Trouble with half-blind dovetails


I have used through-dovetails on a very few occasions in the past, which I cut by hand with OK results. However, I'm faced with making about 20 drawers, so I bought a dovetail machine (Porter Cable 4212) to speed the process along. I've been experimenting with half-blind dovetails and I've gotta tell you, I'm not having a lot of luck. I seem to be able to get them so that they're so tight they can barely be squeezed together with a clamp, or too loose. When they're too tight, I reduce my depth of cut, and vice versa. I seem to be able to make just about anything except for a tight-fitting joint that goes together with slight pressure.
There don't seem to be too many variables to this equation, which is why I can't figure out why I'm having so much trouble. If anyone has some wisdom to share, I'm all ears. In the meantime, I'll be playing with it some more.
todd
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wrote:

I don't know if this applies in your case or not (you know how some people always try to relate others' problems to their own? :-) ), however, you may want to verify that your router bit is truly centered in the guide bushing. I found (after making 12 drawers with less than stellar success) that the bit in my Bosch is off-center by approximately 1/32 or better relative to the guide bushing. This caused all manner of difficulty in getting reasonable fit. Even when making sure that I held the router in exactly the same orientation for all cuts through the machine, I still had issues because I couldn't exactly match that with the mating piece.
It's probably worth checking just to eliminate that as the source of your difficulty.
+--------------------------------------------------------------------------------+ If you're gonna be dumb, you better be tough +--------------------------------------------------------------------------------+
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I couldn't get my Harbor Freight jig to work to save my life. I bought an omnijig and it worked very easily. I presume your 4212 is more like an Omnijig than like a HF?
Assuming you are following the directions.... The wood has dead flat or you will get uneven results. Unless you face planed the wood, you can expect difficulties.
I don't think the centering thing is likely. If you run your router through in the same orientation every time, it should automatically compensate.
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Ok, every DT router bit has its sweet spot. For half blind DT's you must use the exact same depth setting for a proper fit regardless of material thickness. Different bits of the same size will have different sweet spots. Use a shallower cut to loosen the fit and a deeper cut to tighten the fit. DO NOT rotate your router while cutting to minimize the effect of the guide bushing not being perfectly centered with the bit. Make SLIGHT adjustments and DO NOT try to repair previous cut tails and pins. Start with new surfaces. Once you have the "perfect fit" keep a scrap as a guide to set the bit depth for future projects. Also different types of wood will fit differently than others.
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wrote in message snip DO NOT rotate

Gee, this sounds like making them by hand might be simpler. :-)
Seriously, if you've developed some dovetail hand skills, it makes adjusting machine cut ones easier. You might adjust tails that are too tight with a sharp paring chisel. If they are too loose, a sliver of wood glued in the joint will tighten them up.
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I don't know. We are talking Half Blind here not Through.
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Just a follow-up. I went back out and did some more experimentation, taking careful notes of the changes. I had also been rotating the router while cutting the dovetails. This time I kept the router position fixed while cutting. I was able to converge on snug dovetails in just a few tries. I now have half-blind dovetails that I'm happy with. Thanks for your help.
todd
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wrote:

Playing with it some more might be the best answer. and document what you did each time so that when you hit the sweet spot you will have it in the future.
I've had similar problems using the predecessor to the 4212 and an import 12" dovetail jig that was so bad I returned it. Went through what you have gone through. Most trouble was doing different species (Cherry drawer fronts, poplar sides, I think the poplar might even swell a little after the cut). I even experimented with turning a few thousandths off of some guide bushings to try to get it closer. If I get to the point I can touch the pins with sandpaper and then tap them together with a dead blow hammer without breaking off the corners, I'm happy.
Frank
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Then you are doing something wrong. I set my omnijig, made a dozen drawers, and they all went together perfectly. Okay, I have only used it once, but presumably I didn't just get lucky.
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Well a little presumptuous on your part. The predecessor (now discontinued) to a 4212 is not nearly an omnijig. I also have a 7116 Omnijig and do not have problems with it other than matching up tapers with the sliding tapered dovetail template. I find that tricky.
I set my omnijig, made a dozen drawers,

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Went through this with Bay Area Dave a while back. He, and perhaps you, overlooked a parameter - bit slip. Dovetail bits want to "climb" out of the router collet, making deeper cuts. Doing dovetails, blind or through on a router table the easy to remember rule is Heighten To Tighten, Lower To Loosen. If the bit slips and "climbs" you end up with tight or too tight joints.
Check the collet for crud, dings etc. as well as the bit. Clean 'em up, tighten the collet firmly and you're problem may go away.
OR
You could be setting the bit too low in the collet and the collet's trying to grab in the tapered part of the bit below the lower cutting edge. Since the collet has a very limited range to work properly, a little movement down the taper can raise hell with dovetails.
Set the bit a little higher int he collet and then adjust the depth of cut.
OR
You''re parts may have a slight cup or twist. Doesn't take much to go from snug to too tight.
OR
The ends may not be square
OR
You might want to look into a lock miter bit
Like many things in life, what seems to be simple often isn't - until AFTER you figure out what you've overlooked the first half dozen times.
charlie b.
.
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