Router Sled - Fretboard Radius Jig

Discussion in 'Luthier's Corner' started by TerribleTim68, Jan 15, 2022.

  1. Wanted to share this as a separate thread, in case it helps anyone else. I'm in the middle of building a fretboard radius jig for my router, in the hopes that I can radius fretboards much faster than block sanding with a radius block. I've done that, it's a LOT of work!

    Step One, Construction Of The Radius Part -
    I chose to use MDF on as much of this as I could, because it is flat and true, and I can use CA glue to glue pieces together. The first step was to create two "side pieces" with a matching radius on top and a "step" in the bottom for the sled to pass under -

    I taped these together so that I could use various spindle sander, disk sander, etc. to keep them identical.
    Then I cut some "end pieces" to create the "box". All four parts shown here -

    Then we glue them all together, keeping the bottom flat and the sides square and parallel -

    Okay, I think this will work. On to the next part!
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  2. Step Two, Creating The Router Carrier -
    The next thing we need is something for the router to ride on. The first part was to create a base for the router to bolt to. Then, using the offcuts from the radiused top of the above pieces, I cut a couple "rails" for the sides, and then a couple more pieces to act as guides on each side. All five parts shown here -

    The router base has holes drilled to match the bottom of my router, countersunk so my screws will reach the router base. We then start the glue-up, of course using Starbond Thick on all of this -

    After gluing everything together, we have a nice little router base with radius guide rails -

    Okay, on to the sled that this needs to ride on!
    Last edited: Jan 15, 2022
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  3. Step Three, Building The Sled Itself -
    This is a fairly simple part, it's just a big flat bed for everything to ride on. I took a 6" x 36" piece of MDF, marked a centerline on it, then glued it to a 10" x 36" piece of MDF, making sure to keep it all centered. Then we set the router carrier on it and we have something that looks like this -

    Here's a shot with the fretboard blank sitting under it, so you can kind of see where I'm going with this -

    The idea here is to slide the radiused section to a given spot along the sled, then move the router side to side, creating a radius in the fretboard blank. Then scoot the carrier along the length of the sled, move the router side to side again and keep going, much like how you'd use a slab flattening sled, but with a radius on top.
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  4. Here's Where Everything Went Off The Rails -
    So, once I got it all together I figured out the problem. My router won't lower enough to actually reach the fretboard blank -

    So, now I have to figure out how to modify this sled/jig to lower my router and/or raise the fretboard bed. So, it's a great idea in theory. But somewhere I messed up and missed the depth my router needed to actually make it function. :meh: :banghead:
    So I've got some thinking to do and then some modifying to do once I figure out a solution here. Moral of the story - Don't Be Like Tim, He's An Idiot! :mad: :rage:
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  5. Oh, a couple other notes that I somehow forgot to include in the above info -
    1. I built this with a 21" radius, in the hopes that when I get the router bit to fretboard cutting depth it will basically be a 20" radius, roughly. Close enough that when people ask, I'm telling them "It's a 20" radius board".
    2. When I cut the radius pieces, I used a router radius jig that I built out of some scrap plywood. I don't have a picture of mine, but Rockler sells this one, which basically looks like mine but way nicer and laser cut instead of freehand by some long haired hippy dude and a ShopSmith - Router-circle-cutting-jig-.-thumbnail-no-text-1200x676.jpg
    3. The router base rails were literally just the "other side" of the piece I cut, because the radius matches, right?
    So there you go. As soon as I figure out how to fix my mistake on the router depth, I'll post an update. I've got a couple ideas, but I have an equal amount of concerns too. :meh:
  6. I wondered about the bit depth but only because I did the same thing first time around on my router planing jig. I’m sure you’ll get it done. I’m looking forward to seeing your solution.
    Beej and TerribleTim68 like this.
  7. RichterScale


    Feb 21, 2021
    I can't remember all the details, but I feel like I did the same thing. I think that's why I did the aluminum angle and rollers (just 1/4-20 bolts with plastic spacers that came with TV wall mounts). It lets the router sit lower. Maybe this will give some ideas. I also measured out so that the end of the router bit would end up at the right radius.
    Oh, one of the wheels was a hair off. I just wrapped tape around the plastic sleeve until all 4 wheels made contact with the sled.
    Laguna burger build
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  8. That's actually super helpful, thanks. :thumbsup:
    I don't know that I can use aloominoom angles, just because I wouldn't have a way to mount it to my router base. But it does give me some ideas on how I might lower the whole thing. I think I should have glued the radius slides on the outside and higher up. -
    Screenshot 2022-01-15 220010.jpg

    I'll need to cut it all apart, but that was probably gonna happen at this point anyways. It's just CA glue, I can probably just pop it apart with enough pressure.

    Again, thanks @RichterScale. :cool:
  9. Okay, yup, that was the cure. I pulled all the pieces of the router base apart and then re-glued them, moving the guide pieces out on the end of the plywood base, and the radius rails up on the side of the guide pieces. It now looks like this -

    So the router rides much lower, down inside the radius guide block. This puts my bit touching the top of my fretboard blank, with some adjustment left as well -


    Now I will need to make some new pieces for the ends of the radius guide block to glue that back together, but this is gonna work.

    So yes, @RichterScale is some sort of evil genius.
  10. I kind of feel like I need to figure out how to attach everything with screws or something. I'm not convinced CA glue is gonna hold up once I start routing with this thing. So I'm pondering that. But it's hard to screw into 1/2" plywood end grain and make that work. And my brad nailer only uses 5/8" brad nails, so nailing 5/8 through 1/2" doesn't leave much to actually do anything. I might try drilling with my recess bit to remove some material, then brad nail into the recess. Who knows. Lots to ponder while I can't sleep tonight.
    Beej likes this.
  11. 2F/2F


    May 26, 2009
    Los Angeles, CA
    Were you hand planing before radius-block sanding, or doing 100 percent of the work with abrasive paper?

    If the latter, no wonder it took you forever! Sandpaper makes a lousy replacement for a properly sharpened plane.

    I have found fretboards to be quite quick to radius using sanding blocks. The tricks are:

    1. Carve your radius in when the neck is still a blank, i.e. before you have tapered the neck from heel to nut. For separate fretboards, they should already be glued on to the neck shaft before applying the radius.

    2. Hand plane close to the arch first, as mentioned above. Use the sandpaper solely for the finish work.
    Last edited: Jan 16, 2022
    Bent77 likes this.
  12. Flying B

    Flying B

    Apr 29, 2018
    Also you might find you get a nicer finish if you rout longways along the fret board rather than across (if your jig allows this)
    Bloomfield likes this.
  13. dwizum


    Dec 21, 2018
    I had the same depth challenge with mine and solved it similarly to how you solved yours.

    The magic in getting a smooth finish with these is to have everything tight - no wiggles - but still sliding smoothly. If there is any play at all you'll get an uneven result. On the other hand if things are too tight and don't slide smoothly, the router will be jumpy as you reposition and you'll end up with gouges. Get it all just right and the fretboard will only need a couple passses of sandpaper to result in a perfect surface.

    I "tuned" the fit on mine by adding layers of tape to bearing surfaces as shims until it was nice and tight, then I rubbed everything down with lots of paste wax so it would slide. It made a world of difference to spend half an hour doing this.

    Great job showing the whole build process in photos, this is a very useful thread!
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  14. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Hey Tim;

    You're off to a good start, but you have a couple of problems to work out still. As the other guys have said, the whole rig needs to be fairly tight fitting, but still able to slide smoothly.

    Here's a look at my fingerboard radiusing rig. It's built to hold complete necks (with the fingerboard blank glued on), but you could make a shallower version for just cutting unattached fingerboards.

    The router base has the two MDF side plates with radiused undersides, like yours. The base slides and rolls on a pair of parallel round aluminum rods on either side of the fingerboard. This eliminates that lower base that you have, and makes things simpler. The neck (fingerboard) can be finely adjusted up and down vertically, separately at either end.


    The geometry is laid out so the final cutting radius is exactly 1/4" below the curved side plates of the base. In the setup above, I'm cutting 7 1/4" radius. The router base side plates are 7 1/2" radius. The router is set to "0" with the face of the bit sticking down 1/4" below the curved side plates. That's the "0" point.

    The fingerboard is raised up so that its final intended thickness ends up at that same "0" point (exactly 1/4" below the curved side plates). For example, if the fingerboard blank is 0.300" thick and I want it to end up at 0.250" thick at the center, then I adjust its vertical height so the top surface is 0.050" above that "0" point. If I want to put some vertical taper into the fingerboard, I can by raising the nut end a little bit more. If I raise the nut end to 0.080" above "0", then the fingerboard will end up tapering from 0.250" thick at the heel to 0.220" at the nut. That's a common thing to do to make the side edges of the fingerboard look more even in height.

    To start cutting the surface, I raise the router head up in its own base maybe 1/16" (0.063"). Cutting a hard board like ebony, I cut the surface in a series of passes of about 0.020" each, with the last pass at about 0.010". Bring it right down to the pre-set "0" mark. The fingerboard will be right on 7 1/4" radius, and at 0.220" x 0.250" thickness.

    Making the cuts, I go down the length of the board, in a climb cut direction, starting at the outside edges, working in to the center. In the picture, I go down the left side, away from the camera, taking about 1/4" wide off the edge. Off the end at the far end, tilt the base over to the other side, pull it back this way taking 1/4" off the right side. Keep going like that, 1/4" strip on the left side going away, 1/4" on the right side coming back. Down and back, narrow strips working in toward the center. The final pass right down the middle. That was all at one depth of cut pass. The first pass will only cut at the edges and not touch the center. Adjust the router head down 0.020" and make the next pass. The final pass should be about 0.010". For reference, the 1/64" marks on the router base are 0.016". So, 0.010" is about 2/3 of 1/64".

    Over the years, I've made up a whole set of different router bases to cut different radiuses from 4" up to 20". I believe I currently have 4, 7 1/4, 9, 9 1/2, 10, 12, 16 and 20. One of the bases has quickly interchangeable side plates, so I can quickly make up other radiuses if I need them. They are all made to the same reference geometry of the router bit being 1/4" below the radius of the base side plates. The base to cut 12" has an actual radius of 12 1/4"; the base to cut 20" has an actual radius of 20 1/4".

    Here's some pictures of the rig cutting a 7 1/4" radius on an SSB neck.



    Here it is doing a 4" radius. For this setup, I unbolt the one side rail of the fixture and move them closer together. You can see one of the simple wood gauges I use for setting up the height of the neck.

  15. I did the same things @dwizum did as mentioned above.

    I'm glad you figured out how to get the bit to be lower or low enough. I pondered this one for quite a bit, and drew a side view desired picture before hand and came to the same conclusion to have the router base sit in a carriage kind of concept.

    A few pictures after cut, and dry fitting with clamps

    Radius Router jig2.jpg
    Radius Router jig1.jpg

    Due to the router I was using and length of bit, I decided to use one of these Freud bits (04-32)

    My first board came out very nice needing minimal sanding (I believe it hit it with 400 grit).

    Before and After pics


    I will make mods to the carriage to have skate board bearings so it'll be interchangeable with other radius bottom carriages.

    Speaking of making more for other radius (not sure if you're doing one or want to do others). I've come to the conclusion that a band saw circle cutting jig will be the best way to go for making multiples. I'm certain I'll waste less wood and it seems to be a simpler/faster option (if you have a band saw that's big enough).

    Here's one:
    and a radius jig with skate board bearings:

    Hope this helps, or gives you some ideas to build off of.
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  16. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    That's a good visual explanation of the difference between cylindrical and conical (compound radius) cut fingerboards, but they didn't finish the story. One picture shows the high-center problem when the outer strings are off-axis, but then they left it there and didn't get into how the geometry has to be corrected to fix that. That's the whole Hourglass Correction thing we talk about on other threads.
    tr4252, Andyman001 and Beej like this.
  17. Plain Old Barry

    Plain Old Barry Supporting Member

    Mar 1, 2018
    Longtime furniture / cabinet maker here...

    Skip the plywood plate, as you don't need it in this use. Rotate the router 90 degrees and insert rods through the curved carriers and the slots for the edge guide. Most home centers or hardware stores stock steel rod of suitable diameter.

    You can also skip the curved side pieces that match the radius base, allowing the rods to ride directly on the curved rails. Add end caps to keep the router on the rails. If you do this, additional radii will only need one side of patterns made. Wax the MDF edges where the rails will ride.

    Another idea might be to include an attachment for a shop vac on the opposite side of the jig from the operator. It probably won't get all the dust, but it may keep a lot of it out of your face and lungs.
    tr4252, Durham52, Andyman001 and 2 others like this.
  18. I went in the opposite direction. I made a router jig, but would end up with random low spots. And when you have a low spot, you have to lower everything else to match. I switched over to hand planing followed by the sanding block.
    Huw Phillips likes this.
  19. All good stuff guys, I appreciate it. Bruce, your jig is amazing, but a bit more extravagant and intricate than I have the "stuff" on hand to build.
    Yes, I'm aware of using a hand plane, but not entirely skilled enough with one to trust it yet.
    I'm also aware of radiusing the fretboard after it is glued to the neck, but before the neck is shaped. But this current build will not allow that, so I'm going a different route on this one. Thus the simple, quick, down and dirty jig.

    But, lots of great info here, appreciate each of you! Gives me a lot of ideas for future reiterations. :cool:
    Matt Liebenau, chinjazz and dwizum like this.
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