Truss rod slots: curved vs. straight

[Jason Rodgers]

Mark Wybierala's thread on "Anyone want any LP info?" http://www.mimf.com/phpbb/viewtopic.php?f=4&t=3259 began to over-focus on the type of truss rod slot used. We are moving that tangential discussion here... go!

[Peter Wilcox]

I'll bite. I've only made 15 necks, so I am just a novice.

I've never done a straight compression type rod at the back of the neck. I can see that it would work, offsetting the torque of the strings above the neck, but it seems you might need a lot of torque tightening it down, which may not be very easy on the threads or the nut.

I've made a couple of double straight rods per Cumpiano. They are only single action, and seem to work well but need a straight channel 1/2" deep and weigh more than a single 3/16" rod.

I've made a bunch of Fender style 3/16" curved rods, most that adjust at the headstock and a couple that adjust at the heel, most from the back with a skunk stripe and a couple from the front under the fretboard. They work well too - more so the ones that adjust at the headstock, that seem to apply more force at the thin part of the neck than the others (and easier than removing the neck to adjust.) Fender specs for the Strat neck call for a radius of curvature of the channel of 255" from the heel to the midpoint, and 115" from the midpoint to the headstock. I assume this is to put more force on the thin/narrow part of the neck which is more likely to deform from string force. My problem with these is being able to drill a hole at the proper angle from the headstock to meet the truss rod channel correctly. There are jigs that can be made for this, but I haven't.

My last two necks have double action rods (from China) that work well. They are heavier and need a deeper channel, but it is a straight shot from the headstock into the channel. I will probably continue to use these, as I like the potential for correcting back bow in case I screw up. Just my 2 cents.

[David King]

I spent some time figuring out the single curved rod. It took me a while to find a 3/16" dia ball nose cutter with enough length to make a deep enough slot. That cutter is 4 flute so it bogs down if you don't have vacuum to clear the chips. The 3/16" diameter requires a 3/16" collet for the router so I just used it in the milling machine at low speeds. I made a contraption that clamps to the quill and rides on a curved strip of wood taped alongside the slot. It was slow and cumbersome to use so I opted fore a 3/16" wide table saw blade that can rise 1.5" above the table. I tape the same curved strips to either side of then neck and tape small blocks to the table on either side of the blade. The curved strips are concave and ride on the blocks. It will make a full depth slot in about 10 seconds.

[Dick Hutchings]

I spent some time figuring out the single curved rod. It took me a while to find a 3/16" dia ball nose cutter with enough length to make a deep enough slot. That cutter is 4 flute so it bogs down if you don't have vacuum to clear the chips. The 3/16" diameter requires a 3/16" collet for the router so I just used it in the milling machine at low speeds. I made a contraption that clamps to the quill and rides on a curved strip of wood taped alongside the slot. It was slow and cumbersome to use so I opted fore a 3/16" wide table saw blade that can rise 1.5" above the table. I tape the same curved strips to either side of then neck and tape small blocks to the table on either side of the blade. The curved strips are concave and ride on the blocks. It will make a full depth slot in about 10 seconds.

Nice idea Dave. I'll have to try that one on my next neck.

[Barry Daniels]

I use a 1/4" ball nose router bit in a router table. My 3/16" dia rod gets heat shink tubing that takes it up to a 1/4" dia. I apply a couple of 1/8" thick strips of wood to the router table adajacent to the router bit to make a small riser, and use brown masking tape to build up a small ski ramp on the neck to make it rise up as it approaches the nut end.

Routing the slot with the neck in an upside down position automatically clears the chips fairly well.

[Jason Rodgers]

Bingo! Thanks, folks!

Now, to go along with this, what kind of metal are people making these rods from, and what sorts of hardware? Is the mild steel from the rods and bars bin at the Depot strong enough?

[Jason Rodgers]

This link that Mark posted http://theaccidentalluthier.blogspot.co ... s-rod.html has a really cool solution to cutting the curved slot. No need to guesstimate little shims or ramps with this bearing guide!

[David King]

Jason,
That router winged slotting blade was an idea I tried but I could never get it high enough for anything but a narrow neck so I abandoned the idea. I really didn't like the unsupported outboard end of the shaft spinning at 18,000 rpm with no anti-kickback provisions in the blade. The first time I tried it something happened and I took a chip out of one of the carbide wings. That kind of spoiled the fun for me.
My 6" x 3/16 carbide slotting blade in the TS now has a bevel grind so I end up closer to a round bottomed slot. The blades aren't cheap but they are an off the shelf item. Having the bevels ground cost me an extra $5. Blade itself was about $120 w shipping. I find I use it for a lot of other things besides TR slots.

[Barry Daniels]

Mild steel is more than strong enough. But I use stainless steel just for grins.

[David King]

Hot rolled steel with a 10-32 thread on the end has a tensile strength of about 720lb. Under normal circumstances that should take care of it. Interestingly a larger 7/32" diameter rod with 12-24 threads has the same tensile strength. If you really want tensile strength then you can go with music wire which bumps you up to about 4820 lb but it's much too hard to thread until you anneal it and then you've lost most of that strength. A good compromise is either 304 stainless (2000 lb) or annealed O1 drill rod at 2800lb.

[Jason Rodgers]

Jason,
That router winged slotting blade was an idea I tried but I could never get it high enough for anything but a narrow neck so I abandoned the idea. I really didn't like the unsupported outboard end of the shaft spinning at 18,000 rpm with no anti-kickback provisions in the blade. The first time I tried it something happened and I took a chip out of one of the carbide wings. That kind of spoiled the fun for me.

Couldn't you use one of these in a pin-type setup in your mill?

[David King]

Not sure I follow your meaning but it was in the mill that I first tried it and where it blew up on me. There is a horizontal mill conversion kit for the bridgeport but it's an expensive accessory if you never use it. It wouldn't really help you here without an way to control the Z axis repeatably.

It might work in a pin router.

[Jason Rodgers]

Yes, I meant doing a pin-type thing on the mill, but maybe it doesn't work that way. No more work flying into the wall!

[Michael Lewis]

Cold rolled steel rod is usually smoother and closer to shape and tolerance than the hot rolled stuff. Stronger too.

[Clay Schaeffer]

Another way to make a curved bottom channel for a truss rod is to cut curved pieces off both ends of the filler strip. Glue these into the bottom of the straight slot and you will have a filler strip with the matching curve cut into it.
I make my rods from the stock I buy at the hardware store. I anchor the end so tightening the rod pushes the anchor end down into the neck rather than up into the fretboard (and also epoxy the anchor end in place). I buy the brass nuts from Stewmac so the nut is softer than the rod.

[David King]

Truss rods get stripped when the nut's threads are cut too loose. This has been a consistent problem with all the steel truss rod nuts I've gotten from Fender and may of the other suppliers. The tolerance these companies run is terrible and it's amazing they don't strip out more consistently. I've run into many early 1990's Fender 5 string J basses where the factory had put 12-28 threaded nuts onto 10-32 threaded rods. When I talked to someone there they said it was "impossible" because they've been getting the nuts made by the same company for 26 years. "Oh really?"

[Barry Daniels]

I really like the idea of using brass nuts which will take most of the wear since they are so much softer than the steel rod. And they can easily be replaced when they get worn.

[Mark Swanson]

That makes a lot of sense!

[Bryan Bear]

I've done exactly as clay describes with the curved spline glued into the bottom of the channel. It seems to work okay but I would prefer to not have to glue in wood under the rod. I like using the table saw to cut the slot thoug. David, do you have any pictures of your setup?

+1 on the brass nuts from stewmac. I looked all around town for a local source for a suitable brass nut. Finally, I realized how cheap the stew Mac ones are.

[Mark Wybierala]

I'm communicating w/ MLCS to acquire the right bit/arbor combination to try this out per the accidentalluthier link. The simplicity is just very attractive and I'll come up with a method to make multiple passes and reduce the potential violence if something goes wrong. MLCS has made it very clear to me that 10,000 rpm is the absolute maximum that a 1 7/8 diameter 3-wing bit should be pushed. I like that the curvature template can be easily modified and multiple templates (different curve shapes and depths) are easy to fabricate. I'd like to employ a stationary guide rather than use a bearing. I have a source for 3/8" lexan sheet and that will be the sled.