Measuring thread flank angle on a screw

[ScottK]

Hello metrology experts. I have a situation here that I have yet to run across.
My background: I've spent a number of years in different organizations measuring threaded parts - metals and polymer - on optical comparators.

I have a screw in my hand that has a special flank angle - 48 degrees. Here's the drawing from the product literature.


I have the screws in hand and when I put them on an optical comparator I'm measuring 60 degrees, with the axis of the screw perpendicular to the lens.
The supplier states that to measure this angle you need to turn the screw at an angle to the lens and you will find 48 degrees.

I maintain that the drawing shows the measurement with the axis perpendicular to the viewer.

I've never cocked a screw or other threaded part to measure it, but I've also never purchased a 48 degree screw before.

thoughts on this?

 

[Conrad Hoffman]

Old post but new member- I've done a lot of optical measurements on threads and you need to tilt the part to the helix angle of the thread for the measurement to be valid. Example, if you measure a perfect 60 degree thread without the tilt, you won't get the 60 degrees shown in Machinery's Handbook because their diagrams are cross sections. For many threads it might be "close enough", but for many it won't. Both responses are correct, theirs about what they really meant to show, and yours, pointing out their error of what they did show! I've no idea if there's anything special involved in calculating the helix angle of a tri-lobe screw.