I think it may be time for me to purchase a set of fretboard radius blocks. As a hobbiest, I have managed fairly well up to now but there are times when it would make the job much easier.

I work on acoustic instruments. ( I'm not opposed to electrics but I just prefer acoustics.) If it matters, most have standard scale lengths but every once in a while I find myself with something different. 

What I don't know is what sizes would be best. Do I really need to buy a complete set from Stu-Mac or will I find that I only use a couple of them most of the time and the others will just set on the shelf?  Any advice would be appreciated.   

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My first was a 16ft radius wooden block for StewMac. It worked fine for a while, but after using it a dozen times or so, I began noticing that the fret board was not consistent as I wanted it.  I checked the block and it was no longer smooth or even as it was when I first bought it.  So, I then purchased the aluminum one from StewMac (again 16 ft).  If I were to buy again, I would start with the aluminum one.  It is longer, dead straight and the results that I get now are better.

(Glen, you mean 16 inch, right?)  After too many years without 'em I finally bit the bullet and got the aluminum ones, starting with the 12", the 16" and the 20"... adding as I could afford them, since they're not cheap. 

It would nice to have the luxury of buying 2 of each so that one could be cut-into 8", 4" and other lengths, as they come 18" long... but they're a little pricey for that :)

Someone (StewMac?) used to sell blue 4" heavy cast-resin radius blocks, but I haven't seen them around for a few years now.

Mike, you're right 16 inch!

Found this on some guitar blog... no idea who the author is nor his level of expertise, but it's an interesting idea:


How do you make homemade radius blocks for sanding necks and frets?


Technique 1. I made a set with Bondo.  Start with a wood blank the size you want and attach a couple of runners along the edges that stick out about 1/4".  It should now look like a sled.  Then I put a piece of 1/8" masonite in a vise, squeezed it until it formed the appropriate arch and covered it with waxed paper.  Finally, I mixed up a batch of Bondo and smeared it between the runners on the block.  Make sure to get it smooth or there will be voids on the completed surface.  Squish it down on the bent masonite and let it dry for 15 minutes.  Cut off the excess with a knife before it gets too hard and later sand the sharp edges.  That's it.  Elapsed time is about 15 minutes.  Bondo is pretty stinky so use the usual precautions. 

Technique 2.  I have made radius blocks by casting over a purchased radiused fretboard blank. What I do is lay the blank on a plywood surface, cover it with a sheet of aluminum foil, then nail down some 1x1's around the blank, framing it so that I could pour resin in, at the same time pulling the foil down flat and tight over the curve of the blank. Then, pour in polyester resin with some pre cut fiberglass strips, just to fill up some volume and reinforce the casting along its length, I poured the resin about 1/2" deep, allow to cure overnight, knock away the 1x1's, peel away the foil, file away any edges where the resin seeped, and you got yourself a full wide fretboard length radius block.   Cut them into smaller hand sanding blocks, or, use as a long clamping caul.  I've tried this with epoxy and the result was way too flexible, maybe it was the type I used, but the polyester works great. Make a selection of blocks based on all the radii that you can buy prepped blanks of. 

My first one was a Stew-Mac wooden 16" radius block that worked ok for a while.  Then came the expensive aluminum sanding beam also in 16" since I too at least at that time was only interested in building acoustics.  The aluminum sanding beam is very well made and also has a dual purpose with some waxed paper as a gluing caul for your fretboards if that helps you justify the cost.

These days however I start with a 16" radius fretboard and then use the 1" wide leveling beams to impart a compound radius in my boards.  Simply by sanding in the direction of the string paths you will eventually turn a 16" radius into a compound radius which is my preference these days.

I've also done the bondo thing but to make gluing cauls for different radius end blocks for different body shapes.  Bondo worked very well for this too by simply slathering on a layer thick enough to conform to the desired radius and then with some waxed paper in between clamping the bondoed... caul onto the shape that you wish to replicate.

Check back next week because like all things lutherie methods and preferences seem to change frequently at least in my shop....;)

Thanks Glen, Mike, Hesh for the responses.

I've been wanting some for a while but I just keep putting it off because I don't think I do enough to justify the expense. I have thought about trying my hand at making some but figured that I would spend more on materials trying to get them right than it would cost to just buy them.    

I've heard before this that the wooden blocks seem to wear out fairly quickly but I really can't justify the cost of the aluminum models.This is a hobby and funded on a shoestring. I though about using my router to cut some arcs into plywood, cut them apart than glue them up to form a block but figured that the ply would end up wearing out in a hurry. I didn't think about coating it with Bondo.

 Actually, molding my own with Bondo sounds like something I could do fairly easily. I've worked with it before but never though about it in this context. I think I would rather shape an actual form for the process but the forms could be done fairly easily and that way I could make different lengths for a lot less than purchasing aluminum blocks.  It may even be possible to incorporate a bit of mesh to reinforce the bondo where it is thicker on the edges. I'll have to think on this.   I have spend a lot of time finding ways to work around the tool I don't have but it's always a pain. Maybe I can get my block this way and apply my budget to something else.  ( I have a line on some diamond fret files that have only been used once that I can get for half Stew-mac's price, IF they aren't gone already. )  

I started with a full set set of the Stew-Mac 8" wood ones and eventually got the 4" ones too. Over the years I've purchased extra individual ones to cut down for various clamping cauls. I managed to score a set of the no longer available Koontz resin blocks from a guy who wasn't using them. They have a few sizes not available in the wooden sets. Never regretted owning any of them. 

From my experience in full-time repair, for acoustic guitars the radii I most often use have been 12'', 16'' and 20". However, if there's a chance you'll ever work on anything Fender you'll wan't to have a 7.25" and 9.5". A 12" will get you by on most Gibson electrics and their copys, the shredder/metal guitars will be flatter 16"-20". And then there are the exceptions, and compound boards, etc.

That reminds me of a hard lesseon learned on assumption a while back. I shot from the hip quoting refretting and nuts on 3 early '80s Ibanez electrics from the same customer. Their "legal" takes on Gibsons' Flying V and Explorer, and an Iceman. These were really nicely done guitars, even if not your cup of tea. Flame maple tops, good looking cherry bursts, gold tuneomatic/stop tailpieces, very Gibson-y. They're going to be 12" radius, right? I mean, after all there was that Gibson lawsuit and all, they'll be 12" radius!

I'm not really sure what they were. They were flatter than a 7.25", rounder than a 9.5", unlike anything I had ever encountered. Weird sized fret slots that were much shallower at the ends. I ended up using the 9" block that came only in the resin set to level the boards, it was the only thing even close to the original radius. Any flatter threatened the large abalone/pearl inlays. After levelling it took a butt-load of both prep work on the slots and levelling the frets, as the boards did strange things under string tension. Oh, and we were going from vintage-style jumbo to modern railroad-tie frets. I certainly gave away a good part of the farm on that one.

But I digress, my point is in my case it didn't hurt to have all the various sizes available. If you intend to work on acoustics only, the 12",16" and 20" should cover most of your needs. And while I really admire the guys who make their own tools, I prefer to just buy them and do the work. Unless, of course, the tool's not available.

I do see those hot new Stew-Mac aluminum units in my future...

Thanks for the response, Scott.  Over the last few years, I have learned to respect the opinions of the denizens of this forum. Last night I was thinking that I may go ahead and purchase the "trinity" of blocks that everyone seems to agree would serve me best on acoustics, then experiment with making the odd sizes. I really like making my own jigs and tools but I would also like to get on with the rebuild I'm doing right now too. 

I'm surprised to read that some people here have managed to "wear out" the wooden radius blocks from SM (which I also use, I have the complete set)

How can they wear out? They are made of hard maple, and the only thing that should wear out is the sandpaper you're using with them: I use 3M's gold stick-it with them, and after you've peeled it off the block, the block looks like new, as long as you take appropriate care with them, they should last for ever.

That said, I'd still like the aluminium ones SM have, because they'll be heavier than the maple, but when I calculate what they'd cost me to have them sent to Germany (including customs duty and sales tax), I think: the maple ones are nice enough, they do the job.

I don't think any respondees said they wore out. Mine didn't wear out.  If I put a straight edge under the board, it is not level.  I guess the best description is it warped or swelled in the center.  It did what wood does and aluminum doesn't.

Seems like something that luthiers solve all the time - install a truss rod! Okay, not an adjustable one, but a metal bar would straighten those puppies right out.

Hello Comrades,

Pretty much use one radius in electric building these days and don't compound fretboards.   We use the SM Aluminium radius blocks because of the total accuracy that these long units provide. The capability to have all points on the fretboard relative to each other provides a good start to a fret job and the long stable blocks do this well.  Our boards are machined in house to within a few thou and the blocks finish them cosmetically (although I have used them to do the odd radius from scratch).   I use Stikit in various grades with these blocks for Rosewood and Maple and our ebony is further fine finished by hand with a soft rubber (auto paint finish) "black and blue" block.

I also use them for repairs when replacing fretboards and in conjunction with a rectangular long block and some shims I dial in some positive neck relief to compensate for the neck response to gluing (which sometimes causes back bowing - a problem with one way truss rods) with Titebond.  The long alumium caul flexes 20/30  thou with no troubles whatsover and return dead flat when the relief is removed.

I use the wooden blocks which I still have for spot finishing but as they tend to rock and roll and skew in use which is the reason I don't use them for full board work.   The other advantage of the long aluminium blocks is for accurately  putting a gentle drop away into the top end of the board simply by leaning on the block for a few licks as the board is finished. 

They are excellent in use, very durable, and easy to clean adhesive/glue from.  But I think they are too expensive to buy a full set of - even for the larger repair shops and in this particular case I will complain about the cost for these otherwize excellent units.



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