Doweling Steel Plates to Aluminum Die-Sets

Their main disadvantage is that the thermal expansion of aluminum is about twice that of steel, which can create a serious problem for precision high-speed tools. Another major difference is the modulus of each metal. The lower modulus of aluminum simply means that for the same physical dimensions the aluminum is easier to bend. But with dies used in high-speed presses, where the ram of the press completely covers the upper die shoe and the lower die shoe is fully on the bolster plate, this is of minor concern.

This leaves us with the thermal expansion issue. To examine the thermal expansion problem, let's consider a tool that has a plate with dowel pins and assume that the tool will be used in an environment where the temperature ranges from approximately room temperature up to body temperature. The distance between the dowel-pin holes will change depending on the temperature change and the coefficient of thermal expansion.

In this example, assume the holes are precisely located 10.0000 in. (254.000 mm) apart when the materials are at room temperature (70°F). Then at 100°F, the holes in steel would be 10.0019 in. (254.048 mm) apart, and in aluminum they would be 10.0037 in. (254.094 mm) apart: a difference of 0.0018 in. (0.046 mm). What is commonly done is to just ignore this difference and the stresses it causes, as well as the bending of the assembled plates and the dowel-pin hole elongation that result.

However, using a doweling/locating system that allows for differences in thermal expansion can eliminate these problems. Such a compensating doweling system can allow for the growth differences and still maintain a precise, known location of steel tooling doweled to an aluminum die set.

A blade-and-slot solution

One method for a compensating doweling system uses a conventional dowel pin at one location. That location will be the "stationary point," or the point where the steel and aluminum will have no relative movement associated with thermal expansion. The other dowel location will use "blade and slot" locators, instead of a conventional dowel pin (see Fig. 1 below).

---

This is one corner of a test die set of QC-7 aluminum and D2 steel plates. The blade and slot locators shown are used at all corners of the assembly.

---

The blade component precisely fits into the slot component allowing no movement perpendicular to the blade/slot centerline, while allowing movement in the direction parallel to blade and slot. Since any movement associated with thermal expansion will be toward or away from the stationary point, the blade and slot dowel must be aligned with the stationary dowel pin.

With this system the steel plate will be precisely located at all times, and the doweling system will allow for the distance between the dowel holes to expand at different rates due to the differences in the coefficient of thermal expansion. The doweling system will remain stress-free while the steel and aluminum expand at different rates, and the "stationary point" will remain stationary.

For the system to remain stress-free, the steel would have to be able to slide on the aluminum as the materials expand with temperature. The sliding will be the greatest at the point furthest away from the stationary point. If the size of the plate being doweled causes this to be a problem, the best solution would be to have the stationary dowel pin centrally located. This can be accomplished by using three or more blade and slot locators, ideally located uniformly in a circular pattern around the central point.

The concept presented here eliminates the problem of damaged dowel-pin holes and the shear stresses on dowel pins caused by the difference in thermal expansion between steel and aluminum. Although they are more costly than dowel pins alone, they eliminate the costs associated with damaged and elongated dowel-pin holes that often occurs when using aluminum die sets.

--Jim Martellotti

Jim Martellotti is president of Interplex Nascal Inc. (Tustin, CA; jmartellotti@interplexnascal.com). This article is a shortened version of his SME technical paper MF02-100.