Seven Machining Metrics that Matter Most

Manufacturing economics should be “Job One” for any plant manager or owner, and this approach should infuse operations all the way down to the shop floor. There should be full understanding that reducing time and increasing throughput are the keys to driving proper and efficient production. Is there a way to do it twice as fast? Is there a way to drive 30% out of your cost? Sometimes the solution is about the cutting tool, how it is held and how it is driven. Sometimes it is about how the part is held.

Getting the Process Started

Finding a solution is no easy task, particularly if old habits are ingrained. To get this process started, first identify and standardize the correct metrics to properly control and assess time, cycle time and throughput improvement. To begin that process we offer seven key metrics:

1. Analyze Setup
Analyze the current average setup time to see who takes the longest time, and what the average setup time is within the shop. Then calculate a median which becomes the shop standard.

For example, we worked with a mid-sized machine shop making automotive accessory parts, and for years they had focused their improvement efforts on a single milling cutter perceived to be the biggest problem on the job, simply because it chewed up a set of inserts every 20 minutes.

Instead, the company analyzed every aspect of the setup to gain a better view of the real causes. That led to changing the depth of cut slightly and how they entered and exited the part. These tweaks helped double the output of the tool. Instead of taking two weeks to run the lot, it took a little over a week. Net-net, after analyzing all the processes for this part, a 45% throughput improvement was realized.

2. Examine the Materials Mix
The material mix impacts decisions made on the style and geometry of cutting tools, the toolholding, and the best fixturing method.

The best way to assess the mix is to analyze a list of all materials machined under the shop roof, with their percentage of total revenue, quantity, and lot size. Include the materials form: billet, extrusion, casting, etc. This data will help the shop make rational decisions on the expendable tooling side of the equation, as well as grade and coating choices.

3. Average Cycle Time by Family and Part
Let’s say a shop produces 20% aluminum and 80% high-temperature alloy components. Break those blocks down into materials by specific family or part. If perhaps the company builds a lot of large items but with some small castings, bucket the materials by type and by process.

The average cycle time per part will drive a lot of decisions too. What if the mix is 50% milling of rectangular billets, and the part has a critical feature and tight tolerance on a ledge? It takes 30 minutes just to rough it, but the overall cycle time on the milling operation is 40 minutes. In that case, look for any new technology or method that can drive time out of the milling process.

4. What is the Spend on Expendables?
When looking at manufacturing economics, the average spend on expendable cutting tools should be anywhere from 3 to 6% of top line revenue, depending on material mix. Look at the top 20% of spend and quantify the cutting tools in that group and how many of them are used. If you’re going through thousands of $2 inserts, it will be just as important to look at that number and drive it down as it would be to look at why it costs $20,000 a month on 200 end mills!

Just as critical, analyze the number of tool changes that stop the machine, because that drives added costs and results in a profit drain. When efficiencies go down, costs go up!

5. Average Number of Tool Assemblies
Are your tool assemblies firmly controlled and documented to maintain consistency so that every tool is always set up the same way, held the same way in the same class of holder each time you run the part?

How about milling the face of a part? Do the folks on the shop floor have access to that information in a library, programming software, and if so, is it modeled? Is this access available consistently and across-the-board, so that lessons learned can be applied to every part?

For example, a company was using an older machine and a newer machine to make the same part (an hourglass shape out of a billet). Even though both machines were making the same part using similar materials, one had 39 tool assemblies and three operations to complete, while the other had 22 toolholder assemblies and two operations to complete.

Which way is better? Here again ask, “Are we maintaining a control? When we learn the best way to produce a part, do we apply that knowledge to how the next product is machined?”

This should be documented and controlled as much as the cutting tool. If it’s always going to be a 1″, seven-flute 35° helix, corner radius end mill for roughing profiles, and it’s always going to be held in a hydraulic adapter, then control it all the way down to the brand, geometry and holding mechanism, including the collet.

6. Analyze Scrap Rates
It is imperative to analyze scrap rates by percent of topline revenue, and how this rate relates to the number of parts run. A shop making 100 parts per week with a 10% scrap rate is alarming. Conversely, a shop with a $10 million monthly run rate but which is only making six parts per month with 1% scrap, well, that’s also alarming.

It is paramount to analyze it both ways and develop a team to investigate the causes. Look at things from the spindle down and from the table up. Look at fixturing, toolholder assemblies, the cutting tools, the machine movement—anything that will provide insight into why scrap is occurring.

7. Average of Parts per Lot Size
Some shops produce small batches of high-value items, say four each of an item per month, and they only have seven to ten-part families. While that may seem less complex than producing longer runs, the stakes are higher.

Here you should ask, are we building to drive shipments, or building a lot of excess material for work in progress and/or other needs within the shop? What is the process flow?

Look at the average number of parts per shop order and then look at the orders with the lowest and highest number of parts. It may be better to run smaller lot sizes or longer lot sizes, but first ask, “Have we always done it this way or is there a better way to process plan?” Then, look at the throughput to see what needs to ship versus what can be produced at the lowest possible cost.

From there it would be best to compare reducing the lot size and/or increasing the variable workflow so you can better ascertain what needs to ship, rather than just making lots in a production run.

Process Improvement Goals

In process improvement plans, strive for a 30% or 40% throughput improvement in manufacturing, particularly on the machining side. There is bound to be low-hanging fruit in every manufacturing facility. It’s also critical to constantly look at how things can be done better and explore new technologies that perhaps didn’t exist even six months ago, all in a concerted effort to innovate. Going to the people closest to the process is always the best place to start.

About the author: Phil Canipe has more than 37 years of industrial distribution and manufacturing experience, focusing on cutting tool design, manufacturing/application engineering excellence, and process improvement. Prior to joining Stellar, Canipe spent 15 years as president/co-founder of a specialty cutting tool distributor. He is a member of SME and AMT—The Association for Manufacturing Technology.