Saturday, 13 September 2014

F style Eastman mandolin needs a new truss rod

The addition of a pin to secure the rod to the anchor was it's undoing. This is exactly where it broke. It's a compression style truss rod meaning it pulls against the back of the neck rather than up against a curved filet. The  fingerboard had to come off and a new truss rod needed to be made, the brass nut could be kept.
After careful removal of the fingerboard with heat and a palette knife. I used a chisel to remove the filet piece.
The truss rod broke at the very end where the pin had been installed. It's possible that the truss rod would have been better off without the pin, it was that little bit more that weakened the end. I understand the temptation to put it in. Peening the end doesn't seem like enough. However when you keep in mind that the real torque is in the tightening of the truss rod and that when loosened it shouldn't stress that connection nearly as much.
The bass I did recently came loose because the threading in the anchor stripped. In that case I recut the thread further up the rod and brazed it to the anchor. I'm happy with how that worked out. The bass has a curved filet truss rod and it's longer of course so it's a different situation.
I made a new 3/16" truss rod and 3/8" anchor and peened the end just like the traditional Gibson approach. I stuck with the compression style and put in a straight filet. With a mandolin neck being so short I think compression actually works better IMHO. However I also think it's a good idea to loosen the strings when adjusting a shorter neck. In some cases like when the action of the nut seems to tight, it's worth taking the time to clamp the neck straight and then adjust the truss rod.
 I cut the threads on the rod at both ends with a 10-32 die cutter,  and then fit the anchor to the short threaded end after sizing and tapping it to fit.
After rough cutting a piece of maple for the filet I used my belt sander with a thicknessing table to get it to final dimension. I glued it into place and let it dry over night before joining the fingerboard and the neck. The nut was used to index the neck to length, surgical tubing was used to clamp them together. I like that it wants to center the fingerboard on the neck. A couple of cam clamps held the end down. There was virtually no touch up involved when the clamps came off, just some glue to clean.
In the end it set up nicely and reminded me that I used to enjoy playing mandolin. I should get one or make one.

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