Woodworking with CAD

Woodworking With CAD Index

Dimensioning - Part I

The world is built off of dimensioned 2d drawings. Even in today's world of 3d virtual reality design, parts are made and checked against 2d dimensioned drawings. Fortunately for woodworking, dimensioning isn't too complicated due partly because of the way projects are built.

In real CAD software, objects are drawn 1:1; that is, when a line is drawn 12" long, it will measure (using the software tool for doing so) at 12", not the actual length that would appear on a piece of paper when the line is printed. In reality, this 12" line in the software is actually 12 non-descript "units" in length. The software transposes 12 "units" into the selected measurement system, in this case inches. Depending upon the software, the user can easily switch from one unit of measurement (ie: imperial / metric) to another without doing any conversion. For software without this capability, one would have to do some manual calculations because of the "units of measure" constraint.

How does this all relate to dimensioning? A unit of measurement needs to be selected for a drawing. In some software, this is done when a new drawing is created (via wizards), in others it is done via one of the settings in the software. The units selected will also control the dimension units used in the drawing. The dimensioning format can be modified further as well. For instance, with "inches" selected as the unit of measure, decimal or fractional, resolution, and several other aspects of how the dimensioning will appear can be configured and changed as desired.

TIP:
When setting the precision or resolution of the dimension settings, set the value to one level finer than is normally worked in. For example, if 1/16" is the finest measurement one wants to actually use, set the precision to 1/32" or 1/64". This will serve as a "flag" when misaligned objects are drawn or when the design is requiring a dimension that is otherwise undesired or unique.

Basic Dimensioning
In the real world dimensioning is a science unto itself and there are many standard formats and methods available for use. The type of dimensioning that will be most applicable to woodworking is known as Basic Dimensioning. In this type of dimensioning, there are typically no tolerances given for any particular object. Even though there are no tolerances given on the dimension line itself, that does not mean tolerances are still not involved or need to be considered; dimensioning and tolerances ALWAYS go hand in hand.
Parts of Dimensioning
There are basic accepted components of a dimension. If one uses the dimensioning feature of the software it will automatically include these components. If the drawing is manually dimensioned, they need to be included. The format is common sense so it isn't difficult to figure out.		 Dimension Parts
Using Dimensions
In order to be useful, dimension lines need to relate to a "feature" of a part; this involves the "type" (in this case "Basic") and the style. There are basic common sense rules for using dimensions but unless one is already familiar with drafting, they are not all immediately obvious. One likely good source of information related to how dimensions should be applied is in the CAD software manual itself. It should at least describe all the types of dimension lines used by the program.
Dimension Format
Style
Dimension style deals with the particular type of dimensioning format used in every minute detail. In the real world dimensioning style is a very important issue, for a hobbyist, it is much less important.

There are two levels of dimensioning "style", the first level is how the software displays a particular dimension (ie: filled / unfilled arrow heads, number in-line / above and many other specific aspects). The second level is how one uses dimension lines and draws dimensioned parts. An example of this is how to draw a frame and panel door in a shop drawing. The door can be drawn and dimensioned "exploded" or assembled. As long as all the information is present, there is no particular "right" way of doing it. Only when the information isn't there or it's difficult to understand is a drawing dimensioned the "wrong" way.  The exception to this is that if a particular tolerance needs to be held but is not met with the dimensions given.

FYI:
The reason Dimensioning and its style is so important in the real world is that an organization wants to maintain a consistent standard of how dimensions appear. Many organizations have detailed style guidelines and rigorously enforce them. The dimension settings features of production CAD software has a bewildering array of settings that allow these organizations to set and replicate a style to match their "standard".

Tolerance
Tolerance is the location precision needed for any given "feature" of a part. Everything made by man has a tolerance whether it is unknown, big, small, variable, displayed, or not.

A good example of tolerances applied to woodworking is in a frame and panel door. The panel can be no wider than the assembled frame will allow for but not so narrow that the edges show. Since wood can change in dimension depending upon moisture content and temperature, it is important that the dimension of the panel be properly sized at the time of manufacture.

Part I

Here is how the size of the panel would be arrived at using tolerances:

1. Assume the frame interior measures 12".

2. Assume the frame has a 3/8" deep groove to accept the panel.

3. The panel can be no wider than 12" or it will blow the frame apart.

4. The panel can be no narrower than 11 ¼" or it will show an edge.

5. The panel may not be at it's maximum (or minimum) dimension at the time of manufacture.

Since the panel may actually change in width after the unit is assembled into the frame, tolerance needs to be considered. In this woodworking example, 3/16" is a good rule of thumb. A 3/16" allowance leaves a panel that is 11 13/16" wide, we will now call this the Basic Dimension. IF it was assumed that the panel was already AT its maximum dimension AND it were to shrink by 1/8" (another rule of thumb), it would NOW be 11 11/16" wide. IF it was assumed that the (11 13/16" wide) panel was already AT its minimum dimension AND it were to expand by 1/8", it would NOW be 11 15/16" wide.

Since 11 15/16" is less than the 12" maximum between frame members AND 11 11/16" is wider than the 11 ¼" minimum, it can be reasonably assumed that even though the panel may change in width due to normal wood movement, it would not exceed the allowance of the frame nor would it shrink enough to expose an edge. There is of coarse more to sizing a frame and panel door than is presented here.

This is an example of designing the proper tolerance into the part from the beginning.

Part II

Another less obvious (but very common) aspect of dimensioning and tolerances is known as "stack-up". Stack-up refers to a situation were dimensions are based off of dimension points of OTHER dimensions. Any variation of a preceding dimension WILL throw off the position of the actual feature as measured from a base point.

Even using Basic Dimensioning, the way features are dimensioned in a drawing can have an impact on how accurately a project is built. This can correspond directly to how well parts fit together.
Bad Dim In the dimensioning example to the left, the holes and two ends of the part are dimensioned by showing the distance between features.  All the features are located correctly but using this method opens the door for positioning errors when the holes are laid out and the part is made.
That is because each feature shown is referenced off of a preceding dimension instead of a common point.  If any of these holes are mis-located, any hole after laid out subsequently will also be mis-placed.  Even though this part is dimensioned correctly and only Basic Dimensions are used, the way it has been done can lead to tolerance errors when it is made, in this case a "stack-up" error.

There is nothing preventing   the maker of this part from adding the distances given in the example above to arrive at a base-line position dimension (as shown below).  However, when a manual math operation is introduced into the process, it can convert a correct (but inconvenient) dimension into a human error.
Good Dim Here is the same part but a different type of dimension line is used.  This time the features are all referenced from a common point (zero).  If the hole at 1 5/8 were laid out incorrectly, none of the other features would be affected by this error.  Whenever possible, this is the way multiple dimensions should be used. 

Note there are more ways to display this type of dimension line than shown here.
Summary
This chapter is only an introduction to the topic of dimensioning.  As stated earlier, dimensioning is a science unto itself and volumes could be written on the subject.  The main thing to know about dimensioning is that they need to convey the required information in a clear manner that will tend to reduce errors instead of introducing any of its own.

 

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