Design Guidelines For Machining – An Overview

Machining

What is Machining?

CNC machining is a manufacturing process where all the conventional forms of machining techniques such as milling, drilling and turning are done using computer controlled software. In more technical terms you can call CNC machining as “subtractive manufacturing” process.

As in this, the material is removed from a solid piece of a block called “workpiece”. The removal of material is done by different cutting tools.

CNC machining is an automated manufacturing process wherein the computer reads your CAD file and generates a step by step process of making the finished product from raw material. Few people in the industry also call it CAD/CAM machining, as the drawing and tool path generation is done with the help of a computer.

Before proceeding ahead with the design for machining, we will first help you gain some information about CNC machining and the relevant processes and considerations to help you understand the subject in a systematic way.

CNC Machining:

In CNC, a solid structure is subjected to material removal through a variety of cutting tools. These cutting tools move at faster speeds to create the workpiece based upon the CAD model of the workpiece. You can subject both metals as well as plastics to CNC machining processes to produce machined components with increased dimensional accuracy and improved tolerances. Since it is the most effective and cost-effective way of manufacturing parts and prototypes, it is well-suited for high volume production and real-time operations.

While operating an object for CNC machining, it is important to take into account the considerations regarding tool geometry and other important parameters crucial for the CNC machining. So, let us have a look at some of the key Design Guidelines for machining.

  • CNC Design Constraints

The primary CNC design constraints can be observed in the cutting operations that are related to the cutting tool geometry and tool access.

  1. Cutting Tool Geometry –

Irrespective of it being a milling tool or drilling tool, the cutting tools used in the CNC machining process have a cylindrical structure and a finite cutting length. Along with the expulsion of material from the workpiece, the tool geometry plays an important role because not all the geometries can machine a complicated job, so the for a particular job you need a particular tool geometry that will suit your requirement & again it’s subjective, not absolute because of the parameters involved. To better understand this, you can say that the CNC machined part will have a specific diameter or regardless of the size of the cutting tool utilized.

Internal Radii For CNC Parts
Internal Radii For CNC Parts
  1. Cutting Tool Access –

An important thing to consider while subjecting the workpiece to CNC machining is that the cutting tool operates on the workpiece directly from the top. The structures that do not support such accessibility are also not eligible for CNC machining.

For an effective machining process, different aspects of the workpiece such as holes and cavities need to be properly coordinated with the 6 principal axes and the 6 degrees of freedom. In case of components with increased depth to thickness ratio, tool access is another important consideration during machining. Workpieces with highly deeper cavity require longer shafts and tools for effective machining.

Let us now have a look at the rules that define the CNC design for manufacturing processes.

Pocket Depth - CNC DFM
Pocket Depth – CNC DFM
  • CNC Design for Machining rules

Here, we have listed various design rules for the key aspects of CNC machined components.

  1. Cavities and Pockets

The suggested depth of the cavity should be 4 times the width of the cavity. Similarly, for different end mill tools, the cutting length should be 3-4 times the tool diameter.

 

Pocket Depth - CNC DFM
Pocket Depth – CNC DFM

 

In case of minimal depth to width ratio, various factors such as tool deflection, vibrations, and chip expulsion come into the picture. While designing workpiece with larger depths components with a separate depth of cavity should be used. The cavities with a depth larger than 6 times the tool diameter are regarded as deeper.

 

  1. Holes

For the diameter of the holes, the suggested size is that of the normal drill bit. The viable hole diameter is more than 1 mm. The holes can be CNC machined with a drill bit or with the help of an end mill cutting tool. In the case of rigid tolerances for the machining of holes, we make use of reamers and several boring tools.

Hole Size For CNC
Hole Size For CNC

 

The suggested maximum depth for the holes should be 4 times the apparent diameter. Usually, the largest depth of the holes can be 10 times the apparent diameter. The viable depth to be selected for the machining of holes can be nearly 40 times the apparent diameter. While the holes with an untypical diameter are CNC machined with the help of end mill tools, those with more than the normal depth value require specific drill bits for machining purposes.

  1. Internal Radii

For better internal radii of the workpiece, the suggested vertical corner radius is equal to or greater than one-third of the cavity depth. Determining an appropriate value of the internal corner radius helps you decide the most suitable tool diameter is utilized. For the better surface finish of the workpiece, the internal corner radius can be minimally increased by, say 1 mm.

Internal Radii For CNC Parts
Internal Radii For CNC Parts

 

The suggested floor radius for the internal edges can be 0.5 mm or 1 mm or a zero value. Any radius value can be chosen for the floor radius.

  1. Threads

The minimum thread size should be M2 while the suggested thread size can be M6 or more than that. Taps are used for cutting the internal threads whereas dies for the external threads. CNC threading tools are quite commonly used as they can cut the thread to M6 size.

Thread Sizes In CNC
Use standard thread sizes

 

The minimum thread length should be 1.5 times the apparent diameter. However, it is suggested that the thread length can be 3 times the apparent diameter. By utilizing a CNC threading tool, you can thread the hole all over its length.

  1. Thin Walls

The suggested minimal wall thickness is 2 mm. However, suitable minimal wall thickness is 1 mm. A lower wall thickness leads to decreased rigidity of the material used. This, in turn, raises the machining vibrations thus reducing the feasible accuracy. Since plastics easily warp an increased minimum wall thickness is suggested during their machining. A rigid wall thickness would be 2.5 mm.

Minimum Wall Thickness - CNC
Minimum Wall Thickness – CNC
  1. Tolerances

The acceptable tolerance for CNC machining is 125 microns whereas the normal tolerance value can be 50 microns. The workable tolerances for machining purposes can be 20-50 microns. The tolerances for various parts are dependent on two factors – base dimension and part geometry.

General Tolerance of CNC
General Tolerance of CNC
  • Machine setups and part coordination for CNC machining

Along with the rules and constraints for CNC design for machining purposes, the machine setups and part coordination should also be considered accurately. For even machining of all the surfaces of the workpiece, it has to be rotated a thousand times. For each of the rotation made by the workpiece, subsequent re-calibration of the machine is done and a new orientation system is set.

However, continuous rotation and readjustments done can be highly costly and time-consuming. Thus, these can be only applied to situations where the workpiece has to be moved not more than 3-4 times. For obtaining ultimate relevant accuracy in the location, the set up should have the capability to machine two aspects simultaneously.

Now that we have understood CNC machining and its important aspects, let us look at the recommendations to be considered while designing for machining.

Recommendations for the design for machining operations

  • It should always be kept in mind that machining processes should be avoided whenever possible. Rather, you can make use of casting or forming operations that will be quite cheaper.

  • By making use of the maximum flexible surface finish and relevant dimensional tolerances in all the key machining processes you can eliminate the need for associated machining activities such as reaming and grinding that is quite unnecessary.

  • Making use of stock dimension will help you eradicate the need of machining processes even on the extra surfaces.

  • Because of the ease of using simpler tools, rectangular workpieces are usually preferred over various tapered or contoured surfaces.

  • The components should be designed such that they are capable of fixturing and properly holding up during the CNC machining activities.

  • The machining processes used for one-point surfaces should be associated with constant cuts. With interrupted cuts, there are chances of a decrease in the tool life.

  • Any sharp corner or a single point found in the cutting tool is likely to crack. So, these aspects should be avoided as far as possible in the design for machining purposes.

  • The number of shoulders and their sizes should be decreased as these are basically associated with additional processes and materials.

  • As far as possible, the stiffer and harder-to-machine parts should not be machined excluding the situations where operational requirements are so.

  • You can also make use of stamped processes for the machined parts. These stamped parts are specific to particular costs and are thus quite cost-effective.

  • For cutters bushing and various fixture components, some extra open room should be available.

  • Undercuts should be eliminated to discard the need for particular ground tools.

  • By making use of the same surface for consequent machining processes or similar diameter in case of the cylindrical structure, the number of machining activities is highly decreased.

  • Separating lines or draft structures should not be used for clamping or orienting surfaces.

  • For both the rough and finish machining, there should be ample allowance available to the stock. Stress relieving should be associated with both rough and finish machining. For design for manufacturing processes, the suggested stock for finish machining operations is 0.4mm.

  • For the burr elimination process, some extra space should be offered.

  • The workpiece should be constructed in such a way that we can make use of normal clutters.

To reduce the cost of a  machined part, the specifications of design for manufacturing purposes can be understood as follows.

To reduce the cost of a  machined part : 

A cost-effective CNC machined part can be designed by the below features –

  • Drawings and Prints

For an effective drawing and print, you should select exclusive names and component numbers, specific material surface finishes, and be particular about the material to be used. You should also have a 2D print which is properly dimensioned or at least partially dimensioned. The CAD file geometry should corroborate with the drawing dimensions. The drawing title block should be properly benefitted. Tolerances should be properly considered as well. The datum point or the identical edges should be used for dimensioning.

  • Opt Rapid Prototyping, Stereolithography, and other 3D Printing solutions when dealing with complex plastic parts to reduce the cost drastically.

Read this article on when to you CNC & when to use 3D Printing which will give you a comprehensive idea when dealing with it in your product development cycle.

Rapid Prototyping comes in handy for low volume production of complex plastic components. It makes use of completely automated equipment to generate 3D models from the available CAD files. Stereolithography, Fused Deposition Modelling (FDM) methods, and Laser Sintering processes can be used for creating components from plastics or polymer materials.

Learn how to revise & use versions in CAD models the right way.

  • Design for Machining purposes

While considering the design for manufacturing processes the inside corners should properly acknowledge the endmill radii. Sharp and square shaped inside corners should be avoided while designing the parts. Since endmills work effectively when stiffer deep pockets and inside corners having minimal radii should be avoided. Nowadays, any radius of the end mill can be milled with the help of a normal mill size. It should also be ensured that the inside corners are bigger than the endmill radius. With deeper tapped holes, it becomes quite difficult to put the threads in tapping holes. For the highest strength, you should have the depth of tool which is 1 or a maximum of 2 times the apparent diameter. The tool holder and spindle clearance aspects available to a high wall and associated component feature usually create issues during CNC machining.

Internal Radii For CNC Parts
Internal Radii For CNC Parts
  • Saving huge on setups and fixturing aspects

You should always restrict the risk aspects associated with machining processes by designing relatively lesser complex components. You can make use of Vise fixturing which acts as a precision tool. For effective soft jaw fixturing, you can create increasingly complex parts. When there are no parallel edges in the components soft jaws can be utilized. The parts that are highly cost-effective can be machined easily in a single setup. This eliminates the need for extra setups and time for designing parts.

If you are interested in knowing how to reduce machining cost, kindly refer to this article – Click here.

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Conclusion:

Thus you just had an overview of designing for machining which in itself is a very debatable topic. While everyone has many opinions and standards. Hope you have gained some knowledge, please do implement the tips and also subscribe to our newsletter for more contents.

At chizel, we are trying to optimize the process and machining the right way, the most optimum way that manufacturing can be done, all of that through a cloud-enabled platform. Check out our website and resources for more answers to all your manufacturing questions.

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