Technical Drawing for CNC Machining

Modern CNC machining system is able to interpret parts geometry directly form 3D CAD design files. Although Technical drawings is not necessary for quotation request, it is still very important and widely used in industry, in order to improve communication of technical requirements between designers and engineer or machinist. We recommend you to prepare a technical drawing in you CNC order, and guarantee our smooth cooperation in further step.

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Why Technical Drawing is Important

As your technical drawing in order always including following element:

Threads (internal or external)
Feature with exceed standard tolerance
Individual surface with specific finishes

All these requirement is difficult to transfer in 3D CAD file. Even your CNC design doesn’t involve the above element, it is a excellent practice to accompany 3D CAD file with technical drawing once place a CNC order. The 3D CAD file is applied for CNC machine programming, and technical drawing is applied for machining process reference. As most CNC service providers prefer technical drawing over 3D CAD files in reason of:

  1. They are trained to interpret parts geometry form 2D drawing.
  2. It is easier to identify parts main dimensions, function and critical features form 2D drawing.
  3. It is easier to assess parts manufacturing cost.

There are different standards and practices for technical drawing draft, the main focus is to communicate clearly about the technical requirements. As in this article, all our example drawing is recommended not necessary. You can annotate your most important feature to be measured in technical drawing.

Technical drawing anatomy

A typical technical drawing consist of following parts:

  1. Title bock
  2. Isometric or pictorial part view
  3. Main orthographic part view
  4. Section or detail view
  5. Notes of manufacturer

Title block

Title block contains basic information of part, including part name, material, finishing and color requirements, designer name and company. All these information will inform manufacturer about part function. Other technical information, such as drawing scale, dimension and tolerance standard. Angle projection is another element in title block, which determine the view way and arrangement in drawing. Typical drawing drafts use ASME standard of 3rd angel projection and ISO/DIN standard of 1st angle projection.

Isometric or pictorial view

We recommend you to add one or more 3D pictorial views to your drawing, which makes your drawing easier to understand.

Isometric views combine illusion of depth with undistorted presentation, vertical lines remain and horizontal line at 30 degree.

Main orthographic view

Most geometry information is transferred in main orthographic views. These 2-dimensional depictions of 3D object represent the exact shape of parts form one side at one time. All parts edges are drawn in this way to clarify communication of dimensions and features. For most parts, two or three orthographic views can describe whole geometry accurately.

Section view

Section views are applied to present parts internal details, cutting lines in main orthographic view show parts cross-section area, and cross-hatch pattern in section view indicates material removal regions. There are multiple section views with two letters linking each cutting line in each section view, such as A-A, B-B. Cutting line arrows indicate your vision directions. Although hidden internal features can be represented with dashed lines in orthographic view, section views will make it more clear.

Detail view

Detail views are applied to highlight complex or difficult dimension areas in main orthographic views. They are typically placed offset to avoid confusion, and annotated with single letter links detail view with main drawing like A, B and so on. Detail views can be placed anywhere on drawing with different scale to clarify communication.

Notes for manufacturer

Notes of manufacturer can convey additional information, which is not included in technical drawing. Such as: instructions of sharp edges break or dedurr, specific surface finish requirement.

Sometimes symbols are applied instead of text.

Technical Drawing in 7 Steps

Once you plan your technical drawing draft, here is a summary of following steps:

  1. Define the most important views and place relevant orthographic in the center of drawing, left space to add dimensions.
  2. For internal features or complex and difficult to dimension area, add section views or detail views accordingly.
  3. Add construction line to all views, including center lines, center marks, center mark patterns.
  4. Add dimensions to drawing, starting form the most important dimensions.
  5. Specify all threads location, size and length.
  6. Add tolerance to features with high accuracy than standard.
  7. Fill title block, guarantee all relevant information and exceed standard requirement in notes.

When your drawing is finished, export a PDF file and attach it to your order.

Regardless this basic structure of technical drawing, we also need to specific of dimensions, annotations and tolerances.

Dimensions, Tolerance & Annotations

Critical dimensions

Once you parts design is accompanied with 3D CAD file, the dimensions on technical drawing must be critical for manufacturer to check. Especially for important features, in order to avoid errors in manufacturing process.

Tips of critical dimension display:

  1. Placing overall dimension of parts.
  2. Add critical dimensions for functional purposes.
  3. Add dimension to other features, starting form same baseline.
  4. Dimensions should be placed on views with most clear description.
  5. For repeated features, add only one of them, and indicate the total number of repeat features in current view.

Hole callouts

Holes are common features in CNC machining, and usually machined by drill slots with standardized dimensions. Secondary features, like cunterbores (⌴) and countersinks(⌵), we recommend to add a callout instead of individual dimensions. In the example below, the depth symbol (↧) is used to instead of an additional dimension to drawing.

Thread

Once your parts contain threads, these features must be clearly specified on technical drawing. Threads can be defined by simply standard thread size like M4 instead of a diameter dimension. We recommend to define threads through callout, it can not only add clarity to drawing, but also specific different length of pilot holes and threads. In this case, we should define pilot hole dimensions from standard tables at first, then thread dimensions and tolerances.

Specifying tolerances

Tolerances define acceptable value range for certain dimensions of parts, which are especially important for feature interfere with other components. Tolerances have different formats with and can be applied to any dimension both linear ant angular on drawing. The simplest tolerances are bilateral tolerances, which are symmetrical around base dimension as±0.1 mm. There are also unilateral tolerances and interference tolerance in technical table.

Tolerances are only required once exceed standard value, our CNC machining standard tolerance is ±0.125 mm or ±0.005”.

Runsom recommend technical drawing once your parts contain threads, tolerance or specific surface finishes. Fully dimension is required to avoid errors. For more detail information, contact our engineering team for your next project.