I started by measuring to size and cutting on a abrasive chop saw. I can't think of a project that would require a clamp larger than 7', so I will make 4-7' long and 2-6' long to go with the 2 I already have.
All cut and ready to thread. After cutting, both ends get a light reaming and one end gets a slight 45* bevel on the disk sander. The other end gets a much shallower light bevel to give an easier start to the threading die.
Here it is all threaded and ready for the clamp fitting. When threading reverse the die a full turn every 1 1/2 to 2 turns to break the swarf. I don't thread pipe enough to keep threading oil in stock so I use a squirt of light gear oil every couple of turns. A light wire brushing and its ready to assemble.
Here are all the finished clamps including the 4 I already had. A piece of threaded 3/4" pipe 4' long at the hardware store costs almost as much as a 21' length from a steel supplier so you can save alot of money threading your own pipe.
I'll do the 7-1/2"clamps in 4 and 5' lengths when I get my steel order. There's a saying that goes "you can never have enough clamps" with these and between 2 and 3 hundred lbs. of C and F clamps, I think I will be close.☺