Learning to Cut Threads

[Todd3138]

I posted last week asking about thread cutting tooling and appreciate the feedback you guys gave. I have the benefit of having access to a lifelong machinist who works for a friend of mine. He and I have become friends over the handful of visits he has made to my shop to help me begin to understand a little bit of what I can do here with the machinery I've got. Yesterday, he helped me out with cutting outside threads and showed me in very meticulous detail the exact process he uses. He brought a piece of scrap aluminum round bar because it's more forgiving of a hamfisted novice like me when I might crash my tool into the work! I found a random nut that turned out to be a 1" - 8 and he walked me through the process of turning the bar down to the right diameter and then how to set up and cut threads. This was our finished product and the fit was nice just like the two pieces were made for one another (which, funny enough, I guess they were!).



Tonight I decided to do it myself with a 3/4' - 16 nut. While I'm generally happy with how it turned out, I think I got just a few thou too small on turning the bar down and ended up with play between the nut and threaded bar. It's not enough to pull the nut off, but there is that probably 2-4 thou of play. It does thread on as it should otherwise, but my takeaway is to be more diligent and detailed on my measurements. I have never done a lot of precision measuring in my life so learning to properly use and read a caliper and micrometer has been a learning process. I look forward to the day it is second nature as I'm sure it is to most of you. Anyway, here is my finished product from tonight.

 

[markba633csi]

You're getting the hang of it- now you should try steel- with plenty of cutting fluid
-Mark

 

[snoopdog]

Well, If you ain't turnin' you aint lernin, good job!

 

[Todd3138]

You're getting the hang of it- did you use any cutting fluid? While you can often cut aluminum dry, it does help prevent "tearing" of the metal when cutting threads.
I like to use liquid wrench but there are many other fluids you can use
-Mark

Hi, Mark. No, I did not but I get your point. I have a gallon of threading and cutting oil sitting right by the lathe so not much excuse for not using it other than thinking the aluminum was soft enough that I didn't need it. I did notice tonight that the threads were not quite as clean as they were on the larger piece I did yesterday. I'll use some next time and compare.

Thanks for the input!

Todd

 

[Todd3138]

Well, If you ain't turnin' you aint lernin, good job!

That's a great point! Thanks!

 

[MrWhoopee]

Looks pretty good for your first try without adult supervision. For cutting fluid try WD40 or kerosene for aluminum.

 

[SLK001]

Your thread profile looks "off". What does your tool look like? Looking at the thread, your crests look too sharp and your "valleys" look too rounded. A poorly cut thread can seem like a perfect fit for a nut. Did you measure the final product (thread mic or wires)? You should run a triangle file in the threads to clean up the flanks and a flat file to clean up the crests.

 

[vocatexas]

While I find WD-40 pretty much useless as a lubricant, it works pretty good as a cutting oil on aluminum.

Your first two pieces look a LOT better than my first two. Good job!

 

[benmychree]

It does not much matter if the OD of a relatively coarse thread is undersize; there is little loss of strength with a truncated thread crest, you just have to be careful to not cut too deep and get an undersize pitch diameter. Most threads will be undersize on the OD from fileing the crest to eliminate burrs; I did a lot of 1 - 1/8 -7 threads on disc plow axles, I purposely cut the ODs undersize to eliminate the burr problem and also to lessen the tendency of the thread crests to become damage with rough handling, I was able to finish a thread in only 7 passes, using the "oilfield thread technique".

 

[Todd3138]

Your thread profile looks "off". What does your tool look like? Looking at the thread, your crests look too sharp and your "valleys" look too rounded. A poorly cut thread can seem like a perfect fit for a nut. Did you measure the final product (thread mic or wires)? You should run a triangle file in the threads to clean up the flanks and a flat file to clean up the crests.

I'm using a 60* indexable carbide tip. No, no final measurement with wires as I don't have any yet. I did not try the mic though I don't have a "thread mic" either, just what I assume is a normal outside mic set (1", 2", 3"). Good idea on the triangle file. That I DO have!