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I am trying to cut some 3/8" sliding dovetails into the edge and end grain of different 1" thick hardwoods. The problem I am having is the bits are breaking at the neck just above the collet when I am trying to cut the female groove. I am really going slow and goodness knows I have enough horsepower and a stable platform (Porter-Cable 75182 in a Woodpeckers VRL II lift and a Woodpeckers Incra LS fence system on a custom built table with a 2 hp dedicated dust control system).

I have tried cutting the slot partway, vacuuming out the slot, and restarting the route, but as careful as I am there always seems to be an irregularity around the place where I stop and restart that makes the sliding dovetail catch and not slide smoothly.

Open to suggestions including successful techniques or recommendations on a different hardwood to try. I have been trying to do this with Bird's Eye maple and walnut.

Thanks,
Kurt
 

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TRy making a saw kerf or two down the center of the dovetail before you run the dove tail bit down the cut.

Also make sure you are not trying to cut too deep with the dove tail bit. there are no cutters on the shank and if you are allowing the shank to rub in the bottom of the dovetail slot it will add extra heat to the bit and the bit will snap off. It is the same as the "T" slot bit, it needs a place for the chips to go.


Herb
 

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Listen to Herb...
instead of saw kerfs.. cut a full sized slot w/ a straight bit..
use furniture wax on your slide for better performance...
 

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Frank
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I just completed a sliding dovetail for the lid of a blanket chest. It was for the end molding. Mine was a 1/2” dovetail so I used a 5/16” straight bit like Stick’s recommendation. It really made cutting the dovetail easier. I will posting pictures soon.

Frank
 

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Table saw kerf or straight bit first before you try hogging out the whole dovetail. (upspiral bit is good at clearing material for this as well as great for mortises)

Also, you have the option on wider sliding dovetails of using a 5/16, 3/8 or 1/2" inch shank straight bit (with proper collet sleeve) if your router is capable, you'll get less shank deflection under heavy sideloading in hard maple than with a 1/4" shank bit.

Slow feed speeds, keep chips clear (this is where upspiral straight bits help and you're unlikely to get tearout in hard wood) and don't let your bit overheat and scorch/burnish the wood. Gotta be SHARP.

Lee Valley has some great Onsrud upcutting spiral bits in the 5/16" and 3/8" shanks in both HSS and carbide.
 

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Yes, clear the bulk of the wood out of the center of the joint and then just use the dovetail bit to make it the correct shape. Dovetail bits don't handle heavy loading, as you found out. Up-cut spiral bits or table saw can be used to get the bulk of the wood out of the center part of the joint.

The dovetail bits won't have any trouble making the tails part of the joint, but any warping of the board receiving the tail will bind in the dovetail slot, requiring you to clamp the board flat before installing it. If this joint will be one time assembly, just use the clamp or very slightly narrow the width of the dovetail enough to get it together. If you want the joint to slide together and apart easily, use a really flat and stable board.

I cut my sliding dovetails on my Leigh D4R jig. It holds slightly cupped boards really flat when cutting the joint due to the bar clamps on the jig. The cut comes out perfect, but trouble frequently arises during assembly when the board has relaxed and slightly cupped again. I made a pair of clamping cauls using 3/8 all thread, nuts, and washers that I put on the tail board and I force it flat again with this clamp before sliding the joint together.
I add glue to the last inch or so that will be the end of the joint that is seen in the finished project to keep that end of the joint correct during expansion and contraction from humidity changes.

If the joint must be loose so you can always slide it apart and back together easily, again, use a very flat board with stable dimensions, cut the joint a little loose, and add paste wax to the joint surfaces. Add surface finish to all surfaces of the boards to seal out as much moisture as possible.

Charley
 

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I read an article not too long ago in FWW that suggested that a sliding dovetail can be nearly impossible to assemble so the article showed how to taper it so that it only got tight at the very end of its travel.
 

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You can combine the table saw and straight router bit. First, tilt the table saw blade to match the angle of the dovetail bit; make one pass, rotate the board and then a second pass. Now waste the center part of the dovetail with a straight router bit. Unfortunately, the more operations you perform, the more chances of screwing it up. Good luck.
 
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