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Another Radius Block Jig


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No offense to Anthony Setchell. His radius block method is definitely usable. But I had 2 issues with it.

1. The router is used on its side. Routers are not meant to be used that way. Now I agree that it is attached to a jig so it makes it a LOT safer. But I'd rather not go there anyway.

2. The router being on its side causes the bits edges to "dig in" as it sweeps the arc. This leaves some unnecessary cleanup.

I've built this one that takes 3/4" mdf blocks and routes the into a perfect arc.

The issue with this method was lining up the arced pieces to be glued together.

I did that by glueing and then air nailing the pieces together so they are even. There is still a little cleanup and to do that you can use the "other" piece of the blocks that get cut off by the router. This way the blocks maintain a uniform radius.






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I think it's a good concept, but having that many pieces that need gluing together afterwards leaves a lot of room for error. It appears you have 10 pieces in a completed block. This gives 9 glue joints and chances for error. If that radius surface isn't nice and dead flat you are going to have inconsistencies in your block. Worse yet, unless your blocks are perfectly identical and you glue them together on a perfectly flat surface spaced EXACTLY the same from the edges, you will end up with basically a warped block. If your surface you line them up on has a warp, with each piece you glue together, you'll end up with the same warp.

Not knocking your idea, but I'd at least try doing them out of 2" thick pieces and many passes with the router. I also wouldn't use MDF for your block, it's to soft.

I guess the big question is, although this gives you the opportunity to make any radius you want, after the time invested in building the jig, and the time it takes to make all the pieces and glue them up, is it really worth it over buying the machined blocks from stew mac? It might be, not knocking it, but I would question the accuracy of the completed glued up blocks.

Anyway, cudo's for thinking of a new method to do them, just not a method I would personally use.

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When I came up with my jig, I considered the approach above, and discarded it for more or less exactly those reasons. The technique I use (available on the PG mainsite) produces an accourate, consistent block, provided you start with a well squared up blank.

I'm thinking of setting it up again and making a few more blocks using laminated plywood for extra stability and durability. The original block I made is still in use, but I need to replace the paper on it.

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LGM- Thanks for the insight. My issue is more with money than time. Time is cheap for me...at least for now. I like the idea of larger sections because there is some deviation. The only other thing I can think of is to peg the sections so they are in the same spot when routing and when they are assembled. I'll try that tomorrow it think.

This is a picture of the smaller blank with a straight edge on it. If there is any twist to it I don't know because there is no accurate way to measure. You'll notice a little dip at the 4" inch mark. I can force a .008" feeler gague under that spot so its probably more like a .007" dip. I'm doubtfull that it will make a difference while sanding.

I think pegs will be a better approach to lining them up.

My reservation about the Stew mac wooden ones is that they are wood. Even quartersawn the wood is going to fluctuate with humidity and temperature. Thats why i choose MDF. UHMW is an idea as well but then cost is an issue.

The aluminum ones are awesome but the price matches.


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My reservation about the Stew mac wooden ones is that they are wood. Even quartersawn the wood is going to fluctuate with humidity and temperature. Thats why i choose MDF. UHMW is an idea as well but then cost is an issue.

Well, lets blow that argument out of the water...

MDF absorbs a LOT more moisture than "real" wood. Trust me, i worked with the crap for 10 years.

Edited by rhoads56
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My reservation about the Stew mac wooden ones is that they are wood. Even quartersawn the wood is going to fluctuate with humidity and temperature. Thats why i choose MDF. UHMW is an idea as well but then cost is an issue.

Well, lets blow that argument out of the water...

MDF absorbs a LOT more moisture than "real" wood. Trust me, i worked with the crap for 10 years.

Hmm... yes its paper, never looked at it that way. But perhaps more uniform expansion?

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Your jig is pretty much the same as mine, for making radius *gauges* from plexi (or other thin material). My jig was first published in Stew-Mac's 'Trade Secrets' volume 13, first published in 1993, I think (?)

Which is now in book 1 of their 'trade secrets' books.

I wish my information giving days were over, but I will tell you that this same jig is just fine for making fret-press radius cauls.

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By the way, you can check your blocks for twist, by placing them on a thick piece of *flat* glass ( I use 2 pieces of 1/4" very flat glass stacked together, and try to lay those on a surface that's fairly flat). I don't make radius blocks like yours, I just mean I use the flat glass for checking the flatness of a lot of things.

Just by looking at the front edge of your picture, it looks like there is some uneveness.

I agree that using such blocks made of something that will probably shrink, expand, and/or warp is not good for the job they are for.

(can't edit my post, which is why I had to add this one)

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You would have been better to use hardwood (maple, etc.) Soap does have a point, as you could use it to make homemade cauls, but gluing together small pieces of wood to make a uniform sanding radius block would be risky to me. I personally wouldn't trust it for precision work. One good thing about a duplicating machine is that you can buy one of the small radius blocks, 8" or so, make two more of them. Then take the original and the other two and align it perfectly on the stylus side and you can cut out a long radius block perfectly that way. The only thing is that most people don't have a wood duplicating device.

The best ones you can get would be the ones Stew Mac just came out with, the 18" long aluminum ones. The only problem is they are big time expensive, but would be worth it though.

Aluminum Radius Sanding Beams..

Now If you could just get $818.65 together, you could buy all of them.. haha

If that is the case I'd rather buy the Grizzly Radiusing Sander so you can get some compound radiuses too, for only $1,007.25... :D

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That's a great, cheap effective jig.

It can be used to make more than one thing. This is a good thing.

I'm always amazed at guys worrying about minute variations in size when working in wood. There are limits in the precison that you really need because the darn stuff expands and contracts like crazy. If it didn't look so good and sound so good there are all kinds of more stable things than wood to do this stuff with. Ovation guitars with plastic bodies look like crap (I think) and sound only slightly better, but boy are they stable and precise.

MDF is much more dimensionally stable than regular wood, unless you dump your coffee or Coke on it or throw it in the pool. At least the furniture grade is.

You can also shoot a couple of coats of sanding sealer on it , or paint it with glue size. That's why most of the furniture makers that I know who use a lot of veneer use it as a substrate. I have twenty year old jigs made from MDF that are still the same size as when I made them, and Richmond has a humidity of about 85%.

To glue up the pieces parts all you need are some stiff cauls and everything will line up just dandy.

If you're really worried about diminsional movement in a jig that you're going to be using by hand, bum some Corian scraps from a coutertop shop and rout 'em and fuse them together.

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I just remembered that waaaaay back, when I first came up with this type of jig, I actually did make some radius blocks out of 1" thick wood. No "radius blocks" existed at the time, in any of the guitar shop catalogs. I thought I was a real hot-shot, for coming up with this. What I would do, is level the fret-board with my Ken Donnel glass 13" x 2" glass surfaced leveler (with 'bondo board' sandpaper strips clamped on it) from end to end, then I would have sandpaper on the 1" thick radius block, and I would just evenly "draw it" along the fret-board to "perfect" my radius, which it did , but might have thrown the end to end leveling of the board a little off. which is why I stopped using it, and also why I still seldom use radius blocks, because the end to end flatness of the board is what is most important. ( I do have a full set of regular wood radius blocks that I made several years ago in the 4" and 8" widths with a whole other special jig I had made).

My current "radius block " of choice :


Ok, not really a radius block, but it does work with any radius, and is DEAD flat. More flat than the Stew-Mac bars ( I had 2 of those).

in the photo is the 8" one, but my main weapon is the 19" version (doesn't fit in a photo as well) they are 1" wide.

I can't imagine anything coming along that would make me stop using these things.

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Well, I put my head to the grindstone and figured out how to mill the arc for a radius block.


For those of you who took Trig or calculus in the past you may have seen the equation of a circle. R^2=X^2+Y^2 (R=radius)so I plotted this in Excel and came up with the measurements for each pass with the 3/8 ball end mill.

Turns out to be 62 passes to get an acceptable finish. The peaks created (the ripple effect you see were calculated to be .003" and the sand paper when attached sticks well and evens out the surface.

The only snag I ran into was when testing on some MDF. At the point where the depth changes from down to up the mill has a LOT of backlash in that spot. so the head drops about .01" ....really bad. So I got around this by cutting both sides at the same time and meeting in the middle.

Total cutting time for the alum was about an hour, prob 1.5 hours with an 8 inch piece of stock.

Heres some other pictures


Leveled off old block

Cutting 1

Finished Alum

Sorry no inbetween pics of the aluminum...I got carried away.

I cant wait to get some stock to make a bigger piece.

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  • 3 months later...

How about making one thin block and using it as a template for a big one? Just use a template bit with your jig-made radius-template clamped up on a big piece of wood and cut it down using a... lol, i don't know the name know, you know, a template bit with a template mounted on the end.

I also read about a method to radius a fretboard that is used by Robert Koch who wrote a book about building guitars which is very popular amongst guitar builders here in Germany. Ill post a building tutorial as soon as i worked it out for myself. There is no difficulty creating compound radiuses with this as I think, so ill try it.

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  • 1 month later...

Well I got around to milling some radius blocks today.(necessity is the mother of invention) Its just wood this time out but judging buy the results I can see aluminum ones on the horizon.







Completed radius

With sandpaper

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