Design of Parts for CNC Machining Taking into Account Technological Possibilities

July 26, 2020

Design of Parts for CNC Machining Taking into Account Technological Possibilities

What if you could change the design of a part to make the part much easier to manufacture without diminishing its utility? There is a whole system of techniques on this topic, which are described below.


Material type. The material type of the part will significantly affect manufacturability. Plastic, aluminum and brass are very easy to use compared to, for example, titanium and hard steel alloys.

If the part still needs to be steel, use low carbon hot rolled steel instead of cold rolled steel. It is more stable and cold rolled steel will deform and you will need to process it more than once.

Select materials whose physical properties match (but not unnecessarily) the needs of your part.
Then select materials that meet those needs with the lowest material and machining costs.

Sometimes you have to use a more expensive material, but you will save more time reducing your machining costs with a material that is easier to process.
For example, prefer 303 stainless steel over 304 because it handles better than 304. Alternatively, you can use a tougher aluminum alloy such as 7076 instead of some steels. Although aluminum is more expensive, it is processed much faster than steel on most CNC machines.

The hardness of the material. Harder materials are often more difficult to handle. Study the different degrees of hardness of different materials and their alloys and conditions. An example of the condition of a material is varying degrees of hardness.


Form and Dimensions of Workpiece

Workpiece shape. Consider which forms are cheaper to obtain and which forms are closer to the finished part and therefore require less processing.

Workpiece size. When sizing a part, consider the relationship to the dimensions of the rough stock. You want a machining stock that does not require too much build-up in the roughing stock so that you don’t waste time and money turning that stock into a cut.

Preparations of Black Stick

The cheapest form of material removal is often during the rough stock (raw material) preparation stage. For example, if you can start machining on a workpiece that can be waterjet cut, you may only need one pass rather than a large number of rough milling passes before finishing.

Do Not Extreme Tolerances!

The tighter the tolerances, the higher the manufacturing costs. Do not set tight tolerances unless you really need them. One of the most expensive tolerances is thread depth, and often does not matter.

Depth of Cut and Angle Radius

Keep in mind the radii of the corners in the grooves. The stiffness of the tool changes with the third power of the length and the fourth power of the diameter. A tool that is twice as long will have only 1/8 of the stiffness of a short one. A tool that is twice the diameter will be 16 times stiffer. Therefore, do not design parts with deep grooves and small radii. My advice is to maintain a 3: 1 depth to diameter ratio (2x the radius of curvature). Therefore, a 6mm radius depression should be no more than 38mm, otherwise you will significantly increase production costs.

Here's another tip: set the radius of the rounding slightly larger than the end mill that will be used to cut the groove. This will reduce the stress on the end mill by reducing tool nip angles, and will also reduce your production costs by either feeding the end mill faster or extending the run time.


Through and Deep Holes

Make through holes whenever possible as they facilitate chip evacuation. This is especially true for holes to be drilled or tapped.

Deep holes are much more expensive. For maximum efficiency, try to keep the L / D ratio less than 4 (no holes greater than 4 diameters). Any hole deeper than 10 diameters is likely to be problematic.

What About the Edge

It is generally cheaper to chamfer the edges than to round them.

Avoid Mirror Parts

Mirrored parts are usually used in pairs in an assembly. If an assembly can be designed so that both parts can be the same, there can be huge savings.

Avoid Thin Walls

As well as thin partitions and the like. Thin walls and baffles are prone to vibration (which slows down the processing speed), deformation (therefore difficult to maintain tolerances with them), and are more easily damaged during manufacture.

Avoid Undercutting and Anything That Requires Special Processing

In most cases, undercuts are a serious problem for CAM programs and processing, so make sure you actually need them before you point them to parts.

Make Clearance Between Tool and Workpiece when Turning

A 90-degree shoulder joint will have less tool clearance than a tapered shoulder joint, so you will face more problems. Also, if you subtract an area to achieve a tolerance and the perpendicular shoulder joints border the area, you are more than likely to run into bumps.

Thread and Thread Cutting

There are many ways to minimize your threading and tapping costs, for example:

  • Minimize the length of the threaded portion in the hole. An outer diameter of 1.5x provides sufficient strength.
  • Avoid blind holes whenever possible. If you need to thread in a blind hole, allow 1/2 more OD than the thread at the bottom of the hole.
  • Do not overdo it with the percentage working height. A 75% thread gives 95% of the strength of a 100% thread, but only requires 1/3 of the torque - the chance of damaging the outlet is much less.
  • Avoid tight thread depth tolerances, they will be too expensive to use.


Depth Bottom Radius is Less Than Wall Round Radius

It is possible to model in CAD, in which the radius of the dot from the bottom to the edge of the wall will be the same as the radius of curvature, but this will be more expensive, since it will probably be necessary for the spherical cut to make the radius of the bottom, which entails an additional pass with an additional tool ... Set the radius of the bottom that the ball end mill can handle. It can be used immediately to complete a pocket.

Minimize the Number of Installations

The production of a part should contain a minimum number of workpiece installations, ideally, everything should be done from 1 installation. In the case of turning, try to define all exact passes so that they can be processed in 1 pass without rearranging or inverting it. In particular, avoid re-clamping parts when machining surfaces that need to be concentric.

Designing in the Case of Multiple Plants

If you need to use multiple rigs, follow design techniques that reduce costs.

If a part requires multiple installations, design the special fasteners and the parts themselves so that the option of incorrectly inserting the part into the fastener is impossible. This may imply the addition of a special. grooves or symmetrical elements in the cradle

Not being able to insert the part incorrectly into the mount will make the job easier.

Better yet, make each piece symmetrical so that no matter which side it faces in the mount, it will be processed correctly.

Minimize Instrument Requirements

Please note that the machine has a limited number of trays at the tool changer. Try to design the part to use as few different tools as possible. For example, you can use a center drill to countersink flat head cap screws. You may be able to reduce the number of drill diameters required by using an end mill and hole milling. If you are working with a very expensive pre-assembled structure that has a sea of ​​threaded holes, consider thread milling instead of tapping - if the thread mill breaks, it won't get stuck in the hole. Each of these ideas has pros and cons that should be evaluated in order to determine which one will actually reduce the cost of production.