Bolts in dual dimensioned drawing

[dozer]

We've provided dual dimensioned (Imperial and metric) drawings to our client and now they want us to show Imperial and metric call outs on bolts. We're a US company and we use U.S. standard structural bolts. I'm arguing against call outs such as 3/4" (19mm) bolt because a 19mm bolt is not a 3/4" bolt or vice versa. We will be providing 3/4" bolts. I think this needs to be abundantly clear to the client so they will know to have SAE wrenches and sockets on hand. Besides, is there any such thing as a 19mm bolt? (Don't answer that, just musing.) Anyway, I'm wondering if there is any written documentation (industry standard or whatever) that would support my viewpoint. It just seems like common sense to me. If you don't have a feel for what a 3/4" bolt is, is it really that hard to multiply 0.75 by 25.4 and get 19.05 and say, "Oh, so that's about the same size as a 20mm bolt."?

 

[briancpotter]

I tend to agree. A bolt size is more than just a dimension - it indicates a specific bolt that needs to have specific properties. It seems inappropriate to give a dimension in alternate units, as if that's all the properties of the bolt that you need to know.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[Archie264]

If they force the issue could you list a 3/4" bolt as (19.05mm)? Or, conversely, a 19mm bolt would be listed as (.748") That might help the user know which bolt it really is. Having said that, though, wrenches come in metric and imperial sizes for a reason.

 

[Ron]

Don't do it. Dual dimensioning with soft conversion is confusing...error prone.

 

[connectegr]

I am going through this right now on a project. Unfortunately the conversion and rounding results in variations in dimensional tolerances. This results in tolerance concerns for accumulated 1/16" adjustments. For example 5 1/2" = 127mm, but a typical metric gage may be 130mm, which allows for even division of 75mm. Detailing standards for gages and spacings are different for metric and imperial projects. A decision of hard metric or hard imperial is required. The primary dimensioning should follow typical standards. The secondary dimensional information is insignificant and variations in conversion and rounding are irrelevant. If fabricated in the US typically hard imperial is the preference. As mentioned above shear values, tensile strength, bolt diameter, and hole diameters will vary slightly. This is a design and detailing concern. If it is clearly understood that imperial bolts will be used, the metric information is cosmetric only. Similar to the variations in dimensional conversions, the metric bolt information is for record only and will not be used for design, fabrication, or erection.

In my experience combined use of imperial/metric can cause problems. Making a clear choice at the beginning removes the problems.

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[shobroco]

I'm in Canada where our official system is metric, most people think in imperial, and we build in a bit of both. As briancpotter said, the bolts are either metric or imperial & there is no calling one by the other's name. You specify the bolts & that's that.

 

[dozer]

Thanks everyone for confirming my opinion. I just wish I could point to some "standard" to convince the paper pusher that adding metric conversion is not a good idea. Evidently common sense isn't as common as I had hoped.