Wednesday, November 14, 2012

Video-Routing Dog Holes


This week at Benchcrafted we're building a Split Top Roubo for a long-overdue website update. We need to take some fresh pics of the STR with our new handwheels, and the Crisscross. We're also taking the opportunity to shoot some video of a few aspects of the build that we'd thought potential builders would find useful. 

First up is a video on how we cut square bench dog holes. You might be surprised how many folks think these are done with a chisel and mallet. They are actually milled into the side of the dog hole board (strip), capped with a thin piece, then glued to the rest of the top.

Getting all that wood out of the way is not easy. Even for machines. Okay, a dado head and a sliding table saw is simply the best way to do it, but not everyone has access to a machine like that. The next best route, ahem, is a router. 

Here's how we get excellent results, without burning or blowout, efficiently and safely. (You might want to read this post before you read this one further, we've been over some of this before.)

First we lay out every dog hole and draw its extents clearly with a pencil. Then we mark a big X where each hole goes. Once you get rolling and moving the template around its very easy to mess up locations. The big X really helps. 

Then we use a square and marking knife to scribe across the grain at each hole, on the top edge and bottom edge of the dog strip. This helps prevent blowout from the router, and makes a really clean, crisp opening. Don't skip this step, although we say its sort-of optional in that older post. Ignore that. 

We use two routers. The first one uses a template guide that keeps the bit about 1/8" away from the template. This is a hogging cut, but requires care to execute. It get lots of material out of the way so the second router can work at ease and make a fine cut without a lot of load.

The other line of defense in eliminating blowout is to use climb cuts--that's when you move the router in the same direction the bit it rotating. It's dangerous if you're unfamiliar with the technique, so read up and practice if you're uneasy. Two of the four arrises in each hole are favorable for anti-climb ("regular" against-the-rotation) cutting, and two require climb cutting (anti-climb cutting would blowout the unsupported grain). If this all sounds confusing, just keep this in mind. When riding the template (with both router setups) you ALWAYS take a cut by entering from the outside and moving INTO the hole. NEVER route by entering from one end, then riding the template out the other end--you'll blow out. Entering in an anti-climb cut, you'll need to move away from the template about 2/3 of the way through the cut, exit, then re-enter the same side--you'll now be climb cutting. DO NOT start routing with a climb cut. By beginning each side anti-climb and routing away almost all of the material, you leave little chance for the router to "bite" when you re-enter for the climb cut, since there isn't much material left to bite. 



Here's the final line of defence against blowout. It's a cutter I call "super bit". A machinist friend gave me this bit some time ago when it became too dull to do quality milling in metal. Its a solid carbide, four flute, center cutting end mill. And it is simply amazing. A couple months ago I took this bit to Kelly Mehler's School of Woodworking to teach a class on building workbenches. This bit saved the class. I bet we milled close to 100 mortises with it, in hard maple. I still have not sharpened the bit. I just  clean it. That's it. The cutter is 1/2" diameter and the smooth shank is exactly the same diameter as the cutting diameter. So I use the smooth portion of the shank as a bearing. It's the best pattern bit I've ever used.

The cutting geometry of the bit allows it to cut wood, especially hard, dense, figured wood, with virtually no regard to grain direction. It will cut end grain maple and leave a silky-smooth surface in its wake. It is in a word, awesome. Make sure you click on the image below and appreciate the quality of cut in this hard maple.


  And finally, here is the video.



6 comments:

  1. Jameel,

    How about clamping sacrificial pieces along the edges so you don't need to be quite so concerned about blow out?

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  2. Jameel,

    I'd love to know the brand (and model number if available) of your "super" bit. Sounds like it would do a great job routing out neck mortises and pickup cavities, on electric guitars, too. Feel free to email me directly, if you'd rather. No worries if nothing is available.

    Cheers,

    Lee

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  3. A good source for end mills: http://www.american-carbide.com/index.php

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  4. Tico,

    Sacrificial pieces are okay in certain situations, but here, since so much material is being removed, the sacrificial pieces would have to be quite large to stay together after several dog holes have been routed. Plus, additional clamps would be necessary, slowing down the setup for each hole. I like to have as much control of the blowout as possible, and that means the combination of the "fire line" of scribe lines and router control.

    Lee,

    Most four-flute end cutting end mills will do the job. Tico's source looks good.

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  5. Jameel,
    Love this post, and everything you write. As a former machinist, I just want to make a couple comments.
    An "anti-climb" cut has a name--it's called a "conventional cut." At least, that's what every machinist I've ever worked with calls it. On manual mills (like a Bridgeport), it's a common practice to take a heavy conventional cut, followed by a light climb cut--exactly what you're doing here.
    Also, while I love carbide endmills in a router, I've never understood why woodworkers go for 4-flute endmills. In a machine shop, you use 4 flutes for hard, ferrous materials (steel, stainless, etc) and you normally use 2 flutes for softer stuff like aluminum or brass (or plastics, like delrin). The reason is that when you're cutting softer stuff, you feed a lot faster, and 2 flutes gives you much better chip clearance, and thus a better finish. In a router, the bit is spinning much faster than in a mill, and you should be feeding pretty fast, so 2 flutes would seem to make more sense to me. Just my 2 cents!

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  6. Thanks Steve. Conventional cut, I'll have to remember that.

    I can't explain why super bit is so, well, super. But I wouldn't give it up for any 2 flute bit I've used. And I've used top quality ones for years. They work, but super bit leaves the best finish and affords a sense of control that no 2 flute bit I've ever used can.

    ReplyDelete