An In-Depth Look at Milling (Metal Fabrication)

As we reviewed in a previous article, Metal Fabrication 101, milling is the most common type of process for removing material. The milling process includes a rotating drill bit that removes material from the part. We will go into a more advanced look at the overall process for CNC milling.

Materials Used

When parts are moved to the milling station, they come in a few forms. The main forms are in pieces of sheet, solid bars, hollow tubes, and snapped beams (i.e. I-beams). Other processes, such as castings, extrusions, and castings can also be milled down after their primary process.

When choosing a material, there are a number of factors that should be considered. These factors include your target cost, strength, machinability, and resistance to corrosion. The most common types of milled materials are aluminum, steel, magnesium, brass, nickel, thermoset plastics, zinc, and titanium.

Process Cycle

When estimating the cycle time for milling, there are four steps that need to be taken into account. These four steps start when the tool is being placed into the machine and ends when the part is taken out and everything in-between, including idle time. Calculating cycle times can be broken down as follows:

1. Load & Unload Time: There are two parts that are associated with loading and unloading. These are the times that it takes to load the raw material onto the fixture and the time that it takes to unload or to remove the part from the fixture after the part has been processed. The load time mostly depends on the size and weight of the material being loaded and unloaded.

2. Cut Time: This is the time needed for the drill bit to make all of the necessary cuts. When the drill bit is removing material from the part, it is considered cutting time. However, when the drill bit is approaching or retracting from the part, it is not considered cut time because no material is being removed from the part.

3. Idle Time: This is the time in which no value is being added to the actual part. Idle time can include the drill bit approaching and retracting from the part that’s being processed, changing the machine settings, changing tools and anything else that isn’t adding value to the part.

4. Tool Replacement Time: This is the time in which you need to replace the tool that has outlived it’s lifetime. Tools can’t last forever and they are built for a certain amount of cycles. After these cycles are complete, you will need to exchange the tool.

After the process is complete for milling, there is no post-processing that is needed. However, usually there is is a secondary process that will make it corrosion resistant and improve the look.

Cutting Operations

During the cut time, there are multiple different types of cuts and drill bits that are used in order to machine the desired part. The following operations are the most popular methods of machining down a part.

End Milling: For this operation, the cuts will either be peripheral or slots which will be determined in order to get the desired shape. Depending on the depth, you will usually need to make multiple passes rather than just one pass.

Counterboring: This operation will first need a hole to be drilled into the part. The counterbore tool will then cut and expand on the pre-drilled hole. The main reasons for this process is to provide enough space for a head of a screw, fastener, etc.

Chamfer Milling: This operation makes a cut that is on the edge of the part that creates an angled surface. The edges usually are a 90° angle and the chamfer cut will create a 45° angle.

Countersinking: This operation is similar to counterboring but instead of having a 90° angle cut that’s on the inside of the part, the angles are different. After cutting, there is a cone-shaped hole that is used for screws, fasteners, etc.

Face Milling: This operation takes a rectangular or square part and it shaves off the top face. Therefore, the length and width of the part will not change, just the depth/height of the part will change. The cut will be smooth and flat.

Reaming: For this operation, a hole is first drilled into the part. Then the reaming tool removes a minimal amount of material for the desired shape. This will leave the hole smooth and very accurate.

Drilling: This operation takes a drill bit and drills a hole into the face of the part.

Tapping: For this operation, the drill bit cuts internal threads into a pre-drilled hole. This operation is preparing for a screw to be put in during assembly.

Boring: This operation takes a boring drill bit and cuts an internal hole into the part. Usually, a hole will be drilled and then a boring tool will continue to cut the desired shape.

Cost Drivers For Product Unit Cost

The costs of a milled part are pretty straightforward. The main components that drive costs are the following:

1. Material: The material cost is correlated with the quantity that is purchased. If you are purchasing the raw material in higher volume then your prices will be a bit lower. Other factors that play a part with the price of the material is the quality, grade and shape of the material.

2. Process Time: As discussed above, the process time includes load & unloading time, cut time, idle time and tool replacement time. Minimizing the times for any of these will decrease the unit cost.

3. Labor: The hourly rate of an operator that supervises the CNC milling machine or adds any additional value to this part. For labor costs, usually, these only refer to production operators, while quality is considered overhead or extra costs.

4. Extra Costs: The extra costs can also be interchanged with overhead. These costs will include the electricity needed to power the machine, cooling water used, lights used and more. Another factor here is the quality team. Depending on the manufacturer, quality control will also be covered in this cost. If the product is more complex and needs the quality team to pay more attention to it then your overall overhead costs will be higher.

The contract manufacturer will add all of these together and then add on their profit and provide you with the quote. If you feel as if the price is too high then you and the manufacturer can work on cost down projects that would look into the raw material price and also looking into how to reduce the process costs.

For more information on the milling process or to discuss an upcoming project, please contact us to schedule a call or meeting.