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 chipmaking 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 the loading 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.
Of course it’s equally important to minimize the chipmaking portion of the cycle time by optimal selection of tooling, tool paths, and machining parameters. That topic will be covered in a later article.
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 restowing 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 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 amortized 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 nominal 10 minute cycle time, of which 1.5 minutes are consumed in all of the steps of the part load/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.
So, assume it takes a total of six minutes -- one minute per part. Once the machine is started, it will run for somewhat less than 51 minutes -- six parts times 8.5 minutes per part of machine-on time, since the tool changes only need to occur once per batch instead of once per part. Therefore, the run time for the batch of six parts will be 45 minutes of actual machining time (6 parts times 7.5 minutes per part), plus 1 minute for tool changes, plus 6 minutes for loading and unloading, for a total of 52 minutes.
Now, the effective cycle time for each part is 8.67 minutes: a 13.3% reduction. Additionally, the operator will now have 46 minutes of uninterrupted time to effectively tend to other machines or engage in other tasks.
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.
Consider an 8-hour workday plus a 45 minute lunch break. The single vise setup will be able to make 48 parts in a day. The improved workholding system allows the same machine to make 11 batches per day, or 66 parts (8 hours @ 52 minutes per batch = 9 batches, plus one batch during lunch, plus 1 batch after closing time).
Therefore, production is increased by almost 38% 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 optimal selection of workholding systems can dramatically increase productivity and profitability of machining operations. However, there are a huge number of types of workholding systems and components available, and making an intelligent selection can be overwhelming.
In future articles in this series we will look more closely at the physics of the machining process and how that impacts selection of workholding system components. Using this knowledge, we will then look more closely at the types of components available, and their respective advantages and disadvantages.
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