What do you think about this GD&T CBT training material?

Greetings:
I am wondering if anyone here has used this GD&T CBT (computer based
training courses) GD&T Trainer Professional Edition
Based on ASME Y14.5M-1994 from ETI
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If
so, what do you think about it and is it any good for beginners? If
not, can someone please recommend a GD&T reference material for
beginners? I need to understand the why, how and where we use them.
Your additional comments and recommendation would be greatly
appreciated.
Reply to
njchen24
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Haven't used it (don't need it), but the price is not out of line considering what individual classroom training may cost. I've thought about teaching a course myself for a set price per seminar rather than per person, but I've got a problem with certification (which I think costs a lot and isn't easy to get anyway). People would just have to trust that I know my stuff ... and people tend to NOT trust any more than absolutely necessary (understandably). I know just READING the standard is a bit difficult, especially if it isn't written in one's native language. It CAN be done (coming to understand the whys and wherefors that way), but the standard isn't written as a tutorial. I'm sure there are other written tutorials out there that are cheaper and easy to read. Years ago Lowell Foster (member of the standard's committee) used to publish his own interpretive book, but I don't know if he published one on the '94 edition. It wasn't cheap, but it wasn't $900 or anything close. I used to actually correspond with the guy sometimes, but I lost track of him years ago and I don't even know if he still lives (he wasn't young way back then). I can give you a few clues about practical usage without getting too involved.
Some would argue that GD&T should be used in all possible situations. I wouldn't say that, mostly because you're likely to run into problems with machinists understanding the symbology. Instead I would say that GD&T should be used mostly in VOLUME production documentation WHEN it will buy you something in terms of: 1) clarity and/or 2) ensuring fit and/or 3) preventing unnecessary rejection of parts
Particularly locational tolerancing is useful for the 2nd and 3rd items and it's the most widely used tolerance method of everything included under the umbrella of GD&T. Using it, and especially granting bonus tolerance for departure from MMC (in most conditions) or LMC (in a few conditions like preventing hole breakout in bosses), is helpful not so much to a machinist, but rather more to a QA department which has to inspect (accept/reject) parts. It's helpful to a company subbing work out when there needs to be a formal way to return parts if they don't fit, based on a set of tolerances which give as much grace as possible to the manufacturer without giving so much grace that ill-fitting or non-fitting parts result. It's also useful in helping to create valid statistical data for SPC (statistical process control) in which the actual position of things like holes can be realistically correlated to the probability of proper fit.
Contrasting locational tolerancing -- what was at one time referred to as "true-position" tolerancing -- and normal plus and minus position tolerancing (what I tend to refer to as Cartesian coordinate tolerancing since it depends basically on an X tolerance and a Y tolerance): Cartesian coordinate tolerances typically describe a square or rectangular tolerance zone for the location of the feature (usually a hole). The square or rectangle is as wide and high as twice the given tolerances and is centered the theoretically correct location of the feature (because you use +/- in front of the tolerance value). One has to realize that the worst case for the position of the feature is the hypotenuse of a right-triangle with adjacent sides that are equal to the values of the X and the Y tolerances. In other words, by the Pythagorean Theory if you have a +/-.005 tolerance in both X and Y axes, then you have a possible actual worst case positional error of .00717 (the square root of X squared plus Y squared = the hypotenuse). If you're actually counting on the feature being out only .005 worst case then you may be in for an unpleasant surprise. One can say, though, if parts will fit together when the hole is at the farthest X and also Y tolerance allowance -- when it is actually .00717 from theoretical correct position in the above example -- that you've wasted some tolerance zone that could have been used and you may actually end up having good parts (which would fit) rejected. Positional tolerances use a round tolerance zone, which corresponds with the real-world actual allowable zone in which features can be positioned and create a fit scenario. It is more accurate because it utilizes the entire usable tolerance zone instead of just part of it, and/or it allows you no illusions that your parts may fit when they actually may not. It also allows you to use a bonus tolerance for conditions in which you've got the largest hole or the smallest pin or stud or fastener (whatever) that goes through the hold. It doesn't take much thinking to realize that when you've got a large hole it's going to be easier to make mating parts fit together than when you've got a smaller hole, simply because you don't have to be quite as precise in the position of the hole. Using MMC you can say that if you get a larger hole then you can allow the tolerance of the position to be a little larger also. The opposite it true for LMC, which as I said is useful when you want to prevent breakout. The smaller the hole in a boss the less likely you'll get a breakout condition on the side of the boss, right? So, if you give bonus tolerance for the smaller hole you haven't increased the probability of breakout. Hopefully, needless to say, giving a bonus tolerance for LMC might conflict with the purpose of insuring fit if you're talking about a situation wherein the hole is used to connect two parts together via screws or bolts or pins (or whatever), which is why LMC is less often used than MMC for allowing bonus tolerance. When giving LMC would conflict with the ideal of fit, one shouldn't give any bonus tolerance at all. That is to say, the tolerance should default to "regardless of feature size", which in the standard that came out BEFORE the 1994 standard one actually had to specify with a circle S (as opposed to a circle M for MMS or a circle L for LMC).
Did that help?
Mark 'Sporky' Stapleton Watermark Design, LLC
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Sporkman

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