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An In-Depth Look at Turning (Metal Fabrication)

January 8, 2019

As we talked about in a previous article, Metal Fabrication 101, turning is another one of the most common processes for removing material during metal fabrication. Turning is used to produce rotational parts that have multiple different features. Below we will take a more advanced look at the overall process of CNC Turning.

 

Materials Used

When parts are moved to the turning station, they come in a few forms. The main forms are in round bars, round tubes, and custom extrusions.

 

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

 

Process Cycle

When estimating the cycle time for turning, there are a few 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, plus everything in-between, such as idle time. Calculating these cycle times can be broken down into four steps:

 

  1. Load & Unload Time: This is the time 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 needed to replace the tool that has outlived its expected life due to wear. Tool wear describes the gradual failure of cutting tools due to regular operation. Tools don’t last forever and they are built for a certain number of cycles. After these cycles are complete, the tool should be exchanged.

 

After the process is complete for turning, there is no post-processing that is needed. However, usually, there be 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. For turning, there are two operations, external operations and internal operations.

 

EXTERNAL CUTTING OPERATIONS

 

Turning: The turning tool moves in the path of the part to remove material. There are multiple features that can be cut, such as steps, tapers, chamfers, and contours. The process starts on the face and cuts are made moving towards the center.

 

Facing: The turning tool moves up and down to remove material on the part. The cuts are usually only along the front or back face of the product which creates steps, chamfers, etc.

 

Grooving: The turning tool moves up and down to remove material that's in the middle of the part.

 

Cut-Off (Parting): This process is similar to grooving. However, the cuts are not made to go around the entire circumference but just around a specific area.

 

Thread Cutting: The cutting tool (usually a 60° nose) moves along the part to create threads. The threads can be cut to a specific length and pitch.

 

INTERNAL CUTTING OPERATIONS

 

Drilling: The drill bit enters the face of the part and cuts a hole that’s the size of the drill bit.

 

Boring: The boring drill bit enters the pre-cut hole from the drilling operation to create additional features internally, such as steps, chamfers, etc.

 

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.

 

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.

 

Cost Drivers For Product Unit Cost

The costs of a turned part is pretty straightforward. The main parts that drive the 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 of material and the shape.
     

  2. Process Time: As discussed earlier, 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 turning 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, any 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 manufacturer will add all of these together and then add on their profit and provide you with the quote. If the price is too high then you and the contract manufacturer can work on cost down projects that would look into the raw material price and also look into how to reduce the process time.

 

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

 

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