5 Key Benefits of One Piece Flow in a Low Volume Machine Shop

May 3, 2021

Author: Paul Van Metre

When our shop was about 6 years old, we went through a massive Lean Manufacturing transformation. We went all in – Kaizen events, 5S, Kaizen Newspapers, Value Stream Mapping events, and more. One of the most impactful things we did was the change to one-piece-flow for nearly all our jobs for the subsequent 11 years (and still to this day with the new owner). In a low volume, high mix CNC job shop like we were, the process is perhaps a bit different than what is traditionally thought of. However, the impacts were huge and it worked really well for our shop.

It eliminates a ton of different wastes:

  1. Inventory (WIP)
  2. Waiting
  3. Defects
  4. Overproduction
  5. Motion
  6. Transportation
  7. Overprocessing

Wait a minute… is that all 7 of them as defined by the Toyota Production System?? You bet! Flow processes can reduce or eliminate all 7 wastes.

We called the process Bar-to-Box. Here is how it worked:

  1. We put a very small, right-sized bandsaw next to the CNC machine. The machine operator would cut a piece of material needed to load into the machine. (Sometimes we would start with pre-cut plate if that was required. Also, if we needed a dovetail on the material we would cut that on a custom router table before it went into the machine).
  2. The material would be put into the machine and machined completely in multiple setups, or in one on a 5 axis, and sometimes even flow directly to another machine which the operator was running as well. The main point is we’d machine the part completely without any batching.
  3. The part would come off the machine, and get deburred, washed, and rinsed.
  4. The operator would do an in-process inspection, with equipment right there at the machine. (Or if a CMM was needed, they would walk it over and put it on the CMM.) The results of the inspection would be put into ProShop’s inspection results field which would validate the results and alert of any out-of-tolerance issues.
  5. If there was assembly work to be done, such as installing helicoils, or adding masking plugs for anodizing, the operator would do that as well.
  6. They would put the part into the appropriate packaging such as bubble bags, plastic bags, egg crates, etc., and put them directly into the box ready to be shipped.

That’s the process. Bar-to-Box during the machine cycle. We designed the process to make sure it can fit in the machine cycle. Sometimes that means machining multiple parts at once, which can make sense if it’s a small part and we can machine a strip of them to optimize cycle time and tool changes as well. It’s also more fun for the machinist who gets to see finished parts every cycle, stay more engaged in a variety of tasks, and feel more ownership of the parts they are making.

Here are 5 key benefits of doing a one-piece-flow. (There are many others as well):

  1. You can dramatically reduce the labor cost of a job because one person is running many resources at the same time.
  2. You can ship partials almost immediately once the job is set up. This helps clients who need parts faster than you can deliver if you are batching them. They’ll love you for it!
  3. It’s easier to ensure you’re making good parts. If you only can inspect a half-done part because you’re batching op1 and op2, sometimes you can’t be confident that the part is good. Therefore scrap rates are much lower when flowing. There is also less risk of a pile of half-finished parts getting damaged.
  4. You get your cash faster! Jobs through the facility flow much more quickly than with batching, so you can ship and get paid faster from your client. This can have a big impact overall on company cash flow.
  5. Employees enjoy it more. It’s more interesting, they have full ownership of each part they make, and it helps the day go faster.

A few other thoughts to help this work.

  1. Put all your small equipment on wheels. Band saws, granite tables, assembly benches, bead blasting cabinets, wash stations, etc. Make sure you can reconfigure your work area in a matter of minutes. This really adds the ability to be flexible and configure your flows.
  2. Start with a simple project and train your staff on all the required steps.
  3. Put together a Standard Work Combination Sheet to see how it works out with the timing.

How can ProShop help?

In the part module where the routings are defined, it’s possible to outline a flow process by linking operations together so ProShop understands that the operations are part of a flow process. When this is done, ProShop will schedule those operations and resources together at the same time and will understand that completely finished parts are being created with each “takt” time. This makes it much easier to schedule this and have everyone understand what is going on.

In this example below, we can see that all the operations are being flowed with Operation 65 and it takes 26 minutes to complete that full flow including the bandsaw, 2 CNC machines, and the assembly bench. Astute viewers will also notice that we’re doing this flow at a 25% labor target, or about 6.5 minutes of labor which is the longest manual segment of time in this flow process. This enables nearly 20 minutes of free time per cycle for the operator to run some other machines or do other value-added activities somewhere else. So essentially, we’re making a part every 26 minutes even though it has 46 minutes of cutting time, and only using 6.5 minutes of labor. Also, we get finished and inspected parts off each time, that can start shipping to the customer or to an outside process right away.

There are certain situations where a flow like this is more difficult or impossible. However, with a bit of creativity and brainstorming it’s remarkable how many parts can be made into a flow process with an amazing amount of upside in terms of cost, quality, and lead time improvements, and waste reduction.

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