Measuring an internal thread

[f350ca]

Needed to make a 55 x 2 mm nut for a hay rake. How do you measure an internal thread? The shaft wasn't available so I had to make a test shaft that I used the 3 wire method to measure then cut the internal thread using the test plug to check the PD. Is there a beter or easier method? In this case he needed the part, none available in north america and an unknown delivery time from Germany.

[TomG]

Greg,

To do it properly you would need a go-no-go thread plug gauge, but one that size would probably cost you $300 or more. The way you did it will work fine. I usually make my gauges to the upper extreme of the tolerance to ensure that the internal thread ends up large enough.

Tom

[f350ca]

Thanks Tom, thought there might be gauges that were thread specific. A pain to machine a test plug for a one of nut but guess thats the business.

[Mayhem]

C'mon Greg, you know the rules: It didn't happen without pictures!

[f350ca]

Oops, he picked it up still warm off the lathe, needed to get back to being a farmer out standing in his field. or something like that.

[TomG]

I recently cut a special thread using this very technique. It was a .900-32 3B thread, so I calculated the pitch diameter of the external thread and made a gauge to the high side of the 3A tolerance, then cut the internal thread using the gauge to size it. Worked like a champ.

Tom

[Mayhem]

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[dallen]

ones things for sure once you make the gauge you will have it for the next time.

[TomG]

Not me, it went right back into the scrap bin. If I ever need to make another .900-32 thread, I'll just crank out another gauge. It might be nice to have some standard sizes on hand though, maybe even a go-no-go version of them.

Tom

[f350ca]

The nut is off making hay but here's the test plug.



As per Tom its back in inventory.