...I may have passed the classes a very long time ago but I was not the teacher......... Just thinking, we have still not discussed extra fine threads...and I'll pass on that discussion. It is also interesting that there are no fine threads in structural bolts that hold buildings together.... .
th Now for the mind bender, fine threads are not as deep as coarse threads So get ya slide rule out and get it figured!
I went to Mr. Google's house and asked the question....... What is the difference between fine and coarse threaded bolts? Bolts with coarse threads have a larger pitch (fewer threads per axial distance) compared to fine threads. A coarse threaded bolt is specified for most applications unless there is an overriding reason to use a fine threaded bolt (e.g. thorough thread adjustment is crucial for the application). Furthermore, fine thread fasteners are more difficult to obtain. Negative sides of fine threaded fasteners: Fine threads are more susceptible to galling than coarse threads. They need longer thread engagements and are more prone to damage and thread fouling. They are also less suitable for high-speed assembly since they are more likely to seize when being tightened. The potential benefits of fine threads are: Size for size, a fine thread is stronger than a coarse thread. This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter). Because of the smaller pitch, they allow finer adjustments in applications that need such a feature. Fine threads can be more easily tapped into hard materials and thin-walled tubes. Fine threads require less torque to develop equivalent bolt preloads. Fine threads have less tendency to loosen since the thread incline is smaller and hence so is the off torque.
Years ago my boss signed me up to attend an Engineering Seminar on Fasteners. I was LESS than thrilled to go and told him so - he asked that I humor him so I reluctantly did. Well I have to say, after it was all said and done I was quite happy I attended and would jump at the chance to attend another. Fasteners are something that we typically take for granted and often what we "know" is based more on anecdotal evidence rather than science. As part of the seminar the Fastener Engineer gave us this example: He had a customer that had a steam flange that over time would start "popping" bolts. Initially they went to a higher grade fastener - Result? No improvement. Next they INCREASED the bolt size - result ? No improvement. Next they DOUBLED the number of fasteners - Result? No Improvement. That's when they brought in the experts. The experts immediately noticed that they had broken some of the "golden rules" to fasteners - that being the ratio of the fastener diameter to the clamping distance and parallel clamping surfaces. The experts suggested that they GO BACK to the ORGINAL DESIGN (number and size of fasteners) BUT machine the flange with a spot face AND add a spacer to get the ratio more favorable. The customer thought they were crazy but eventually gave in and decided to try it. Result? Success. The customer couldn't believe it. In reviewing the actions they took - "just" increasing the diameter actually made the ratio worse so that while sounding good was doomed to fail. Imagine the fasteners threads having to BEND as the bolt exits the threaded part? If the BOLT was really long that bending stress would be spread out over that longer distance (distance meaning the measurement from the head to the mating threads) - now imagine if that bolt was infinitely SHORT (like a 1/2-13 used to bolt some thin sheet metal in place and those threads were crooked - that bending stress would be very much concentrated. Add some thermal cycling and boom a formula for disaster. So While I will leave the coarse fine argument alone what I would suggest is that you look into some of these "rules" and see where you can apply them to your build - you might be pleasantly surprised. BTW many industries suffered from those faked chi-com grade 8 bolts. I could tell you a few first hand disaster stories from bogus materials!!!
Extra fine thread is what the best women's underwear is made of, and in my experience, it can sometimes be seriously difficult to remove.
Worked on aircraft for a bit. Replaced more than a few bolts that had thread damage due to wear. Was told there grade was similar to grade 9 which Fastenal carried. Only used aircraft grade hardware. In the automotive field, I used grade 8 a lot and learned about the grade 5`s stretch before breaking. The grade 8`s don`t stretch, they just break. Use them way less now. Mainly for installing seatbelts, fine thread too. As far as the O P original question. I would weld the battery box to the firewall. The bolts might break or the metal will start to tear out. Use a large washer for a good footprint. One other thing. Grade 8`s are the most overused bolts in the hobby field. No, the box stores hardware is the most overused in the hobby. People use what`s laying around to do a job. They need to use the correct quality hardware to make a repair. This is were breakage occurs.
Somewhat related. I worked for 30 years as an industrial machinist making plywood mill machinery, it was common to engineer in fasteners that forced the customer to have to buy replacement parts directly from us. While still an apprentice I was given a job to machine 50 one and one half inch diameter shoulder bolts with one inch fine threads but the problem was they gave me an outdated blueprint (our company goes back about 150 years) that simply called out a one inch NF thread. The print should have been spelled out as either 1"-12 or 1"-14, having a foggy memory now I can't say which it should have been but I do recall loading a rather heavy box on a forklift to put them in the steel scrap dumpster.
Concerning fine threads. The Head retaining studs on a Nuclear reactor I worked on several times were fine threads. The studs were 8inches in diameter and had about 12 inches of thread on each end of a 5 foot stud. The studs were turned into a flange face to a monitored depth. Then they were stretched 15 thousandths with a hydraulic apparatus that would also torque a retaining nut to 2500lbs while holding the stud in stretch. after the torque was met the machine would then release the stretch on the stud. There are some 56 such fasteners that secure the reactor head to the fuel containing lower section of what is essentially a really large high pressure boiling pot. There would be 3 crews and machines working to remove and reinstall the retaining studs in concert with one another. Not auto related but I enjoyed the task and found it interesting.
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