# Dovetail Jig: design feedback sought



## PhilBa (Sep 25, 2014)

I have been designing a jig to cut through Dovetails. I looked at a lot of the commercial ones out there and like the capabilities of the Lee Valley super jig - particularly variable width spacing. I certainly don't like the price point. Here is my design. I would love to get feedback on it. Suggestions for improvement are welcome. When I finish, I will produce fairly complete plans for people to build their own.

Going in, my requirements (and non-requirements):

Must support variable spacing
Must allow simple fit adjustment (tightness of the joint)
Only for through Dovetails
Possible to make with standard shop tools
24" width
Inexpensive, using off the shelf hardware and common materials - $50 cost max
Uses a 7 degree, 3/4" DT bit, straight bit and standard PC style bushings but easy to adapt to other bits
External clamping (i.e no built in clamp)
Mounts on workbench
Minimum DT spacing of 2" (more on this in a bit)
Dust extraction not necessary (use router DE)

The LV Jig has a lot of features and I decided that to make this buildable I would eliminate anything not central to making through DTs or could be done externally. So, a clamping mechanism, dust extraction, sliding DT features went.

The fingers attach to a platen that can be moved to adjust the tightness of the joint. The side cutouts in the base allow easy clamping to a workbench. The workpiece would be clamped to the base sides. 

The fingers are the key to this and need to be fairly precisely cut. I plan on cutting them with a CNC machine but think a careful person could make one finger and use it as a template to make the rest. With that in mind, I have designed the fingers to have only straight lines and tolerate 1/8" radius inside corners. I am considering making them out to 3/8 lexan/polycarbonate which is easy to mill with simple wookworking tools. When I get my all-singing-all-dancing new CNC machine, I might cut them out of 1/4" aluminum. However, I think the polycarbonate will be plenty strong. The main disadvantage is it scratches easily. It may be possible to use plywood for the fingers - I've seen some designs on the web like that but probably won't try it.

About the spacing. To keep the fingers sturdy, I have made the minimum spacing 2". This is probably overly conservative. With aluminum, I can get to 1" spacing. With Polycarbonate, maybe 1.5"? I don't feel a need for really tight finger spacing so I'm going with 2" to start. Also I have shown spacers in my design - in the picture, there a 1" spacers making the example DTs have 3" spacing. I have 1/2", 1", 2" and 3" spacers drawn up. However, the LV jig doesn't seem to use them so maybe they are unnecessary. I do like the idea of easily getting exact spacing of the fingers.

So, in lieu of an additional 1K words, pictures.


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## Nickp (Dec 4, 2012)

Phil...I don't see stop blocks to align the two workpieces...did I miss them...?

Also, I gather you will be using clamps to hold the pieces to the jig while cutting...?


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## PhilBa (Sep 25, 2014)

Yes, clamps. The stop blocks need to get added. Still thinking how I ensure they get aligned. Probably something that hooks into the T-Track on top. Might require a lengthening of the platen.


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## Nickp (Dec 4, 2012)

PhilBa said:


> Yes, clamps. The stop blocks need to get added. Still thinking how I ensure they get aligned. Probably something that hooks into the T-Track on top. Might require a lengthening of the platen.


...or a slotted piece on each end of the sides to allow left/right adjustments. The slotted could be held with a 1/4" machined bolt/screw/knob, etc. I suspect you're right...would need to extend the lengths.

Nice design...

Have you looked at the General Pro jig...? might provide some additional ideas for stops...

I like your design...could give the big boys some competition...

Edit...just had a thought...you don't really need an adjustable stop since your fingers are variable...just something to make sure the two boards are at the same starting point...a pin straight through would do it...


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## Bstrom (Jan 2, 2020)

This is a nearly verbatim copy of the Sommerfeld jig - minus the squaring pins on the fingers and the aforementioned clamps and stop blocks. Dunno if its trademarked...


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## PhilBa (Sep 25, 2014)

Nickp said:


> ...or a slotted piece on each end of the sides to allow left/right adjustments. The slotted could be held with a 1/4" machined bolt/screw/knob, etc. I suspect you're right...would need to extend the lengths.
> 
> Nice design...
> 
> ...


Here's what I have but I'm unhappy with it. Top piece would be Lexan, lower pieces MDF glued up. It looks kind of big and clunky but at least does solve the problem of keeping the two sides in alignment. If the stop is attached to the finger platen, then it needs to overhang. I don't want to have to align each side's stop individually. That's asking for trouble. I like the idea of a pin going through both sides though I'll need to make sure it leaves room for the clamps.


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## PhilBa (Sep 25, 2014)

Bstrom said:


> This is a nearly verbatim copy of the Sommerfeld jig - minus the squaring pins on the fingers and the aforementioned clamps and stop blocks. Dunno if its trademarked...


Doubt it's trademarked though I'm not selling anything so it's a moot point. To be honest, I never even looked at the Sommerfeld jig so either it's a copy of the Lee Valley or the other way around.


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## PhilBa (Sep 25, 2014)

Actually, I think I'm guilty of overthinking the stop. Since the fingers are movable, the stop can be fixed. Duh. The only issue is getting the initial alignment right.


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## Nickp (Dec 4, 2012)

Initial alignment of the stops could be done by drilling or machining the ends of the two sides at the same time...tap some threads in the sides and screw the stops to the sides. This would make the two sides exactly the same... You could also mill a groove at the same spot on both sides and lay the lexan or other material in like an inlay...

...or make the stops exactly the same and install them both to the edge of the sides...


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## Nickp (Dec 4, 2012)

PhilBa said:


> *However, the LV jig doesn't seem to use them so maybe they are unnecessary. I do like the idea of easily getting exact spacing of the fingers.*



Not really required as the pins and tails align to each other since they are on the same finger. You could get by with one set of spacers and adjust each finger with the same spacer(s)...tighten it down and go on to the next. You could also leave out all the spacers and make tight dovetails.

Or you could give it the handmade look and just guess at the spacing...:smile:


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## PhilBa (Sep 25, 2014)

Yes, drill them together. Stop will be 1/4" thick to avoid interference with the clamps.


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## Nickp (Dec 4, 2012)

Best of luck with it...anxious to see what you come up with...

I agree with you...not plywood for the fingers. I like aluminum better...reduces scratches, easier to clean with all that toxic stuff...:grin:


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## PhilBa (Sep 25, 2014)

Nickp said:


> Best of luck with it...anxious to see what you come up with...
> 
> I agree with you...not plywood for the fingers. I like aluminum better...reduces scratches, easier to clean with all that toxic stuff...:grin:


Yeah, Al is the best bet but my current machine is pretty unhappy cutting aluminum so Lexan is my compromise.


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## gmercer_48083 (Jul 18, 2012)

I would think many of your parts could be printed on a 3D printer. Just a thought.


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## PhilBa (Sep 25, 2014)

gmercer_48083 said:


> I would think many of your parts could be printed on a 3D printer. Just a thought.


Yes I'm sure one could 3D print parts for it. I have a printer but I'm not sure the accuracy would be there. At least, I've never been able to get my printer to the level I'd need.


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## DesertRatTom (Jul 3, 2012)

Shopsmith just did a TV show on making a finger joint jig, very much like this dovetail jig. It's a video project and might be helpful. https://www.woodsmithplans.com/plan/finger-joint-jig/


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## PhilBa (Sep 25, 2014)

Interesting. Thanks for the pointer. Easy enough to make fingers to go with it. There is an inherent limitation to how small the fingers can be with this design. 1/4" may be too small, even for aluminum fingers. I'm thinking 1/2" is the smallest size. Gives me some ideas though.


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## PhilBa (Sep 25, 2014)

Here's a quick rendering of box joint fingers. I think you could get down to 1/4" with AL and _maybe_ Lexan. I could use straight fingers that would be easier to make but then the limit is probably 3/8" fingers due to the bolt hole needed to lock them in the track.


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## Nickp (Dec 4, 2012)

PhilBa said:


> Here's a quick rendering of box joint fingers. I think you could get down to 1/4" with AL and _maybe_ Lexan. I could use straight fingers that would be easier to make but then the limit is probably 3/8" fingers due to the bolt hole needed to lock them in the track.



Nice design on the graphic...I would think you could make the fingers for box joints one piece. For variable, just pick the finger section that suits your spacing. Don't think you need to make individual fingers due to the nature of the box joint. Since you can't cut both boards at the same time, you could save on alignment and manufacturing issues by using one side for both boards. Just offset the second board by the width of the pin against your alignment pin...just a thought on saving some time making your fingers...


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## 1fizgig (Feb 11, 2018)

This is all looking great Phil, looking forward to the finished and working design!
I'm hoping it will be easy enough for someone less experienced like me to build.


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## PhilBa (Sep 25, 2014)

Nickp said:


> Nice design on the graphic...I would think you could make the fingers for box joints one piece. For variable, just pick the finger section that suits your spacing. Don't think you need to make individual fingers due to the nature of the box joint. Since you can't cut both boards at the same time, you could save on alignment and manufacturing issues by using one side for both boards. Just offset the second board by the width of the pin against your alignment pin...just a thought on saving some time making your fingers...


Yeah, that's true (easy to make all one piece). Variable spacing would need spacers to direct where the cuts should be. (thinking that spacers are needed to avoid mistakes when doing DTs, also)

I probably won't make the box joint fingers because there are lots of ways to make box joints. I use an Incra LS which is fast and easy (and cuts 2 boards at once). Though, anyone could make them if they wanted. I consider this an "open" design.


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## PhilBa (Sep 25, 2014)

1fizgig said:


> This is all looking great Phil, looking forward to the finished and working design!
> I'm hoping it will be easy enough for someone less experienced like me to build.


Thanks for the feedback. It is absolutely a goal that high skill is not needed to build it. Only the fingers need precision and I'm playing with ideas to simplify the number of cuts and a jig to make them. Sort of a metaJig!


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## Nickp (Dec 4, 2012)

PhilBa said:


> Yeah, that's true (easy to make all one piece). Variable spacing would need spacers to direct where the cuts should be. (thinking that spacers are needed to avoid mistakes when doing DTs, also)
> 
> I probably won't make the box joint fingers because there are lots of ways to make box joints. I use an Incra LS which is fast and easy (and cuts 2 boards at once). Though, anyone could make them if they wanted. I consider this an "open" design.



Other equipment for box joints is a good idea...


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## Cherryville Chuck (Sep 28, 2010)

Looking at the finger joint jig makes me wonder if using 1/2" key way stock for both fingers and spacers would work? Key way stock is very accurately sized. 1/2" is the smallest size pattern bit I'm aware of so I think that would be the minimum size.


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## PhilBa (Sep 25, 2014)

Cherryville Chuck said:


> Looking at the finger joint jig makes me wonder if using 1/2" key way stock for both fingers and spacers would work? Key way stock is very accurately sized. 1/2" is the smallest size pattern bit I'm aware of so I think that would be the minimum size.


That's a great idea. All that would be needed is to cut the two shallow(1/16") dados to lock onto the T-Track and a 1/4" hole with a counter bore (though counter sink is OK, too). None of those require much precision other than the dados being at 90 degrees. I'm also looking at how one would use a bolt smaller than 1/4" - I think #10, M4 or M5 would work fine.


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## PhilBa (Sep 25, 2014)

Here how it would look using simple square stock (like for keyways). Note this is with a new version of the base. It includes holes in the sides for clamps (for shorter workpieces) and generally reworked positions of the alignment holes and T-Track. This requires using a pattern bit.

I think it would be fairly easy to cut accurate finger stock on a table saw. But note that as long as the pattern bit is smaller than the width of the finger stock, the actual width doesn't matter. Consistency does, though. All in all, fairly low precision is required.

Note also, I've drawn the workpieces with a sacrificial backer board to prevent blowout. This also works for DTs.


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## PhilBa (Sep 25, 2014)

I spent some time thinking through how to make a jig to cut the dovetail fingers. The only precision operation is to get the 7 degree angle correct. To that end, I set up the dimensions so they are fairly coarse standard Imperial values. There is one slight imprecision of 2.1 mils (21/10000") which can be ignored. I believe this angle can be cut fairly accurately on a standard taper jig by carefully marking the lines on the blank and aligning them on the taper jig. The rest of the cuts can be made on a router table. Once the jig is done, the finger blank is rough cut to shape and the bolt hole is drilled. The hole/bolt serves to ensure that the blank is correctly positioned. Then the blank is mounted on the jig, one side of the finger trimmed with a straight bit, flipped over and trimmed on the other side of the finger.

Of course, this has to be tested but I'm pretty confident it can be made to work.


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## Nickp (Dec 4, 2012)

PhilBa said:


> I spent some time thinking through how to make a jig to cut the dovetail fingers. The only precision operation is to get the 7 degree angle correct. To that end, I set up the dimensions so they are fairly coarse standard Imperial values. There is one slight imprecision of 2.1 mils (21/10000") which can be ignored. I believe this angle can be cut fairly accurately on a standard taper jig by carefully marking the lines on the blank and aligning them on the taper jig. The rest of the cuts can be made on a router table. Once the jig is done, the finger blank is rough cut to shape and the bolt hole is drilled. The hole/bolt serves to ensure that the blank is correctly positioned. Then the blank is mounted on the jig, one side of the finger trimmed with a straight bit, flipped over and trimmed on the other side of the finger.
> 
> Of course, this has to be tested but I'm pretty confident it can be made to work.



Phil...I think you lost me on the need for the angled finger...are you trying to cut the dovetails with a straight bit rather than using a dovetail bit...?

The shape of the finger and the dovetail bit going all the way through the work piece is what would make the angles on the work piece...

see this jig as an example... https://www.woodcraft.com/products/...-for-13-inch-woodriver-dovetail-jig-woodriver (this one happens to be 8 deg, but you get the idea)


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## PhilBa (Sep 25, 2014)

Yes, for through DTs you need to cut on an angle with a straight bit. All the jigs that do through DTs use both a DT bit and a straight bit. That's how both sides can have flat bottom cuts and a perfect fit. That template you pointed to uses both straight and DT bits.

Blind DTs can be cut with just a dovetail bit and have gaps (though they are hidden if done right).


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## Nickp (Dec 4, 2012)

PhilBa said:


> Yes, for through DTs you need to cut on an angle with a straight bit. All the jigs that do through DTs use both a DT bit and a straight bit. That's how both sides can have flat bottom cuts and a perfect fit. That template you pointed to uses both straight and DT bits.
> 
> Blind DTs can be cut with just a dovetail bit and have gaps (though they are hidden if done right).



Okie, doke then...anxious to see what you end up with...good luck


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## DesertRatTom (Jul 3, 2012)

Uniformity of the fingers and blocks is critical. Why not mill a block of aluminum to the exact shape, drill the block, then slice off each finger. You're likely to get more uniform pieces that way, and you might be able to do it on a table saw if the aluminum is fairly soft.

I think there are several precision measurements involved that may not seem apparent at first. The angled fingers not only have to be exactly the same as the DT bit angle, but the inside edge must be a precise depth, same with both ends of each finger. They must line up precisely with the body of the jig so the bits depth of movement is just right. 

If you can locate a woodworking store that has one of these jigs, I'd go and give it a good looking over. I think you might also download an instruction sheet for either commercial jig. I know the instructions for the Sommerfeld jig are quite explicit and will answer a number of questions for you. Most such instructions are on websites as pdfs to download. This will get you to pictures of the Sommerfeld jig: https://sommerfeldtools.com/sommerfeld-s-dovetail-jig I posted a picture showing the bar with the stops.

Youtube has two videos by Sommerfeld showing the jig in use. I think it will help. But to be honest, at $240, I think I'd rather just buy one.


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## PhilBa (Sep 25, 2014)

DesertRatTom said:


> Uniformity of the fingers and blocks is critical. Why not mill a block of aluminum to the exact shape, drill the block, then slice off each finger. You're likely to get more uniform pieces that way, and you might be able to do it on a table saw if the aluminum is fairly soft.
> 
> I think there are several precision measurements involved that may not seem apparent at first. The angled fingers not only have to be exactly the same as the DT bit angle, but the inside edge must be a precise depth, same with both ends of each finger. They must line up precisely with the body of the jig so the bits depth of movement is just right.


My CNC router isn't going to do a big enough block of aluminum so that's out for me. Though, it has pretty good repeatability - I have confidence that I can cut 10 fingers and have them all come out very close - 5 mils or better.

I think you are referring to where the fingers line up with respect to the workpiece and determines the width of the pins. The "depth" of the finger (i.e. the width of the pins) can be adjusted by moving the platen as shown in the picture. That's how you tighten/loosen the joint. The LV jig has a similar adjustment. Also, you would use the adjustment to handle different backer thicknesses. You can see a line on the fingers that indicates the nominal position for the inside edge of the workpiece. In practice, I expect some amount of tweaking to be done to get the joint exactly tight. All the through DT jigs out there have some sort of way to adjust this and their instructions all talk about adjusting for a perfect fit. I'm thinking of putting ruler tape (measure tape?) on each side to allow visual adjustment. Would prefer some sort of screw adjust - thinking about how to do that.


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## gmercer_48083 (Jul 18, 2012)

Phil, I still think a 3d printer would be the way to make the pins. Since they would be the most likely item to get nicked by a router and could be easily re printed as a set or individually. As far as precision they need to match each other as a set.

I watched a video where a guy made a cube 1"x 1"y 1"z in a file, then printed and measured the cube after printing. The cube had the x y and z axis printed on the cube, so when he measured the result...he then knew which axis needed adjustment.


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## PhilBa (Sep 25, 2014)

Design update. Family issues have slowed me down a bit and several funerals have eaten time (go to other people's funerals, otherwise they won't come to yours) but finally got some more time to work on it.

Here's the latest. Biggest change was to add screw adjusters to control moving the finger platen. While it adds a little cost ($10ish), I think it makes it easier to use. I also did a lot of spec work on HW and some test cuts for fit. Cleaned up my drawings, hopefully much clearer. Started on a bill of materials though it's not done until the fat lady sings. Cost of hardware looks to be about $25. Anyway some pictures. As was said before, I'm interested in your feedback and questions.

Next steps are build the base, platen and cutting test fingers with 1/4" mdf (saving the lexan for when I'm done with the tweaks). Still need to figure out the tension springs for the platen adjuster.


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## Bstrom (Jan 2, 2020)

DesertRatTom said:


> But to be honest, at $240, I think I'd rather just buy one.


I agree. I did. Super simple to use and comes with the correct router bits.


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