Canadian Plastics

Making D-Bits the easy way

By Mark Benoit   

One of the most ingenious, yet hardly recognized inventions, is the Single Lip Cutter Grinder S0, better known as a Deckel grinder.What is a Deckel Grinder? The deckel, duplicator, and pantograph were...

One of the most ingenious, yet hardly recognized inventions, is the Single Lip Cutter Grinder S0, better known as a Deckel grinder.

What is a Deckel Grinder? The deckel, duplicator, and pantograph were once the workhorses of the machine shop floor. This equipment is used for diesinking cavity, roughing tools down, engraving, enlarging, reducing, finishing, prototyping, development, and repair of tooling that had been welded for rework using a worn out workpiece, model, or template.

The cutters needed for this equipment were by and large special handmade D-bits from carbide or high-speed steel. A skilled tradesperson required an excellent working knowledge of cutter geometry and experience honed over the years on the Single Lip Cutter Grinder to generate the vast array of D-bits required. The tradesperson who possessed this knowledge had a skill that allowed him or her the flexibility to take on challenging tasks and build complex cavities. As CNC usage grew, use of the Deckel, pantograph, and automatic duplicator began to fade, however the ability to generate a variety of handmade D-bits is still relevant in today’s toolroom.

Most of the early CNC programs incorporated the use of single lip D-bits with flat, bull nose, ball nose, and tapered profiles. While there is still a large number of shop using these D-bits, there is a trend away from this technique because of the capability of CAD/CAM and standard off-the-shelf tooling to cut the complex profiles required.


Most tradespersons have a good understanding of how to make D-bits with radius, tapered radius, or flat cutters, but are less knowledgeable about how to make a compound four- or six-sided D-bits. This type of D-bit is useful because it has more cutting edges and generally lasts longer in the machine.

How do you calculate the correct machining angles when making compound four-sided draft cutters? Making four-sided drafted D-bits requires some mathematics to calculate the correct machining angle for the Deckel grinder. Once this angle is set on the grinder it will produce the angle you require on the edge of the cutting tool.

To help illustrate the concept, we will work through an example. I do not have enough space in this article to explain the operation of the Single Lip Cutter Grinder S0. You will need to partner with a journeyperson if you need training on the operation of the grinder. It is a small, complex grinder with tremendous adaptability.

Problem: You have to make a cutter for engraving a logo into a plastic injection mold. Because of the letter spacing a cutter with a 0.020 inch diameter with 10 degree draft is needed. On a four-sided cutter the 10 degrees is the result of two angular surfaces intersecting. The result of these two intersecting surfaces will create our 10-degree draft along the cutting edge. So the real question is what angle do we need on the four-sided cutter to create the 10 degree angle along the cutting edge of the tool?

The equation needed for working with compound angles is the following:

cot C = cot2 A + cot2 A

Angle “A” is the finished angle we want on the four (4) sided draft cutter . Angle “C” is the angle we must set the compound indexing slide on the grinder to create the intersecting angle of 10 degrees.


cot C = cot2 10 + cot2 10

cot C = (5.67218)2 + (5.67218)2

cot C = 64.32687

cot C = 8.02040

C = cot-1 8.0204

C = 7.10708 or 7 degrees, 6 minutes

Once the correct angle has been established (which in this example is 7 degrees, 6 minutes) set up the cutter grinder and grind down the cutter taking equal amounts off all four faces. This keeps the cutter on center. Measure the smallest diameter on a shadowgraph and continue removing material until the bottom of the cutter measures 0.020 inch. The resulting intersection of these surfaces, which is the cutting edge of the tool, will measure 10 degrees.

Mark is the chair of manufacturing and transportation at St. Clair College in Windsor, Ont.

One mold cooling technology that can cut plastic injection molding cooling time by as much as 30 percent is being introduced to North America through an alliance of Bayer Corporation and Innova Engineering GmbH. Called Contura, this technology features mold cooling channels that follow the part shape to allow more uniform and faster heat transfer. In effect, Contura turns a standard mold into a heat exchanger.

Contura is a patented technology that is available through license. The license fee depends on the clamp force of the machine that will run the mold. Innova and Bayer will design the mold cooling channels and the shop building the mold will make it according to Innova/Bayer specifications.

The moldmaker will make slices of the mold core and machine in the cooling channels. These slices will be shipped to Innova’s bonding shop in Pittsburg to be vacuum brazed together, then returned to the moldmaker for finishing. Turnaround time for shipping and bonding is expected to be about a week.

Contura molds will be, as a rule of thumb, about 10 to 20 percent more expensive than conventional tools. The technology is applicable to virtually any type of mold, from small parts to large door and instrument panels.

Innova is an engineering firm based in Menden, Germany, that started out in vacuum brazing for aircraft parts. In the early 1990s, Innova applied this technology to the manufacture of injection molds. There are currently more that 3,000 Contura tools in operation, mainly in Europe. About 80% of them are for automotive parts, but the technology also has relevance to a wide spectrum of markets such as appliance, telecommunications, consumer electronics, medical and power tools.

Bayer’s contribution to the alliance is the company’s expertise in engineering thermoplastics and capabilities in thermal imaging, mold and part design and performance testing.

Customers can visit Bayer’s Technical Center in Pittsburgh to see a Contura technology demonstration and evaluate how the technology can be applied to their tools and operations.

D-M-E Company is marketing a rapid-cooling process first developed by researchers at the Massachusetts Institute of Technology and now owned by Extrude Hone, a company based in Irwin, PA. The process, called MoldFusion 3D Metal Printing, uses special machines at Extrude Hone’s facility to build powdered-metal mold inserts. The inserts include special cooling lines that quickly remove heat during molding. Extrude Hone says the inserts can reduce cycle time by 30 to 50 percent. The insert technology costs approximately $250 to $300 per mold cubic inch, according to a company spokesman.

Precision Optical Manufacturing, based in Plymouth, MI, is offering an advanced mold cooling technology, called conformal cooling. The process uses direct metal deposition to fabricate conformal cooling channels and imbed high conductivity heat sinks integral to the die cavity. The company says the technology can reduce cycle times by 50 percent or more.


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