Cautionary Tales XXXIII

Lean Manufacturing
in the Spring Industry

By Mark Hayes

Let me have men about me that are fat…Yond Cassius has a lean and hungry look” is what William Shakespeare wrote to voice Julius Caesar’s fears for his future due to the ambition of one of his team members. You can take these clever words many ways, but with a bit of a stretch it is possible to apply them to lean manufacturing.

Toyota successfully applies lean manufacturing and demands that its suppliers do likewise. Read a text on the subject, and you’ll soon be convinced that this is the way forward for the manufacturing industry. Toyota and others want their suppliers to be lean and hungry so there is no chance that their position might be threatened unless competitors set up leaner supply chains. However, the question is, “Can lean manufacturing work all the way down the supply chain to the spring manufacturer?”

In IST’s experience, lean manufacturing cannot be completely adopted by spring manufacturers because the quality of the raw material is not good enough. The quality aspect that is of concern here is the elastic/plastic response of the wire at the point of coiling.

National, international and commercial specifications for spring wire do not provide information about how the wire will respond during coiling. Until they do, springmakers will not be able to rely on the next coil responding exactly like the last coil. You will need to make some adjustments at the coiler to get the springs to the middle of the customer’s tolerance band.

Lean manufacturing is partly about minimizing costs in the supply chain – a laudable goal, but let us consider the customer who calls off x-thousand springs a week on a JIT (just in time) basis. Which will be cheaper for the springmaker?

1. Buying wire each week (from stockists), making the required quantity of springs and having zero stock of wire or springs.

or

2. Buying a larger lot of wire (from a mill), making the required quantity of springs each week and being confident that the wire will behave when you make these springs next week.

or

3. Buying a larger lot of wire, coiling it all in one go and supplying springs from your stock.

Option one is lean, option two a little tubby and option three is fat, but I leave readers to decide which is cheapest.

The technological requirement for lean manufacturing of springs is having material suppliers provide information about the elastic/plastic and surface friction properties of their product so that springback is accurately known and consistent. Only wire suppliers who have a wire coilability machine (such as IST’s Fracmat) are able to do this.

Since it is not likely that all wire suppliers will adopt this method of characterizing their wire, it is worth looking at what could be done to be more certain the coil of wire you purchase will respond the same as the last one when you thread it up on the coiler and set the CNC controls to the same values you used last week.

The first instinct of many springmakers when wire doesn’t respond exactly as they expected is to check the tensile strength. Disappointingly for them, they nearly always find it to be as shown on the material supply certificate. It is a curiosity that the only test value supplied is tensile strength, yet the one thing a springmaker is never going to do, hopefully, is break the wire.

The procedure IST recommends to springmakers when first-off samples aren’t accurately on the mean values required is to use a computer program that will tell you what to adjust to bring you back to the mean. An example of such a program is IST’s Compass module.

A typical printout from this program is shown (right) for an unground compression spring.

This shows the dimensions off the coiler, as well as after stress relief and prestressing under “Measured Characteristics.” The load test shows the two loads are in spec, but are well below the mean value.

Without this program, springmakers would adjust either the free length, outside diameter or the number of active coils using their experience to determine the adjustment.

IST’s Compass software module indicates what spring parameters to adjust to bring loads back to the mean.

However, the computer program says that if all these parameters were changed (increase the outside diameter, increase the free length and take almost 0.4 coils out) then the loads would be very close to the mean the second time around. This program calculates the required dimensions off the coiler, knowing the dimensional changes that will occur due to stress relief and prestressing. It helps to minimize setup time on the coiler, and hence the springmaker’s chance to become a little leaner.

This Cautionary Tale advises that full implementation of lean manufacturing is not realistic for springmakers, but there are technologies that will help you approach “leanness” (i.e. Fracmat and Compass), and this may be as near as you can get to being able to rely on the response of your next coil of wire.

However, referring back to my original quotation, you could interpret Caesar’s words another way: Beware the lean, hungry and ambitious industries of China and India; they will take your market if you don’t invest as far as you can in lean manufacturing.