External thread and matching internal threads

devils4ever

devils4ever

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So, I want to try cutting an external 3/4"-16 (UNF) thread in 6061 Aluminum rod and a matching internal thread in 6061 Aluminum hex (nut). A couple of questions/comments:
  1. What size should the hex be? I see both 1-1/8" and 1-1/4" listed as standard nut sizes for 3/4" threads.
  2. I'm thinking I should cut the external thread first since I can measure the pitch diameter using thread measuring wires and a mic.
  3. Then, I should cut the internal thread so I can keep checking the fit with the external thread previously cut. I don't have a 3/4" bolt to use as a reference.
  4. For the internal thread, I'll use a drill bit smaller than the minor diameter (probably 1/2") and use a boring bar to get the diameter in the middle of the tolerance range for the minor diameter. For 3/4"-16, the minor diameter range is: 0.682" to 0.696". So, shoot for 0.689".
  5. Then, use the internal threading tool bit to cut the thread. How to remove raised burrs? Round file?
Thanks!
 
How about buying the thread gauge, making a part to fit that and then make the second part to fit the first?
 
Buy 3/4” nut and use it to gauge the threaded rod and then make the matching nut at the end.
Pierre
 
devils4ever said:
So, I want to try cutting an external 3/4"-16 (UNF) thread in 6061 Aluminum rod and a matching internal thread in 6061 Aluminum hex (nut). A couple of questions/comments:
  1. What size should the hex be? I see both 1-1/8" and 1-1/4" listed as standard nut sizes for 3/4" threads.
  2. I'm thinking I should cut the external thread first since I can measure the pitch diameter using thread measuring wires and a mic.
  3. Then, I should cut the internal thread so I can keep checking the fit with the external thread previously cut. I don't have a 3/4" bolt to use as a reference.
  4. For the internal thread, I'll use a drill bit smaller than the minor diameter (probably 1/2") and use a boring bar to get the diameter in the middle of the tolerance range for the minor diameter. For 3/4"-16, the minor diameter range is: 0.682" to 0.696". So, shoot for 0.689".
  5. Then, use the internal threading tool bit to cut the thread. How to remove raised burrs? Round file?
Thanks!
Click to expand...
I think you're on the right course. As far as the hex size, either will work.
 
1-1/8" has always been the common nut size for 3/4" threads. McMaster Carr sells extra wide 1-1/4"nuts for high pressure or structural applications.
 
For lack of a better way to express it, lack of any chart and such, when I was working in a steel mill under high strength conditions, a Grade 5 fastener had nuts of the more common size. Such as 1/2-13 having a 3/4 wrench. A Grade 7, or more, of the same size had a 13/16 or 7/8 wrench. The difference having to do with the tensile strength of the fastener. Thicker wall, thicker nut, more threads, et al. In most, but not all, cases, a fine thread such as 1/2-20 is of higher tensile strength, so would have those higher ratings. Since you are cutting fine threads, 3/4-16, my opinion would lean toward the heavier, thicker nut. That's just my opinion, now.

Next, assuming a "standard 60 deg thread, drill size would be fastener size minus the thread pitch. i.e. 3/4" minus 1/16"=11/16". If you can swing it, cost, or time, or whatever, a factory nut would be a good purchase. Thread wires are useful, but for "standard" threads a waste of time. A thread gage to check your work is all you need. Cut the bolt first, check the OD, and then make the nut to fit. If you can lay your hands on a factory nut, cut the bolt to fit. Otherwise, cut it to "standard" and then make the nut to fit.

.
 
A tap would solve most all of your problem; the internal thread can be roughed out neatly to size, and the tap used to finish the thread, and the external thread cut to fit the nut.
As to hex sizes, I like to use heavy hex nuts on jobs that may be frequently adjusted, such as on machine tools, the larger hex size does not tend to round over with use, and standard hex nuts do.
 
DiscoDan: Do you mean a thread plug go/no-go gage? They are a little pricey.
pdentrem: I'm trying to find a local source for a 3/4"-16 nut. Not easy to find and shipping one nut is not cheap either!
RJSakowski/Bi11Hudson: I think you're correct. 1-1/8" is the standard size and the 1-1/4" size is for heavier applications. In any case, I'm using 1-1/4" hex stock, so I'll just stick to that for now.
benmychree: A tap would work, but I'm trying to learn to cut internal and external threads.
 
You can still go through all the motions to cut the internal thread, and use the tap for final sizing. You should be able to find a nut at most any well appointed hardware store or auto parts. When I use a nut for sizing an external thread, I nearly always run a sharp tap through it to remove any burrs that may be present.
 
Aluminum threading into aluminum will likely gall and seize up. Using the male thread as a gauge will not work. If you do not want to buy a bolt to use as a gauge, machine a male thread in any metal but aluminum and use that as your thread gauge

To get rid of ID burrs cut a lengthwise slot in the end of a wood dowel. Stick a piece of abrasive cloth through the slot. Run that back & forth in the bore.
 
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