Angle for dovetails

[Dan Gill]

Okay, I'm ready to start playing with hand cut dovetails. I know the traditional 1:8 ratio for hardwoods. My question is - what angle is that? You can tell I'm a complete newbie at this.

[Bob Smalser]

Draw a 1:8 slope on graph paper...or better yet, a dovetail gage profile so you can see it.

Drawing layout problems to scale then taking angles from them is an important technique.

You can even make yourself a set of 1:6 and 1:8 dovetail squares by cementing that graph paper drawing to sheet brass and sawing/filing them out.

I make them the width of my TS slots so I can use them as a dead-square gluing jig:

http://pic3.picturetrail.com/VOL12/1104763/2594266/68779301.jpg

(Note the engraving on the back of that scrap brass. ;) )

[Dan Gill]

Bob, you're a good teacher. You don't give me the answer, you tell me how to find it on my own.

This leads to another question, though: Why do we speak of this as a 1:8 ratio instead of just an angle? There has to be a reason that goes back to the Dawn of Woodworking Time. That's just my natural curiosity coming out.

[Marc Hills]

This leads to another question, though: Why do we speak of this as a 1:8 ratio instead of just an angle?

Basically, because when that convention was established, a carpenter or cabinet maker was unlikely to own or use a protractor. Even an accurate rule was often a precious and rare commodity.

The journey man woodworker probably never knew what the dovetail angles were in terms of degrees of arc, nor would he have cared.

If you know a dovetail angle in terms of a ratio, then you can accurately set your bevel gauge to the correct angle without being dependent on any measuring device.

In times of old, woodworkers commonly made do without precision measuring tools, or even plans, and their work was often more accurate than it is today. For example, using a story stick allowed woodworkers to transfer critical measurements directly to the wood to be worked, which drastically reduced measuring errors.

It's a mind set that the modern woodworker would do well to adopt. The other day I was laying out mortises in each of four table legs. 2" long, beginning 20 1/2" from the top. I took out my tape measure and measured from the 1" mark because I don't trust the first inch with the hook. I almost made the "too short by an inch" error (don't tell me you don't know what I'm talking about!) and then had to force myself to put it away after marking the first mortise location. At that point it was just a error waiting to happen. Instead I lined up all four legs and used a square to transfer the marks from the first leg. Faster and more accurate.

Not to be a Ludite, but sometimes the old ways really are better. :)

[Chris Padilla]

The 1:8 ratio is simple math, Dan. Good ole trigonometry: rise over run.

For a rise of 1 (inch, for example, the units don't really matter), you want a run of 8".

Make yourself a right triangle with one leg at 1" and the other at 8". Take the arctan of 1/8 and you'll get 7.125°. 1:6 will get you 9.462°. Make or buy your own gauge and use whatever you like best. There is no correct angle.

[Dan Gill]

. . .simple math, Dan. Good ole trigonometry . . .

"Simple math" and "trigonometry" are contradictory, or at least they are in my book. I was a victim of "New Math", and I've never recovered. But I can draw a 1" x 8" right triangle.

Thanks for the help, folks.

[Bob Smalser]

You don't give me the answer, you tell me how to find it on my own.




Important stuff for next time.

Think you could build an entire boat accurately working just with a table of offsets and no other pictures?


http://home.att.net/~shipmodelfaq/smf-q078a.gif



Sure you could....you just lay it out full-size on white-painted plywood sheets, drawing each offset measured upwards from a baseline at each "station" or measuring point you draw at the bottom of the sheets. Done in two and sometimes even three full or partial profiles, every size and angle off odd-shaped pieces impossible to visualize otherwise can be got off of that "lofting" with surprising accuracy.

But that's also one of the reasons boatbuilders were some of the last professional fonts of hand tool technique before the Great Craft Revival....because the bigger, odd-shaped hunks taken off those loftings don't fit in machines and usually have to be hand fit to the structure with hand tools.

Even in uptown cabinetmaking, folks often ask about "formulas" for obtaining stave angles for curved work. Shucks....even if I had a formula, I wouldn't trust it and would simply draw it to scale to get my angles.

[Keith Christopher]

The 1:8 ratio is simple math, Dan. Good ole trigonometry: rise over run.

For a rise of 1 (inch, for example, the units don't really matter), you want a run of 8".

Make yourself a right triangle with one leg at 1" and the other at 8". Take the arctan of 1/8 and you'll get 7.125°. 1:6 will get you 9.462°. Make or buy your own gauge and use whatever you like best. There is no correct angle.

The other thing about ratios is they are independant of distance or size. So larger and bigger still maintain this ratio and it will work.


Keith

[Chris Padilla]

Keith,

Correct, they are unitless so as long as whatever you are using for a measurement is consistent (5 pinkies, 4 hands, 1/2 furlong etc.), the angles are all the same.

[Rick Hoppe]

Structurally, anything from about 1:5 to 1:8 will work with hardwoods. Beyond that, for me, it’s more of an aesthetic consideration.

Let’s say I’m dovetailing a small drawer from 3/8”or ¼” stock. A 1:8 slope will look like a box joint. 1:5 will give me more of a definite triangular shaped pin.

On the other hand, if I’m dovetailing a chest from 1” stock, a 1:5 slope will look too broad and triangular for my taste. A 1:7 or a 1:8 slope will give me that “fine pin” look that I want.

[Alan Turner]

The angles of dovetails in 18th century furniture varied greatly. I am building a reproduction piece, for the first time, and the carcase and drawer blades are cockbeaded, instead of the drawers. First time I have ever done this type of detail. So, from photographs, and using an angle finder, the sliding dovetail joint at the drawer blade/carcase intersection is 18 degrees, which is pretty steep. These were fun handwork.
From the photos, it appears that all of the dovetails on this piece, from carcase joints to drawers to the pullout gallery/writing surface are at this same angle. I think this presents too much short grain for the structure, and so backed off the hidden joints to a more typical 1:6, and the standard drawers to a 1:8. The original had pine drawer sides, but I make drawer parts from hard maple.
Following rules has always been a bit of an issue for me.
Alan

[Bob Smalser]

Following rules has always been a bit of an issue for me.


Then try some of these Bermudan Dovetails....none of this is cast in stone once you understand wood. Also one of the reasons for that high-tension coping saw.

http://pic3.picturetrail.com/VOL12/1104763/2594265/71825009.jpg

[Dan Gill]

Important stuff for next time.

Think you could build an entire boat accurately working just with a table of offsets and no other pictures?

Sure you could....

Hooray! Bob said I could build a boat! Now I just have to convince my wife . . . :D

[Keith Christopher]

Then try some of these Bermudan Dovetails....none of this is cast in stone once you understand wood. Also one of the reasons for that high-tension coping saw.



Bob,

What is your reference for these ? I'd like to see an upclose detail of them.


Keith

[Chris Padilla]

Also one of the reasons for that high-tension coping saw.
Bob,

How do your boat building buddies like your coping saw? If you have any to sell, please drop me a line! :D I think it is simply wonderful to support those of you making tools for SMC/yourselves/buddies. I have a marking knife from Dave Anderson and if Leif or you have some stuff to sell, I'm all ears! :)

[Pam Niedermayer]

Bob,

How do your boat building buddies like your coping saw? If you have any to sell, please drop me a line! :D I think it is simply wonderful to support those of you making tools for SMC/yourselves/buddies. I have a marking knife from Dave Anderson and if Leif or you have some stuff to sell, I'm all ears! :)

Here's a chisel set on ebay (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=303&item=6127228595&rd=1&ssPageName=WDVW) Bob's selling through the elizabeth seller. No coping saws yet.

Pam

[Bob Smalser]

Kieth,

The Bermudan Dovetail diagram came from a museum in Bermuda and was passed on to me by a boatbuilder there. That's all we have of it, but it's likely an old ship's carpenter's tool chest from a month or so of being becalmed in the doldrums somewhere. A future project....playing with those decorative dovetails.

Nice thought, Chris, but the coping saw article was for y'all to make your own, not for me to sell them. Way too many seperate operations involved to ever be able to produce them at that level of shape and finish for anything other than a special gift...even though I've made dozens of frame saws over the decades, usually in a modest run for extras to trade or give away.

http://pic3.picturetrail.com/VOL12/1104763/2594266/71906896.jpg

My blacksmith friend and I may make a larger run of mesquite/tool steel caulking mallets later this winter for sale to boatbuilders, but they'll go for $200 or so each with about a fourth of the labor involved in one of those little saws.

http://pic3.picturetrail.com/VOL12/1104763/5313557/67908857.jpg

As Pam points out, I do sell excess on Ebay occasionally. When I get a request I can't turn down from a tradesman pal (most of whom don't use computers) for an old-time set of whatever tools with custom handles, I buy up large lots and sell off what I can't use for a set. Making something more useful out of the give-away shorties and a myriad of unneeded handles below is fun, and I recommend it...why pay 30-80 bucks each for modern chisels when even the best classics average me less than 6 bucks in lots over time?

http://i18.ebayimg.com/02/i/02/b3/41/df_12_s.JPG

Moreover, folks....why should you buy that mortise chisel set I made up for what may be another ridiculous sum when you can do all this yourself on the cheap?

http://i9.ebayimg.com/02/i/02/aa/94/e8_1_b.JPG

Here's a 30-dollar lot of junkers I got last week with two of those valuable mortise chisels hidden therein. 5 bucks each...and one is a Swan.

[Rick Hoppe]

Bob, the photos of your home-brew and rehabbed tools are beautiful! You’re an inspiration.

Keith, here’s a one-page article from issue #35 of Fine Woodworking, apropos to Bermudan dovetails:

[Chris Padilla]

Very cool...doesn't look too terrible to do...with the right tools of course! :D

[Pam Niedermayer]

Jeff Gormon's treatise on Moorish/Bermuda dovetails (http://www.amgron.clara.net/dovetails/moorishdovetails/moorishindex.htm) is a little more informative/graphic.

His site is overall one of the most useful woodworking sites I've found. He hangs out on the Old Tool list.

Pam

[Keith Christopher]

Ohhhhhhh I LIKE THIS.....My scrap pile is going to suffer until I get this right !

[Marc Hills]

Jeff Gormon's treatise on Moorish/Bermuda dovetails (http://www.amgron.clara.net/dovetails/moorishdovetails/moorishindex.htm) is a little more informative/graphic.

His site is overall one of the most useful woodworking sites I've found. He hangs out on the Old Tool list.

Pam

Pam:

I second your opinion of Jeff Gormon's site. He has an absolute wealth of information and I keeping coming back to it. It is one of the best neander resources on the 'net.