Thread wire measurement consistency

[brino]

@SLK001 ,

That was my first thought too; "that's cheating by taking the work out of the lathe!"

.......but then I figured that maybe he was looking at a practice piece and just setting-up for the run of real parts.

Brian

 

[f350ca]

I put a dab of light grease on the threads to hold the wires. Im sure there's a thin layer between the thread and the wire but doubt you could measure it.

Greg

 

[mickri]

I was getting lousy inconsistent results every time I tried using three wires. Always felt like I needed a couple of extra hands. Then one time I happened to put the two wires on the top of the thread. They just sat there. I then put the mic on the wires with handle hanging down. It just sat there balanced. With slight downward pressure I carefully tightened the mic while holding the one wire on the bottom of the thread. Wrote down the mic reading and tried it again. I got the same reading. I now get consistent readings when using three wires.

 

[Craig1514]

Even with the 1 wire method I am getting inconsistent micrometer readings. The problem seems to be my wobbly hands! In Machinery's Handbook it says one can (should) use a floating micrometer. Well, don't have one. But... I thought, maybe I could put my micrometer in a toolholder for my lathe? So took my Shars micrometer 1-2" and put it into an AXA-XL toolholder, with a piece of aluminum on top of it to protect it from the screws. Tightened the screw only enough to hold the mic in place. Seemed pretty stable. Used the nut of the QCTP tool holder to adjust height, and adjusted the micrometer to be square to the spindle.

Adjusted the anvil to just touch the wire and hold it vertical against the thread. Then turned the micrometer to touch the thread on the opposite side and hear it ratchet. Tried this whole procedure a couple of times, and the answer repeated to within tenths, rather than thousandths. Got to admit, it looks like a kludge, but it works. It works well enough that I'd consider making a real fixture for this.
View attachment 400472

Thanks, I’m going to try this.

 

[RJSakowski]

Trying to measure the pitch diameter of my 1-3/4"-8 threaded lathe spindle. First, the thread wires don't seem to be identical, two are 0.07175" and one is measuring at 0.07185". Thought I would 0.0718" as W. (Average is actually 0.071783")

But more annoyingly, is the measurement M is different every time I measure it. Each time, it has gotten a little smaller, by about 0.001". It's like a three ring circus doing this measurement, but somehow haven't lost any wires, or even dropped them. Is there something I can do that might help with consistency? I'm pretty sure I'm on all three wires, I do check for that. Also lock the mic before removing it from the wires and spindle.

The largest measurement I made was 1.7676". That gives me a pitch diameter that is 1.6605. My smallest measurement was 0.002 under that, which gives a pitch diameter of 1.6585". Class 2A external threads are 1.6688 max, 1.6591 min. Not that I'm expect a Grizzly lathe to meet Class 2A.

Anyways, I'm trying to copy my spindle, so I'd like to get this pretty close. Any advice on reducing the measurement variability?

I can try the 1 wire method, but that feels like cheating. Would be good to learn it the standard way.

For the 1 wire method, if I recall correctly, PD = 2T - D - k, where T is the measurement with a single wire, D is the measured major diameter, and k = 3W - 0.86603*pitch.

FWIW, I checked my 602 spindle pitch diameter and came out with 1.655" by both the three wire and one wire methods. Lathe spindle threafs are intentionally loose as you don't want them biasing the fit to the registration surfaces.