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I originally introduced the idea of a compound scarfing jig waaaay back in something like 2007-2008. A few people around ProjectGuitar.com have successfully used the idea, and a few people around the interwebs have taken it on also....some clearly took it directly (including images and zero credit) however convergent evolution means it would surface of itself at some point anyway. It's all cool. Rising tides floating all boats and that.

The idea was based off the established idea of a router scarfing jig, but improved to allow for twisted headstocks and even string pull for multiscales that do not have a nut perpendicular to the centreline.

Normally the treble side of a multiscale is pulled backwards, causing the headstock to "twist" clockwise as you sight towards it down the fingerboard. The higher the difference between the two outer scales, the larger the twist.

This creates problems for necks with tilted headstocks as the scarf needs to incorporate a new compound tiltback angle. Thankfully, there is a simple solution to this which isn't much more difficult than the standard router scarfing jig.

Firstly, let's look at the standard router scarfing jig:

standard scarfing jig.jpg

 

At their most basic, they consist of a box bounded by two guiding rails. On top of this rides a router with a wide sled which ensures it maintains contact over both rails. The neck and usually the piece being scarfed are cut at the same time. Depending on the final orientation of the scarf, one piece has the glue joint surface cut whilst the other has a facing surface finished:

scarf types.jpg

 

Pretty standard fare so far. To make a compound angle, the sleds are simply offset from each other. Rather than riding on the faces of the sidewalls the sled now runs on the edges; the inner wall on the furthest forward and the outer wall of the furthest back. The correct offset corresponds to a line drawn from each contact point on a flat plane:

angledscarf1.jpg

 

When glued up, both halves produce the expected compound scarf.

angledscarf2.jpg

 

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The two main downsides to the simple scarfing box jig is that it is not adjustable for various headstock angles and there is no ease of repeatability. If you're only planning on making one or two scarfed necks, then that's fine. No need to go to the time and expense of making up a big shining jig.

I'll update after I've made changes to the Solidworks model....

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I made the sled today anyway. I need more Perspex to make the rest. I couldn't resist making some sweet Karelian Birch handles and soaking them in Linseed oil....

IMG_7840.JPG

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Thanks guys. I'm going to try to make the side rails from plywood or HDPE in lieu of Perspex. Once I get the idea proofed I'll make the finished parts. I need some bolts also.

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Base and lower platens made....

IMG_7843.JPG

 

I made the base from two stacked pieces of thick plywood. I saw that one of the facing veneers was buggy Birch. I couldn't resist it....

IMG_7842.JPG

 

Note that one of the mounting holes on the left-hand lower platen is offset from the rear so that it doesn't drop out the T-track.

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Like all of my jigs, this will have a mounting flange attached to the base so that I can clamp it up in my bench's leg vise.

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I have no way to work metals right now. Perspex/acrylic is easy with woodworking tools if you slow right down. I found that the Colt M2 HSS FCE bits (review pending) are clean and quick in Perspex. Amazingly so. I'd rather stick with something clear eventually purely for light and visibility in the cut.

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I was thinking about the visibility aspect as well when first I had that thought and so kept it to myself. but then you mentioned plywood....

SR

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Sure. Well, with plywood I can copy that straight to the Perspex instead of working in the plastic direct. It's fine for things like platens and sleds, but for weird angles and such I think that having a template helps. At least, in this case. I tend to change my mind on how I do things depending on which way the wind is blowing, tool availability and some practical aspects. Mostly the first of the three.

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Adjusting in the changes to geometry, sizing and a bit of arrangement this is more or less the finished prototype model:

test_render6.JPG

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More progress today. Fits as expected. I need to buy more Perspex for the side rails....hardware is on its way. I drilled the sled to fit my Makita palm router....

IMG_7845.JPG

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Queries:

Should the sled the router gets mounted on have some form of side-to-side travel stops to prevent the router bit being accidentally slid into the side pieces? On straight scarf cuts using a template bit it shouldn't be an issue as the bearing will keep the bit from traveling too far left or right. But as soon as you shift one of the sides backwards to introduce the angled scarf cut, the router will lean marginally to one side and the risk of running the tips of the bit into one of the side pieces increases.

How do you prevent the bit from being driven into the base of the jig at the bottom of the scarf cut? Sacrificial plywood base perhaps?

How does the workpiece get secured in the jig?

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I'll calculate out those variable with the bit. The sides have bolts which act as stop limiters. Maybe having more than one will allow it to be set up in a more safe manner. The workpieces should be fitted onto a sacrificial base. Two ways of mounting the workpieces spring to mind. Directly clamping to the jig or double-stick tape. I plan on drilling two slots to accept the T-shaped end of my smaller clamps.

Great questions Andrew.

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Well, here's the completed sled. I designed it to work with the Makita. I'm not sure how well it will work with it being so narrow. It might be a little tippy. Still. I'm sure it'll work with care taken. The handles act as stops to prevent the sled going too close to the sidewalls. Whether larger bit extensions and compound offsets negate this "safety" feature, we'll have to see. Really, the bit extension shouldn't bee too much anyway. Advancing the pieces being scarfed forward prevents excessive bit extension.

The handles were countersunk and drilled, the sled drilled and tapped. 50mm M6 hex head bolts make this a nifty little sled.

IMG_7930.JPG

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Very clean and clever work.  I like Scott's thought of using aluminum for the side pieces.  It machines easily with woodworking tools and could have openings machined in it to allow clearance for the bit in the event of too much skew. . . . Your designs always inspire me to go out and build something.

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Good point about cutouts. I'll see what materials end up on my doorstep as I progress this one. Thanks for the positive comments too! The whole reason I share these things is to inspire people to make something. I take that as mission accomplished!

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Hey guys so my question is after using the jig and making the scarf. This slightly angles the face of the head stock right?

Edited by Pestvic

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It does, yes. If the break angle of the headstock is not perpendicular then it twists the headstock. A real compound angle!

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Thanks Prostheta! I knew I had it right. For some reason part of me kept saying "Are you sure its not supposed to be flat somehow?? haha Man I feel dumb. Anyway, im waiting for parts to come in then its off to building some more :)

 

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