Dovetail ratios

[Vince Shriver]

Two dovetail questions:

1. What does 1:8 ratio mean exactly?

2. How does one determine what ratio is appropriate for any given piece of work?

Thanks. I really appreciate the information I get from the many talented folks on this site. Vince

[Dave Mount]

1:8 is an alternate means of describing an angle, as opposed to "degrees". It means the same angle as would be described if you laid a straight edge across a framing square, with the edge touching at 1" on one leg of the square and on the other leg touching at 8". If I remember right, this is about 7 degrees.

Roof pitches are described in a similar way, rise over run. A "5/12" pitch roof rises 5 feet vertically for every 12 feet of horizontal distance.

With regard to what to use, a common recommendation is 1:8 for hardwoods and 1:6 for softwoods. These recommendations are based on some arguments related to strength. However, people use a lot of different ratios depending on application and aesthetics. In most applications, the specific angle is not that fussy a thing.

Best,

Dave

[harry strasil]

As I do living history demos and subscribe to the old school theory, I use what was recommended in the mid 1800's 10° for hardwood and 20° for softwoods, I don't really care for the little thin pins, so don't do them. I work mostly softwoods, pine etc and like to use sliding dovetails and I do use a tailed router for them at 14°.

[Pam Niedermayer]

20° in pine would chip away due to grain runout, at least in my experience, can't hold a tail in pine at even 10° or thereabouts. Of course, maybe you're talking about 20° another way?

Pam

[mike holden]

1:8 = for each unit over, you move eight units up
This is from when tradesmen did not receive formal education in mathematics.
They may not have even been able to read.
However, they *were* schooled in geometry and the ability to measure pitches, runs, divide angles equally, etc. using nothing more than a pair of dividers.

As regards the ratio to use on various woods, dont get your knickers in a knot.
Anything from 1:6 to 1:9 looks good and works well.
Too little rise leaves corners with short grain that easily breaks off, too much rise looks like a poorly made box joint.

As long as it looks good, and assembles without breaking, its a good joint.
Mike

[Mike Henderson]

20° in pine would chip away due to grain runout, at least in my experience, can't hold a tail in pine at even 10° or thereabouts. Of course, maybe you're talking about 20° another way?

Pam
I agree with Pam. A 1:8 ratio would be about 7* and a 1:6 would be about 9.5* A 10* would be about 1:5.65 and a 20* would be about a 1:2.75

I'd have a hard time keeping the the side of the tail from breaking off at a 20* angle. Perhaps we're misunderstanding what you mean by 20*.

Mike

[harry strasil]

Maybe I misunderstood the 20° in old time books I have. These 3 tool boxes have logged thousands of miles in a cargo trailer going to demos and being unloaded and loaded again every day and occasionally getting dropped. and are still as new. The constant bounce of the trailer is one of the worst torture tests on most anything.

http://img.photobucket.com/albums/v81/irnsrgn/wood/box1B.jpg

http://img.photobucket.com/albums/v81/irnsrgn/wood/box2B.jpg

http://img.photobucket.com/albums/v81/irnsrgn/wood/box3.jpg

of course there is a world of difference between old wood and the new stuff.

[harry strasil]

The two dovetail marking gauges I use all the time.

http://img.photobucket.com/albums/v81/irnsrgn/wood/dovetaillayoutjigs1.jpg

http://img.photobucket.com/albums/v81/irnsrgn/wood/dovetaillayoutjigs2.jpg

[Wilbur Pan]

I once read a blog post by Glen Huey where he mentioned that he always uses 12 degree dovetails, regardless of wood type. By my math, that's a 1:5 ratio (actually 1:4.7). I asked him why he didn't use the usual 1:6 for softwoods/1:8 for hardwoods rule, and his reply was:

1. He likes the way 12 degree dovetails look.
2. He has never had a problem with corners chipping out or the joint failing due to the angle.

He also pointed out that there are dovetail router bit sets (I know, I know) with 14 degree angles, and that there's the issue of making a drawer with softwood sides and a hardwood front, so which rule do you use?

So I guess the bottom line is, use whatever you think looks good, as long as your corners stay intact.

By the way, if in the future, I hear people saying something along the lines of, "Oh, that must be a Vince Shriver piece -- note the signature 15 degree dovetails," I'll be expecting my cut. ;)

[harry strasil]

This is my only attempt at precision laid out dovetails with modern angles to the tails. It looked to machine made so I don't try that anymore. Most of the time I just scrib a depth line and just start cutting without laying out any angle and I mark the pins from the tails, there is no way you can duplicate my dovetails with a tailed router and it gives it that old hand done look. I usually start on both ends of the piece and work toward the center and just use my eye to space them out. Even when I do use the little layout jigs I just lay them out according to my eye, never measure. If there is enough room in the center for one more, fine, if not that's fine too.

Sorry I don't conform to the ideas and ways good WWers think things should be done.

http://img.photobucket.com/albums/v81/irnsrgn/wood/sewingbox.jpg

http://img.photobucket.com/albums/v81/irnsrgn/wood/swngboxback.jpg

[Pam Niedermayer]

Jr., I don't layout dovetails, instead I cut the tails freehand and mark the pins from the tails. When I've built shop furniture in pine and have actually made dovetails approaching 20°, they've blown out. Immediately. So I'm speaking from experience, not some theory or other.

Pam

[Wiley Horne]

Vince,

You probably gathered this from context, but maybe it's worth a mention anyway....The ratio 1:8, or any other ratio, refers to each half-angle of a dovetail. As Dave Mount said, 1:8 works out to a little over 7 degrees, and that's for each side of the dovetail. The total included angle of the tail is a bit over 14 degrees.

One reason why specific ratios are not magic, is that a given ratio will look entirely different depending on the pin entry width, and the length of the pin. A ratio like 1:8 or 1:7 can look very dramatic if the pin entry is tiny, like 1/32", and the pin 3/4" long or more. But the same ratio will look very uninspiring if the pin entry is, say, 1/2", and the pin length is 5/8". You can check this for yourself on the back of an envelope by drawing a long V to represent the dovetail and then visually isolate the narrow end, and then isolate the open end--and compare. It's the same ratio, but the look is totally different.

Wiley