Engineering notes For hobbyists
Theaded holes
Threaded holes are considered to be called out to minimum depth. If a threaded hole needs to be held to a +/- tolerance, then a measureing method must be agreed upon by both parties. Two different thread gages may give a different amount of turns and off-the-shelf hardware will give a different number as well. The same goes for broach depth.
Unless specified on the print, threaded holes may be thread milled. made with a cutting tap, or formed.
Please note that a form tap may not be used on sanitary parts. The reason is that a form tap will typically leave tiny "bacteria birdhouses" at the crest of internal threads. (see photo to the right)
Surface finish
Fine surface finishes require special processes, more time, and of course, extra work. Our finishes are generally 64 Ra or better but if the print says 250 finish, then there may be a spot on the part which is rougher than other spots. If you absolutely need/want a specific finish, we are happy to provide it with an appropriate increase in price.
Tight tolerances
Tight tolerances are no problem. They can equal a significant increase in price per part though so only call them out if you need them. Our title block tolerances are pretty typical at +/- .010" for three place decimals, +/- .030" for 2 place decimals and fractions, (please avoid fractions except for stock callouts) and +/- 3 degrees for angles.
Holes
Holes are probably the most common machining feature. Be aware that any time a hole is required to be deeper than 3X its diameter, some problems can crop up with roundness, parallelism, runout, perpendicularity, bell-mouthing, etc. Special processes and tooling much be used to create precise holes beyond this 3X diameter threshold with a subsequent increase in price.
Sharp inside corners
Lets say you need a square hole on your part. There usually has to be some sort of radius in the corners of that square and (generally) the bigger the radius allowance, the bettter. This radius is limited by the tool used to make the corner. Even an EDM or laser with leave a small radius. A broach can make a very square corner but this is a time consuming operation and will cost more.
