Can the metal lathe cut threading into 1/2” to 5/8” steel rod?
Yes. Depending on the alloys and hardness you may have to buy special inserts.
I’m probably looking at grade 5 maybe 8. I may just buy bolts for the threading I need, but I was wondering if the lathe could cut these course threads.
Would I need special inserts for either of these higher tensile strength grades?
Especially if you are looking at grade 8 performance, I would just buy the commercial piece of mind.
short answer is yes. You can look at the front faceplate to see the different imperial and (blah!) metric threads it will cut.
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The harder the alloy the type and geometery may change. Research what you need online based on the material (you’ll need to reasearch what hardness, etc grade 5 and 8 are and is there a standard alloy or is it based on physical properties that meet the requirements, e.g tensile strength, hardness/brittleness etc.) Once you know these you can find out what the best threading insert would be.
If you use the wrong inset two things will happen: You will probably have lousy thread form and the insert will fail quickly. Defintely want to flood with coolant.
Heere’s an example of tech data to select insert" Insert selection
Other companies like Sandvik, ISCAR, etc. also have data.
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I’m seeing several references to medium carbon steel, quenched and tempered. 150Kpsi tensile. Cutting threads before hardening and tempering is the easy part. Not building stress risers in the root of the thread can be challenging, and getting both heat treating processes right is where nobody gets it right the first time, and doesn’t do it repeatably without temperature controlled furnaces.
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Very interesting. A lot of good data, thanks. A lot to digest as well.
I’ll have to research if there are any rule of thumb advice on threading different grades and the inserts needed.
I am fairly certain grade 8 would require rolled threads, not cut threads. There is also a good chance that grade 5 would require rolled threads as well.
Cut threads significantly weaken the core metal, particularly is a proper threading tool (form with radius for that specific thread pitch) is not used.
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I tend to “over-engineer” my needs. I might just try a grade 2 bolt and see if it can hold the shear force I need.
If so, maybe that low to medium steel grade is good enough. I think the tensile on that is in the 60k range(?).
You can buy B7 all thread rod, which is essentially grade 8.
Strength wise you have several factors other than tensile.
How weldable is that rod?
It wouldn’t be rated if you welded on it. You’re messing with the temper.
Let’s say, I have about 8” of it and I weld one end of the rod. Would that ruin the other end or degrade it significantly?
Curious, what is the fastener securing? If anything safety related, buy the proper bolt. Rolled threads have the best form, surface finish, and strength as the process work-hardens the thread and deforms the grain structure of the base material so you aren’t openly cutting across grain patterns.
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If you are engineering a solution, say flying equipment overhead:
- Use a rated bolt.
- Do not weld joints unless you are a certified welder. (This joint would also need to be engineered.)
- It is usually easier to engineer non-welded solutions. (Find a way to use mechanical fasteners.)
- Use the right safety factor for the job. (If it could hit anyone in the head use a safety factor of 5x the designed load.)
- Make sure any existing structure will support your load. (With a safety factor.)
Interestingly, aircraft safety factors tend to be the tightest, but they analyze all of the conceivable loads and loads tend to be more predictable.
Definitely not for anything that will be flying…other than when it doesn’t work and I may throw it. 
I’ve never welded a grade 8 bolt. I may try one and see it I can break it after welding. I’ll do a constant by putting one in a vise and getting a 6’ pipe and see what happens, then weld a plate to one end of another and see what happens then.
I’ve never seen an engineered joint with a welded high strength bolt. Have seen a few welded nuts on tooling where the nuts have been lightly tacked to hold in place due to accessibility. But not the bolt heads.
I may cut the bolt heads off and then weld the end through a pipe or just put the bolt through a pipe and then weld the bolt head to the pipe. I’m looking to add threading to a larger diameter round stock (pipe, round stock,?)
What’s the plan for it? There may be easier ways.
I’ve welded grade 8. It usually messes the threads up.