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Discussion Starter · #1 ·
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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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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Discussion Starter · #3 ·
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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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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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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Discussion Starter · #6 ·
...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...
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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Discussion Starter · #7 ·
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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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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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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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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Discussion Starter · #13 ·
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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I would think many of your parts could be printed on a 3D printer. Just a thought.
 

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Discussion Starter · #15 ·
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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Discussion Starter · #17 ·
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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Discussion Starter · #18 · (Edited)
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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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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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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