Drafting technical drawings for CNC Fabrication

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Here is an article written by our engineering team that will guide you on how to make the most out of your drafting experience. We hope to give you a succinct yet complete depiction of the steps required to create a technical drawing, which is an especially broad topic.

Importance of Technical Drawings

Technical drawings are required to be submitted along with the CAD files if the parts contain:

1. Thread specifications (internal or external)
2. Different surface finish requirements for different surfaces of the part (e.g. surface roughness, etc)
3. Tolerances of any feature that are different from the standard

It is not possible to indicate these requirements accurately through the CAD model alone.

In practice, it is generally better to include technical drawings with 3D CAD file submissions for CNC manufacturing, even if it does not contain any of the above-mentioned features. The 3D CAD files are used to program the CNC machine while the drawings are used as a visual reference by the operator. In fact, CNC manufacturers can machine and fabricate parts using the technical drawings alone with a large number actually preferring them over 3D CAD models. The main reasons for this are:

• It is easier to visually assess the parts without having to load it into complex CAD model viewing software, making it easier and quicker to provide cost quote estimates.
• Manufacturers are trained to quickly identify the part using 2D drawings.
• It is easier for them to identify major dimensions and critical components of the parts.

There are many different standards and practices used for drafting a technical drawing. The technique used does not matter as long as all the important technical requirements are indicated clearly.

Components of Technical Drawings

A typical technical drawing consists of the following different components:

• Title block:

The title block, as shown by the red box in the sample drawing above, is an essential part of every technical drawing which contains basic information about the part, including its name, material, finishing, scale, dimensioning and tolerancing standards as well as information about the part’s designer and/or company. The title block helps manufacturers understand the utility and function of the part presented, allowing them to better comprehend the required specifications.

• Isometric view of the part:

The isometric view of the part provides a 3D representation of the part making it easier for the reader to visualize and understand the part quite quickly. Isometric views are used for this purposes, as they combine the illusion of depth with the undistorted presentation of the part’s geometry (vertical lines remain vertical and horizontal lines are drawn at 30 deg).

• Dimensioned orthogonal views of the part:

The main orthogonal views represent more detailed 2D depictions of the 3 dimensional part, exactly as seen from the outer side of a bounding box one side at a time. Only the edges of the parts are drawn this way to allow for the clearer communication of dimensions and features. These views are primarily used to depict all the detailed dimensions, features, and specifications of the part such as length, surface roughness, tolerancing ranges, feature descriptions etc.

For most parts, the entire part can be visualized and fabricated using two or three orthogonal views.

• Section and detail views of the part:

Section views can be used to depict the essential internal features of the part, especially those features which are obscured in the main orthogonal and isometric views. The cutting line in a main orthographic view shows where the part is cross-sectioned, and the cross-hatch pattern of the section view indicates regions where material has been removed. The arrows of the cutting line indicate the direction you are looking at. For drawings with multiple section views, the cutting line can be named with alphabets such a A-A, B-B etc. to associate each section view with its respective cutting line. Usually section views are placed in-line with an orthographic view, but they can also be placed elsewhere in the drawing. The part can be sectioned along its whole width, along half its width or at an angle. The red square in the bottom of the drawing above is an example of a section view.

Detail views are used to highlight complex or difficult to dimension areas of a main orthographic view. They are typically circular in shape (placed offset to avoid confusion) and are annotated with a single letter that links the detail view with the main drawing (for example A, B and so on).

Detail views can be placed anywhere on the drawing and can use a different scale than the rest of the drawing, as long as this is clearly communicated, as indicated in the top red square above.

• Special notes to manufacturers about fabrication:

Notes to the manufacturer can be added on the technical drawing on the bottom left to convey any additional important information not included in the technical drawing. For example, instructions to break (deburr) all sharp edges, common fillet radius, overall surface finish requirements, or a reference to another component the part in the drawing interacts with can all be added to this section.

Sometimes symbols are used instead of text. For example, surface roughness is commonly annotated using symbols.

Adding critical dimensions

Important dimensions, center-line, countersink and counter bore.

The dimensions on drawings must match those on the part uploaded. This makes for a smooth assessment and quoting process and allows us to ensure that we can completely assess your part for DFM issues, if any.

Here’s the steps we suggest you pursue to make a sound engineering drawing:

1. Add the basic part dimensions that define its boundary values.
2. Add the dimensions for important features that are essential for smooth working of the part. This can be a slot, a hole or a dowel pin.
3. Add the remaining dimensions must now be added. It is advisable to have dimensions added against a datum to ensure uniformity.
4. For multiple features of a same kind, such as those in a pattern, it is acceptable to add the description of the feature, in addition to the number of features of that particular kind (e.g. 2X or 6X , as seen in the next image).

Hole Callouts

Holes can be CNC machined and consist of many variations such as through holes, threaded holes, counter-bore and countersink holes. More often than not, you will find yourself using standard dimensions.

A hole callout

Adding Threads

Threads are used to place locking features that fix the alignment or position between two or more entities. It is possible to define a thread by mentioning its external dimensions or standard thread size (e.g. M3/M4/M5).

Perhaps the soundest way of defining a thread is through a callout, much like other key features. This is primarily because the callouts enable the viewer to look at features, such that they are mutually discrete, clear and concise.

So in a crux, adding the thread dimensions is a two-step process. First, add the diameter of the hole and follow this up by adding the thread details, in addition to the different kinds of tolerances. It may also serve as a good documentation procedure to add a cosmetic thread to the drawings, that enable our suppliers to assess your drawing more accurately. For further details, you may refer to this open-source document by MIT.

Specifying Tolerances

Tolerances specify how much a dimension can vary and, in an essence, what is the range of allowable measurements for a feature. They tell the manufacturer the level of detail and time he needs to spend to control a specific feature. A tight or close tolerance refers to a dimension which cannot vary much, and a looser or wider tolerance refers to a dimension which can vary significantly. It is important to note that these values are not quantitative but are rather qualitative. They can be compared to each other depending on the kind of process and equipment being used. They can be applied to any sort of measurement, dimension or feature such as holes, angular sections and even diagonal sections of parts.

The first kind of tolerances are bilateral tolerances. These are symmetrical around a nominal or baseline dimension (for example +-0.2mm here). The second and less common kinds of tolerances are called unilateral tolerances. These are defined based on the upper and lower bounds separately(such as +0.02 and -0.01 here). The third most common kinds of tolerances are interference or fit tolerances which are defined with respect to the degree of overlap between mating parts and can be found in standardized tables.

Example of hole callouts

Perhaps the most advanced kind of tolerances are GD&T tolerances which can specify each feature, curve and dimension to be within an allowable range.

GD&T: Specifying Tolerances

GD&T works on the principle of specifying a theoretically exact dimension and then following this up by defining all other dimensions with respect to this particular dimension. This is an add on to basic drafting and will not be covered in this document. An excellent resource for this can be found here.

Thanks for reading!

About Factorem:

Factorem is Southeast Asia’s first on-demand, one-stop portal to custom manufacturing. We are making hardware procurement and prototyping processes smarter, faster, and more efficient. Through our trusted partner network, we help hardware teams get high quality customized parts at their optimal price and lead time, alongside DFM feedback. Our partners’ capabilities include CNC Machining, Sheet Metal Fab, 3D Printing and more.

Drop us a message at hello@factorem.co — we’re happy to help!

Citations:

Frey, D., & Gossard, D. (n.d.). Design Handbook: Engineering Drawing and Sketching. Retrieved July 19, 2020, from https://ocw.mit.edu/courses/mechanical-engineering/2-007-design-and-manufacturing-i-spring-2009/related-resources/drawing_and_sketching/

SOLIDWORKS Inspection Drawing & Report Automation Software. (2020, February 10). Retrieved July 19, 2020, from https://www.javelin-tech.com/3d/technology/solidworks-inspection/