Tightening the reins on dimensional tolerances in aluminum green sand castings.

Inside This Story

* The key characteristics to achieving tight dimensional tolerances in aluminum green sand castings are evaluated.

* A total system evaluation is needed, including the molding sand, tooling and equipment to achieve tight dimensional tolerances. Detailed within is what to look for in these critical areas to produce the necessary results.

Metalcasters have a tendency to be control freaks. It stands to reason--the tighter the process controls, the better the chances are of producing a high-quality cast component. Tightening the leash on parts of the metalcasting process is vital to a metalcaster's success. But the stakes are even higher when dealing with aluminum green sand castings that require tight dimensional tolerances.

The definition of what constitutes tight dimensional tolerances has obviously changed over time. Because of improvements in molding equipment and less variation throughout the entire process, tighter tolerances are now easier to achieve.

"It has changed remarkably in the last 10 years--even in the last five years," said Larry Smeltzer, Littlestown Foundry, Littlestown, Pa. "Conventionally, we looked at [+ or -] 0.030 under 6 in. But with the newer molding equipment we have today, we can get at least 50% of that."

With tighter tolerances becoming more common, the focus on process control becomes increasingly more important.

"The primary consideration when producing this type of casting is process control--from the initial design through coremaking, molding, pouring, and cleaning and finishing," said Geary Smith, Manufacturer's Brass & Aluminum Foundry Inc., Blue Island, Ill.

The difference between producing a typical aluminum casting and one that requires tight dimensional tolerances is not that great. Mainly, it entails doing what you are doing--only better.

"When you are talking about casting these types of components, you are not going to change your procedure drastically, you are just going to make sure you are doing it better and are in control at all times," said Matt Granlund, metaclasting facility consultant.

In the past, it was not uncommon for metalcasting facilities to overcompensate by adding in more machine stock. But recently, the philosophy has shifted toward eliminating the excess material that would just be machined out later. In doing so, metalcasters can save on material and machining costs.

Controlling the Sand

The building block of all aluminum green sand castings is the sand. If there are problems with the base sand, it will affect the quality of the resultant cast component.

"It's hard to hold dimensional tolerances if you have problems with your sand," said Scott Strobl, Simpson Technologies Corp., Aurora, Ill. "In green sand casting, one big thing we look at is eliminating mold shift. You have to have better control over the squeezes and movements of the molds when you close them."

That control over mold movements comes from regular preventative maintenance on the molding machine to make sure it is not out of specification. If it is not delivering repeatable pressure or the right squeeze pressures, it will affect the hardness of the mold.

As for the base material itself, olivine and silica are two of the most common choices used in green sand molding. Both are fine sands (olivine is 90-120 mesh and silica is 80-120 mesh) and will require extra attention at the mulling stage.

"Both olivine and silica give excellent casting dimensions, but they are two extremes in molding media and machines," said Vic LaFay, The Hill & Griffith Co., Cincinnati, Ohio. "With both of them, because the sand is so fine, you have to have good mulling. The finer the sand, the more mixing you will need."

Removing variation from the mulling process is essential to gaining control of the base material. By eliminating human error and automating the process, metalcasters can reduce variation and achieve tighter process controls.

"Controlling the properties of sand is very important, and more specifically controlling the variation of the green sand is important," Strobl said. "Automatic controls at the muller take the human operator out of it and reduce the variation. Also, one of the things we always stress is that it is a system approach. You need to have good mullers, molding machines and shakeout facilities. It is a combination of all those components."

To reduce the variations and eliminate problems, a system overview must be performed. If any one component is off kilter, the whole process will be off.

"Procedures must be in place, understood and followed to ensure the proper ratio of moisture-to-green compression strength and compactability of the molding sand," Smith said. "And the same goes for proper pouring temperature, alloy composition and grain refinement."

All of those properties are specific to each metalcasting facility, given the type of equipment being used and the requirements of the resultant casting. The only way to be sure there are no variations is through routine testing.

Controlling the Tooling

Equally as important as having a solid sand foundation is starting out with quality tooling.

"Tooling is the lion's share of how you are going to hold tight dimensional tolerances," said Mike Stahl, Olson Aluminum Castings, Rockford, Ill. "We will not cut corners on tooling for something like this, We will spell out what we want for a pattern material and the specific layout we want. Also, something metalcasting facilities often do is let the patternmaker decide how the runner system will be laid out. You should not leave something that important up to somebody else's best guess. Especially when holding tight dimensional tolerances is critical, metalcasting facilities need to take more control."

With tooling being so critical to success, CNC patterns become a more logical choice because the model will provide greater accuracy. Also, the material of the pattern comes into play. Softer materials may cut faster, but they tend to be more porous, are more susceptible to damage and do not hold dimensional tolerances as well. Stahl said metalcasting facilities, if possible, should look into harder materials, such its foundry board or even aluminum.

"If you can cut a pattern out of aluminum, that would be ideal," he said. "Aluminum is more robust and less susceptible to damage. If the tooling is for a high-volume job or it needs longevity, softer materials will show signs of wear that affect its ability to hold dimensional tolerances."

During a recent prototype project at Manufacturer's Brass & Aluminum, the firm realized the mounting bracket for a piece of electrical switchgear equipment (Fig. 1) required a stronger pattern. The tolerances were beyond standard matchplate tolerances, so a master pattern with an extra 1/8 in. stock overall was cast. The master pattern was then welded to a plate and finish machined on the CNC. The scrap rate on the component is well below 1%, and the only machining that is required is for threaded bolt holes and bushing bores.

[FIGURE 1 OMITTED]

"A CNC machined pattern and corebox equipment is mandatory to get tight tolerances," Smith said. "We use a high-pressure, automatic molding process and practice smooth and accurate mold handling and closing to hold the parting line tolerance to a minimum."

Smeltzer also puts a lot of stock into starting out with quality tooling.

"My first concern is to make sure the tooling is indicative of the tolerance you are trying to control," he said. "If you need 0.030, a matchplate would be OK. If you need 0.010, you want to go to a machined pattern, where you know that every pattern is the same so you are not using up the tolerance from pattern to pattern."

Littlestown also has the benefit of being in a position where it can cost-effectively use trim dies to help hold tight dimensional tolerances.

"The dies are machined so that each impression and each part, along with runners and gating system, are literally identical," Smeltzer said. "Really, this was unheard of in the past because of a lack of control in our sand systems, and pattern equipment from impression-to-impression did not lend itself to it. Also, the expense involved in trim dies is high, but every part you put in there is going to come out in that configuration."

Because of the consistency and accuracy of the trim die, Littlestown cut production costs. The firm studied a 1.5-lb. gear housing and learned it could drastically cut machining time and costs with the trim die.

"We studied one job that we have that is about 150,000 pieces per year, and each piece had to be machined," Smeltzer said. "We discovered that we were removing an average of only 0.07 in. during the machining operation. We don't need the machine stock that we used to think about. We would always put extra here and there to make up for the absence of control. The fact that we can control those things more closely today allows us to keep that excess metal and reduce machining."

That reduction in machining could prove to be important once the casting hits the cleaning and finishing room. Smith pointed out that special consideration also must be taken to produce castings with tight tolerances.

"The finishing room must have the training and instruction for those particular castings," he said. "Too many castings can turn into scrap in the grinding room because they weren't handled properly and too much was ground off. The process must be controlled tightly."

Back to Basics

Producing aluminum green sand castings that require tight dimensional tolerances is nothing new for the metalcasting industry. The practices are essentially the same that are required for casting other components. The difference is that more control is needed to achieve the desired results. So, why is there sometimes a problem?

"Metalcasting facilities sometimes tend to blow hot and cold," Smeltzer said. "We set up the parameters, and then we won't police them on a regular basis. Consequently, the sand system is up and down. The key in any process is to have a system that can be verified on a regular basis. It sounds pretty basic, but it is a matter of doing it."

Table 1. Dimensional Tolerances for Aluminum Green Sand Castings

                             A

Between two points in the same part of mold;
not affected b parting plane or core.

        Specified
      Dimension (in.)                     Tolerances (in.)

       Up through 6                        [+ or -] 0.030
       More than 6                   add [+ or -] 0.003 in./in.

                             B

Across parting plane. A-type dimension plus
the following chart:

      Projected Area                          Additional
        of Casting                          Tolerance for
[A.sub.1] x [A.sub.3] (in.)               Parting Plane (in.)

       Up Through 10                         [+ or -] 0.020
          10-49                              [+ or -] 0.035
          50-99                              [+ or -] 0.045
         100-249                             [+ or -] 0.060
         250-500                             [+ or -] 0.090

                             C

Affected by Core. A-type dimensions plus the
following chart:

      Projected Area of                       Additional
     Casting Affected by                       Tolerance
Core, [A.sub.3] x G (sq. in.)                for Core (in.)

      Up through 10                          [+ or -] 0.020
         10-49                               [+ or -] 0.035
         50-99                               [+ or -] 0.045
        100-249                              [+ or -] 0.060
       250-1,000                             [+ or -] 0.090

More than 1,000 Consult metalcasting facility
Minimum diameter of cored holes is
0.025 in.

                             D

Allowance for finish

         Maximum                                Nominal
     Dimensions (in.)                       Allowance (in.)

      Up through 6                           [+ or -] 0.060
          6-12                               [+ or -] 0.90
         13-18                               [+ or -] 0.120
         19-24                               [+ or -] 0.150
      More than 24                           Consult metal
                                           casting facility

For More Information

"Dimensional Variability of Aluminum Castings," W.C Faustine, R.C. Voight and F.E. Peters, AFS Transactions, 1999, paper 99-201.

"Aluminum Casting Technology 2nd Edition," AFS, 1993.