measure dovetail

[altnickee]

Hello,

I need to mill a female dovetail in a tool, in witch a extra part of the tool needs to be fixed (press fit)

I know there is a way to measure with a two small wire rod.

In the Machinery Handbook i found a explanation,but i don't understand some of it
see link for picture

In the Machinery Handbook they say "To obtain dimension x" , but i don't need to obtain, i can measure with my caliper!!

I think being not American/ English i don't understand the explanation.

Any help appreciated

gr Anthony

 

[altnickee]

Hello again,

I forgot, please a explanation with some measurements as a example .

gr Anthony

 

[MikeHalloran]

There is a typo in the text. Where it refers to dimension alpha, it means dimension a.

You're right, you can just measure the dimension over a pair of arbitrary balls. The text is written as if you've designed the part, know dimensions a and b, and need to compute dimensions x and y, so you can put them on a drawing. Which is not exactly what you're trying to do.






Mike Halloran
Pembroke Pines, FL, USA

 

[altnickee]

during the milling of the female cut, i want to measure the cut, so i don't make the cut over sized compared to the male dovetail

So how do i measure the male cut and how do i measure the
female cut to get e press fitting ?

gr Anthony

 

[MikeHalloran]

You algebraically rearrange the given equations so that you can measure and input dimensions x and y and compute dimensions a and b.





Mike Halloran
Pembroke Pines, FL, USA

 

[eromlignod]

If I understand you correctly, you are trying to machine a press-fit dovetail joint? This is a little tricky.

The angled contact of the flanks cannot provide the fit alone. You must have pressure from the top or bottom of the joint (but not both, since each literally prevents contact with the other) to force the angled flanks into engagement, otherwise they simply fall apart and the joint is loose.

You probably want to use the top of the male to bottom of the female junction for your press fit (as opposed to the outside faces of the female to the outside faces of the male, which is also possible). So the critical dimension would be the width b, both for the female part and the equivalent dimension of the top of the male part.

Think of it this way: imagine that the flank lines in the diagram on the right are extended upward until they form a triangle. Then imagine that you are trying to press fit this triangle into the female dovetail. If the triangle is too narrow, it will just rattle up and down in the groove, it has to be oversized just enough so that the bottom of the groove provides upward pressure and forces the angled flanks into a pressed engagement.

Rearranging the formula for b gives:

b = y + D * (1 + cot(alpha/2))

for the female member. For the male member, you can simply measure the width with calipers.

Note that the height h doesn't really matter to the press fit, however it does determine what gap will be left above the joint.

Don
Kansas City

 

[eromlignod]

I just thought I'd add another tip.

You probably will not want to machine the acute edges of the male member sharp, as this will probably just cause problems. You should blunt these corners to ensure that the three faces of the joint are causing the press fit, not minor interferences at the sharp corners.

In this case the width you use for the male part will have to be the *theoretical* width, which would be (if I did my algebra right):

w = x - D * (1 + cot(alpha/2)) - 2 * h * cot(alpha)

Alternately, you could first machine the corners very sharp for measurement. Once you have your mating dimensions where you want them, you can blunt the corners and the joint should still fit tight.

Don
Kansas City

 

[altnickee]

Hi ,


The dovetail must be like a adjustable gun sight fit

I want to use a formula (as in my link), so that i can
measure the width of the flanks of the male dovetail
by using the round rods.

link

With the measurements i get out of this formula, i can
control the cut i am making for female dovetail.

Meaning,i start cutting a undersized (female) dovetail
cutout, than use the round measuring rods inside this
dovetail to measure the flank width ,and compare to the
outcome of the formula, and see how much more i need to cut
out !!

the only problem i don't understand the formula in my link

Thanks,

Anthony

 

[downwash]

if you are not good with formulas . it would be easyiest to draw on cad with whatever pins suit. get a dimension and measure. the female dovetal should be measured with gauge blocks between pins