My second fret slot cutting jig is complete.  My driver for building this jig is that I am manually challenged to cut vertical fret slots. This jig will ensure my fret slots are vertical. This second version has some helpful enhancements.

Rather than have a depth stop on the fret saw, this capability is built into the jig. This more easily accommodates a supply of varying thickness fret boards. More later on this. 

There is hopefully sufficient info here to produce and build your own design. As always, please do feel free to ask questions or suggest improvements. Apologies for the excessive spacing in this post - I have tried but could not get it any tighter. It seems to display better on mobile devices..

Here is a high level view of the finished jig.

Here is a view with a fret saw and fret board in place.

Inexpensive toggle clamps securely hold the board in place and allow one to focus on the sawing.

THE BASIC STRUCTURE

To start, there are five parts to the basic structure:

- 2 sides both the same height

- the jig base; the sides must be parallel to each other

- the top piece

- the support base (plywood or hardwood is recommended, not particle board which may warp)

Hardwood is recommended. If you are screwing anything together, hardwood will not have the give that softwood or plywood may have, which may upset the accuracy of the jig.

A sixth part, the stop piece, clamps down on the shims that set the depth of the cut. Wing nuts hold down the stop piece.

Screw or glue the sides to the jig base.

Drill the peep hole (optional) in the top piece.

If the depth stop is being installed, drill the holes in the top piece now. And to ensure the bolts can be installed afterwards to hold down the clamp, drill holes in the jig base large enough to accommodate the bolt head to allow the bolts to be fed into the top piece from the bottom of the jig base.

Screw or glue the top piece to the sides. You have completed the basic structure.

The next step is Very Critical Step Number One. Using a table saw or chop saw, cut the assembly in half with the cut at a perfect right angle (90 degrees) to the sides and base of the jig. 

SCREWING THE JIG TO THE SUPPORT BASE

For the depth stop, install the two bolts now before you screw the jig base to the support base. If not done yet, drill holes in the jig base large enough to accommodate the bolt head and feed the bolts into the top piece. These bolts are needed to clamp down the stop piece.

Use four screws for each side. Place the screws towards the edge of each side. Ensure the heads are below the top surface of the jig base.

First, align and screw the left side (or right) to the supporting base. Easy enough.

Second, align and screw in the other side. This requires patience and accuracy. This is Very Critical Step Number Two.

Fold a piece of paper over the teeth of the fret saw. Bring the remaining side over to the first and place the blade between the two sides. The saw must be snug, not floppy, but not tight. The paper allows clearance so you can fit and remove the saw easily during operation once the second side is screwed in place.

I used screws to temporarily tack the right side in place until I got a fit I was happy with. It took a few shots. I then drilled pilot holes into the jig base and supporting base and screwed in the permanent screws. 

HOW DEEP A SLOT DOES THE FRET SAW NEED TO CUT?

This photo shows the stop piece which clamps down on the shims. The height of the top surface of the stop piece establishes the depth of cut.

An adjustable stop allows varying depth of cut. If your fretboards vary in thickness, this is very handy.  I have access to scrap mahogany and ash boards which are  very close to standard fret board thickness. What I don't have is access to a planer to ensure they will all be exactly the same thickness. And occasionally with purchased fret boards, again they may be a different thickness.

If all of your fret boards are and will be identical thickness, screw a permanent depth stop of the required thickness in place to get the slot depth that is needed.

This photo shows how the stop piece does its job.

The position of the stop is set by adding veneer shims to ensure the fret slot depth will be exactly what is needed. One side is sufficient; the frame of the jig will guide the blade and keep the blade vertical.

The prototype model used screws rather than wing nuts to hold down the stop piece and the shims. This proved a bit of a nuisance and this solution should be easier to deal with.

THE PEEP HOLE

The Peep Hole was added to help align the cut line with the slot in the jig. It's doable without the peep hole but this allows a bit more light on the area. 

WHAT CAN YOU USE FOR SHIMS

Veneer is an obvious choice. It is about 0.6 mm, 1/32 inches. 

I have a supply of tongue depressors which are about 2mm, 5/64 inches thick. 

Stack the shims as needed. To fine tune the thickness, layer masking tape or painter's tape on the shim. This will increase the thickness in small amounts to get the thickness you need.

SECURING THE JIG WHILE CUTTING

There is a temptation to clamp the jig down to the workbench to secure it. Depending on where the clamps are placed, this may cause the slot which guides the blade to spread open. It is better to secure the support base to a work surface using bench dogs. My simplistic solution, as I have a very old work surface for a bench, was to drill two holes through the support base into the work bench, and drop in a 2 inch common nail to prevent the support base from sliding around. 

I hope this has been helpful. Please feel free to ask any questions or offer improvements.