Why do we use DATUM?

A datum is a point, line, or surface from which dimensions are taken. It is the place to start. This is important to note when marking out a job. The datum can be a single point eg the centre of a hole or face of a feature to a component.

A datum is used to reduce errors when marking out, and measuring by reducing the accumulation of errors.

Can you find the datums on this drawing?

Why use Draft Angle?

If you’ve ever wondered if draft angles were a must, we’ve got the answer for you

The justification for draft angles comes from the nature of the injection molding process, and the ever present issue of mold shrinkage. Injection molding is a high-pressure process. These high pressures force the plastic into intimate contact with all surfaces of a mold’s cores and cavities. This high-pressure packing of the cavities makes it difficult to get the part out of the mold, or “eject” the part.

Draft facilitates the removal of the part from the mold and is very important in injection molding where the molds are straight pull only (no side actions).

The guidelines associated with the number of degrees of draft required will vary with geometry and other part characteristics (e.g. surface texture requirements), but in general the more the better. Draft is your friend when building molds.

Here are some rough guidelines to follow:

• We ask for at least 0.5 degrees on all “vertical” faces
• 2 degrees works very well in most situations
• 3 degrees is minimum for a shutoff (metal sliding on metal). Without this, the
  mold would lock up and be unable to open
• 5 or more degrees is required for heavy texture

Remember, draft is your friend when designing molds.

Geometric Dimensioning and Tolerancing: Why use it?

Geometric Dimensioning and Tolerancing (GD&T) is the precise language of engineering drawings. In the United States the standard for defining this language is ASME Y14.5M Dimensioning and Tolerancing. The first Y14.5 standard was released in 1954 with the latest revision being released in 2009. While a good portion of our customers use GD&T on a regular basis, others are unfamiliar with this powerful communications tool. Section 1 of the standard defines its scope as “This standard establishes uniform practices for stating and interpreting dimensioning, tolerancing, and related requirements for use on engineering drawings and in related documents.” It does this by utilizing both the more familiar rectilinear system of dimensioning and tolerancing and overlaying it with a system of symbols (or language) to convey precise geometrical relationships between features.

The rectilinear system of dimensioning and tolerancing utilizes a plus/minus system of tolerancing based on a two dimensional rectilinear coordinate system supplemented by notes to define a part’s shape and features. While adequate by itself for many applications, the rectilinear system often fails to fully define more complex relationships between part features. The limitations of this system are especially apparent in determining positional and size tolerances for assembly.
GD&T uses datums, tolerances of location and tolerances of form, profile, orientation and runout to communicate through its symbolic language the precise relationships between part features and to define assembly requirements. To those unfamiliar with GD&T conventions and symbols a drawing utilizing GD&T can be daunting. Through application and use the language becomes familiar and GD&T’s ability to define requirements becomes apparent. Once understood, drawings utilizing GD&T simplify determining required manufacturing steps and inspection requirements.
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We at Meyer Tool are often faced with fabricating complex welded chambers that require post weld machining. The proper application of GD&T informs us what is critical to the application. This allows us to determine the best manufacturing processes and inspection processes to achieve the desired outcomes.

Figure 1. Illustration of Real World Object where GD&T defines the requirements.

To assist those unfamiliar with GD&T in gaining the basics, we plan to write a series of articles explaining basic concepts. We will start in the remainder of this article by discussing two concepts of GD&T that are key to understanding its use. The first is a simple but often misunderstood concept, the “Basic Dimension”, and a more involved subject, “Feature Control Frames”.
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BASIC DIMENSION: Defined in Y14.5 as “A numerical value used to describe the theoretically exact size, profile, orientation, or location of a feature or datum target.” A basic dimension can be a rectilinear dimension, a diameter, an angle, etc. It is indicated on a drawing by a box around the dimension value as shown below.

The key to the basic dimension is to understand it has no tolerance itself. As stated it represents the exact size, the number of digits to the right of the decimal place represent no implied or additional tolerance. The tolerance of a basic dimension is defined by the feature control frame associated with it.
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FEATURE CONTROL FRAME: Defined in Y14.5 as “A geometric tolerance for an individual feature is specified by means of a feature control frame divided into compartments containing the geometric characteristic symbols followed by the tolerance.” The feature control frame is the long rectangular box, filled with smaller boxes that contain the specific sequence of symbols, tolerances and datum identifiers that define the tolerance of a basic dimension or geometric feature.

A feature control frame can apply to a single or multiple features. It must contain at minimum the geometrical characteristic symbol of form, orientation or position (e.g. true position, flatness, etc.) and a tolerance; they often contain tolerance modifiers (e.g. regardless of feature size, maximum material condition, etc.) and datum references. Feature control frames can be attached to a surface, axis or centerline. The method of attachment determines what feature(s) control is limited to. Like a well constructed sentence in a story, the feature control frame is used to communicate the full requirements to the reader. Once you have mastered the application and interpretation of feature control frames you will have learned the language of GD&T.

WHAT IS TOLERANCE AND WHAT IS IT USED FOR?

Imagine you are working on a design for a high efficiency windmill. Due to its complexities you need various custom parts, so you send out manufacturing drawings to various vendors for them to be made. Several weeks later you receive all the parts, but some do not fit.

One of your special shafts that should be 7/8 in. in diameter does not fit in its mating bearing. What happened? All the manufacturers were reputable and dealt with precision components, often used in aerospace applications. So, you grab your Vernier caliper and measure the section of the shaft only to discover that it has a diameter greater than what you requested, but by only 0.004 in. Yes, four thousandths of an inch can make a difference.

Any interference, defined here as the diameter of a hole that is smaller than the diameter of a shaft, will prevent parts from sliding together. They might have to be pressed on. If too large of an interference exists, it will degrade system performance, especially in bearings.

You specified the diameter of the shaft as 0.875 in., but the machine shop made the part to a 0.879 in. diameter. Why the difference? Some machine shops will apply a standard tolerance of 3 decimal places (±0.005) to un-toleranced dimensions, especially if they do not know the design intent.

Now, you’ve lost weeks of time while you wait for reworked parts.

Such a scenario can be avoided. While many machine shops use due diligence to verify non-toleranced dimensions, it is critical to understand the importance of tolerances, and how to use them correctly. Since parts need to be made either from larger pieces of material or built up from a powder or liquid, there’s no guarantee they will be exactly the size you want.

Fig 1. “Tolerance stack” will affect a part. Although every length dimension has the same tolerance, the tolerance between surfaces B and D can be as large as ±0.15 ( 1(b)) or as low as ±0.05 ( 1(c)), depending on the placement of the dimensions. It is up to you to decide which lengths are critical to the part’s function.

ASME Y14.5M defines tolerance as “the total amount a specific dimension is permitted to vary.” Tolerance is the difference between the maximum and minimum limits. This can be shown as upper and lower limits or an allowable amount above and below a nominal dimension. Either of these methods define the same range of allowable dimensions. In this example, a finished part is acceptable when its dimension is anywhere between 0.2498 and 0.2500 in.; outside of this range, it is rejected.

This range of allowable dimensions is the tolerance band. The larger the difference between the upper and lower limits, the larger the tolerance band, also considered a “looser” tolerance. Conversely, the smaller the difference, the smaller the tolerance band, also considered a “tighter” tolerance. Tolerances should always be used. Always. Ambiguity is not your friend. If you leave a dimension without a tolerance, no one else will know the importance, or the unimportance, of that dimension.

Benefits
When used correctly, you have much to gain when using tolerances. Parts with proper tolerances will fit as desired, be it a sliding fit, or a press fit.

It can also reduce costs. With unnecessarily tight tolerances, parts become more expensive to produce; there is no reason to apply a ±0.0002 tolerance when ±0.002 will do. Also, while some manufacturers apply their own set of standard tolerances to non–toleranced dimensions, many will not begin making parts until all features are defined, consuming valuable time and possibly pushing out delivery time.

Expecting parts to be made to the machinist’s best effort is not acceptable. The machinist does not know how parts interact, nor is he or she responsible for knowing. Furthermore, one machinist’s “best effort” may be maintaining the feature to within a few ten thousandths to the dimension indicated, whereas another may make the feature 0.015 in. larger or smaller than indicated.

Tolerances should not be used with hesitation. Just because a larger tolerance band is used, it doesn’t mean that parts will be sloppily made. In fact, depending on the manufacturer’s standards, shipped parts might have even tighter tolerances than you’ve specified. One good example is a bore in a gear. The specification might be ?0.250 +0.000−0.002, but the manufacturer may manufacture the bore to a tighter tolerance of ?0.2500 +0.0000−0.0005 simply because it is the standard to the particular manufacturer and this tighter tolerance is critical to the gear cutting process.

Additionally, by using proper tolerances, the liability of making the part correctly goes to the manufacturer. If the part is within tolerance and it doesn’t fit, the manufacturer cannot be held accountable. Dimensions without tolerances, however, leave the acceptable limits open. The manufacturer is not responsible for the design intent of the parts being made, and therefore cannot determine what an acceptable tolerance should be.

Proper application of tolerances
While tolerances are important, it is just as important to apply them correctly. One of the most important considerations when applying tolerances is fit. This is how shafts will fit into bearings or bushings, motors into pilot holes, and so on. Depending on your application, you might want a clearance fit to allow for expansion due to heat, a sliding fit for better positioning, or an interference (press) fit for holding capability. Information on limits and fits (among a plethora of information) can be found in the 28th edition of “Machinery’s Handbook” (ISBN 0831128003) for both U.S. Customary units and standard ISO fits.

Another consideration is how “tolerance stack” will affect the part. Suppose you have a shaft with four sections, each of different diameters, as shown in Figure 1. Although every length dimension has the same tolerance, the tolerance between surfaces B and D can be as large as ±0.15 in 1(b) or as low as ±0.05 in 1(c), depending on the placement of the dimensions. It is up to you to decide which lengths are critical to the part’s function.

Be careful when applying tolerances to radius or diameter dimensions. A tolerance on a radius will be doubled when measured as a diameter. A tolerance on a radius might be looser than intended, while one on a diameter might be tighter than intended. This effect is illustrated in figure 2(a) and 2(b). If 2(a) is used to manufacture the part, a hole diameter of 0.502 is acceptable.

Fig 2. A tolerance on a radius will be doubled when measured as a diameter. A tolerance on a radius might be looser than intended, while one on a diameter might be tighter than intended. If 2(a) is used to manufacture the part, a hole diameter of 0.502 is acceptable.

Also, you do not need to assume measurements will be rounded when determining if a part conforms to the specified tolerance. If a part is measured to be 0.2502 using a dial micrometer or other device, and the part’s dimension is supposed to be 0, the dimension is not rounded down to three decimal places, it is considered a nonconformance. ASME Y14.5M states dimensions “are used as if they were continued with zeros,” even if not shown.

You should also take into account any plating or finishing processes the part requires. A note should indicate if dimensions apply before or after them.

When using either MIN or MAX limits, ensure that if the dimension approaches infinity (in the case of MIN) or is zero (in the case of MAX), it does not hinder the design of the part. In figure 3(a) a MIN tolerance is used, possibly to ensure that there is a radius for reduction of stress concentration. However, figure 3(b) shows a dimension that is within tolerance, but may hinder the part’s function. Other features should clearly define the unstated limit.

Fig 3. With MIN or MAX limits, ensure that if the dimension approaches infinity (in the case of MIN) or is zero (in the case of MAX), it does not hinder the design of the part. A MIN tolerance can ensure that there is a radius for reduction of stress concentration. However, you can receive a part with a dimension that is within tolerance, but that may hinder the part’s function.

Both the location and size of alignment holes (such as for dowel pins) should not carry the same tolerances as clearance holes (such as for screws to go through). A certain deviation from nominal in the location of a dowel pinhole may cause your assembly to be impossible to assemble, while the same deviation in the location of clearance holes will likely cause no effect to the entire piece, except for perhaps a near-imperceptible aesthetic oddity.

While thoroughly dimensioning parts is important, take care to avoid redundancies. They may cause conflicts in inspection because certain features will be defined more than once in more than one method. If a dimension that will over define the part is desired, a reference dimension should be used, between parentheses and usually without tolerances, like dimension (M) in Figure 1(b). This dimension is derived from others or is repeated, usually in a different view.

Dimensional tolerances are key in getting parts you want. Using them appropriately will save time spent coordinating with the manufacturer, circumvent design issues, and reduce unnecessary costs.