Dovetail geometry - Router Forums
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post #1 of 14 (permalink) Old 03-22-2005, 06:00 PM Thread Starter
 
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Cool Dovetail geometry

Are there any good primers on the relationships between dovetail angle, diameter and depth of cut vs material thickness, etc.

I'm getting ready to cut some dovetails and want to learn some more about the basics of the joint.

TIA
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post #2 of 14 (permalink) Old 03-22-2005, 10:50 PM
 
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The reply depends on whether you intend making hand cut dovetails or use a router to produce them.
Hand cut dovetails usually have a 7 or 8 angle depending on the material.
Dovetail cutters are usually similar angles and the fit depends on the depth you set the cutter at.
Tight fit, lower the cutter slightly.
Loose fit, raise the cutter slightly.
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post #3 of 14 (permalink) Old 03-22-2005, 10:57 PM Thread Starter
 
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What is the relationship between material thickness and bit size, same different??

Does material 3/4" thickness require minimum 3/4" long cutting section on the bit??

Does the template determine the bit angle ??

I have a Stotts and his wbsite talks about an 8 Deg. bit but what about 7 or 9 deg.??
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post #4 of 14 (permalink) Old 03-23-2005, 03:43 AM
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Zxxer, I dont think there is per say a standard for dovetails. Many of the jigs work off similar dimensions but I think that is mostly a copycat situation. Some of the nicest looking pieces have high angles or double dovetails, spacing set to all different patterns to create unique designs. Obviously a jig is designed to work in a certain set up, with a certain combination of rules. This does not mean it wont work when you change some of the parameters, just that you will need to confirm the results are what you are looking for.

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post #5 of 14 (permalink) Old 03-23-2005, 02:02 PM
 
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Quote:
Originally Posted by zxxer12
What is the relationship between material thickness and bit size, same different??

Does material 3/4" thickness require minimum 3/4" long cutting section on the bit??

Does the template determine the bit angle ??

I have a Stotts and his wbsite talks about an 8 Deg. bit but what about 7 or 9 deg.??
It depends on whether you want through dovetails or half blind dovetails.
The angle is personal preverence when hand cutting dovetails.
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post #6 of 14 (permalink) Old 03-23-2005, 10:56 PM
 
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With regards to the stots jig, look closely at the jig. The taper on the tapered fingers is 8 degrees, just like the 8 degree bit you use with the other side of the template. A 7, or 9 degree bit with this jig, will not produce tight fitting joints.
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post #7 of 14 (permalink) Old 03-24-2005, 05:40 PM
 
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Zxxer12,
Router cut dovetails are determined by available cutter angles. So, I think this begs the question, are you going to custom grind dovetail cutters or have them custom gound? Or, are you looking for visual relationships? Perhaps a design manual that talks about proportions would help. Can't help you there. I have a Craftsman Dovetail Jig where all the spacing and angles are fixed. I precut one dovetail, measure the width and angle with a dial caliper, refer to trig tables, move dovetail jig with dial caliper measurements, move router depth with dial caliper measurements, finish cut. It's worked perfectly so far. Good Luck with your search and have fun.
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post #8 of 14 (permalink) Old 11-07-2009, 08:10 PM
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Default There is a simple relationship

Hi. I'm a brand new member here, and this is an old thread. I don't really see an answer to the original question here. So it seems I have something to offer.

Many decades have passed since my F in shop class, I have 4 college degrees, and I am doing much better now. I am a "re-beginner" at woodworking.

Here is the math. If the explanation below is hard to visualize, please tell me. I can put an illustrated version on my web site.

Draw a sketch of the interface between the two dovetail-joined parts. It should look like a jagged line of opposing trapezoids. I'll try to do that with ascii art here (if you used a fixed-width font, it might work):

----- ----- -----
\ / \ / \ /
----- ----- -----

It doesn't matter whether this is a blind or through dovetail joint. The analysis below focuses only on the geometry of the joint interface. (How that lines up with your workpiece determines what kind of dovetail joint it is.)

Define some variables:

h = depth of cut (distance from top to bottom of my diagram above)

d = diameter of router bit at widest point

p = period of the dovetail pattern (distance from corner of one tail (or pin) to the next tail (or pin)

e = angle of dovetail bit. This is the angle between any of the slanted lines in my diagram and the vertical, and corresponds to the way bit angles are defined in typical bit specifications. (The total angle subtended by the bit, a pin, or a tail is 2e.)

The universal relationship between these variables is

h sin(e) = d - p/2

I just purchased an Incra Incremental Positioning Jig Model 25971. It comes with a couple of templates, one for box joint and another for dovetails. These templates are only good for two particular dovetail bit geometries and a couple of stock thicknesses. Rather than buy more templates, I can use this formula to make my own, for any bit geometry and stock thickness.

The distance between any line and the next in an Incra template is p/2, and the distance between any A line and the next A line or any B line and the next B line is p.

All I have to do is check and maybe calibrate the scaling accuracy of my printer and I can crank out any template for any geometry with a simple computer program. The printer scaling can even be slightly off, and the Incra incremental positioning jig will get it perfect as long as p is an integral multiple of 32nds of an inch.

Thomas D. Shepard, Sc. D.
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post #9 of 14 (permalink) Old 11-07-2009, 08:21 PM
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Hi Thomas and welcome to the forum. Thanks so much for such an excellent reply even if the thread is old. I am sure many here will benefit from the time you have taken to do this.

We hope you will elect to participate often, you have much to offer it appears.
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post #10 of 14 (permalink) Old 11-18-2009, 09:04 AM
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Quote:
Originally Posted by tdshepard View Post

h sin(e) = d - p/2
Oops! Slight mistake here. That should be a tangent function, not a sine function. That is,

h tan(e) = d - p/2

For the small angles typical of dovetail bits, it doesn't matter too much, because then sin(e), tan(e) and e (if in radians) are all approximately the same. And you're probably going to begin by making a test cut and adjusting the bit height (h) to get a good fit anyway.

Also, while I am splitting hairs, it wouldn't surprise me if the angles given on the bit packages are actually rounded to only 1 significant digit. When I set up to do dovetails on a router, rather that using trig functions, I just use the approximate relations

tan (14) = 1/4
tan (10) = 1/6
tan (9) = 5/32
tan (7) = 1/8

Thomas D. Shepard, Sc. D.
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