For many job shops in manufacturing, mold and die work usually refers to making parts that are asymmetrical. Sheet metal components are of various shapes, and injection molded plastic parts range from telephones to computer cabinets.
However, there are, in the world of die and mold making, a great many parts that are round. Caps, seals, gaskets, to name just a few, are parts that require die and mold components that are round.
Much has been written about machining mold cavities and die sets for asymmetrical parts. In this article, the focus is hard turning as applied to making mold and die components to produce the numerous metal and plastic parts that are round.
To find out about this manufacturing niche, we visited Reliance Tool and Manufacturing (Elgin, Illinois). It's a 50-year-old job shop that has developed a specialty in hard turning and uses it to make mold components and die sets that produce round products.
We talked to Dale Schrieber, plant manager, to find out about how Reliance has benefited from hard turning in its production processes.
Progressive And Compound Dies
Die sets incorporate a number of components. These are often individual parts that are mounted to the top and bottom of the die plates. This design allows adjustments to be made on the individual components of the die set rather than on the whole unit.
In a progressive die, coils of sheet metal are fed between the upper and lower halves of the die set. Each time the press closes, a complete part is ejected at the finish end of the press.
A progressive die may have two, four or even 16 different stations to generate a complete stamping. After the press completes a stroke, the coil feeds from one die station to the next. Some die sets are designed to progressively stamp a flat shape and then fold the stamping into a 3D part. This is all done in one press on one progressive die.
Compound dies differ from progressives in that they create a complete stamping with a single stroke of the press. Compound dies will form, punch holes and shape the workpiece all in a single hit from the press.
Like progressive dies, compound dies are also made from components that are mounted on the upper and lower plates. It's these components that Reliance manufactures and assembles into complete sets. For most of their customers, these components are round.
Why Hard Turning?
About six years ago, Reliance began experimenting with hard turning. The idea was to augment the traditional grinding operations with a process that was more flexible and one that could put more parts through the shop.
Prior to implementing hard turning, Reliance manufactured its die sets and mold components in the traditional way. They would rough turn the part leaving grind stock, then heat treat and finish grind.
Like any job shop, pressures to deliver parts ever faster drive much of the hunt for alternative technology. The only stipulation Reliance imposed on looking at new ways to manufacture die sets and mold components was to give its customers parts that were as good or better than those that had been ground.
"At first our approach to hard turning was trial and error," says Jeff Staes, who developed the hard turning process at Reliance. "The advantages over grinding were immediately apparent for some of our applications."
Most of the work earmarked for hard turning has tolerance requirements of 0.0005 inch. But with radii as small as 0.005 inch and the need toblend tapers to radii, single point turning has distinct advantages over grinding. Tapers, both ID and OD, which are necessary for draft angles and part geometry, are also easily generated on a turning center.
"Trying to maintain a tiny radii or generate tapers with a grinding wheel requires frequent truingwhich slows the production process," says Mr. Schrieber. "Finish grinding the last couple of thousandths is also time-consuming because the wheel must be trued more frequently. The finer grit of the finish wheel loads faster than a roughing wheel that restricts metal removal rates."
What They've Learned
The key to successful hard turning starts with soft turning. Achieving consistent surface finishes and dimensional accuracies on hardened workpieces requires the right amount of stock left after rough turning prior to heat treating the blank.
Geometric features such as thin wall sections often need to have more stock left for hard turning because of the tendency to warp slightly in heat treat. The idea is to heavy-up the thin sections to have enough stock to allow for dimensional changes from heat treat.
"Our heat treater does a good job keeping warpage to a minimum," says Mr. Schrieber. "They use ceramic plates for thin cross-section workpieces along with other techniques to control dimensional change. We even rack our parts before we ship them for heat treating to control consistency."
Maintaining the same stock allowance for hard turning is important because of tool wear. Generally the hard turning process is two steps: one or more roughing passes followed by a finish pass. Removing the least amount of stock on the roughing pass saves production time but more important, it saves on the rough hard-turn tool. "Rough, hard-turn tool wear will affect surface finish and size on the final pass," says Mr. Staes.
The cutter of choice for Reliance is CBN (cubic boron nitride). For very short runs or interrupted cuts ceramics are used, but do not wear as well as CBN for larger production runs.
"CBN has great versatility," says Mr. Staes. Insert selection is a limiting factor however. "We design inserts and have them made especially for us by our tooling suppliers. We create other insert geometries by reworking existing inserts into shapes we need," Mr. Staes continues. "There isn't yet the variety of insert shapes and geometries for CBNbut it's getting better."
Tool Steel In The Cut
Much of the material used by Reliance is A-3 and D-2 tool steel hardened between 48 and 64 Rc. CBN insert cutters work well on these hard turning applications.
While CBN is the second hardest material, after diamond, inserts made with CBN can be fragile. Some turning processes need to be modified from what is considered conventional to allow for this fragility.
In general, a CBN insert is made from brazing or otherwise joining a pre-formed edge of CBN material to a carbide substrate. This joint tends to be the weak point for the cutter.
Die and mold components need to have high dimensional accuracy and excellent surface finish. To achieve these goals using hard turning, Mr. Staes has developed a few guidelines.
"The direction of cut into a shoulderon an OD or IDcan be a factor if close tolerance on the depth and wall finish are required," he says. "Rather than come into the face from the ID or OD, it is better to face down the wall first to remove excess material. Leave between 0.0005 and 0.0010 inch of stock for a finish pass.
"With the roughing pass complete and most of the stock removed, the finish pass can then be made into the wall. Removing the remaining 0.0010 inch or less of metal puts lower tool pressure on the insert, particularly if a small, lead-angle tool geometry is used," says Mr. Staes.
On boring operations, the center height of the insert is a critical factor for successfully holding size and finish. If the bar flexes, it could drop the cutting edge below the center line, resulting in shorter tool life and poor finish. Mr. Staes recommends solid carbide boring bars for hard turning. "They will reduce the flexing problem and give greater depth capacity," he says.
Temperature And Workholding Considerations
By the time a workpiece is hard turned, its value has increased significantly. Accumulated costs include material, rough machining time, and heat treat. This is definitely no time to scrap out such a part.
When Mr. Staes turns these pieces, he likes to keep an eye on the tools, especially when using CBN. Some applications can be cut dry, but others are adversely affected by elevated temperatures from the machining process. To control this, while still allowing clear view of the work zone, Reliance uses shop air or a cold air gun to maintain part temperature.
Workholding is another consideration for making round die and mold components. Roundness is a critical dimension for many of these parts, which are frequently very thin walled.
"We use a six-inch manual three-jaw chuck held by the hydraulic chuck that is mounted on the turning center," says Mr. Staes. "The soft jaws on the manual chuck are bored on the machine within 0.0005-inch concentricity."
Much of the die and mold component work that Reliance does is relatively easy to inspect for dimensional accuracy and surface finish. They have all the necessary inspection equipment, documentation and traceability techniques in place to qualify for ANSI/ASQC Q9001-1994 and ISO 9000.
But some of the workpieces produced are not that easy to measure. Reliance has developed an interesting technique to verify hidden features in IDs and other hard-to-reach places on a workpiece.
"For ID and OD forms, we used to periodically cut a piece in half to check the features," says Mr. Schrieber. "Now we use dental material [a high viscosity vinyl polysilicone] to take an impression of the feature. That material is then checked on an optical comparator. It's a technique that's worked well for us and is very accurate when done correctly."
Reliance has built an impressive reputation for manufacturing round mold and die components. "Many of our best customers are other tool shops that need our expertise in producing round parts," says Mr. Schrieber. "Especially if an application that is difficult to grind comes along. We get a lot of business for hard turning."
Call it partnering or specializing, shops that might have once considered each other competitors work together to deliver good parts to customers. It's a trend in contract shop operation that reflects the realities of today's manufacturing environment.
Shops like Reliance provide specialized services to other job shops that need those specialties. Likewise, when Reliance comes up against a process problem beyond its capability, it will solicit help from other shops.
Standardizing process documentation using ISO 9000 techniques and others helps shops do business together. It's readily apparent by reviewing process documentation if a shop can do what it says it
Reliance, like most every other job shop, is under extreme pressure from its customers to reduce lead times. This pressure is generated by shortened product life cycles and continuous improvement programs within these companies.
In addition to making die set and mold components for other businesses, Reliance has a nice in-house fabrication business. They make numerous stampings of their own.
"Hard turning has helped us reduce deliveries to our customers from an average of 16 to 18 weeks to six," says Mr. Schrieber. "It has also gotten us business from other die and mold shops that don't have the turning expertise but see the advantages of hard turning over grinding for some applications.
"Our own stamping operation has become, in effect, a beta-test site for new processes and techniques. We try them ourselves before trying them on a customer's application," Mr. Schrieber adds.
For Reliance, hard turning appears to be working. Sales are up 44 percent, the customer base is expanding, and growth is projected over the next few years between 15 and 20 percent.