Sine bar protractor

[toastydeath]

Setting the sine bar over 45 degrees is usually done for compound angle fixturing, such as setting a rotary table on top of it to bore or grind holes in a perimeter, at an angle. In that application, small errors can make a difference in the first thing to be attached to the table itself, as now you've starting to stack objects and accumulating error.

It's also done in some inspection applications, where you want to get the specific angle between some feature and another without interpolating it from other features (and thereby increasing the error of the measurement).

And unfortunately, there can be more error in the method detailed in fig. 33 than is suggested by that text, by nature of a somewhat awkward setup.

[LES A W HARRIS]

Marv, Nice job, thanks for sharing.

Tweeked a bit, rolls at 5.000" C.D. & zero gap at zero angle.

[lazlo]

Marv, Nice job, thanks for sharing.

Tweeked a bit, rolls at 5.000" C.D. & zero gap at zero angle.

That's very clever Les. So Theta simplifies down to:

Θ = 2 Sin-1[d+D/10]

Now that's something you can do in the shop without a bunch of keypresses!

[oldtiffie]

Correction

Edited/deleted-out

[LES A W HARRIS]

That's very clever Les. So Theta simplifies down to:

Θ = 2 Sin-1[d+D/10]

Now that's something you can do in the shop without a bunch of keypresses!

Minus the Base Angle Constant!






2*INVSIN ((.5000"+3.3858")/10) - 5.731968deg
40.00038deg
Closer than my eyeball!

Drawn at 40:00:00

[dp]

I think some math can be eliminated by measuring a pin and zeroing your caliper at that dimension offset, no?

[LES A W HARRIS]

I think some math can be eliminated by measuring a pin and zeroing your caliper at that dimension offset, no?

The original layout used an inside measure, and therefor, one adds the pin dia to get the center to center dimension (the chord dimension). I presume one could zero at pin dia and measure across the outside of the pins giving the center to center distance, (chord).

[oldtiffie]

Edited/deleted-out

[miker]

Keeper

Thanks for posting this Marv.

Now I HAVE to learn TRIG!!!!!

Thanks a LOT!!!

Rgds

[John Corden]

Gentlemen,

I came across this thread while searching for details of a sine protractor I acquired - see photos below.








There are no instructions or details with the device. However, by measuring the height below the arm for various angles, and doing some trig, it would appear that :

the arm length is 100 mm
the height to make the arm parallel to the base is 5 mm.

So from these figures I can use it as a (very nice) sine bar.

Oddly enough, the height of the machined surface (on which to stack the gauge blocks) is 27.69mm - a strange dimension.

My question is: what was it used for, and have I missed something obvious?

I assume that I calculate the height of the stack required for a given angle, then add 5mm?

Any thoughts or more information would be greatly appreciated.

Thanks,
John

[Bob Fisher]

Should call it an isoceles protractor. It solves as two right angles one half the target angle. Bob.