Metric Fastener System

Having grown up with Imperial (british) then when I got into cars I had to learn american imperial and they are quite different. The later I had to learn metric as Australia changed over from a mixture on imperial, (brit and amer) to metric. I am now fully conversant with them all and see no real difference other than the actual dimensions.

The argument that one is stronger than the other is largely based on a preferential bias. Remember its difficult to compare because they are not the same size. A half inch bolt is a bigger diameter than a 12mm one, so you can't compare. The biggest difference is actually between the two imperial systems. The british use a 55 deg thread while the americans use a 60 degree system which is the same as metric. Also various standards call for radius tops and bottoms while others cal for flats and points.

As to which is better it's rather a moot point, just use what you have, it's not worth arguing about.

Consider the history of threads which were first made in any quantity in britain during the industrial revolution. Every town had it's own standard. and even after a long program of standardisation we still have BSW, BSF, BSP, BA, Brass, Bicycle and a few others that i can't think of right now. Just to add to the confusion we also have Acme, Square, and Buttress threads.
 
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RJSakowski said:
I'm not sure where the author arrives at his conclusion about the 3%. There is no practical difference between the material specifications for metric and UN fasteners which leaves only mechanical properties. Strength of a fastener derives from the tensile strength of the core of the fastener and from the shear strength of the threads. Shear strength will decrease as the the thread pitch decreases so while a UNF thread has a greater tensile strength than its UNC counterpart, its threads will be weaker. In applications like head bolts, this can be offset by having a longer thread engagement. Metric fasteners tend to have finer threads than similar sized UN counterparts which might weaken the fastener for a given thread engagement but not significantly.
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I’m also not sure about the specifics of his conclusions. If I was younger I’d probably read up more on the subject but at this stage I’ll just take his word for it... METRIC Fine thread FASTENERS aren’t markedly stronger than metric course thread fasteners. Maybe it’s 3% or maybe it’s 15%. But I doubt that they are 15% stronger similar to the way UNF is 15% stronger than UNC THREADS. In any case the author isn’t just any Bozo.
Here’s his bio or resume http://www.lewmerrick-pe.info/resume/
Below are just a few of his accomplishments

  • Journeyman’s rating as a tool & die machinist through the German Machinist’s Guild.
  • Formally trained and experienced in machining, hand & die forming, casting, molding, and lay-up of metals, plastics, ceramics, and composites.
  • Formally trained in sketching, drafting, and engineering drawing creation and maintenance.
  • Formally trained in tolerance and allowance application to mechanical design.
  • Well experienced in location and selection of Commercial Off The Shelf hardware components.
  • Formally trained in the properties of materials such as: metals, plastics, composites, and ceramics and application of those properties to mechanical design.
  • Formally trained in hand and computer aided analysis of: stresses, strains, dynamic responses, and fatigue of structures and components.
 
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Years ago tinkered with rv . chopper made in USA but metric hardware.

Manufacturer stated metric was used because the thread pitch was finer, more threads per inch.

Consider a gear drive, more turns to same distance so the finer thread provides greater compressive force with same twisting force.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 
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Tim9 said:
I’m also not sure about the specifics of his conclusions. If I was younger I’d probably read up more on the subject but at this stage I’ll just take his word for it... METRIC Fine thread FASTENERS aren’t markedly stronger than metric course thread fasteners. Maybe it’s 3% or maybe it’s 15%. But I doubt that they are 15% stronger similar to the way UNF is 15% stronger than UNC THREADS. In any case the author isn’t just any Bozo.
Here’s his bio or resume http://www.lewmerrick-pe.info/resume/
Below are just a few of his accomplishments

  • Journeyman’s rating as a tool & die machinist through the German Machinist’s Guild.
  • Formally trained and experienced in machining, hand & die forming, casting, molding, and lay-up of metals, plastics, ceramics, and composites.
  • Formally trained in sketching, drafting, and engineering drawing creation and maintenance.
  • Formally trained in tolerance and allowance application to mechanical design.
  • Well experienced in location and selection of Commercial Off The Shelf hardware components.
  • Formally trained in the properties of materials such as: metals, plastics, composites, and ceramics and application of those properties to mechanical design.
  • Formally trained in hand and computer aided analysis of: stresses, strains, dynamic responses, and fatigue of structures and components.
Click to expand...
I never said that the author was any kind of a bozo. However, in the more than fifty years of my professional life, I have run across enough instances of supposedly highly qualified people making erroneous statements to question any statement that offhandedly seems wrong. The statement "... the gain in mechanical properties is usually in the 3% range."is questionable in my mind.

In looking at actual proof strengths of UN and metric fasteners, UN fasteners increase in proof strength going from coarse thread to fine between 10.1% and 14.8% with 12.3% being average over the size range from 1/4" to 3/4". Metric fasteners going from standard thread to fine thread range between 6.5% and 13%, averaging 7.9% over the size range from M8 to M18.
 
I have never found any of the threaded fasteners in any of the above mentioned systems to be deficient if used within their specification range. The biggest problem was and still is the stupid difference between 1/2" BSW and UNC Why the americans chose to use 13 TPI is beyond me.

I don't even understand the argument between 55 and 60 deg thread form. It has never been explained to me in a way that actually makes sense. I only use one cutting tool and it's 60 deg. if I'm cutting BSW, UNC or metric it does the job well enough. If I was making a precision thread forming tool then I would have to do it correctly, but for the work I do it's good enough.
 
I wasn’t accusing you of calling him a Bozo....I was only stating fact that I don’t think he is a Bozo. For what it’s worth.... in most cases all quality fasteners will do the job they are designed for. The biggest problem with fasteners IMO nowadays is that it’s not that easy to find quality fasteners..
So many stores only carry Chinese crap and it’s really hit or miss with them. There’s just not much quality control in China. Damned shame really. It’s hard to get quality bolts.
Same thing with something as simple as underwear. Years ago... I could buy high quality Jockey underwear made in USA. Now no matter what brand I buy.... my underwear looks like it’s 2 years old after one washing. It’s just all crap IMO.
 
Here in Australia we "went" Metric in 1972. What a confusion its turned into. We get fasteners measured with a combination of metric and imperial. Particularly self drilling screws for wood work etc. The big hardware shop sells them with the length in mm's (which is easy) and the dia in some weird gauge size. Why they can't just put the dia in mm on the packet is beyond me. We also get small screws and bolts that are a metric length but the thread is Whit or UNC. Or the head of the bolt is in imperial sized hex but the nuts are a metric hex (or vice versa) Aaaaaaghhh. (this is mainly at hardware shops - specialist fastener supplies have their acts together)
The Caravan/RV and Boat industry are still stuck halfway as well. Length is in feet & inches whilst width, height and weight are in metric. Go figure. Funnily enough, even though we changed to decimal currency only 6 years before going Metric, no one gives a price in pounds shilling and pence and have not done so since about 1967. Fancy that :)
peter
 
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