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Re: CTF (critical to function) on mechanical drawings
Re: CTF (critical to function) on mechanical drawings -- joeli	Post Reply	Top of thread	Forum

Posted by: traingdt ® 01/25/2007, 17:39:10 Author Profile eMail author Edit

Another more common term for what you are asking is CTQ (critical to quality). It is common in the auto industry, but not exclusively. These are flagged dimensions (or geometric tolerances) that are often subdivided into two groups: fit/function and safety. They indicate more than just an inspection dimension; they require variable data to be taken and statistically monitored. This is where other terms like Cpk and six sigma come in. Unfortunately, each company uses their own symbols to identify these CTQs (or "critical characteristics", or "significant characteristics"). Sometimes it's a diamond, or an oval, or simply an asterisk next to the dimension. The ASME Y14.5 standard offers the "ST" symbol, but its usage is limited (and not very well understood!). Perhaps you can do a Web search with the keywords critical, characteristic, and quality.

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Re: CTF (critical to function) on mechanical drawings
Re: CTF (critical to function) on mechanical drawings -- joeli	Post Reply	Top of thread	Forum

Posted by: Kelly Bramble ® 01/25/2007, 17:27:20 Author Profile eMail author Edit

I suspect that you are reffering to Critical Feature Drawings or Limited Dimensioned Drawings. This is a concept drawing used in industry which only delineates the most critical or CTF features. ASME Y14.5M - 1994, ISO/R1101-1969, and MIL-T-31000 do not mention this practice - and probably will not in the future. In many ways this practice is nothing more than an inspection drawing except it is created in the front-end of the design process. See the book "Geometric Boundaries" by me, for details on how to intergrate into your organization.

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Re: Re: CTF (critical to function) on mechanical drawings
Re: Re: CTF (critical to function) on mechanical drawings -- Kelly Bramble	Post Reply	Top of thread	Forum

Posted by: randykimball ® 01/26/2007, 01:24:22 Author Profile eMail author Edit

My preference is to use BASIC dimensions. For those not accustomed to them, they are dimemsions with a rectangle drawn around them. I explain to my shops my intention is that if you must stray off a dimension... do not miss these. (they are critical to the function) I use them most on prototypes. Often on a prototype, function is the key results you want, the shop sometimes has to vary a dimension from what is written in order to correct function in a manner not caught in design processes nor the CAD system. These activities are best accomplished after an engineer and a shop have had some experience with each other. Quality feedback from the shop about these values is critical if you ever intend to get a successful product into full production . The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

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Re: Re: Re: CTF (critical to function) on mechanical drawings
Re: Re: Re: CTF (critical to function) on mechanical drawings -- randykimball	Post Reply	Top of thread	Forum

Posted by: traingdt ® 01/26/2007, 18:04:16 Author Profile eMail author Edit

Randy, if I may clarify -- basic dimensions aren't necessarily critical dimensions. They simply establish target or ideal dimensions, but a tolerance is given in the form of GD&T (with the exception of gage dimensions and datum targets). For example, a simple clearance hole can be located by basic dims from the edges of a part, but then a position tolerance gives a cylindrical zone. The hole itself may not critical to function. If the hole is critical, then some sort of marker is placed next to the feature control frame, not the basic dimensions. A critical characteristic is something that the product engineer deems important enough to be tracked statistically, and that ongoing statistical data may be used to monitor tool wear, or it can even be audited for compliance by the customer if that is written into the purchase order. But yes, in a prototype or custom-made situation, I suppose the tolerance may be open-ended.

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Re: Re: Re: Re: CTF (critical to function) on mechanical drawings
Re: Re: Re: Re: CTF (critical to function) on mechanical drawings -- traingdt	Post Reply	Top of thread	Forum

Posted by: Kelly Bramble ® Administrator 01/27/2007, 10:18:49 Author Profile eMail author Edit

A Basic Dimension when not used with a Feature Control Frame is considered toleranced at Standard Tooling Tolerances. Standard tooling tolerances should be understood before assigning, as these mean something different to each manufactruing organization.

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