Prioritizing Workholding Density Versus Simplicity

Reader Question: We do a mix of small-batch and high-volume production for our customers. We’ve recently started utilizing fixtures which hold more parts to improve productivity. However, for some jobs it doesn’t seem to make sense to invest the time into setting these up. Can you help us discern when it makes sense to increase parts per fixture and when to keep it simple?

Miller’s Answer:

You’re right, high-density fixtures do increase productivity in a general sense, but when deciding whether to process parts this way, a deeper look into the details is required. The purpose of increasing part density on a fixture is amortization of tasks for the machine and operator, or even a robot.

When it comes to the machine, the amortization of tasks mostly refers to tool changes. Instead of making constant trips back and forth to the tool changer, the machine can stay low in Z and jump from part to part with each tool. This saves overall rapid traverse time per part, as each tool can execute more nearby versus constant full retracts to and from the toolchanger. It also spreads the toolchange time over more parts, again reducing per part cycle time, even if the overall cycle is longer.

For the operator, this amortization of tasks means only engaging with the machine once for a longer period, but the machine has more work to do, thereby improving “walk away time.” Again, the operator is engaging in a longer overall task to load more nests with parts, but the machine can then run longer without intervention.

The benefit of this amortization is dependent on the process. A process with a quick load time and lots of short running tools will benefit more than a process, which takes a long time to load each part and runs each tool for a long time. An example might be an aluminum part clamped in a simple vise with a lot of quickly drilling cycles versus a detailed hard metal part with complex 3D surfacing.

This amortization of tasks should be viewed as helpful but diminishing. A 6-second chip-to-chip time for one part becomes 3 seconds if you were able to put two parts in the work zone. At 12 parts it becomes 0.5 second. At 15 parts is becomes 0.4 second. As you can see, the per part reduction begins to approach zero over time. For the operator, the time used to load parts means the machine is not cutting parts. The breakeven point of productivity is exactly where the lowest overall time per part for the total cycle exists. The total cycle meaning the time to load all parts, cut them and unload them and start again.

Benefits of Simplicity

In a production environment with a multiyear contract on a part, the time to develop and study the right number of parts exists, with the benefit paying off in more parts overall. For this type of shop, a high-density fixture may be an obvious choice. However, in shops that handle a mix of volumes for shorter runs, there is more we should consider and there is a lot of benefit to simplicity here as well.

First, one thing that may get lost is “time to first article.” If you decide to do a heavily nested fixture, there is time in production and programming all those nests in addition to cost for all the individual stations. Compare this to the time it takes to program a simple single part flow of operations, setup time, quality and then running the rest of the job. By the time you’ve developed the high-density process and confirmed quality on all the various process streams, you may have been done with the job as a single part flow even if the per part time is slightly longer.

Risk is another thing to consider. Depending on the part, tool life in that material and tolerancing, you may want to mitigate risk by managing parts one-by-one versus scrapping an entire pallet of parts due to a chipped end mill.

Lastly, what happens to this fixture after the job is done? Is it worth making a high-density fixture for a job you may only run once? You could have all those fixtures and soft jaws collecting dust in the shop just in case, but that pain is worse when you’ve sunk tons of time and cost into that fixture.

The Impact of Automation

If your shop has automation, this changes the equation a lot, because you no longer need to consider tying up an operator’s time in loading and unloading the fixture. The type of automation you have (for example, a robot loading a chuck or a pallet system) also changes this equation. In the case of a robot loading material, single piece flow keeps things simple and can be quickly programmed with each job. For a pallet system, you may want to consider the number of pallets a job will consume, the density of each of those pallets and what other work you need to balance within that automation with other concurrent jobs. The benefit may be that you gain the overnight run time with simpler workholding versus wasting energy and effort building a highly productive monolith.

As you tackle this decision with each job, the first thing to determine is if your concern is strictly cycle time, strictly walk away time or a balance of both. If you are striving for fastest cycle time, there are lots of ways to optimize in addition to the workholding. If you are starved for walk away time to increase your operator’s productivity, you may be ready for a cobot, which achieves the same goal with a simpler workholding solution.

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