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Tutorial: Dimensioning and Tolerances The following material addresses those dimensioning and tolerance principles necessary for you to detail the

design and manufacturing processes used to make your team's Pipsqueak engine.

1. Accuracy, Clearness, Completeness

Accuracy Process equipment depends on reference points, lines and surfaces (typically planes in space) that

can be calibrated using equipment processes. Calibration means that the process equipment deter-mine a reference location and/or orientation to make relative process moves, such as moving a cut-ting tool relative to metal. When making volumetric parts, as in the Pipsqueak project, process accu-racy requires one or more calibrated surfaces. We call this reference plane a datum plane. A set of da-tum planes form a coordinate reference for the part. In 3D manufacturing the coordinate system would be an X-Y-Z reference frame (or coordinate set). A datum can be lines or specific points as well.

A calibrated datum(s) is what a machine needs to make relative moves to create features such as holes, countersinks, and cuts. It is important to have a datum for all dimensions. For each dimension that you have you need to have a tolerance associated with it. Tolerances are measurements of acceptable dimensional error or variation. This is especially important for parts that may fit into another (e.g. a pis-ton or a bushing). If you have a shaft that fits too loosely within a bushing bearing surface, because your bushing tolerances were too large, there can be lots of unwanted vibrations, which will cause premature failure. Or if your shaft is too big and can’t fit through the bushing, an additional manufacturing process will be required. What might it be? Many issues can be fixed by using a common datum and applying acceptable tolerances.

Figure 1.1 Courtesy of: http://www.designworldonline.com/why-its-important-to-always-use-tolerances/#_

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In Figure 1.1, how far away is datum B from datum D? For 1(b): distance=(J ± 0.05) + (K ± 0.05) + (L ± 0.05) = (dimensioned length) ± 0.15 For 1(c): distance=(dimensioned length) ± 0.05 1(b) could lead to tolerance variations of ± 0.15 while 1(c) leads to a smaller tolerance variation.

0.15 variation is more than enough to bind up a pipsqueak engine. The accumulation of tolerances shown in 1(b) can occur because of the production processes you use, or inconsistent datum(s). Your tol-erances will be determined by the machining process that you use, by the requirements of the part, and by the cost associated with the part. Common machine tolerances can range from +/-0.02” to +/-0.0005”. Also note that if you choose multiple datum's you will have to recalibrate your mill or lathe, which takes time.

From the reference in Figure1.1: "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 consid-ered a “looser” tolerance. Conversely, the smaller the difference, the smaller the tolerance band, al-so 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."

Clearness

All drawings should be clear. If you are unsure about how to draw a dimension, as a rule of thumb go with what is cleaner to the eye. For a FULL list of dimensioning and tolerance standards see:

http://www.eng.fiu.edu/mme/robotics/EML4501MechDesignII/Catalogs/ASME_Y14.5M- 1994_Dimensioning_and_Tolerancing.pdf Common dimension symbols (like diameters, threading and countersinks) will be discussed in another

section. Another way to make the drawings clearer is types of views, which will be discussed in greater detail in the readability section.

Completeness

Compare the pictures below. Assuming these are flat disks, where would you make the holes (the small round circles)?

Good Bad

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Figure 1.2 courtesy of:

http://www.eng.fiu.edu/mme/robotics/EML4501MechDesignII/Catalogs/ASME_Y14.5M-1994_Dimensioning_and_Tolerancing.pdf

The right drawing in Figure 1.2 is an example of an under-dimensioned drawing. This is confusing to

manufacturing personnel and the drawing will be sent back for editing. Another type of error corresponding to completeness is having a drawing that is over-dimensioned.

An example of over-dimensioning is shown in Figure 1.3. Over-dimensioning a drawing is just as confus-ing as under- dimensioning.

If you have dimensions that are not consistent you will create confusion and manufacturing person-nel will have to pick a dimension or ask for clarification, which delays the production process and costs $$$.

Figure 1.3 Courtesy of: http://www.engineeringessentials.com/ege/tol/ex_over_dim.png

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2. Dimensions and Callouts

Clarity and Efficiency in Drawing Dimensioning should follow these guidelines:

• Accuracy: correct values must be given. • Clearness: dimensions must be placed in appropriate positions. • Completeness: nothing must be left out, and nothing duplicated. • Readability: clutter, intersecting

lines, and wordiness should be min-imized.

Dimensioning Conventions

Dimensions should be placed on the face or view of the object that describes the feature most clearly. Avoid dimensioning to hidden lines and between dissimilar surfaces (i.e. flat to round).

Holes, radii, and threads should be dimen-sioned on a view facing the observer. Other-wise this can lead to feature ambiguity. Cen-terlines and axes should be shown to clearly mark the center of a circular feature. Dimen-sioning to these lines is not only allowed, but encouraged.

If possible, dimensions should be stacked using one datum. Stacking dimensions allows for the use of one datum for multiple features. Repeatedly measuring from one point to an-other will lead to inaccuracies; see Accuracy, Clearness, and Completeness section. It is of-ten better to measure from one end to vari-ous points. This gives the dimensions a refer-ence standard.

It is helpful to choose the placement of the dimension in the order in which a machinist would create the part. This convention may take some experience. This saves time for the machinist and also adds more clarity to the drawing. Using the same datum for multiple dimensions allows the machinist to edge find or “zero- out" the machine just once before making several features. (For more info, visit:

http://www-mdp.eng.cam.ac.uk/web/library/enginfo/drawings/index.html)

It is not necessary to place a dimension on every feature. If a feature, such as a hole or a notch, is repeated on a part, preferred procedure is to only dimension one of the features. This reduces

Figure 2.1 Courtesy of: Source: MIT OpenCourseWare: http://ocw.mit.edu/courses/mechanical-engineering/2-007-design-and-manufacturing-i-spring-2009/related-resources/drawing_and_sketching/#Dimension

Figure 2.2

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the number of dimensions on a drawing without reducing the amount of information, reducing clutter while preserving clarity.

There are several different ways to indicate a repeating feature on a drawing. In Figure 2.2 a repeat-ing dimension may be indicated by a number followed by an X, to denote “this dimension repeats four times.” Another method includes putting TYP. or TYP. OF (for “typical of”) followed by a number, denot-ing the number of features that this dimension should be applied to.

If there are several repeated features, make sure to help those that interpret the drawing clearly distinguish which features correspond to which dimensions. Options for this include marking certain holes or features with matching letters, including a dashed or dotted pitch circle for holes in a circular pattern, or including dashed or dotted lines between duplicate features.

Now examine Figure 2.3 and see if you can tell where each feature’s duplicates are located. From what you have learned so far, is this a good drawing? Why or why not?

Figure 2.3 Feature dimensioning (For more info, visit http://www.maelabs.ucsd.edu/mae_guides/cad/dimensioning/dimensioning_fundementals.htm)

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Callouts Callouts are brief, informative descriptions located on the drawing itself. Callouts provide needed

additional information without causing clutter, so symbols and abbreviations are used quite frequently. Typical symbols, abbreviations, and their definitions are found in Table 2.1:

Table 2.1 Callout symbols

(For more information, visit http://learning.hccs.edu/faculty/a.nemirovskiy/dftg2302-2/reading-list/threads)

While most CAD programs will automatically cre-ate callout notes complete with these symbols, it is still strongly encouraged that designers familiarize themselves with these symbols, their definitions, and uses, especially when custom or non-standard fea-tures are being made.

For threads and holes, thread dimensions should be attached to the hidden line of the feature. (dashed circle surrounding the circle indicating the hole diame-ter in Figure 2.4).

Thread and hole notes should include the nomi-nal diameter, thread specification, and depth. Blind hole depth can be defined with the DP (deep) symbol. If a through hole is desired, THRU can be included in the callout. However, if no depth is defined in the note, a through hole is assumed by convention.

Figure 2.5 shows some examples of hole callouts. Figure 2.4 Callout for threaded hole

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Figure 2.5 Callout examples See if you can read the all of the callouts in complete sentences. For example, the first (left) callout

says, “Drill a 10 mm diameter hole through this location, then counterbore the hole at a 20 mm diameter down 8 mm.” A cross-sectional view of how the features look is given as well.

From these examples it is very easy to see the usefulness of callouts and dimensioning symbols. A lot of information is conveyed using only a few characters.

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3. Process Sheets

Why Process Sheets?

As clear, concise, and informative as drawings can be, process sheets are still necessary to successfully make a part, as design drawings can be confus-ing; see Figure 3.1.

Process sheets provide instructions for machinists to follow in order to quickly and correctly make each part, to avoid process problems; see Figure 3.2. More Process Sheets

Process sheets come in all shapes and sizes; see Figure 3.3. We will be using a specific template provided on Learning Suite.

Process sheets should include: • Clear sequence of processes • Material specifications • Process equipment to be used • Short description of all opera-

tions, in proper order • Equipment settings (such as

feeds & speeds) • Tool types, names and numbers

Figure 3.2 Process gone wrong!

Figure 3.1 Drawing confusion

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Figure 3.3 Process sheet example

Homework Question Write out a process sheet (using the Template on Learning Suite) for changing a tire. Print out your pro-cess sheet and turn it in.

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4. Readability

When making engineering drawings, readability refers to using the correct views in a way that is both easy to read and transmits all of the required information to build the part. In this section we will pro-pose important principles that you need to apply when making your Pipsqueak engine drawings.

Cross Section

Figure 4.1 shows a cross sectional view with an additional side view to visualize the inside of the part. The correct formatting of this, as seen above, shows a dotted line along the cross sectional area with two arrows perturbing from the end. These arrows point towards the cross section. The cross sec-tion is then labeled “Section A-A” corresponding to the letters on the drawing.

Figure 4.1 Cross section

Isometric Views Isometric views are also used on a drawing page and the exploded view to provide a 3-D model visu-

alization. These are often located on the top right hand side of the page as shown in Figure 4.2.

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Figure 4.3 Isometric view Exploded View (Assembly Drawing)

Another page that you will want to make is an exploded view, as shown in Figure 4.4.

Figure 4.4 Exploded view

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This view will be a guide for you and the reader to make sure that you have all the parts that you need. Also important in Figure 4.4 the list of parts with their quantity and a balloon showing where they fit within the drawing assembly. This list does not replace the Bill of Materials that you need to make because a Bill of Materials talks about the stock parts from whence your parts are made. The exploded view shows the finished parts and how you will assemble them. Lastly for the exploded view don’t for-get to show your isometric view of the assembled design.

Bill of Materials (BOM)

Another important part of readability is a Bill of Materials (BOM). A Bill of Materials is a tabulated list that lets the machinist know what materials and components needed to make your part. These stock components will then be used with your drawings and your process sheets (sheets that explain how to make the part) to create the individual parts. The stock components for the Pipsqueak will be given to your team so be sure you use those stock parts on your Bill of Materials.

Table 4.1 Sample BOM

Really good suggestion: Remember that cleanliness is next to Godliness. Keeping everything clean and orderly means that your part will be made correctly and ultimately your team will receive a good pro-jects grade.

D&C 88:119: "Organize yourselves; prepare every needful thing; and establish a house, even a house of prayer, a house of fasting, a house of faith, a house of learning, a house of glory, a house of order, a house of God;"