Offtopic technical question

[mark_m]

I just bought a digital vernier gauge that is supposed to be accurate to .01mm. I figured it's a good investment because with that acuracy it doubles as a micrometer. When I got it home I decided to test it on the 8x20 machine screw I bought that goes into my visions banjo fitting when I bleed the brake.

To my surprise it measures 20mm lengthwise, but exactly 7.90 mm when I measure the width by putting the vernier around the thread. 7.9mm seems like too round a number to be an inacuracy in the vernier gauge.

Does anyone know why a metric screw that is supposed by be 8x20 is actually 7.90mmx20mm?

Thanks.

Mark.
ps: The screw was bought at OSH in their metric screws section.

[Blake]

haha

when i got my set from harbor freight (6" digital calipers) i measured the same thing.

7.9mm.  (best i can think of, which cooresponds with my friend the machinist) is when they turn the bolt for the threads they take a little off, and plus you dont want it tooo tight, or else the bolt wont slide in.  basically a little "slack".


or thats what i was told.  to me seemed to make sense.



Blake

[Junior]

That is correct. It was a 8X20 until the thread pitch was tooled on the Outer Diameter bring the thread to correct size. The same on Inner Diameter threads. If you tap a 1/2" hole with a 1/2" tap, it should check a little larger. OD threads are normanly cut with a slight minus tolerance and  inner diameter threads with a plus tolerance. The plus and minus tolerance is normaly determined for the application it is design for. ( I Think ::)

[mark_m]

Awesome, thanks guys! Thanks Junior - that makes perfect sense.