Milling Services

Milling Machines and Milling Process

Milling is the process of machining using rotary cutters to remove material by advancing a cutter into a workpiece.

It requires a milling machine, workpiece, fixture, and cutter.

The workpiece is a pre-formed block of material that the milling process will turn into the finished part.

The workpiece is fitted to a fixture in the milling machine, to lock it in place and allow the cutter to cut cleanly and precisely.

Cutters rotate at high speed while secured to the movable parts of the milling machine.

Moving the workpiece into the cutter shaves small chips of the material away and creates the desired part shape.

The finished asymmetrical parts that have many features, like slots, holes, shells, pockets, and 3D surface contours.

Parts created completely through milling are often used for limited quantity needs, for example prototypes.

Tooling for other processes is also able to be made, like 3D molds.

Refining or adding features to parts from a different process is also a common milling function.

Milling also offers high tolerance and clean surface finishes.

 

The milling process

 

The time to make a quantity of parts includes the initial setup time and the cycle time for each part.

The setup time includes preparing the machine, planning the tool movements, and installing the fixture device into the machine.

This is often greatly shortened by using a CNC machine as the movements are all managed by the computer.

Cycle time depends on these four steps:

Load/Unload time: getting the workpiece into the machine and unloading it after the part is finished.

Cut time: the time it takes for the cutter to run the total length of the multiple cuts as well as the speed of the cutter as it makes each cut.

Idle time: any tasks that do not have the cutter actively removing material, including the tool approaching and retracting from the workpiece, tool movements between features, changing tools, and changing machine settings.

Tool replacement time:  worn or non-effective tools must be replaced, especially on large production runs or with more durable materials.  

After the cycle there is no post processing, scrap material in the form of small chips are thrown away from the surface of the part by the spinning cutter or lubricant spray.  

If the surface of the part needs further improvement additional steps can be required.

The scrap from the process is collected and is discarded after the machine operation.

Cutting parameters

 

The speed and motion of the cutting tool are defined through several parameters, determined by material, tool material, tool size and additional factors.

Cutting feed: the distance advanced by the cutting tool during one revolution of the spindle and tool, measured in inches per revolution.

Cutting speed: the speed of the workpiece relative to the edge of the cutting tool, measured in surface feet per minute.

Spindle speed: revolutions per minute of the spindle and tool.

Feed rate: the speed of the cutting tool movement relative to the workpiece as the tool makes a cut.

The product of cutting feed (Inches per revolution) and spindle feed (Revolutions per minute) measured in inches per minute.

Axial depth of cut: the depth of the tool along its axis in the workpiece as it makes a cut, can create high load and reduce tool life.

Radial depth of cut: the depth of the tool along its radius in the workpiece as it makes a cut, if the tool is engaged the radial depth will be at least equal to the tool diameter.

Operations of a milling machine:

• End milling
• Chamfer milling
• Face milling
• Drilling
• Boring
• Counterboring
• Countersinking
• Reaming
• Tapping.

Description of Operations

 

End milling makes peripheral or slot cuts, determined by step-over distance, across the workpiece in order to machine a specific feature.

These may be profiles, slots, pockets, or complex surface contours.

Depth of these features may be machined in a single pass or multiple passes.

Chamfer milling is making a cut along the edge of a workpiece or feature to create an angled surface, known as a chamfer, these are typically 45 degree angles and can be machined on the interior or exterior of a part following a straight or curved path.

Face milling creates a smooth finish by creating a flat surface on the workpiece, this is usually done by removing a small depth of material from the edge.

Drilling creates a hole with a diameter equal to that of the tool, extending through or partially through the workpiece.

Boring tools cut along an internal surface to form different features, often used after drilling to create more precise dimensions or enlarge the diameter of the hole.

Counterboring enlarges the top of an existing hole to the diameter of the tool providing space for a fastener head to sit flush with the workpiece surface.

Countersinking creates a cone-shaped opening in the top of an existing hole, creating space for a fastener head, like the top of a screw, to sit flush with the surface of the workpiece.

Reaming enlarges an existing hole to the diameter of the tool, it often removes a minimal amount of material and is performed to obtain a more accurate diameter and a smoother internal finish.

Tapping cuts internal threads in an existing hole, the threads may be cut to the full depth of the hole (through tap) or a specific depth (bottom tap).

Other Considerations

 

Possible defects in the finished pieces can be caused by incorrect cutting parameters, dull cutters, or unsecured workpieces.

Milling is one of the most commonly used processes for making custom parts to precise tolerances.

These tolerances include +/-.005” for local tolerances for most geometries, +/-.10” for plastics with variation dependent on the size of the part, 0.03” minimum wall thickness for metals, and 0.06” minimum wall thickness for plastics.

Article refrences:

https://www.custompartnet.com/wu/milling

https://en.wikipedia.org/wiki/Milling_(machining)