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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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A different single-point threading question....
All,
I also have a question concerning single-point threading. I recently started practicing making a few *small* parts for a miniature hit n miss engine I'm going to build (never had to make stuff this small before). One of the pieces is a downsized "grease cup" as is seen on some bigger machinery for greasing the bearings. Simply a threaded fixture to screw into the bearing cap, and a reservoir with a grease cap to be tightened to force grease down through a small hole into the bearing. As I very seldom had to thread mating parts without having the "other half" to try for fit, I'm in the process of "bettering" my accuracy so when I make the female thread, the male thread will fit as it should. I have a nice threading book (using 29 degrees angle) from Sears/Atlas showing a lot of info, but I also wanted to try something. Example: 1/4-20 threaded male piece. Charts show the major diameter of .2500 with a minor diameter of .1850 which corresponds to the National Form Tool Double Depth of Thread of .0650. (.2500 minus .0650 equals .1850). The manual I have shows depth of compound feed for the thread to be .037. So all is well and good so far. The chart also shows calculations for VEE form tool being Double Depth of Thread of .0758. Therefore, .2500 minus .0758 leaves minor diameter of .1742. The compound feed is shown to be .043 for the VEE thread. Again, okay so far. Now... I have Guy Lautard's bedside reader showing a formula for diameter reduction and also length reduction. Using the VEE form example above, I need to reduct the diameter of the shaft by .0650 in the threading process. Dividing this number by 2 for my lathe, I need a "crossslide" infeed of .0325 for the depth of thread. Using Lautard's system for small crossslide infeeds by using the compound, his formula is: Amount of crossslide infeed needed divided by sin of compound angle setting. Therefore .0325 divided by sin(29 degrees) should give me the correct amount of compound infeed to equal the amount of crossslide infeed I'm attempting to get. Formula yields .0670. Wrong answer..... what am I overlooking? The charts in the manual work well and the fit is good, but I was just trying to use a method which prevents looking up everything. Thanks. Ken. |
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