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View Full Version : Fastners Metric or Standard does it matter


chris saintdenis
Tue 15 September 2009, 13:51
Humor me…..

I am going though the steps to determine a bom for fasteners.
I would like to go with inch bolts over metric for cost savings.
Have I exhausted all the consideration necessary to use inch hardware?
What am I missing?

I gather I will be needing m12, m10, m8, m6 bolt selections.

I did the conversions from metric to inch.

m12 = 7/16" which is .475mm smaller than metric equivalent
m10 = 3/8" which is .8875mm smaller than m10
m8 = 5/16" which is .0625mm smaller the m8
m6 = 1/4" which is .35mm larger than m6

Hmmm, not quite a perfect match. The first three inch bolts above are slightly smaller than their metric equivalent and the 1/4” bolts are a little bigger.

I already have purchased the B3X bushings and the GW3X guides so I guess I have committed to m12 fasteners. What about the B3X bushings? I think the 5/16” bolts will work. They seemed to work on the skate once I torque down the cams. There is a little slop with 5/16" and B3X. Using 5\16" bolts are cam B3X going to rattle louse when used on the spider. Probably apparent by now I don't have a feel for these things.

I have already purchased grinding skate hardware.

If I use inch drills and taps it seems as though there will not be any issues.

What about the laser cut parts? Drill out any M6 tight spots if I find them.
What about slop in the M10 holes?

Any thoughts?

sailfl
Tue 15 September 2009, 14:45
Chris,

I think you should include both. mm are not that hard to obtain for those that would like to stay with mm.

Gerald D
Tue 15 September 2009, 20:57
Looks like you have to stay with the bigger M12 and M10 sizes in metric - there are not many of them. (You could drill the 10mm laser holes out for 1/2" if you really wanted to).

The smaller sizes are close enough.

inventall
Tue 15 September 2009, 21:16
If you have ever worked on American cars in the past 20 years they all have both. That was how I justified to my self to use standard fasteners. I already own all the wrenches taps and dies for both.

Gerald D
Tue 15 September 2009, 22:44
What are the popular pitches of the imperial threads? I get the impression that the common pitch of metric threads are a bit finer than the common pitch on imperial threads. (A finer pitch tightens, and holds its tightness, a bit better. Also taps easier).

The standard (common) pitches for metric screws are:
M5: 0.8mm
M6: 1.0mm
M8: 1.25mm
M10: 1.5mm
M12: 1.75mm

inventall
Tue 15 September 2009, 23:02
Also strips out and cross threads easier. And takes longer to tighten the bolt.
UNC(more common coarse thread)
1/4-20
5/16-18
3/8-16
7/16-14
1/2-13
UNF(fine thread)
1/4-28
5/16-24
3/8-24
7/16-20
1/2-20
Off the top of my head. (can you tell I am a mechanic)

bfauska
Tue 15 September 2009, 23:10
There are fine and coarse threads for the imperial system. The most common are the coarse.
They are called out in threads per inch. For example a 1/4"-20 bolt has a diameter of 1/4" and there are 20 threads per inch.

Unified Coarse thread UNC and Unified Fine Thread UNF are the two common standards
These are the most common sizes:

1/4"-20 UNC
1/4"-28 UNF
5/16"-18 UNC
5/16"-24 UNF
3/8"-16 UNC
3/8"-24 UNF
7/16"-14 UNC
7/16"-20 UNF
1/2"-13 UNC
1/2"-20 UNF

(Edit; Pete is quicker than me. Oh well, I'll leave mine here anyway)

Robert M
Wed 16 September 2009, 05:30
(can you tell I am a mechanic)....
Pete, I could really tell....especially for the 7/16....most commonly used in automotive ! ! :D

chris saintdenis
Wed 16 September 2009, 09:38
I am now looking over the drawings and fastener bom for a better understanding, and then to the web to price check and compare.
I will probably be asking another question soon.

It's is great to do the research and really get consumed with an element of the build. Best part is asking you guys for you advice and then seeing all the considerations and possibilities I missed get brought to light. Thanks for that!!

Makes me feel a little guilty I don't have more to share.:(

I noticed the different Imperial pitch that were available and that the price can double for the bolts depending on what pitch you choose.

Thanks everyone.

Gerald D
Wed 16 September 2009, 10:12
Finer pitch bolts are often only available in a higher grade steel, made to a better quality (tolerance) standard - that adds to the price.

AuS MaDDoG
Mon 05 April 2010, 04:30
Question!!

I will be using Hex head metric bolts where I can. What would be the best type of bolt to use?? Zinc plated - Gal - Stainless - or HDG ??

Thanks.
Tony.

Robert M
Mon 05 April 2010, 04:45
Tony, no real repercussion on a built.
Use / buy what suites your budget / taste.
A regular low cost graded w/ no platting or zinc will do just fine !

AuS MaDDoG
Mon 05 April 2010, 04:59
Thanks Robert,

Much appreciated.

Cheers
Tony.

Gerald D
Mon 05 April 2010, 05:10
My default choice, for all machines is zinc-plated ("electro-galv.) grade 8.8. I would avoid stainless steel fastners.

AuS MaDDoG
Mon 05 April 2010, 06:00
Thanks Gerald

Cheers
Tony.

liaoh75
Mon 05 April 2010, 12:28
Gerald, may I ask why you would avoid stainless steel fastners? Is this only a price issue, or is there more to it?

Gerald D
Mon 05 April 2010, 12:46
Stainless steel does not like sliding on stainless steel. It has a habit of welding to itself under high pressure/friction. When you tighten a stainless steel bolt & nut, you don't know if the joint is getting tighter, or if the bolt & nut are busy "welding" (galling) to each other. There are exceptions of course, but it is a complex subject, needing special attention to material grades, thread lubricants and speed of assembly.

http://www.google.com/search?q=stainless+steel+galling&sourceid=ie7&rls=com.microsoft:en-za:IE-SearchBox&ie=&oe=&rlz=

liaoh75
Mon 05 April 2010, 22:57
Thanks Gerald, that was an interesting read. I had never heard of that phenomena before with stainless steel fasteners.

Zouave
Tue 06 April 2010, 13:55
Nevermind, answered my own question with some google-fu.

Now I'm left with this question: IN YOUR EXPERIENCE... Is there a need for the Grade 8s, or will Grade 5 suffice? I've been looking at the shear strengths and tensile strengths on both, and they seem pretty stout overall, but I'm not sure if any of you guys have ever encountered any bolt failures on the Mechmates...

Gerald D
Tue 06 April 2010, 23:18
Grade 5 is sufficient. But, if you use standard wrenches/spanners, you will not break a grade 8.8 with hand tightening. With grade 5's (or unmarked screws) there is always the risk that the screw will break before you think it is tight. In my factory it is safe to tell unskilled labour to go and tighten a bolt really tight. :)

Zouave
Wed 07 April 2010, 00:51
Grade 8 it'll be. Out of curiosity, do you know why its referred to as 8.8 in countries outside the US, but just Grade 8 in the US? Is it just because we're grossly incompetent and have our silly english system? Or is there an actual reason for dropping the .8?

Gerald D
Wed 07 April 2010, 01:00
Copied from http://en.wikipedia.org/wiki/Screw:

"The numbers stamped on the head of the bolt are referred to the grade of the bolt used in certain application with the strength of a bolt. High-strength steel bolts usually have a hexagonal head with an ISO (http://www.mechmate.com/wiki/International_Organization_for_Standardization) strength rating (called property class) stamped on the head. And the absence of marking/number indicates a lower grade bolt with low strength. The property classes most often used are 5.8, 8.8, and 10.9. The number before the point is the tensile ultimate strength (http://www.mechmate.com/wiki/Tensile_strength) in MPa (http://www.mechmate.com/wiki/Megapascal) divided by 100. The number after the point is 10 times the ratio of tensile yield strength to tensile ultimate strength. For example, a property class 5.8 bolt has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.
Tensile ultimate strength is the stress at which the bolt fails. Tensile yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2 % offset strain (http://www.mechmate.com/wiki/Yield_strength#Definition). When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region; whereas elongation beyond the yield point is referred to as operating in the plastic region, since the fastener has suffered permanent plastic deformation.
Mild steel bolts have property class 4.6. High-strength steel bolts have property class 8.8 or above."

I realise now that your grades 5 and 8 do not relate to the ISO grades 5.8 and 8.8.

An 8.8 will start to give at 800MPa and the last load it will hold before it snaps is 640MPa. Compared to a 5.8 which gives at 500MPa, the 8.8 is 60% stronger - that is a big difference.

Zouave
Wed 07 April 2010, 02:01
I just noticed the word difference between 'Grade 8' and 'Class 8.8'. Am looking up the definition of Grades...

So, looking it up, it appears that Class is defined by ISO standard R898, with an 8.8 having a min. yield strength of 92K psi, and a min. tensile strength of 120K psi.

Grade falls under a variety of standards, but straight Grade 5/8/whatever appears to be under SAE J429. A Grade 8, for example, has mins of 130K and 150K, respective to earlier. (It also appears Grade 5s have tensile strength of ~120K psi, which puts them equivalent, roughly, with an 8.8)

Interesting research to be done!

And thus we illustrate why using the correct terminology is critically important... Mixing up Grade and Class will get you two significantly different products...

Regnar
Thu 08 April 2010, 15:50
Not only for the reason mentioned by Gerald but Stainless also speeds up corrosion with dissimilar metals. They only way to prevent it is by not using it but you can slow it down with corrosion preventative compound.

Almost all bolts have a working torque range. This allows you to go to the max torque for that bolt before destroying it. This is just one chart but there are others out there.

http://www.portlandbolt.com/technicalinformation/bolt-torque-chart.html?gclid=COiZ4bSG-KACFU-E7QodHENBvg

http://www.efunda.com/designstandards/screws/calc_bolt_torque.cfm

http://home.clear.net.nz/pages/national.torquetools/Converters/bolt-torque3.htm

taps
Sun 19 December 2010, 06:53
I run the parts department at an equipment dealer in Texas, and we sell Kubota tractors. As we are about the only place in town that stocks a decent selection of metric bolts in more than one pitch, we get a lot of people (that rarely use metric) looking for metric bolts.

As has been said, the rough equivalents that will get you in the ballpark for grades you are likely to find/look for (without checking the actual specs): grade 8 is about 10.9 / grade 5 is about 8.8

On a side note, if you find 307a, that is roughly equal to grade 2 (non-marked) bolts floating around most hardware stores. 304a is actually a bit weaker until you get to 7/8" or so, but most people will only use either of these ratings in non-critical apps. We mainly use them as sheer bolts. And since it comes up from time to time, 304 stainless has nothing to do with 307a.

melissa
Fri 07 January 2011, 17:16
I'm in the midst of making the metric/SAE decision on my MechMate. Whilst looking at the conversions in the first post, the numbers didn't look right.


m12 = 7/16" which is .475mm smaller than metric equivalent
m10 = 3/8" which is .8875mm smaller than m10

The correct conversions:

m12 = 7/16" which is .8875mm smaller than metric equivalent
m10 = 3/8" which is .475mm smaller than m10

Probably just a copy/paste error, but on a table where millimetres count, that makes a difference!