Achieve required Cpk values in plated machine parts

Modern Machine Shop, April, 1997 by David M. Udler

Monitoring and reporting machining process capability and performance is a common practice for many machine shops. Particularly for shops serving the auto industry, the ability to meet target values of dimensional capability/performance indices becomes a competitive advantage. But for the overwhelming bulk of machined parts requiring subsequent plating, capability measurements of the finished product reflect the effects of both machining and plating. And so quality professionals must learn to account for the variability in both processes if they hope to achieve the final objective of good finished parts.

Production part quality planning now faces a double challenge - continuously improving in-house machining capability while managing the ill effects of the plating process's inherent instability. Plating is often done by a subcontractor, and that doesn't make controlling quality output any easier for a machine shop. But that's exactly what this article is designed to help shops do. The following is a simple and practical method for controlling dimensional output of the total machining-plus-plating process.

Common Pitfalls

When a customer requires a certain Cpk value for the final product, most shops begin by machining parts to final product spec - hoping to hit the required Cpk, and then praying that plating will not be much of a factor. More often than not, however, plating results in significant part growth and increased dimensional variation. As a result, even if parts were in spec after machining, the final plated product is rejected by the customer.

For machine shops, typical dimensional tolerances range from 0.020 to 0.005 inch for multispindle machining and 0.006 to 0.0002 inch for CNC lathes and machining centers. For the most popular cadmium and nickel-chromium plating, dimensional buildup can amount to 10 to 30 percent of a blueprint tolerance spread (Figure 1), A buildup of such magnitude cannot be ignored and must be incorporated into the manufacturing strategy.

When a shop discovers that previously good parts went out of spec after plating, it blames the plater first. However, arguing with platers is not much help - they cannot control plating thickness with any great precision. Switching platers usually produces the same results. The shop ends up having to adjust to plating and not the other way around.

The next step is an attempt at adjusting engineering specs for the machining operation. This looks like a good idea, but the question remains of how to determine what the new spec should be. Since parts grow during plating, shops first try bringing down the upper spec to leave room for plating. But that moves down the nominal and results in moving the final size average close to the lower spec, and, again, kills the final Cpk. One might also try squeezing both upper and lower spec limits towards the nominal by a couple thousandths of an inch. That may look like a winner until the process is shut down by the shop's SPC system.

Selected specifications for plating thickness.

Basis                Minimum Thickness (mil.)
              Cadmium         Nickel        Chromium

Steel        0.15-0.50       0.20-1.0          0.01
Brass        0.30-0.50       0.10-0.50         0.01

What's playing tricks on us here is the intricate way in which average sizes and variations of consecutive processes interact with each other to produce the final Cpk. As we will see shortly, to correctly compensate for the effects of plating, we must adjust both upper and lower spec limits and introduce a new Cpk target for the machining operation.

Compensation Strategy

The problem we are dealing with is control of dimensional output of a two-step process - machining followed by plating. After machining, size measurements can be characterized by their average and spread (sigma). Plating buildup can be described in terms of average thickness and thickness variation. The Cpk of the final product is the result of combined characteristics of the two processes [ILLUSTRATION FOR FIGURE 2 OMITTED]. But the big question is this: If we cannot control variation in plating buildup, and the customer wants a certain Cpk for the final product, then what can we do during machining to hit that final Cpk?

Before attempting to compensate for plating, we should first do a quick check to see if plating buildup is a really a problem. That's done by looking at the ratio of an average plating buildup to the tolerance window for a dimension in question. The rule of thumb is that if this ratio is less than 0.1, the effect of plating can be considered negligible for all practical purposes, and the shop should concentrate its efforts on machining. If the ratio is higher, then plating is a problem, and the shop needs to come up with a compensation strategy to manage the effect of plating on final product Cpk.

The next step is data collection. Through capability studies we can obtain dimension-specific values of average size after machining, X(m); after-machining variation, [Sigma](m); average finished size, X(f); and variation of finished product, [Sigma](f).