Cutting Threads for the first time...SB 10k

AJ

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Hi all,
I'm about to cut my first set of threads for a project I'm working on. 1/2-20 both right and left hand. I'll be going by the Machinerys Handbook to learn. I know the basics, compound at 29 1/2 deg. 60 deg tool, major and minor diameters, cutting fluid, machine feed rates(20tpi), not to sure about spindle speed (I think it's just cold rolled 5/8 rod so 4 x 100 / dia. ?) but that's about it. I will be doing a couple test pieces to make sure I get this right. Also, I think I'll be grinding my own tool for the first time instead of using one my dad gave me... Any advice ? The pic is of a couple test passes just to get use to the lathes controls... No ryme, reason or size to the threads in the pic.....Any advice ?20180908_224717.jpg
 
Use a fish tool to grind and set up your tool. Buy one or make one; it doesn't matter. Just set it up and get the angles correct. Watch a quick video to make sure that you are measuring the 29,.5 degrees from the right place. If you get this wrong, and lots of people do, it is better to just go straight in. Starrett makes a nice fish, but you can just download a template and make one out of some junk sheet metal.
 
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You're fishtail has to be 90 degrees to the cutter while it's flat is against the part, just a thought.
 
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I like full profile inserts(only do one thread size per insert) which requires less depth, gives a stronger thread and needs less machining passes.Downside is you need one for each type of thread you want to cut.
I would also recommend machery's handbook(buy an earlier edition,27 or 28 on ebay to save money,dont get the guide) and a screw thread micrometer(chinese is ok).

Try to cut class 3 threads to learn quality threading. Then cut double start and triple start threads for a challenge!
faster speed 200 rpm or more can give a better finish on the material.
I would start out with free machining steel, delrin and brass to learn the ups and downs.
 
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Threading can be challenging, but you don't seem to frightened of it. forget minor diameter, there's no way in God's green Earth to measure it, short of an optical comparitor.
For setting you angle of the compound, (since numbers can start at any quadrant) visualize a 4 pitch thread, visualize the right side of the cut, set the right side of your tool parallel the right side of the 4 pitch visualization. clamp the compound there.
Make the first cut a fine scratch, compare it with a known thread or a thread pitch gauge. Make sure it's the pitch you want.
Make sure there's some lubricant on the thread and take a lot of passes to get to where a nut will screw on easily.

Don't sweat the small stuff, it'll come eventually, that's why we suggest first practicing on plasltic or Ledloy (12L14) (Free Machining Steel)
 
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Hey AJ, your threads look pretty good for a first timer - congrats!

I have a few suggestions:
  • Grind an accurate 60 degree threading tool. I would recommend 15 degree relief angles. This will give you cleaner threads. Don't forget to stone a small flat at the very tip or you'll crack the tip off. Be fussy about your angles and also when aligning the tool to the work.
  • Unless the work is really short, less than 2 X the diameter, use a live center if you can. This has a significant impact on fits.
  • Make your thread relief about 2-3 thread widths wide and about 0.003-0.005" deeper than the minor diameter.
  • Don't forget to put a 30 to 45 degree chamfer on the starting end of the work. Make this about 1-2 threads wide.
  • Stage your cuts as you progress. Try 0.005 to 0.009" for the first pass, then 0.005" for the next two, then 0.001" less for every pass until you are taking only 0.001" deep passes. Check for fit early and often. Use thread wires or a thread mic if you need a precision fit; otherwise, a nut will do.
  • Use cutting oil. Any threading oil or cutting oil works.
Go slow, take your time and it will go well.
 
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I'll second Mickey and what you said about getting used to the controls. There is a rhythm to threading, feed levers, backing off the cross slide, adding to the top slide, return to zero on the cross slide, engage, disengage. One of those all senses involved and concentration relaxing things about machining. See the process, do the process. Scary at first, not so much later - until threading to a shoulder on an internal bore with a screw on chuck where you can't run the lathe backwards (see Joe Pie's video).
 
I'll give you my take on the speed, generally the faster you go the better finish you will achieve. I don't end in a relief most of the time as it keeps me in practice and the rpm is governed by my reaction time to pull out at the same point rather than the correct tool speed. Even with a relief and using carbide your reaction time to pull the half nut tends to limit your speed rather than the correct tool speed. Coarse lead screws and fine threads help. Try and run as fast as you are comfortable with and it will probably be OK.
 
Have at it, this part does not hold the wings on a commercial aircraft I suspect.
Chuck stock, set tool as square as possible, touch it off to determine a tool position then thread away until the part measures either over wires or to a ring gauge set.

You likely do not have the equipment to measure a thread in every possible physical aspect of its form.
 
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