Dovetails Measuring Calculator



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Entry fields for the dovetails You enter the S, D, H, and Clearance values, the calculator figures out the Y, T, and missing S values for you. All values are unitless. Doesn't matter if you use inches or mm as long as you are consistent with all entries.

Enter the inner distance between the two dowels here. You can use a digital caliper for this or you could use adjustable parallels, which you would lock into place between the dowels, and then measure with a micrometer. If you don't have one of the parts, for example if you don't have the female part, but you want to make it by taking measurements off the male part, just leave the female S value blank. Once you get the male Y and male T values, you can use them to figure some reasonable values for the female Y and T values. All you have to do is make the female Y a bit larger than the male T and make the female T a bit larger than the male Y. How much larger is up to you, depending on how tight a fit you're looking for. Leave this blank for now if you don't have the missing part, and then use the Target S button to figure out what you need the S value to be.
Enter the S Female (top red line) Measurement:
(will be calculated if left blank)

Enter the diameter (D) (green line) of the dowels:
Enter the diameter of the dowels here. I'm calling them dowels, but they can be anything cylindrical shaped, such as dowels, pins, pipes, drill bits, so long as they have a known diameter and come reasonably close in size as to match the scale as depicted.

Enter the angle (usually 60 degrees) of the dovetails:
Enter the angle of the dovetails here. These are usually 60 degrees, but you might find some 45 degree ones, too.

Enter the S Male (bottom red line) Measurement:
Enter the outer distance between the two dowels here (outside to outside). If you don't have one of the parts, for example if you don't have the male part, but you want to make it by taking measurements off the female part, just leave the male S value blank for now. Once you get the female Y and female T values, you can use them to figure some reasonable values for the male Y and T values. All you have to do is make the male Y a bit smaller than the female T and make the male T a bit smaller than the female Y. How much smaller is up to you, depending on how tight a fit you're looking for. Leave this blank for now if you don't have the missing part, and then use the Target S button to figure out what you need the S value to be. (will be calculated if left blank)

Enter the height (H) (yellow line) of the dovetails:
Enter the height of the dovetails.

Enter the desired clearance (example: enter .010 if you want .005 clearance per side):
Enter the desired clearance (both sides combined, e.g. .010 = .005 per side):


Calculator Output Precision: decimal digits


Instructions:

Complete all fields above except for either the S Male or the S Female. Leave one of those empty and fill in the other one. The one left blank will be calculated when you press the Go button.
T and Y Values (read only) Check these measurements against the ones produced below as a crosscheck of the calculator. If the calculated values below aren't very close to the ones you get when you measure yours, then something isn't quite right. If the numbers don't match, then maybe you entered something incorrectly or maybe there is a bug in the calculator. Keep in mind the numbers below include the clearance value you used above.

The T female value is:
The Y male value is:
(Male Y should always be slightly smaller than female T.)

The Y female value is:
The T male value is:
(Male T should always be slightly smaller than female Y.)

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What are dovetails and why would I want to measure them? Dovetails are types of joints used to connect two pieces (such as wood or metal) together for a (usually) tighter fit. Often they are used where the two pieces must slide relative to one another in only one of the 3 axes (x, y, and z). For example, you might see a dovetail joint used on a vise so the movable jaw can only move either towards or away from the fixed jaw, but in no other direction along any other axis. In some machinery when it is expected to be put into heavy use, small spacers (called gibs) are often placed inside the dovetail, the purpose being to allow the joint to be tightened (by adjusting the gibs) periodically as the parts wear. Of course, the reason you would want to measure dovetails would be preparatory to cutting them.

Dovetails modeled in FreeCAD: stl.stl

But why would I need a calculator to make some measurments? Well, you could certainly measure them without this calculator, but where great precision is needed they can be difficult to directly measure accurately enough. For example, if a machinist is making an accessory for his metal lathe that will extend an existing dovetail, he would need some very accurate measurements of the existing dovetail in order to fashion a piece that would mate with it properly. The primary difficulty is in getting the measuring device into the corners. No matter how tiny the measuring tip of the instrument, it will still never quite get into the corner all the way. (Note the circles in the images above, and how they don't reach completely into the corners.) It's far easier to get really good measurements of the red lines in the images above than it is to do the same with the white lines. By using dowels (or other similarly shaped objects) the dovetails can be measured with great accuracy, provided we know with great accuracy the diameter of the dowels and the angles of the dovetails, along with a little bit of trigonometry. This calculator handles all the trig for you.

In the top image you see a female dovetail, the bottom is the male. The white lines and the blue lines color code the matching surfaces. The 2 circles represent the steel dowels used to aid in the measurements. The dowels are not a part of the joint when the pieces are put together. The green line represents the diameter of the dowel. I say dowels, but these can be any rod-shaped objects of any diameter reasonably close enough to match the scale as shown in the images. The yellow line represents the height of the dovetail. It should be the same for both pieces, but if it's different, use the male part's height. (The Y parts of the tricky ones, but you should be able to get decent measurements for the T parts, both male and female, which can be used as a crosscheck for the values calculated with this calculator. If the T values the calculator gives you aren't very close to the ones you're directly measuring, then something isn't right somewhere, so don't just accept these numbers blindly.)

If you want to watch a great youtube video on this process, see this one by mrpete222, who also has a great many other fantastic videos if you're interested in machine shop-related videos.

--Mark Ganson

Send any questions/comments/flames to mwganson at hotmail dot com with dovetails in the subject line.



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