Thread wire measurement consistency

[WobblyHand]

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.

 

[T Bredehoft]

Three wire, stick all three of them in a chunk of styrofoam, you won't have to worry about juggling or loosing them.
Please, don't try to work beyond four decimal places, humans can't stand the stress raised by 5 places. No work done in a hobby shop is critical enough to work that close to spec.

 

[mikey]

Are you using quality thread wires or imports? Pee Dee is a good brand and the wire diameter is typically consistent.

One thing that I recall helping, back in the olden days before I discovered thread micrometers, is that using the friction or ratchet mechanism causes inconsistencies in the readings. I found that ensuring the faces of the spindle and anvil were in good contact and then taking my reading without tightening further (without engaging the friction or ratchet) was the most consistent. I took hundreds of readings like this so I know it works. You might try it.

Then, when you get sick of manipulating things and doing calculations and searching for your wires in the chip pan, buy a good thread mic and be done with it. They are direct reading so no calculations, nothing to drop and you get to keep more of the hair on your head.

 

[WobblyHand]

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.

 

[WobblyHand]

Three wire, stick all three of them in a chunk of styrofoam, you won't have to worry about juggling or loosing them.
Please, don't try to work beyond four decimal places, humans can't stand the stress raised by 5 places. No work done in a hobby shop is critical enough to work that close to spec.

Thanks for the styrofoam idea. And, no, I'm not really trying to work beyond 4 decimal places.

 

[WobblyHand]

Are you using quality thread wires or imports? Pee Dee is a good brand and the wire diameter is typically consistent.

Imports. They seemed ok, at least in the beginning. But looking at one of the wires, labeled 0.0722, (but actually 0.0718) which I used for the first time, it had a significant nick in it. According to the Machinery's Handbook, the wires should be rather hard, like not being cut with a file. I haven't tested the hardness yet, but was surprised to see the nick in the middle of a supposedly hard wire. As for consistency, the 3 wires were within one tenth of each other, in the un-nicked area. No measurable taper of the wires from end to end, as measured on my Digimatic.

One thing that I recall helping, back in the olden days before I discovered thread micrometers, is that using the friction or ratchet mechanism causes inconsistencies in the readings. I found that ensuring the faces of the spindle and anvil were in good contact and then taking my reading without tightening further (without engaging the friction or ratchet) was the most consistent. I took hundreds of readings like this so I know it works. You might try it.

For measuring telescoping gauges, I agree, you can develop a feel for the mic when it just touches the surface. Believe it was you that told me that, and I have been able to make inside measurements pretty reliably now. For this gross immovable object stuff, I don't know, but I'll try it. Habits are tough to break sometimes!

Then, when you get sick of manipulating things and doing calculations and searching for your wires in the chip pan, buy a good thread mic and be done with it. They are direct reading so no calculations, nothing to drop and you get to keep more of the hair on your head.

Hah, I only drop the little wires. I'll look for a thread mic, but at the moment have a tiny tooling budget. Things should lighten up a little by years end, but it's tight now.

 

[ttabbal]

I have no idea if they are available elsewhere, but I bought these a while back and have found them to work well and help get reliable readings.

P D Check Thread Wire Attachment System: Thread Gages: Amazon.com: Industrial & Scientific

P D Check Thread Wire Attachment System: Thread Gages: Amazon.com: Industrial & Scientific

www.amazon.com

Thread mics are nice, I have one from Shars that matches up with the wires and my Mitutoyo mic. My wires say Brown and Sharpe on the package. I can't promise they are anything special, but the diameter on a couple I tested at random is consistent across the length and is within a couple tenths of the stated measurement. I only have 0-1" thread mic, I use the wires for bigger stuff as I don't have a lot of call to measure bigger threads.

 

[mikey]

I think a lot of inconsistency when measuring thread pitch is due to the user. Before taking a measurement, I actually use a wire brush to remove burrs from the crest and in the valleys and give it a blast of air to make sure things are clean before I use my thread mic. Anal? Maybe, but when you're working into the tenths on a class 3a thread, anal counts. If you're leaving chips and debris in the thread when using wires then you might want to think about what you're doing.

 

[SLK001]

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.

They probably do meet a Class 2A fit. It's not hard to hit when you are grinding the threads. mickey hit on something that you should consider, namely the finish on the threads themselves. Unless you are measuring in exactly in the same place each time, expect variations. Also, it is critical to measure exactly on the crest of the wires (or exactly on the top and bottom). Having your mic in a fixture takes away the feel that you need in hitting the crest.

From the numbers you posted above, you're already there. Stop trying to beat a horse to glue.

 

[WobblyHand]

I think a lot of inconsistency when measuring thread pitch is due to the user. Before taking a measurement, I actually use a wire brush to remove burrs from the crest and in the valleys and give it a blast of air to make sure things are clean before I use my thread mic. Anal? Maybe, but when you're working into the tenths on a class 3a thread, anal counts. If you're leaving chips and debris in the thread when using wires then you might want to think about what you're doing.

I agree. That's why I made that crude fixture. FWIW, I cleaned those threads with a wire brush, and pushed a nub of paper towel into the threads and rotated the spindle (by hand) to get every last bit of junk out. Did the paper towel thing 3 times, so there was nothing in those threads.