| Gallery, Projects and General > How do I?? |
| screw-cutting basics |
| << < (3/6) > >> |
| minerva:
Lew I originally started this thread in order to nail down a practical course of action when faced for the first time, with the single point cutting of several different sized threads somewhat larger than those normally encountered . The every day use of the ubiquitous taps and dies it must be said does not encourage one to retain knowledge of each and every size, normally the tapping size drill tends to stick in the mind and one tends to calculate this figure as maj.dia-pitch ( I made a conscious decision to go metric when I started back some four years ago). I have decided to invest in a set of thread data tables as a start to my edification into the mysteries of alternate thread cutting and the application of specific threads for specific jobs, the learning curve seems to have got steeper!! once again many thanks, terry |
| andyf:
Terry, I do it in a somewhat rough and ready manner, using the tap drill sizes shown in tables. Simple example: Thread required is M10 x 1mm. An M6 coarse thread has 1mm pitch, and tables show the tap drill size for it is 5mm, which is 1mm less. So to screwcut an M10 x 1 female thread, start with a hole which is 1mm less than the major diameter of 10mm, i.e a 9mm hole More complicated example, using one of the threads you want: Thread required is 1''x 24TPI BSF. Thread tables like this http://mdmetric.com/tech/thddat8.htm#bsf (BSW is at the top of the page, BSF at the bottom) disclose there is no standard BSF thread with 24 TPI. But a standard 3/16" BSW has 24 TPI. The thread form is the same for BSW and BSF. So (converting to metric) subtract the tap drill size of 3.70mm from the major diameter of 4.76mm. The difference is 1.06mm. The size required for 1" X 24 BSF will be 25.4 - 1.06mm = 24.34mm. The answer would be the same if the instructions called for 1''x 24TPI BSW. As I said at the outset, a bit rough and ready, but it seems to work for me when making a female thread in one part and a male one on another, and neither will be required to mate with anything but each other, or another thread I've made myself. Andy |
| Lew_Merrick_PE:
--- Quote from: minerva on January 04, 2013, 05:45:16 PM ---( I made a conscious decision to go metric when I started back some four years ago). I have decided to invest in a set of thread data tables as a start to my edification into the mysteries of alternate thread cutting and the application of specific threads for specific jobs, the learning curve seems to have got steeper!! --- End quote --- Terry (& all), The best data set on (most) screwthread forms is (U.S. Government) FED-STD-H28. It can be downloaded for free from http://quicksearch.dla.mil/ -- it is a bit complicated, but all the information (more than any sane person would need) is there. It is non-classified, public domain information. For God's sake, do not pay for thread data! There is quite a bit posted in my "page" at ScribD (http://www.scribd.com/Lew%20Merrick) and, if you have trouble downloading it, drop me a line (tangent@olympus.net) and I will e-mail it to you. There is a public domain booklet that was published by the U.S. Government during WWII on the mathematics of cutting screwthreads. My copy is buried (we moved a few years back and I do not have space for my technical reference library, so there are boxes of books sitting in storage), but I will try to dig up the title of it. It was aimed at taking farm-hands and teaching them how to cut threads on a lathe. It was one of the texts used back when I was an apprentice... Please remember that about 15%-20% of the work I do is seriously mission critical (i.e. the bolt circles that held the parts of the Space Shuttle's External Tank together type of stuff) and I tend to think(?) in such terms. Nearly 50% of the work I do must meet very high (medical, military, space, etc.) standards and I tend to think(?) in such terms. This is why I tend to restate the question in my response because I regularly see applications where the quick & common answers are unacceptable. This does not mean that I do not know or use the quick & common answers, merely that much of the time I cannot use them because of the exceptions that exist to all rules... |
| minerva:
Hi Andy, lots of positives here. Firstly many thanks for the link which I feel has most of the information I shall ever require. also the details of their use. your explanation has clarified the whole operation I have always suspected that matching male and female threads cut with the same profile must if machined to fit be approaching something like the definitive version should be. Lew I tried to download from the quicksearch site and encountered some pretty heavy security warnings ,so I have postponed for now and will try again later when I have a little more time. Regards Terry T |
| andyf:
Just remember its limitations, Terry (my idea, not the table). :scratch: Are you sure 1/16" x 24 is right? That will be a bit hard to do on the lathe, because a threading tool to fit down your (approx) 0.5mm pilot hole would be impractical. So would screwcutting the male thread, with its 0.5mm minor diameter, ready to snap off as soon as the tool touches it. You might consider converting the two other threads to metric, if you're more used to that. 24TPI is close to 25.4 TPI, or 1mm pitch. 1" is only a little over 25mm, so you could substitute M25 x 1. 5/8" is only a little under 16mm, so you could substitute M16 x 1. The pilot holes would be 24mm and 15mm. Andy |
| Navigation |
| Message Index |
| Next page |
| Previous page |