Thread depth

Three wire thread measuring:

http://www.emte.ch/English:3-Draht-Gewindemessung.asp

"Billy G"

Hukshawn said:

I have an er32 collet nut in the mail arriving any day now. I have the barrel nut ready to have the nut threads cut. But it will need to be very accurate!
How would I go about accurately transferring that female thread to my male barrel nut? What's the preferred method?

Click to expand...

Okay, I think I see where you're going here, Shawn. You have an ER-32 nut on the way and you need to know how to cut the mating male thread for it, right? You seem to be asking: 1) what dimensions to make the snout of the chuck so the nut will fit properly, 2) what the PD (pitch diameter) is and 3) how to measure the threads so you have a good fit (pitch diameter tolerances). Is that right?

An ER-32 nut has a M40-1.5 thread. The M40 part is the nominal major diameter and the 1.5 part is the pitch of the thread. Ideally, you would also have the tolerance the female thread is cut to and the maker can supply this info for you (unless its Chinese, so good luck).

1) To sort out what size to make the male blank, look here: http://www.engineersedge.com/hardware/metric-external-thread-sizes2.htm. Scroll down to the M40-1.5 row and look for 4g/6g in the next column and you'll see the min/max dimensions for the external snout. You will also find the min/max Pitch Diameters for that class of thread.

2) When you say, "But it will need to be very accurate!" you are talking about tolerance class. Again, this relates to the 4g/6g thing above. A metric class 4g is equivalent to an American class 3a fit, which is a very precise thread used when strength requirements are high. It is a precision fit that requires precision dimensions and precision thread cutting. To get a handle on what this fit stuff means, go here: http://www.katonet.com/article/science.html, and here: http://www.bluetoad.com/article/Metric+Thread+Tolerances/1799736/0/article.html. This will explain what those numbers and letters mean.

3) To cut the thread accurately, you have to be able to measure the thread accurately and the guys have given you a number of options . A thread micrometer is the best way but is also the most expensive way. A 3-wire set is the cheapest but the most tedious. Your call but the Shars thread micrometer set can be had for less than $70.00 and seems like a good way to go because it can be used for every other threading project you do.

All this info is in the Machinery's Handbook, although it takes a bit of searching to find it. And don't even think that you're the first or only one to be confused by this thread cutting stuff because you aren't. Many of the guys who do understand it were right where you are now at some point so hang in there and learn. The thing I like about you, Shawn, is that you're willing to ask ... that counts in my book.

Awesome write up. When I read all the articles, when I have time, it looks like it will answer a lot of questions.
Another question tho... how do you measure, aside from the major diameter, of an internal thread? Wires or thread mic doesn't work, does it?

The cheapest way to know about the dimensions of your ER nut is to ask the maker. Another way is to do as Tony suggested - make go/no-go make gauges and check for fit; this is the most practical way to go. I think I also saw something on direct measuring micrometers but there are really expensive if I recall; haven't looked at this in a very long time.

To make the gauges, you have to make at least two of them. Check the charts for the min/max dimensions and cut them.

Hopefully, someone will know of an easier way to do it.

When I cut Threads, I mount my Dial Indicator on the Back of my Crossfeed and have the indicator measure off the back of my QCTP. As I feed in the compound, the indicator gives me the depth without having to calculate based on angle and compound depth of feed. I just have to remember to divide the difference between the Major and Minor Diameters by 2. (Almost forgot on the last one )

Mikey, I haven't really been talking specifically about the er collet, was just using that as an example. But more so, in any future situation where I need to make a male thread to fit a female pre-made nut.

In my mind, the only measurement I can take from a female nut is to just caliper the major diameter and calculate from there to decipher the male thread.

What I'm trying to sort out is, when you go to make the new male thread, what do you do with that female major diameter? How much smaller do you go to make the male thread? Is there a rule of thumb? Do I just refer to the charts and use the min/max? What do you guys do?

There are more or less direct methods of measuring the PD of an internal thread, but the instruments to do it are not really practical for a home machinist. There is a company called Gagemaker who markets a couple of different systems, along with software to do all the calculations and also some setting standards if you are checking the lead. They have quite a variety of tools for just about any style thread. If it were me in your place, I would wait until I had the nut in hand, thoroughly clean it of all oils, then spray some cooking oil type stuff (Pam) and make a plaster cast of a little less than half the diameter. Do this with the nut standing on edge, with masking tape dams front and back. When it cures, it should come out easily, and you can get a look at the profile and if you have the right tip on a drop indicator, or a thread height gage (yes, they make such a thing) you can measure the facsimile of the thread, albeit a mirror image, so to speak. You can get a direct measurement of the Minor diameter, close enough if you have some good calipers. Then add 2x the thread height and you have indirectly come up with the Major diameter of the internal thread. You also get a look at the root radius, so you know what the crest of your male thread has to look like. Of course, there should be some clearance between the major of the external and the major of the internal. All the contact on this type of thread should be on the flank only.

I have a 50 lb box of dental plaster that sets extra fast, just for such things as looking at features that I can't get to directly. It's pretty handy also if you have an optical comparator to have a good look at the cross section of such a casting (moulage) style.

You have to know what size the female/nut is. That is, you need to know if it is a 1/4-20 or a 1/2-13 nut or whatever. To do that, measure the ID of the nut and check the chart to see what the size is, then confirm the pitch or thread count with a thread gauge. Once you ID the female thread then you can look up the major diameter of the male part and cut it to the size you need.

Many of us know what fits we need. For example, if you want to cut a 1/4-20 male thread to fit a confirmed 1/4-20 nut then you can make the male blank 0.25 minus maybe 0.005" and you'll have an okay fit in a hardware store nut. If you want a really good fit, you have to know what the female thread tolerances are and that is more involved, as we've discussed above.

Download a chart of the common major diameters plus min/max sizes as they relate to tolerance class and put it in a folder. When you need to make a male thread, look up the size of thread you need and turn the blank to suit the fit you need. There are no shortcuts to this as far as I know. You certainly cannot infer these dimensions from measuring the ID of a female nut.

I can give you the dimensions for all the main thread features if you're sure of the thread.

Jeez. No simple solutions anywhere in machining, is there? And hell, it's also really expensive! There's a different tool for everything.

Bare with me, guys. Believe it or not, I am learning a lot.