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Compression Fretting 101

On the older Martin guitars and guitars that use a non-adjustable truss rod, you may need to adjust the relief of the neck. Since there isnít a nut to turn you have to use compression of the fret board to adjust the relief.

I use a variety of tang widths to push against the slot of the fretboard, thus the term compression fretting. What I do and how I do it often depends on the individual neck. In this case the neck also need to be refretted so here is the scenario. This guitar was a D18 and I was doing a restoration. I pulled the frets using a soldering iron. Here are the tools I used for this particular job.

Fretting Hammer (Cobblers Chasing Hammer), File Dressing Tool, Knife File, Caliper, Nipper, Small Nipper

Fretting hammer (cobblers chasing hammer) File dressing tool, Knife File, Caliper, Nipper, Small Nipper and Machinist Parallel

Machinist Parallel

This shows me the neck deflection. My target will be to get about .007 to .009 fall off from the 7th fret to the nut when I install the frets. I plane the board and get it as true to flat as I can and to a 16-inch radius. I will also use fret wire of differing tang thickness so I can control the compression of the neck and thus the fall off. I use tang thickness of .0185 to .0275.


I start by measuring the fret slot with a feeler gauge so I can now work with the tang that I need. I also like to use Fish Glue when installing the fretwire. I work in some glue in the slot and then with the hammer tap the wire into the slot. So that I can get an idea of what is happening with the neck and the reaction of the tangs I start with about .002 wider than I measured. If I measure .022 I try a .0235 or .0245. The lower down the neck, and the thicker the neck , the less effect of the compression. I like to start at frets #4, #6, #8 and #10. I rough cut the length of the fret wire to the area of the board, apply glue to the slot and tap in working from on side to the other.

Nipping the Fret

Here I applied the first fret and nipped off the ends. Then once I have a few frets into the board I can see what the movement is. I use the heavy parallel for this and a feel gauge. I have feeler gauges from .0005 to .030. At this point I want to see some movement. I will put a fret into #1 that same as the slot width, this is my point of measurement.

Measure Neck Relief Measure Neck Relief

Once I can establish control of back bow I continue the compression. I work to get the back bow as controlled as possible from #7 to the 1st fret for that .007 to .009 fall off. Once the frets are installed I will dress off the ends with a dressing block. Here I use 30 degrees and a 2nd cut smooth mill file.

Fret File Angle 2nd Fret File Angle

On the fret board extension I use a tang width just under the slot width and rely on the barbs and glue to hold the frets down. I donít want a great deal of bow in this area. Once fretted I will install the neck and apply some weight to the neck to simulate a string load (I use 10 lbs). I then dress the frets and I am looking for something pretty flat. This will get me about a .004 relief on the neck with light strings.

The most important thing is to have a touch of back bow when you are done and no string load on the neck. Once the neck is ready I set up the nut. Here I want the bottom of the nut slot to be .007 to .010 above the plane of the frets.

Once I have the guitar strung up and under tension I will then fine tune the nut slots. I like to see just a touch of clearance off the 1st fret when the 3rd fret is pressed. This sets the string off the 2nd fret and nut so I can see the fret plane and slot relationship. I donít want buzzing on the 1st fret and fine that about .007 is about right.

Take your time, as compression fretting is an art and you have to work with the strength of the wood of the neck. This is often individual to each guitar as the neck shape glue all play a part in how the neck will flex. These old guitars need to be fretted in this manner. Most non-adjustable rods that I have seen from Martin are the Tee bar, the Ebony reinforcement and square tube. The Tee Bar is the stiffest and is my favorite. Ebony rods can be tricky as they have more flex than the steel tubes. Good luck and don't let this process scare you. Once you do a few, you will learn the cause effect relationship of the compression fretting technique.

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