Designing for Manufacture: Keeping the Production Team Happy

By Dave Lapthorne for Autodesk University

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Basics of CNC Machines and Tooling Geometry

CNC Machine

A computer numerically controlled (CNC), milling machine is used to transform blocks of raw stock into finished parts by cutting away material. There are generally 3 axes to a milling machine:

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Basics of a milling machine.

Types of Milling Tools

Flat Endmills

These are the most commonly available type of cutter. They’re efficient and cheap. Flat end mills are most commonly used for machining of flat bottom cuts and vertical walls.

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Common milling tool side profiles.

Bullnose Endmills

The bullnose profile cutters are similar to Flat Endmill in that they share a common rectangular side profile but with an additional radius at the bottom corner of the flutes. They are great for quickly removing material while leaving a small bottom radius in pockets

Ball Endmill

The ball endmill has a full radius at bottom of flutes. These cutters can be utilized for complex surfacing as the contact point between the tool and material surface is constantly changing with variations in the slope of the surface.

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Example of Ball End Mill contact point versus centerline of tool.

Drills

Drills are used exclusively for making vertical holes in parts. There is no side to side or horizontal motion of the tool when using drills. The finished size of the hole will be dictated by the diameter of the tool itself. Subsequent operations can be used to alter the size of the hole (reaming, etc.) but those tools are not discussed here.

Additional Considerations Regarding Endmills

Besides the side profile it is also important to understand how the number of cutting edges, or flutes, are formed into the tool as viewed from the cutting end.

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Number of flutes versus performance.

Types of Milling Tools

But What about Tool Length?

Tool diameter, profile, and number of flutes are important things to consider. Another critical piece of information to consider is the tool length. Especially the exposed length of the tool that is exposed when the tool is clamped in the tool holder or spindle. This is commonly referred to as the tool stick out.

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Tool holder and stick out.
  • Large step down (changes in Z height) during operations
  • High feed-rates (the speed with which the X or Y location of the tool changes while cutting)
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A collection of endmills.
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Flute length and tool diameter.

Tool Aspect Ratio and Deflection

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Chart depicting deflection due to side loading.

Model Features

Internal Fillets

When considering internal fillets on vertical walls, we can see how the previously discussed topics impact our design process.

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Internal vertical fillet considerations.
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Part and cutter radii.

Bottom Edge Fillets

Often the design will require internal fillets at the bottom of pockets or walls. In fact, parts without this feature can experience unplanned failure of the final assembly as a result of stress risers caused by a sharp corner.

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Bottom edge fillets.

External Fillets

If the design intent allows, it is worth adding fillets to vertical external corners. This will allow for in-process deburring of those edges.

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External fillets.

Chamfering and Deburring

It is preferable to break the sharp edges on parts during the manufacturing process. This is because of the way that milling machines are capable of creating incredibly sharp edges which can lead to workplace injury when production workers are handling parts.

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Top edge fillets.
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Breaking sharp edges.
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Simple text note call-out on drawing sheet.

Holes and Threads

Several factors can impact the manufacturability of hole features.

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Flat bottom versus drill point hole profiles.
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Hole depth considerations.
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Thread depth.
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Hole starting on non-flat face.
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Hole breaking through adjacent wall.
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Holes in sheet metal.
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Holes next to adjacent walls.

Feature Height and Thickness

Components which require fins or ribs of material can present challenges beyond the ones outlined on the previous page. If the height of the machined features exceed approximately four times the width of the feature, the vibrations induced during machining can result in poor surface finish, scrapping parts due to them being out of tolerance, or even tool breakage.

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Feature height versus width.

High Precision Locations

In areas where high precision flatness is required, it is easier to maintain those tight tolerances in smaller regions. To this end it is recommended that the components are designed with raised bosses which will be machined with slower, higher precision strategies while allowing the rest of the part to be machined in a more efficient manner.

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Bosses for high precision areas.

Machining Complex Surfaces

While complex surfaces can be aesthetically pleasing, they also require different tooling and more complex machining strategies. The net result is increased machining time which will drastically impact the cost of manufacturing.

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Undercuts

Undercuts are sometimes required for assembly or fixturing purposes. In these cases, specialized tooling will be required for successful machining.

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Undercuts.
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Example of a T-Slot.
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Alternative designs to eliminate undercuts.

Part Markings

When part marking is required it is recommended to use engraving rather than embossing. Embossing typically refers to text which is raised above the finished surface. To machine small features like this will require very small end mills (assuming the font face used for part marking is around one-quarter inch in height) to be able to get into the small inside corners and interior pockets.

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Embossing example.
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Engraving example.

Interfacing with Square Corners

Often times, when designing fixturing for machining parts the parts that are being clamped have sharp outside corners. In these cases, consider using a dog bone corner on your fixture.

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Dog bone corners.

Milling Setups

Whenever material is loaded onto a machine and clamped in place, this is referred to as a setup. In the example at the beginning of this article, all three axis directions are fixed. This means that if a part has machined features on the top as well as through the sides, two or more setups will be required to completely machine the component.

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Two setups required.
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One setup required.

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