can someone explain to me about tapered gauge pins?

While such a gauge would be good for a quick check of a hole diameter, it isn't a great way to determine the true diameter due to it's inability to gauge anything but the entrance to the hole, which often is oversized. You are also limited by holes that are blind----depending on the depth. They are in no way a replacement for sets of pins, some of which come in tenth increments (Deltronic pins, for example), and can be relied upon to determine true diameter. I'd suggest they would be great for rapid inspection of given holes, with a reasonable tolerance.

Harold

Adding to what Harold said: Hard to get a good reading when the bore is camfered. Mostly useful when checking the diameter of jets.

Nick

I have some tapered pins that I use to check hole sizes. Instead of using some sort of internal measuring tool, I just slip the tapered pin in the hole and use dial calipers to measure the diameter right next to the hole. I do this mostly on holes I am boring on the lathe so am not worried about the hole being camfered. And usually I am boring the hole to fit some shaft. So maybe the dial calipers aren't super accurate as far as absolute accuracy, but they are very good for comparing two outside measurements. Certainly close enough to make the hole a press fit or a sliding fit if that is what is needed.

The pins I have were not made for this purpose. They are some scrap from some tapered centerless ground mandrels.

Dan

What Harold said; what Dan said: Think of them as tapered mandrels. Stick them in the hole and measure across the diameter of the "gage" close to the hole. They'd be barely ok for rapid inspection if you weren't very fussy about accuracy. Think of the jewelers' ring-size gage. He sticks the gage (tapered mandrel) into the ring and reads the size which is engraved on the gage at the point where the gage stops.

Bob Swinney

While such a gauge would be good for a quick check of a hole diameter, it isn't a great way to determine the true diameter due to it's inability to gauge anything but the entrance to the hole, which often is oversized. You are also limited by holes that are blind----depending on the depth. They are in no way a replacement for sets of pins, some of which come in tenth increments (Deltronic pins, for example), and can be relied upon to determine true diameter. I'd suggest they would be great for rapid inspection of given holes, with a reasonable tolerance.

Harold