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Measuring the angle of a dovetail

How about applying some practical trigonometry ? If measurements were taken of the height of the dovetail and the widths at the "top and bottom", there would be enough to calculate the angle of the dovetail:

Call the narrow ("bottom") measurement of the dovetail "A"

Call the wider ("top") measurement of the dovetail "B"

Call the height of the dovetail "Y"

Call the angle the dovetail makes with a vertical line "c"

If we draw the end view of the dovetail and dimension it as per above, the tangent of angle "c" = [ (B-A)/2 ] divided by Y

to get the dovetail angle, take the arctan of tangent of "c". This will give "c" in degrees. The dovetail angle = 90- c

Using the 'nibs' on a vernier caliper and a depth mike, the measurements should be accurate enough to get a close calculation of the dovetail angle. I do not think dovetails were made at odd angles (such as an angle expressed in degrees + minutes and seconds of arc).
 
Measure over the OD of two different size sets of pins or dowels with a Mic & then do the calculation. I'm assuming this is something like the cross slide dovetails on a lathe

Good luck,
Matt
 
In practical terms, all the machine dovetails I have ever had to re-machine were either 60 or 45 degrees. I have an assortment of different diameter dovetail cutters in both angles. The cutters make a decent gage to see if the old part angle is close enough to use the cutter to mill new surfaces. So far, it has never been a problem.

Now, if the question deals with wooden furniture dovetails, all bets are off and you need one of those sets of dovetail gages sold by the fancy woodworking tool stores.

Larry
 
It is the dovetail on the base of the taper attachment for my 14-inch cone head Sidney. I am reasonably certain it is a 40-degree angle and this seems to have been a proprietary measurement used throughout the machine...I doubt it's anything really odd - there simply isn't any reason it would be.
 
Make a copy with some bondo Eighter oil the surface or hold some plastic foil in between
Then use your protractor in that copy piece

BTW The most commen ones are 55Dgr here
Peter
 
I have the Starrett C359 protractor set with the 2 blades and acute angle attachment. The ends of the blades and attachment are angled so they can fit into a dovetail. Not cheap but I sure a machine shop near you will let you borrow one.
 
There are several good ideas here that I hadn't thought of but I took John O's advice and tried my threading "fishtail" in the dovetail...and it fit perfectly. So, it's a 60-degree angle. I suppose that would be obvious to most of you but I have a difficult time visualizing angles so I'm thinking "why didn't I think of that".
 
You could also make a casting of the dovetail with either wax or low melting point metal alloy. Then its easy to measure the angle. This will also allow checking for uneven wear.

Tom
 
Joe M.'s trig process is the best if it does not simply fit a gage you have. Another way to apply it is with two dial indicators, and a straight slide or flat surface.

Position one indicator to read the height of a point near the edge of the dovetail. Make sure the travel of the indicator is vertical (perpendicular to the flat surface of the dovetail. A height gage can also be used for this.
Position another to display horizontal travel parallel to the width of the dovetail. Move the dovetail some arbitrary distance laterally, then read the height change on the vertical indicator, and do Joe's math.
 
Machinery's Handbook, #31, Large print, page 697, has a mathematical version using a pair of rods to measure the dovetail. It may be possible to figure the angle using these measurements and solving for the cotangent.
 
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