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.