fine threads VS coarse threads on set screws...........................

Hello All,

Just stumbled across this thread and it's absolutely perfect timing for me - I'm working on a couple of small internal combustion engines and trying to figure which to use for set screws, fine or coarse threads.

Reading all the posts it seems the pros fall heavily on the side of the fine threads, but I didn't see one thing I was looking for - does the metal involved have a bearing? In this case 7075 Al. My 'guess' is the fine threads, again, would be the better choice to give the biggest possible contact surface but then the depth of the threads is considerably less.

Any help?
 
Hello Gordon,

Thanks for this information - I'm completely unfamiliar with 'full form taps' so assume they are something other than the ones I currently am using.

Found this chart http://www.championscrew.com/form_tap_drill.htm that provides drill sizing for the various form taps, and at McMaster Carr found this information on 'Thread Forming Taps', http://www.mcmaster.com/#standard-taps/=gj75oq so assume this is what you are referring to.

Interesting information - can you say approximately how much stronger this type tapped hole would be over conventional cut thread? I'm a bit concerned about possible breakage of the taps as the sizes will likely be on the small size; 8-32, 10-32 and similar. Do these require more torque than conventional taps?
 
Mike,

Fine threads are generally preferred in steel, but I would recommend using coarse threads in aluminum, even in an alloy as strong as 7075. Fine threads in soft metals are easily cross threaded and are more apt to seize, especially when using stainless steel hardware, plus they require more thread engagement to equal the pullout strength of a coarse thread screw.

Tom
 
In general the threads left by form taps are superior. The metal is cold flowed into the thread profile, which of course does not cut any crystal or lattice structure in the material, and it work hardens material with that possible property. Less torque is generally required to form the threads than to cut them. About the only drawback I have experienced is the only once in a while tap breakage. Since there are no flutes to grab, standard extractors won't work at all, EDM hole poppers leave the outside of the tap complete, with a hole down the center, Tap-Out will get them out of the good stainlesses, but takes so long on alloy steels the corrosion may be objectionable.

The most difficult form tappping I have done is a 2-56 X .375d in BeCU, Alloy 25 Condition AT. Even worse than Inconels I have done. Done literally thousands of 4-40 form taps in 6061 and 2024 Aluminum. Loved it in those. Never broke a tap.
 
Hello Tony,

Yikes! I don't like doing anything at 2-56, if you breath too hard it snaps! Good info re the coarse threads being less likely to get cross-threaded, although in this application I wouldn't expect to do much disassembly/reassembly.

As I said in my earlier post, had no knowledge of the thread-forming taps although I've known about commercial thread-rolling for many years. It seems I'll have a few additional things to throw on my next McMC order.

Oh well, one can never have too many tools.
 
Be sure and follow the hole size carefully. It will make all the difference. I don't think I would try hand tapping with a form tap either. I have only power tapped with them, and most of that with a dedicated tapping machine (Speedicut). The don't even feel right when trying to hand tap. But then, I don't like the feel of spiral taps by hand either. Too springy. With form taps, it's wide open and don't look back.
 
Hello Gordon,

Not a bad idea! I have a cordless power screwdriver that would serve that admirably - no cordless drills though, had several and got tired of the batteries going belly-up on me and costing as much as the entire unit so I bought a corded DeWalt and haven't regretted it one bit.
 
One thing I've found in common situations where people break taps.....I tell them that they turned them too hard!

The cordless or corded drill will work fine as long as you are aligned with the hole. If you are not, then breakage is still likely. Or if following Gordon's method, there will be no torque setting that will drive the tap all the way in if you are very far out of line.

One note on the form taps: If you are drilled to proper size, and still don't quite get the depth you need out of the one shot you have, go in with a cut tap and finish the hole. The resulting threads beyond where the form tap stopped won't be proper, of course, but you may decide you have enough engagement above it to get away with it. I would never recommend it for making parts to print, however. Use at your own risk.
 
As you correctly surmise, there is a question that must be answered when considering the possibility of using form taps. It is the ductility of the material. Obviously, you could not cold flow glass, or diamond, for instance. And a marshmallow would not be a candidate either. To directly answer your question, yes you can, but there are limits. Not all cast iron is ductile enough. There is a family of cast irons that are referred to as "ductile" as a product description. It is, in reality, a fairly complex type of metal.

http://egmrs.powweb.com/EJS/PDF/vo242/151.pdf

Other types of metal in the cast iron family are white, gray, compacted graphite, and malleable. Each has properties that make it useful for some applications, and not so much for others. In general, it is cheap, but sufficiently strong or has other desirable properties that make is a good choice for certain products.

Good read of the basics of cast iron types:

http://www.atlasfdry.com/cast-irons.htm


OK, I digress....there is a large foundry here, and there was two. My grandfather worked at the one that is now closed and gone, so it's a little close to home. I've been in the plants and have seen and machined all too much CI. I don't care for it. In the end, it's not a prime candidate for any cold forming processes, but recently there have been form taps developed that are said to do well in it. I haven't used them myself, but I have never had cause to disagree with Emuge:

http://www.emuge.com/technical/pdf/Threading/Tap/zp10019_gb.pdf

They have form taps for various difficult materials, including GGV, which I would think a very difficult material to cold form.
 
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