The Economics of Workholding Systems
Part 2 of The Complete Guide to Stationary Workholding and Machining Fixtures
Machining is a subtractive process in which material is removed from an oversized workpiece until the final part geometry is achieved. In traditional machining, a sharp-edged tool is caused to move through the workpiece to form small chips.
This chip-making process results in forces being applied to the workpiece that are sometimes quite large. Therefore, all machining operations require a system to hold the workpiece securely enough to resist these machining forces.
In many shops, this takes the form of a manual vise that is more or less permanently mounted to the machine worktable. In this article, we will examine how the choice of the workholding system can dramatically impact the economics of machining operations.
One of the biggest contributors determining the cost to produce a machined part is the cycle time, or the total elapsed time between loading of a raw workpiece into a machine and removal of the finished part.
The total cycle time is generally composed of time spent making chips, i.e. actually cutting material from the workpiece, and other, non-chipmaking, activities. The familiar saying, “if you’re not making chips you’re not making money,” reminds us that minimization of the non-chipmaking portion of the cycle time is critical to the overall profitability of the operation.
The major non-chipmaking contributors to cycle time are:
Part loading and unloading, which may include:
Machine start and stop
Door opening and closing
Blowing chips and coolant away from the finished parts
Clamping and unclamping of the part(s), including locating and restoring of any required tools
Cleaning of the machine workspace and fixture to prepare it for accepting the new workpiece
Part touch-off, if required
In-process inspections, if required
Periodic machine cleaning and maintenance, appropriately apportioned to the number of parts produced between these activities
Loading and setting of cutting tools, appropriately apportioned to the number of parts produced between these activities
One of the most important factors influencing the selection of workholding systems is the total production volume, or the number of identical parts that are going to be made; and the batch size, or the number of identical parts that will be made in a single setup and run.
For one-off parts or prototypes, all of the contributors must be included in the cycle time for each part.
For very large production runs and continuous production, it can make sense to design and build custom, automated workholding systems dedicated to this single part or family of parts since the cost of these can be recouped over a large number of parts. This can greatly reduce the portion of cycle time not actually spent in making chips.
For intermediate batch sizes and production volumes, intelligent choice of workholding systems can greatly decrease effective cycle time by spreading the non-chipmaking activities over multiple pieces, increasing productivity and profitability.
Consider a machine with the typical vise being used to produce a hypothetical, smallish part with a 10-minute cycle time, of which 1.5 minutes are consumed in all of the steps of the part load and unload process. Of the other 8.5 minutes, assume that one minute is for tool changes and 7.5 minutes is for actual chipmaking.
In this example, with the vise as the workholding system, it is only possible to make one part at a time. Once the part is loaded, the machine operator has 8.5 minutes with little to do while the machine runs. This is likely because there is not enough time to tend to another machine or to make meaningful progress on other tasks. So, in one hour six parts will be produced and the operator will be “idle” for 51 minutes. In other words: only adding value for 15% of the time.
If the workspace in the machine is large enough to accommodate a workholding system that can hold six parts simultaneously, the numbers change dramatically. The total part loading/unloading time will likely be reduced to less than 1.5 minutes per part since some of the individual activities will only need to happen once and others, such as part and fixture cleaning, take place for all of the parts more or less simultaneously.
There are additional economic benefits to this improved workholding system. The machine can now run untended for 46 minutes, allowing it to continue production during lunch breaks, and even after closing time.
This drastically increases production with no additional labor cost; and the machine operators have enough time to effectively engage in other tasks, which increases their productivity and reduces the embedded labor cost in each part produced.
This simple example shows how the optimal selection of workholding systems can dramatically improve profitability of machining operations. While there are a huge number of types of workholding systems and components available, making the correct selection for your shop can increase production and improve efficiency.
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