Half Blind Dovetails

[mrdovey]

Dave...

Well, I just did a test and can report that they fit just like they did yesterday.


Hmm. It never dawned on me until just now to add a parameter to specify the width of an optional glue gap. Oh well, back to the drawing board...

...Morris

[mrdovey]

Mike...

For now, I've set up the programs to make multiple passes, stepping down the diameter of the bit for each pass. I could probably get away with stepping down as much as three diameters; but I only had one sharp 1/8" bit on hand.

The part programs adjust step down value and compute points on the toolpath based on the current bit diameter (gotta love access to those system variables!)

Just out of curiosity, what were you thinking?

...Morris

[mikejohn]

Morris
I am looking hard at furniture joint making over the holidays. (Frowning if I trundle off to the workshop from nearest and dearest, so its computer theoretical stuff).
My thoughts so far are:
Precise positioning of materials square to x,y and z axis, and precise to 0,0,0.
Getting a joint that fits well, but not to tight.
Wondering if traditional joints are still the best joints now we have the capabilities of the 'bot.
Wondering if its possible to build a library of .dxf joints to use in different situations.
Trying to balance accuracy with speed.
Which is where the depth of cut came in.


..............Mike

[mrdovey]

Mike...

Ahh... Exploring the zen of woodworking - one of my favorite recreations! Then let me share a few thought fragments...

My positioning efforts have been much facilitated by adding a reference edge to my table. It can be removed when necessary; but for 99% of what I do on the 'Bot, it provides dead-on alignment with the x-axis. Since I machined the reference edge in place, I know exactly where it is and use it as a reference point for y=0.

Dave Rodda inspired me to cut a set of dummy bits from drill rod. I have 3" lengths in 1/8", 1/4", 3/8", and 1/2" diameters. I can chuck one of these into my most-commonly-used collets and use the 'Bot itself as a sort of stop block for precise (to the limits of 'Bot precision) workpiece positioning. You'd probably benefit from making a similar set in metric sizes.

I like joints that almost don't need glue. I've discovered a couple of CNC-only joints that tighten with any change in moisture content without causing fracture - there's one such on my web site. This is the joint that Dave was yanking my chain about (he's visited the shop and seen it first-hand). [That joint came about as a result of Dale Kerr's nudging to produce a tilt-top material handling cart here on the forum.] From my perspective, the only way a good joint can be detected is by the interruption of the grain pattern at its boundries.


The traditional joints are good; but modern glues provide a strength that in the past could only come from the joint structure itself with a much lesser strength contribution from the glue. I like the idea of making high quality dovetails - not because they're (any longer) stronger than other joints - but because by producing them well I feel as if I'm honoring the incredibly skilled craftsmen of the past who didn't have precision robots and glues that bonded more strongly than the wood fibers themselves.

I took a shot at building a library such as you suggest a couple of years back and gave it up because I could imagine no way to relate the drawings to the hugely different behaviors of different woods in different environments. The effort to build a meaningful library struck me as overwhelming. Worst of all - the more I learned, the less I knew.


After all of that, it's got to sound pretty lame when I say that I went with a shallow depth of cut because I didn't want to risk breaking my last 1/8" bit on a preliminary test...

...Morris

[charles_o]

Morris
A couple of questions

How long does it take to cut a joint like the one above.
Have you tried to do this with a dovetail cutter to speed production

Charles

[mrdovey]

Charles...

This first test took almost six minutes on my PRT. I think that could be cut in half with a more agressive stepdown value; but that'd be pure conjecture at this point. I will guess that it could be done some faster on a PRT with a faster computer and (perhaps) very much faster on an Alpha.

I haven't tried to do this with a dovetail cutter on the 'Bot because I already have a conventional dovetail jig. I was after some of the things that aren't generally options with normal dovetail cutters:

[1] The higher strength and greater gluing area of the full rectangular cross section joint elements.

[2] The ability to use dovetails with small angles - one of the hallmarks of hand-crafted dovetails.

[3] I wanted the ability to use dovetails of different geometries in a single joint - which includes not only mixing wide and narrow dovetails; but also asymetrical dovetails.

I've seen photos of attempts to use specialized dovetail bits on CNC machines and I really didn't care for the quality of the resulting joint. The photos Jay sent, for example, show a CNC-produced joint that (1) left a tail-sized semi-circular void hiding behind the tail and (2) the tail part required a shoulder large enough to cover an unsightly top radius.

My reading is that a specialized dovetail cutter could be used to speed production and lower costs - but that the quality of the result would make the product not worth buying.

If I were making speed a priority, I'd go for a second (tandem) z-axis and a second stepper for my y-axis - and if I had the money to spend, I'd opt for a tool changer so I could use a large bit to hog out the bulk of the material and only use the small bit to dress the inside angles.


...Morris

[stickman]

Morris,

What you have got there, looks really good, I like how you used just one bit to get your results. It looks as though you might have something there you could easily convert to a through type dovetail joint. Even with the slight radius, that could be part of the joints feature.

Look forward to seeing more.

[gene_marshall]

Morris Doveytail joints, I love it.

I have a few drawers to make this week and would like to try this.

gene

[mrdovey]

Thanks, both Jay and Gene.

Jay, both through (common) dovetails and full-blind dovetails can already be achieved by changing the existing cutting parameter variables.

I don't particularly care to have noticably radiused edges; but they're controllable to a reasonable degree by choice of bit diameter. With a single bit, there's an inverse relationship between size of radius and cutting time. I rather like that the radii in the finished joint will clearly signal the maker's prioritization of quality.


Gene, there've been photos of my first two prototype jigs in plain sight on my web site for well over a year. The jig isn't complex, but needs to be built and trued with exacting care. My software won't be available until it's been thoroughly tested; but there's nothing to stop you from whipping up a jig and using your favorite CAD package and PartWizard to turn out dovetailed drawers for your project.

You'll probably have your drawers finished and installed before I'm able to satisfy myself that I have a safe and accurate general solution - and I'm not sure what the impact of incorporating a specifiable glue gap [Thanks a lot, Dave!
] is going to be.

...Morris

[mikejohn]

Morris
For a specific glue gap, can you not tell the program your bit diameter is smaller than it really is, by half the glue gap?
.............Mike