If the major diameter is 0.90 in and the thread depth is 0.05 in, what is the undercut finish diameter?

If the major diameter is 0.90 in and the thread depth is 0.05 in, what is the undercut finish diameter?

• A. 0.80 in
• B. 0.85 in
• C. 0.90 in
• D. 1.00 in

Answer: (A)

The undercut finish diameter is the root diameter of the external thread, which is found by subtracting twice the thread depth from the major diameter. The thread depth represents how far the thread profile sits below the crest on each side, so the total reduction from the major diameter to the root is 2 times the depth. Compute: 0.90 in − 2 × 0.05 in = 0.90 in − 0.10 in = 0.80 in. So the undercut finish diameter is 0.80 in.

The undercut finish diameter is the root diameter of the external thread, which is found by subtracting twice the thread depth from the major diameter. The thread depth represents how far the thread profile sits below the crest on each side, so the total reduction from the major diameter to the root is 2 times the depth.

Compute: 0.90 in − 2 × 0.05 in = 0.90 in − 0.10 in = 0.80 in.

So the undercut finish diameter is 0.80 in.