Cost Drivers in Machining

So, you've engineered your new part. Before you sign off on the final design and specifications, be sure to consider several factors that can make a huge difference in the final cost, with no performance penalty. You and your machine shop are on the same team. Careful attention to detail can help your machine shop deliver the best possible product, on time and on budget.

In today's cut-and-paste world, it's not unusual to take requirements from one job, and automatically apply them to the next, without considering what does or does not make sense.

When a machinist receives your specs, he must respond to them verbatim, no matter how long it takes, or what it will cost. After the part is complete, you may discuss why the project cost so much. You can take steps up front to analyze what has been specified, and why. Slight changes can radically speed production and reduce costs.

The major cost drivers when it comes to machining include:

• the raw material cost;
• tolerances;
• internal features;
• surface features; and
• coatings.

Decisions about each of these have a direct impact on the cost and production time for your product.

Considerations

There is little difference in the machinability between 6061 and 7075 aluminum. However, the cost difference is enormous, at $4/lb. for 6061 and $20/lb. for 7075. Structural differences between the two, in certain applications, might only be 10 percent.

So, if you spec 7075 for a part because you always have, re-examining that decision might save you money. If performance requires 7075, the additional cost is certainly justified. However, if 6061 will work, at one-third or one-fourth the cost, consider the change.

Similarly, geometric design features can make or break your budget. Tolerances impact how long it takes to machine a part and determine what machine or machines are used to create it.

For example, when specifying tolerance for an outside/inside feature, an engineer might need +/- .005, but then spec +/- .001 as insurance. The problem, given the law of diminishing returns, is that it costs much more to get to .001, rather than .005, and you don't need that precision.

For internal radii, today's cutter technology allows everything to be done on a milling machine. With old technology, a part was milled, heat treated, put on a jig grinder and then a Sinker EDM machine.

If you need a one-inch-square pocket, how do you handle the corners? If you spec a corner radius of 0.0030, it will have to be milled, and then put on a Sinker EDM machine, an extra step on a second machine that adds time and cost. If you spec a one-eighth-inch corner radius, the entire piece can be milled on one machine, saving time and significant expense.

You can also save costs when it comes to surface finish. For hidden features that no one will ever see, don't spec a 63 finish. A 250 finish works, and saves time, effort and expense.

When the part has been machined and it's time to coat it, consider your needs carefully. Multiple coats of different types, with required masking, take a lot of time and effort. Is it necessary, or are there other options?

Finally, be aware how long it takes your shop to do the things you request. If you need the part in three weeks, don't spec a process that takes four weeks, or a material that takes eight weeks to get. No matter how professional and dedicated the machinists are, you will be disappointed, either on how long it takes, or how much it costs.

Do you have a challenging project? We accept your challenge. Contact us at 303-420-7422 or visit www.faustson.com.