Carbon fiber reinforcement in neck questions.

[mcgeorgerl]

Having some difficulty adapting my existing jigs to the shorter mandolin neck.

I asked a friend at work where he gets his carbon fiber since he uses it building RC aircraft. When I explained what I wanted to use for, he said "Give me an hour."

He came back with several busted "Robot Fingers" made of carbon fiber with an applied urethane gripper. The fingers aren't broken but the applied urethane is worn out, that's why they are sometimes discarded.

I took the fingers and machined away the bolt holes and gripper. I can get a carbon fiber bar that measures nominally 9 3/4 x 3/4 x .200. Reducing it futher in thickness should be no problem other than this time I'll need to take some protective measures... my hands and forearms still itch like crazy!

So the questions are:

1. What typical sizes are you guys using?

2. If one is good, is two better?

3. Do you extend the carbon fiber bar past the nut? I figure my neck will have a volute.

4. What adhesives are you using? I usually use Smith's for instruments but haven't checked yet if my version works with carbon fiber.

5. How tight/loose of fit should I shoot for before I apply the adhesive?

Are there any threads that anyone can recommed?

[Tavy]

1. What typical sizes are you guys using?

I use 12x2mm, which seems to be plenty strong enough for a mandolin. Having the bar thicker won't hurt (other than it may complicate construction), but it's depth that matters. If yours is 3/4" deep I would worry that it's too deep and won't leave enough room for wood.


2. If one is good, is two better?

Not especially, one is enough to do the job...


3. Do you extend the carbon fiber bar past the nut? I figure my neck will have a volute.

I've tried both, but I figure that making the bar as long as possible can't hurt, so yes to that one. The area where the bar runs out of the headstock gets covered by an overlay anyway so there's no cosmetic issue.


4. What adhesives are you using? I usually use Smith's for instruments but haven't checked yet if my version works with carbon fiber.

For carbon fibre I use either epoxy or Gorrilla glue. Either way roughen the CF surface for better adhesion before gluing.


5. How tight/loose of fit should I shoot for before I apply the adhesive

Tight: if the bar is a good push fit into the slot, then the slot itself acts as the clamp while gluing. Less viscous glues may be an advantage here - you can poor some glue in the bottom of the slot and then squeeze/clamp the bar in and the glue gets pushed up around the bar - you should see it squeeze out the top or else you didn't use enough ;)

[mcgeorgerl]

Tavy,

Thanks for the response. I think I'll reduce the thickness of the carbon fiber bar down to 3/16" in width and set it 1/2" deep in the neck. I'll try the Smith's epoxy that I have on hand, it should work. As a rule, I try to avoid the polyurethane glues since I haven't been overly impressed by their adhesion. The neck is going to be wider at the nut than normal so I'm going to have more wood to act as reinforcement too. I figuring that plus the carbon fiber should make for a stiff neck.

I'll use 1 bar instead of two. I did a little research online and found a few references to concerns about expansion/contraction of wood around carbon fiber. I've never heard of anyone having a neck crack/split though. Nor have I seen splits caused by steel, aluminum or stainless reinforcements so the concern may not have much merit.

[slausonm]

Somebody on the site mentioned a while back that http://www.dragonplate.com/ was a good place to buy it at a reasonable price. If you search through their site you will find the sizes they list for musical instruments. Ironically one of my former students used them for a project not related to instruments. They happen to be about 6 miles from my home. They even have a recommended glue/resin for their product.

Matt

[mcgeorgerl]

Actually, dragonplate is the name I couldn't recall that the guy at work usually gets his carbon fiber material. He usually needs thin stuff to laminate between balsa but said they have carbon fiber and supplies in many forms.

Thanks!

[Lefty Luthier]

Dragon Plate is the trade name used by Allred Industires for their carbon fiber materials. They have a 0.5 x 0.125 rod that is just right for mandolin neck reinforcement. I have been using it for more than 10 years with excellent results. One caution is to carefully clean the surfaces of the CF with acetone before installing. I use structural epoxy by Locktite, which seems to work very well.

[kkmm]

To install the CF bar, I understand that the fretboard has to be removed, then the neck grooved so the CF bar fit in it, and flushed with the neck surface. Does the CF bar lay flat under the fretboard or vertical to the fretboard ?
I also heard about the T bar, that's easier to figure.

[Audio Artillery]

TDoes the CF bar lay flat under the fretboard or vertical to the fretboard ?


Perpendicular to the fretboard since that's the orientation that is stiffest.

[kkmm]

Audio Artillery,
Thanks for the clarification. I imagine that the width of the bar is not large otherwise it requires the groove to be cut as deep. A cut too deep will split the neck in two.
And what are the tools to use to cut the CF bar to to desired dimension / thickness / width ?

[Grommet]

It is highly recommended to purchase your cf bar stock at an acceptable final width and thickness to avoid having to reduce it those dimmensions, avoiding hazardous dust. The OP's .20 thickness should be just fine. Also use a proper mask/filter, hand and eye protection if you must cut it to length. The resulting dust and splinters are quite nasty. I would recommend setting it below the finished level of the neck and using a maple filler strip to avoid having to plane, scrape or sand the cf level with the neck. It should make for a light and rock solid neck!

Scott Crownover

[mcgeorgerl]

I managed to get the carbon fiber bar set in last weekend and I hope to do some shaping of the neck Saturday.

I milled it down to .200 in the Bridgeport at work using standard high speed cutters with no issues other then the mess. I don't recommend this!!! The fibers are a literal pain and the resin(?) winds up looking like powdered graphite which transfers to everything. You know those 4th of July "Snake" pellets you put a match to and they grow? That's sort of what happens around the cutter as you take a pass. Again the only reason I took this route is these bars are being "re-purposed" since they wind up getting tossed out in the trash.

A slot was then milled in the prepared neck blank. Used a 3/16" cutter that I modified for deeper cut (standard 3/16" milling cutters usually have a LOC of only 7/16"). Kept taking out a few thousandths off each side untill the bar was just a slip fit. I used the entire length of the bar which will wind up extending about 1" past the nut. The overlay will cover everything up.

Since this neck will be a bit wider and chunkier, I decided to risk setting the bar in deeper so I went a full .600 deep. I may pay for that in the future.

I used my handy Smith's epoxy for oily woods. Since the bar had been machined on both sides, it needed no roughing up. I used the extra epoxy to glue a piece of maple to a second bar that I have. I'll do some unscientific destructive testing to verify the bond but everything I've tried so far has not popped the two apart. Smith's is not your typical 5 minute epoxy and it's used to build boats. The only drawback is that it comes in cans (2 parts) and measuring small amounts is difficult.

After the epoxy had cured for a couple of days, it was back to the Bridgeport to machine away the excess carbon fiber bar and epoxy. Then back home to the sanding board (unused table saw cast iron wing with sandpaper glued on) to flatten everything. This quickly contaminated all of the sandpaper due to the exposed strip of carbon fiber. Very messy and not recommended.

If I were to do this again, I'd definitely bury a 1/2" or less wide bar and cap it at the same time it got epoxied into the neck as recommended in the above post. That way the only exposed carbon fiber would be at the head if the bar exteded past the nut. I'd probably try to wick some CA on that to seal that bit.

Once I get the neck roughed out, I'll post a photo of the progress.

[Wes Brandt]

I would emphasize... be very, very, very, careful not to breath the dust. You really should buy what you want in pre-made dimensions and install it with a wooden overlay for leveling.

I've been watching fellow luthiers dropping like flies with various cancers the over the last 10-15 years and carbon fibers like this could, I fear, have similar effects to asbestos fibers, a known lung cancer causer.

[mcgeorgerl]

Yes, I should say that I usually use a good dust filter mask for most of the stuff I do in the shop, both at work and home. Since I have no decent dust control in the home shop, a well fitting mask (dual filter) is a must for just about any sawing or sanding. I positively hate working with MDF for jigs and patterns. Even with a well place shop vac, you can still see the dust in the air. We also work with other material that are made with resins and fibers; Micarta and Garolite. We use a ton of aluminum also and I still have, in the back of my mind, the early suggestions that aluminum may be connected with Alzheimers.

It's wonder that some of us made it this far. When I was a kid we used to have the wipe the dust off the desks that fell from the ceiling. Later we learned that it was from the asbestos ceiling tiles. Up until about 1985 I used to re-fit dies made from beryllium copper. We'd have that stuff all over us as we we grinding. Then we found out how unbelievably hazardous that stuff was.