Lathe Threading, My Threads Look Wrong

[Paul in OKC]

Calculating thread depths will get you close. A lot depends on the tool tip, and I usually end up going a tad deeper almost every time. Did you use any lubricant? Aluminum can ce pretty gummy in some grades, and the lines could be material build up on the tool. I also rarely do the compound set up for threading, unless it is a course thread, or a very light duty machine.

 

[Wreck™Wreck]

The most important dimension of a thread is the Pitch Diameter.

Calculate this as Pitch Diameter = Major Diameter - .650 X the lead.
In this case the PD is 18.35 +- the tolerances of the thread class, measure the external thread over wires and use it as a gauge for the internal thread. A set of thread wires is an inexpensive tool.

Check the math, I converted to inch then back to MM.

http://www.mscdirect.com/browse/tn/...Pitch-Diameter-Measuring-Wires?navid=12107674

I prefer 3" long wire as they are easier to handle.

 

[Bob Korves]

Futterama said:
(snip) Bob, are you sure it needs to be flat for metric threads? All the drawings show a radius e.g. the attached one from Wikipedia. I didn't try to make any exact radius on there, I just took away the sharp edge really. (snip)

Bob: You are correct about the root profile being rounded preferentially, and mandatory on class 8.8 and stronger. Slight corner radii are acceptable on lower classes of M series fasteners. Thanks!

 

[rlgustin]

Thread wires would be a great investment. Here is some thread info I was taught, compound movement is .75(pitch).
Pitch Diameter for a class 3 fit= Major Diameter- .75Pitch(cosine 30Deg). Flat = .125(Pitch).

Sent from my LG-D850 using Tapatalk

 

[Tony Wells]

Thread wires are a must, IMO. And yes, I can give you a MOW for any wire that will fit the thread. But the ideal wire will contact the flank of the thread at the pitch line. That is in order to minimize any error in the angle of the threading tool. I use Gagemaker software, which is very flexible and will calculate based on whatever you input for a nominal size and class of fit. It also can provide the proper dimensions for making up your own ring and plug gages if you need them to make parts in the future that will interchange with what you have already produced and perhaps do not have access to. It also allows custom, non-standard threads, like if you wanted a 19.33 x 1.3, it would calculate per input sizes and provide standard figures per approved formulae.
I don't have the software installed on this laptop (I don't know why not), so I will get to the shop tomorrow and pull out all the data on that thread, including the specified nose radius for the tool, or flat. Some threads either is acceptable, some are specific to the type of thread. But since you didn't say specifically what type you are in need of, I'll give you the general purpose data.

 

[Futterama]

So my threads are all OK now. Thanks for all your replies.

As a final note, I want to show you my project. It's a new case for a 1.2MPa pressure sensor from ebay. I need to measure the pressure of a small propane container. I need the pressure to be constant during use, so the container is wrapped with a layer of kapton tape and some iron wire is wrapped around it, as a heating element. The nominal pressure is 900kPa at room temperature, but since the propane container is so small, the temperature drops rapidly when consuming the propane, so that's why I need the heating element. The pressure sensor measures the pressure, when it's low, the heating element is turned on, until the nominal pressure is reached or a defined maximum temperature is reached.

This sensor was the only reasonably priced sensor I could find that can take the pressure range needed. Since my application is space and weight sensitive, the original steel housing could not be used. The original housing was also missing a suitable hose nipple input.

Enjoy the pictures