the resin. If laying by hand use a stiff brush and poke the fabric to get the resin to penetrate the fabric. A 1" or 2" cheap bristle brush is fine, and you throw it away after use because the resin hardens and renders the brush unusable. Once the first layer is hardened you can add stiffener material where it is appropriate, this can be wood, urethane foam, or metal. Add another layer of fabric and resin. You have to decide how you want your mold, whether flexible or stiff, but the stiff ones are usually the most precise and hold shape better.

You will need to trim the edges of the mold so you can release the plug. Find the layer of mold release and squirt some water in and blow air if you have a compressor. Add more water, flex it all a bit and add more water until the plug comes out. Carefully inspect the inside surface of the new mold and make any repairs that are necessary, like filling air bubbles, smoothing scratches, etc. Once the mold is declared perfect wax the dickens out of it and spray mold release in it to start the process. Once the inside surface is ready you can apply a "gel coat" or finish coat as the first thing to go into the mold, as this will be the surface you see when the part is removed from the mold. When the gel coat has set up you can add resin and fabric to your heart's content to make up your part. Here is a good application for a vacuum system.

You will have a good long discussion at the place you buy your fabric and resin. They are usually very helpful and can tell you what product goes with what other product, how to mix the stuff, how to handle it, how to clean it up, etc. You need to be ready BEFORE you start mixing anything, so you should mentally go through the process and figure out what you will need and when and where. Nitril or other gloves, solvent for the resin you are using, paper towels, a trash can lined with a garbage bag, cardboard boxes, and lots of fresh air.

This is only touching the surface, I suggest you make a fiberglass rim as a practice exercise, as the materials are less expensive and more readily available, and if you can do a good job with fiberglass your chances of doing well with CF get better as the experience you gain will be tested. Your mold will work for both polyester and epoxy resins. There are many different specific resins so ask at your materials supplier.

First you have to design your pot. Where does it need thickness, strength, shape, etc.

[Dave Sayers]

That's great Michael, and although I have done some glass fibre work I preferred to start this particular project from a point of no experience at all and learning new. You have raise many pertinent points. The mould will probably be two or three piece and held together with cam clamps. The outside does not have to match the inside. At the moment I am thinking of building a roller to apply the mat to the inside of the mould and then use another three piece plug mould inside a spring steel form which expands to hold the carbon mat in place as it dries. I moved an old wood lathe around to the other work area I have I have and that is going to be bench mounted and adapted to operate at low speed for wooden rims anyway. I am quite happy to make several pots and bits and call them test pieces to improve my knowledge.
Reading up on cutting carbon fibre told me I already have a lot of tools that I need and a mini circular saw and carbide blades is on its way even as I write.

Thank you once again to everyone who has contributed.

[Nicholas Blanton]

I've worked a bit with CF, both simple layups and with a vacuum bag. Michael describes pretty well the process of making a mold for something like a kayak or guitar, and if you decide to make a few of them, it's the way to go. I would suggest you might make a positive mold for just the first one, layup fabric around a simple cylinder form. It could be styrofoam, suitably sealed, waxed, and coated with mold release. You don't need to squeeze the layers with a vacuum bag, using a ribbed roller and a body putty spreader to press them down and get rid of bubbles will result in a strong laminate- you'll just use more epoxy than you would if vacuum bagging or pressing it. You'd want a slight taper, or draft, in the cylinder form, so you can get the thing free ( and you have to get it free soon after it sets, but before it becomes rock hard) or you could simply destroy the form to release it, use it once. You could trim it to height with a router jig and carbide router bits, as well as groove it for a tone ring. The outside will have a kind of "organic" finish, but you should at least be able to figure out if you like working with CF, and like CF banjo pots. You may like neither.

If you're just doing the rim, a good way to buy CF might be in the form of tape. Woven CF tape comes in a few different widths, 1 1/2" and 2" seem pretty common, both woven and unidirectional ( unidirectional is maybe not what you'd want to use first). It would save you from having to cut strips from the edge of a sheet, which can be messy.

[Bob Menzel]

+1, but I think the OP is looking for the chatoyance and finished surface that a negative mold will yield (like a Rainsong guitar). Either way (positive/negative molds) the level of quality will only be a good as the mold it is cast from but is much more critical with a negative mold. I've never done one am not sure how one would vacuum bag such a shape (donut shaped bag?). My thinking would lead me to incorporate an inflatable bladder somehow.

[Bob Menzel]

Thinking about this a bit further, I know boat builders bag hulls in negative molds, so it's do-able. Might be a lot of effort for a one-off though.