steel for screws

[kokomokid]

I got a lathe to play with and have been making some gun screws from grade 8 bolts. I used potassium nitrate at 700* to blue them. Would like to know if something else like drill rod would machine better and still blue good? Also will be making some pins?

[oldred]

I suppose you could use drill rod but I use 4140, I think 4140 (someone correct me if I'm wrong) is what is used for grade 8 bolts but for what ever reason I find 4140HT (TG&P) rod to yield a really good finish.

[theperfessor]

I'll second oldred's suggestion. 4140 is extremely versatile and pretty easy to heat treat. If you heat it to 700F you will lose some hardness.

[cwheel]

I've been using stress proof 4130L. It heat treats the same as 4140 and is just a little easier to machine at the smaller sizes, a little easier to get a good finish on the threads. More often than not, it's what you got, used plenty of drill rod as well. Threading becomes a problem when you are at a #4 and smaller and single pointing the thread, a little less surface tension is a good thing, and that's where the leaded helps some.
Chris

[Buck Neck It]

Komokokid, where do you get the potassium nitrate for bluing? Can this stuff be mailed? (I am betting no).

[W.R.Buchanan]

4140 is what G8 bolts are made from, however 4140 machines like doodoo. Most of those bolts are cold headed and the threads are rolled on as a last op. Heat treating is the next operation and then waxing or oiling.

4140 forms real nice and that's why they do it that way. It is also about 10 times faster.

Randy

[oldred]

4140 forms real nice and that's why they do it that way. It is also about 10 times faster.Randy



Cut threads weaken a bolt also vs rolled threads, it's my understanding that a grade 8 bolt can lose 10% or more of it's ultimate strength depending on how sharp the cutter is if the threads are cut instead of rolled. Rolled threads of course displace metal leaving threads with a smooth compacted surface and a fairly uniform and slightly radiused bottom to the thread valleys as opposed to cut threads with an irregular and ragged surface that has had metal literally ripped out of it leaving many stress risers. With a good sharp cutting tool and proper procedure this can be minimized and the surface may look quite smooth unless examined microscopically but more often the case is that the flaws are visible even to the naked eye!


A case in point was huge gear made in sections for a mining machine I worked maintenance on for years, this gear was held in place with 1 1/2" bolts 12" long. The bolts the machine used needed another 3/4" of thread on the shank vs an "off-the-shelf" bolt so for several years the company just sent them out to a machine shop to have them threaded 3/4" higher on the shank, these new threads were of course cut. Major down-time and production (money loss!!!!) was occurring due to these bolts breaking, after nearly ten years of this the gear segments and gear mounting surfaces were damaged so badly because of these broken bolts it required a major rebuild, the engineer sent from Marion power shovel to oversee this rebuild took one look at what we had been doing with those bolts over the years and just about had the big one right there on the spot! He called the mine maintenance supt over and told him that "you may have saved a couple of thousand dollars on bolts but it's costing you nearly two million dollars to repair the damage caused by them"! After the repair the bolt breakage (and the dirty difficult job of replacing them!) became a thing of the past, was that because of the rebuild or due to using the correct fasteners?

[kokomokid]

Buck; you can get it on e-bay and mailed in the lower 48. It did great for me except except where i did not get all the antiseze from test fitting cleaned off the threads. When melting it foamed up and crusted over making a mess that was unexpected. Just hope the 700* soak did not temper screws too much .

[waksupi]

You could most likely use Stump Out from the hardware store in place of the potassium nitrate.

If you want to just do screws, heat your lead pot to the same temp, and do them in the lead. Does exactly the same thing. I blue screws and small parts with a torch. Heat them, and dunk in oil.

[oldred]

Yes that remover does indeed do the job, most brands are almost pure Potassium Nitrate but the key word here is "most". One brand in particular is Bonide "Stump-Out" which is not NO3 but rather Sodium Metabisulfite, I had to look up the actual ingredients but I knew that one and maybe a couple of others are not Potassium Nitrate. It should say on the label if it is or not but "Green Light" brand for one is the right stuff.

[Ballistics in Scotland]

Buck; you can get it on e-bay and mailed in the lower 48. It did great for me except except where i did not get all the antiseze from test fitting cleaned off the threads. When melting it foamed up and crusted over making a mess that was unexpected. Just hope the 700* soak did not temper screws too much .

You might get a bigger choice of sizes and prices if you search for saltpeter. I think that foaming up is because it contains some atmospheric moisture. Keeping it for a few days on top of a radiator, with occasional stirring, might help.

[Zouave 58]

There's a bunch of companies on the internet who sell to the folks that make home made biodiesel. One outfit advertises 99% pure potassium nitrate- one assumes the other 1% are tagents.

[wills]

http://www.skylighter.com/potassium-nitrate-powdered.htm

http://www.skylighter.com/mall/product-details.asp?id=2606

[oldred]

Potassium Nitrate is easy to find, it's even available from several E-Bay sellers in just about any amount you want.


I just checked E-Bay and it came up with 208 listings in various amounts.

[sackot]

Cut threads weaken a bolt also vs rolled threads, it's my understanding that a grade 8 bolt can lose 10% or more of it's ultimate strength depending on how sharp the cutter is if the threads are cut instead of rolled. Rolled threads of course displace metal leaving threads with a smooth compacted surface and a fairly uniform and slightly radiused bottom to the thread valleys as opposed to cut threads with an irregular and ragged surface that has had metal literally ripped out of it leaving many stress risers. With a good sharp cutting tool and proper procedure this can be minimized and the surface may look quite smooth unless examined microscopically but more often the case is that the flaws are visible even to the naked eye!


When I was learning to fly I had trouble reaching the rudder pedals in the old biplane (I'm 5'4"). It was suggested that I could use Cathy's pedal risers. I later learned that the reason Cathy didn't need her pedal risers any more was that she had been flying an aerobatic plane where a cut thread had been substituted for a rolled thread in the tie rod which holds the lower wings together...

I'm sure the problem there will have been the stress risers, I doubt whether a vital component like that has a safety factor of less than two so a 10% reduction in ultimate strength would never have been detected, but fatigue would be critical.

[oldred]

That 10% figure is just something I have heard tossed back and forth over the years, it could be less or more depending on how well the threads are cut. Two otherwise identical bolts could possibly have a great deal of strength difference due to the threading method it would seem, threads with a somewhat ragged surface with metal literally torn out of them vs threads with smooth burnished surfaces and slightly radiused valleys might make only a small difference in failure due to tensile loads but fatigue failures could be substantially greater.

[Ballistics in Scotland]

Long ago when I worked, in a conspicuously unmartial capacity, with the Royal Saudi Air Force, they qualified a student pilot who needed wooden blocks screwed to the pedals. Apparently he was otherwise quite all right, though. It sounds to me like there was something else wrong with Cathy's tie rod, on the basis of the age-old principle that a number of individually acceptable failings combine to cause an accident.


Most of my life I thought of rolled threads as a factory expedient to make them cheaper, but in fact there isn't a thing against them that I know of. They can even be just as accurately sized as die-cut. I believe the danger of failure probably depends on the pitch. In most available threads (Unified, Metric or Whitworth) there is a coarse series with which the two kinds of failure (thread stripping or pulling in two) are close to equal. Then there is a fine series, with which stripping is more likely, in exchange for reducing the effort to tighten, and/or the chances of vibrating loose. Probably one of these would be less vulnerable than the other to the stresses of thread cutting. But which one...?

[Frank46]

Company I worked for had these huge boiler feed pumps that put out boiler feed water at over 2000psi. Come time to rebuild the pumps it was not an easy job to pull the nuts off the threaded studs that held the pump casing together. naturally they has these huge impact wrenches and guys would be standing there with the wrench banging away trying to loosen the nuts. If that didn't work out came the tourch and copious amounts of heat was required before the wrench was put on in another attempt to loosen the nut. talk about nut busting. Frank

[sackot]

Long ago when I worked, in a conspicuously unmartial capacity, with the Royal Saudi Air Force, they qualified a student pilot who needed wooden blocks screwed to the pedals. Apparently he was otherwise quite all right, though. It sounds to me like there was something else wrong with Cathy's tie rod, on the basis of the age-old principle that a number of individually acceptable failings combine to cause an accident.


You seem to be exactly correct:- courtesy of the wonderful internet, it's easier to find out now than it was back then. Chapter and verse here:
http://www.aaib.gov.uk/cms_resources.cfm?file=/Stampe%20SV4C%20G,%20G-ATKC%20%2002-88.pdf

TLDR version. The original tie rod was also waisted to a smaller diameter over part of its unthreaded centre. This allowed it to stretch a little, distributing more stress to other members. Making it 'stronger' helped it break.

Apologies to OP for the thread drift, hope it's all mildly relevant and interesting.

[Ballistics in Scotland]

You seem to be exactly correct:- courtesy of the wonderful internet, it's easier to find out now than it was back then. Chapter and verse here:
http://www.aaib.gov.uk/cms_resources.cfm?file=/Stampe%20SV4C%20G,%20G-ATKC%20%2002-88.pdf

TLDR version. The original tie rod was also waisted to a smaller diameter over part of its unthreaded centre. This allowed it to stretch a little, distributing more stress to other members. Making it 'stronger' helped it break.

Apologies to OP for the thread drift, hope it's all mildly relevant and interesting.

Who, me? I'm not any kind of engineer, but I'm not sure you are right about that, since thread fatigue is clearly the main cause they identified. All I can see they identified is that lack of the waisted tie-rods of the original design, and the use of spring washers, might have imposed bending moment on those threads.

It only adds to the tragedy that a DH Moth should have died with them. Adjusting the allowance of hours seems a pretty haphazard way of dealing with the problem, and I wonder how the aircraft would have been the worse for larger rods from the start. It isn't like they would be making it heavier all over. Maybe its design dates from days when pilots were expected to do a certain amount of dying. Maybe a club aircraft was doing an unexpected number of circuits and bumps in those hours, and a quota of those would have been a fairer measurement.

[oldred]

My understanding that cut threads are inferior to rolled threads far predates the internet and I really hadn't thought much about it, just accepted it as fact without really checking it out. So I decided to ask Mr Google about this and I find LOTS of verification that cut threads are indeed quite inferior to formed threads, some manufacturers even claim as much as 30% increase in strength with rolled threads vs cut threads!

http://www.rolledthreads.com/index.php/aboutus

The best explanation I found was on this site,

http://www.resnapshot.com/MD1298.htm

[deltaenterprizes]

Saltpeter is available at drugstores

[W.R.Buchanan]

Frank46: I worked for SoCal Edison Steam division back when and as a Millwright after I quit them. The bolts that hold the Turbine casings together have a hole thru them and you use a rosebud torch tip silver soldered to a piece of Copper Tubing to heat the studs from the inside out. This causes them to expand, and when they become loose enough you knock the 75lb nuts loose with a hammer wrench and a 20 lb sledge hammer.

Going back together you heat the studs,,, all of which have been pre measured for length and recorded. You put the nuts on and heat the studs up and drive them down and then let it cool. Then you measure it again which tells you the overall length which allows you to know how many more flats you put on the nut to achieve proper torque.

Torque on the stud was calculated by virtue of the amount of stretch you put on the stud and we could get it with in a few Ftlbs. These studs were mostly 8" in dia and 8 feet long and the torque was around 3100ftlbs IIRC. There isn't a wrench big enough to do this and the threads would have problems anyway so they use the heat stretch method.

Kaiser Aluminum in Oxnard CA had a huge H frame press that was bolted to the floor with 4 studs 31 feet long x 12" in dia. We stretched those studs over 1 foot! over 6000 ftlbs! Using the same method.

That was a filthy job as whomever had to work the bottom of the studs was in a hole under the floor and it was pretty gross! You looked like tar baby after a day in that mess. I literally had to strip before I got in my car and drive home buck naked. Luckily I lived 15 minutes away and it was 3:30 in the morning when I got home.

Ended up throwing my cloths in the trash, the black grease would have ruined our washing machine.

Randy

[nanuk]

...
.... Then there is a fine series, with which stripping is more likely, in exchange for reducing the effort to tighten, and/or the chances of vibrating loose. ...


if so, why are fine threads rated for a higher torque than course?


when getting some truck spring Ubolts bent, the guy said the rolled threads were stronger enough that in my province, cut threads weren't allowed for spring bolts. I dunno if he was stringing me along, but it sounded good anyways

[oldred]

Frank46: I worked for SoCal Edison Steam division back when and as a Millwright after I quit them. The bolts that hold the Turbine casings together have a hole thru them and you use a rosebud torch tip silver soldered to a piece of Copper Tubing to heat the studs from the inside out. This causes them to expand, and when they become loose enough you knock the 75lb nuts loose with a hammer wrench and a 20 lb sledge hammer.

Going back together you heat the studs,,, all of which have been pre measured for length and recorded. You put the nuts on and heat the studs up and drive them down and then let it cool. Then you measure it again which tells you the overall length which allows you to know how many more flats you put on the nut to achieve proper torque.

Torque on the stud was calculated by virtue of the amount of stretch you put on the stud and we could get it with in a few Ftlbs. These studs were mostly 8" in dia and 8 feet long and the torque was around 3100ftlbs IIRC. There isn't a wrench big enough to do this and the threads would have problems anyway so they use the heat stretch method.

Kaiser Aluminum in Oxnard CA had a huge H frame press that was bolted to the floor with 4 studs 31 feet long x 12" in dia. We stretched those studs over 1 foot! over 6000 ftlbs! Using the same method.

That was a filthy job as whomever had to work the bottom of the studs was in a hole under the floor and it was pretty gross! You looked like tar baby after a day in that mess. I literally had to strip before I got in my car and drive home buck naked. Luckily I lived 15 minutes away and it was 3:30 in the morning when I got home.

Ended up throwing my cloths in the trash, the black grease would have ruined our washing machine.

Randy

Wow does that bring back memories, just not good ones!!!! Same procedure for some of the fasteners used on large mining shovels and draglines but the part that brings back the nightmares is the guy working at the bottom, in the case of the swing rack on those draglines he (me all too often!) was working in about 4" of "Crater-O" open gear lube -basically tar on steroids! Many times I have stripped down to my underwear and donned an old pair of coveralls and rubber boots, these were just tossed after the job was finished since there was no cleaning them OR ME. Try as I might the remnants of that stuff lasted for weeks and to this day I have a small black "tattoo" on the back of my hand where I got cut while working in that stuff and couldn't get it out, I get reminded of that dragline every time I see that mark that's been there for nearly 25 years now.

[John Taylor]

if so, why are fine threads rated for a higher torque than course?

The threads are not as deep leaving more metal for strength.

Getting back to the original question, I have used old car springs for making screws and machining with carbide. If I buy metal for screws I use 1045, it can be hardened without becoming brittle when heat treating. I have had a few made with 4140 that would crack when heat treating, probably from to cold a quench.