3 Wire thread measurement

[Mayhem]

OK - when I caught up with PixMan last year, he showed me how to measure threads using the 3 wire method. Being able to juggle 3 wires and a micrometer isn't as easy as he made it look.

I purchased a set of Pee-Dee wires from Fisher Machine Shop Inc and have used them to measure up the threads on the spindle of my lathe. My plan is to make a replica of the spindle nose, so that when I get around to making the backing plate for my 3 jaw chuck, I have a standard against which to check the threads.

For my immediate needs, all I need to do is measure the threads using the wires and then cut the replica to match. I don't think that I need to compare the measurements to any specific chart, as these threads would have been cut over 100 years ago and may not match. I'm simply making a copy.

Does anyone see a problem with my logic on this one?

[EdK]

Sounds perfectly logical to me. That's probably the most accurate way of getting an exact copy of the spindle threads.

Ed

[PixMan]

That's the right way to do it. First make the O.D. match (unless using cresting, full form inserts), then be certain the pitch of the threads are going to match. Then cut until the pitch diameters match. Done!

If you are using pitch-specific full form inserts, leave the O.D. of the piece .005" to .010" (.12mm to .25mm) oversize. As you machine the thread, watch to see when the cresting insert starts taking some material off the O.D., then you will likely have a match of the two pieces when the O.D.'s match.

Sorry if I made that look easy. I may have a couple of years experience doing it. Until you're well-practiced at it, you can use aids to keep the wires on the threads while you fiddle with the micrometer. Big wires like those used for a 6TPI thread can be stuck on with chewing gum, modeling clay, or with rubber bands to name a few options.

Cheers,

Ken

[Mayhem]

Thanks Ken,

Yes you did make it look easy but in following your instructions, holding the wires hasn't been much of an issue. You method of correctly placing the bottom wire was a big help. I'm glad you showed me as I would be spending more time searching from them on the floor otherwise!

I think I originally confused myself when I measured the spindle thread, as I checked my measurements against a reference for the 6tpi thread and they didn't match. That put doubt into my mind as to whether I was measuring correctly etc but then I figured that these threads were cut over 100 years ago, so they are probably not going to line up exactly with the standards.

I will be cutting the clone using a HSS threading tool. However, I will cut the internal threads with a Carmex 22IR 6UN BMA full form insert. I picked up an internal threading holder for a good price on eBay and as I am going to make other backing plated and collet chucks etc, I thought it was a good investment. As I only plan on making one clone, it didn't make sense to use anything other than HSS.

[Mayhem]

I was just thinking that three wire thread measuring would make an excellent 'how to' topic for someone to write up

[Highpower]

I have a cheap set of wires, but have yet to use them.

[ScrapMetal]

(04-28-2012, 10:12 PM)Mayhem Wrote: I was just thinking that three wire thread measuring would make an excellent 'how to' topic for someone to write up

Ditto! I'd really love to see the proper way to do that.

-Ron

[PixMan]

A write-up isn't going to work as well as a video. A video is going to be a bit difficult to make because of the small sizes of things involved.

Here's a link to a pretty good write up, although it's not quite the way I choose to do it:

http://littlemachineshop.com/instruction...Method.pdf

The way I do differs only in that I put the pair of thread wires on top and hold them with the thimble of the micrometer. Then I take the third wire, touch it under the left-hand top wire, and trace the third one in the valley of the thread down to the bottom. I'm now holding the third wire as I gently snug-up the micrometer to make the measurement. As long as the thimble is squarely upon the top two wires, they both stay there and the micrometer comes up to bear upon the third.

Much easier to do it than to describe it. If I can get my butt off the motorcycle on the weekend, I'll make a video.

[Mayhem]

You are more than welcome to shoot the video using my spindle thread. Size will not be an issue then!

   

For me it is more a case of what to do once I have made the measurement, which is explained in the link you posted. It is probably a thread class confusion given some of the charts I have seen.

[Mayhem]

One of the things I do is look at the pages that guests are viewing, to see what is getting found via Google and the likes. Today, I stumbled upon an old thread of mine. Whilst reminiscing, I noticed that this would make an excellence video topic for Tom's new web site.

See how I look after you Tom