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The industryaccepted substitute for Helvetica is Swiss 721.
This is included in your Windows 7 default fonts.
I guess I should clarify; often, typesetting or artwork for silkscreen specifies"Helvetica" as the typeface to be used. When drawing, or modeling silkscreens, the Swiss 721 fonts are perfect for this requirement.
If you are doing the actual artwork, of course, you must satisfy the customer, but for a "look and feel" on the product model or detail drawing, the Swiss 721 is close enough to get a realistic look at the final product. There are only subtle differences. If you know what to look for (as a customer should) you would know if someone complied with the requirements.