CNC / machinist calculator
Dovetail Over-Pins Measurement Calculator
Inspect or cut a dovetail to size by measuring over two pins. Enter the dovetail's wide-flat width, the diameter of the pins or wires you are using, and the included angle, and this calculator gives the distance over the pins for a male slide or between the pins for a female slot. It is the standard way to check a dovetail without trying to gauge the sloped flanks directly.
How it works
A dovetail's angled flanks are hard to measure directly, so the shop trick is to seat two equal pins in the corners and measure across them, then back out the true width from known geometry. For a male dovetail you measure over the outside of the pins; for a female slot you measure between the inside of the pins.
The Machinery's Handbook formula uses the flat at the wide end of the dovetail, the pin diameter, and the included angle. The over-pins measurement for a male dovetail is the wide flat plus the pin diameter times one plus the cotangent of half the included angle. The between-pins measurement for a female dovetail is the wide flat minus that same term. A sixty degree included angle is the most common, but the calculator takes any angle.
Two cautions decide whether the number is right: the width here is the flat at the wide end where the pins seat, not the narrow end, and the pin must be sized so it contacts the flank below the top corner. Choosing the wrong reference width is the most common mistake.
Worked example
A 60 degree male dovetail with a 1.0 in wide flat, measured over 0.25 in pins: 1.0 + 0.25 x (1 + cot 30) = 1.6830 in over the pins.
Frequently asked questions
How do I measure a dovetail with pins?
Seat two equal pins in the dovetail corners and measure across them, over the outside for a male slide or between the inside for a female slot, then compare to the calculated value for your flat width, pin size and angle.
Which width do I enter for a dovetail?
Use the flat at the wide end of the dovetail, where the pins actually seat. Entering the narrow-end width is the most common error and gives a wrong measurement, so confirm you are using the wide-end reference.
What included angle do dovetails use?
Sixty degrees is by far the most common dovetail included angle, but forty-five and fifty-five degree dovetails exist. Enter the actual included angle of your slide; the formula uses the cotangent of half that angle.
What pin diameter should I use?
Use a pin large enough to contact the flank below the top corner of the dovetail but not so large it rides above it. Standard gage pins or dowel pins work; the exact size cancels out as long as both pins match and seat on the flanks.
Does this work for both cutting and inspecting?
Yes. When inspecting, compare your measured over-pins value to the target. When cutting, work backward from the target measurement to know when the dovetail is at size, which avoids gauging the sloped flanks directly.
Related calculators
Sources
- Measuring and designing dovetail slides (Engineers Edge)
- Cotangent and triangle geometry (Wikipedia)
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.