Determining an angle of a blind pocket ?

[jcfx]

I was wondering how to determine what an angle would be for a blind pocket ?
Through trial cuts on a lathe I arrived at an angle that fits the pocket, but I was wondering if
there was a better way.

A little info, what I'm doing is converting a Manfrotto 143 Magic Arm from a cam lock to a screw lock,
which is the 244 model, for those that don't know what a Magic arm is they're also called
variable friction arms used in photography, think Noga indicator arm but much larger.
I use one to hold my DRO readout on my mill.
The 143 style arm uses a balls riding in a sloped grooves in the end of a cylinder that pushes
two cylindrical parts together which shifts and wedges another ball in the arms to lock the arm in position,
the 244 model has exactly the same parts except the cylinders, locking uses a knob, and doesn't use the
balls in a sloped race, just a ball bearing thrust washer. Turn the knob to tighten, much easier.

Pictured is one of the original cylinders and the replacement, arrow points to the pocket which looks like a
counter sunk pocket.

[Harold_V]

Sealing wax has been used to determine such features. I've not tried it, however.

H

[whateg0]

Tedious, but a DTI with a long stylus could be used to sweep it. Then a little trig, or CAD to get an angle out of it.

Dave

[earlgo]

Woods Metal which melts at 165°F is often used for this purpose. I have used it for many things including rifle chamber casts.

chamber cast 218 MBee

--earlgo

[Bill Shields]

are you saying that a sphere (ball) goes into the 'pocket'?

and is held there by a screw?

[whateg0]

now I'm not sure I understood the question. A cross section of the part would be helpful.

[jcfx]

Interesting ideas, in the realm of wax I did try a hard styling clay which needs to be heated
to work with but the part was too much of a heat sink to use the clay.

whateg0 - I was wondering about using a DTi but I would have make a needle point probe, the
pocket is only a little over 1/8" deep, when you say sweep is that axially ?

earlgo - I had never thought of low melting point metals, some thing to file away for future use.

Bill - Yes, the ball sits in the pocket, but no screws attach the ball,
the ball is captive between the cylinder with the pocket and a rod that
has a corresponding cup for the ball. There is two opposing each other
as the knob is tightened the dylinders are drawn to each other and the ball that sits
in those taperd pockets ride up and lock. See drawing
( I left some details out but the gist of how it works is there ):

[Bill Shields]

Good drawing..

so what is the angle that concerns you?

From a manufacturing standpoint I would bet that the blind pocket is normal to the cylinder axis.

If you are concerned with the included angle of the drill point..I would cut something at the standard drill angle..dip it in some dykem blue and give it a spin and see where it contacts. Repeat at new angle as needed.

In reality it is the depth of he spotface more than the angle that would concern me..

Or am I missing something?

[whateg0]

I am the master of overthinking things, and you are overthinking this. The included angle of that does two things. Steeper (more pointed) allows more axial force to be applied to the sleeve at the expense of travel of the vertical rods (as shown). I doubt they made a special tool to bore that dimple. More than likely, it's just a 118 degree included angle, so as Bill suggested, spinning a drill in it should get you pretty good contact on the end of both flutes. The more important dimension is the diameter at the mouth of the dimple and the center. It needs to be positioned such that the ball stays between them on that side of center.

Now, if you want to know the angle, a pretty simple way to figure it out, assuming the angled face comes all the way to the surface of the sleeve, is to drop a ball bearing into the dimple. Measure how far above the circumference the ball sticks out and measure the diameter of the dimple at that point. A little trig will get you the angle.

I wouldn't do the math, though. I'd dump the info into CAD and let it figure it out for me.

Dave

[RSG]

Woods Metal which melts at 165°F is often used for this purpose.

Hm! Learn something new every day!

[NP317]

Seems to me you can simply measure the diameter of the circle at the top,
and measure the depth of the hole at those same points. Then calculate the angle.
RussN

[jcfx]

Bill - You're not missing anything, I was just looking for alternate ways of measuring that angle
rather than my way of turning a point using the compounds protractor and seeing if it matched.
or test fitting counter sink bits I had ( were all too acute ).
You're correct that the tapered pocket is on the center line of the part.

whateg0 - Sorry if I came across as over thinking it, agreed it could be a standard angle.