Hey Guys,

I’m a bit confused regarding thread depth and how large to bore a hole that’s to be threaded. I will use the following as an example.

I have a male piece that measures .970” OD at 20TPI and I need to bore a hole and thread the hole @ 20TPI so that the male piece (.970”) will screw into the bored & threaded hole. I am using the following formula to calculate thread depth:

Depth of Thread = 1 / TPI X .866

DoT = 1 / 20 X .866

DoT = .0433”

Double depth = .0866”

If I subtract .0866” from .970” the ID of the hole to be threaded is .8834”

However stamped on the Starrett 60 deg Center Gauge, is a value of .065” (double depth) for 20TPI rather than the calculated double depth of .0866. This would mean that my hole diameter would need to be .905” ID. This is a hole diameter difference of .0216”.

So which do I follow when boring the hole …. Starrett Center Gauge's stamped values or the formula? What is the reason for your choice?

Harold

I’m a bit confused regarding thread depth and how large to bore a hole that’s to be threaded. I will use the following as an example.

I have a male piece that measures .970” OD at 20TPI and I need to bore a hole and thread the hole @ 20TPI so that the male piece (.970”) will screw into the bored & threaded hole. I am using the following formula to calculate thread depth:

Depth of Thread = 1 / TPI X .866

DoT = 1 / 20 X .866

DoT = .0433”

Double depth = .0866”

If I subtract .0866” from .970” the ID of the hole to be threaded is .8834”

However stamped on the Starrett 60 deg Center Gauge, is a value of .065” (double depth) for 20TPI rather than the calculated double depth of .0866. This would mean that my hole diameter would need to be .905” ID. This is a hole diameter difference of .0216”.

So which do I follow when boring the hole …. Starrett Center Gauge's stamped values or the formula? What is the reason for your choice?

Harold

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