Electrode fabrication using a radius cutting technique on conventional equipment

Today CNC, five-axis machining, laser digitizing and machining, high-speed milling and sophisticated CAD/CAM systems have taken over the moldmaking environment. All this technology is great. It enhances flexibility, innovation, adaptability and ingenuity. Without it, we could not compete globally in the manufacturing industry. However, it is important not to lose sight of some fundamental principles that help lay a solid foundation to build on. While I could comment on several of these principles, I would like to focus on one very specific application: radius cutting on conventional equipment.

Why radius cutting and what application does it have in electrode fabrication? For the past ten years I have enjoyed training precision metal cutting apprentices across at basic, intermediate, and advanced levels. When working with the basic-level students I would introduce them to a technique that always had a profound effect on them. Once they were able to grasp the concept it usually resulted in a ” Ah- ha” moment and left them exploring applications on the lathe, grinders and milling machines.

Creating a radius on conventional equipment with standard ball nose tooling is underlaid by some basic math principles. Understanding the math behind the generation of a 0.750 radius on the corner of an electrode also helps to understand how CNC controllers operate. In order to help the reader better understand the principle, I will provide an example and a template chart that will illustrate the concept of radius cutting.

Example:

If you required a standard radius on a corner of an electrode you could just use a 1/2 in., 3/8 in., 1/4 in., etc. radius cutter. But when fabricating electrodes you must consider two other factors: shrink of the part (normal for steel cutting) and overburn (unique to electrodes). These factors leave the electrode fabricator with a radius of an odd size requirement. For example, we need a 1/2 in. radius on the corner of a finished plastic part with a shrink factor of 0.010 per inch.

1) We add in shrink0.500 x 1.010 = 0.505 R

2) We factor in overburn of 0.012 in. per side

0.505 – 0.012 = 0.493 R

Given this, we need to have a 0.493 in. radius on the electrode.

How do we cut this radius on the electrode using conventional equipment?

The diagram at right shows a practical formula that can be applied to any radius using any size of cutter.

Formula:

Rc + R = R1

Rc = radius of cutter

R = radius needed on

the part

R1 = value used for the

hypotenuse

Template chart for creating a 0.493 in. radius

Example: using a 3/8 in. ballnose carbide end mill to create a 0.493 in. radius on the corner of an electrode

Rc = 0.375 / 2 = 0.1875 in.

R = 0.493 in.

R1 = 0.6805 in.

A = you determine the

angle of increment

(I used every 3 degrees as a good starting point)

Determine the angle of increment, then use the formula in the box below to calculate the math values for each tool path. The Total (Down) value represents cummulative change from the starting point of 0.

There are many practical application of this technique. It can be applied to the lathe, milling machine, grinders, CNC milling machine, OD/ID grinders, etc. When I was teaching apprentices both in the shop and during their formal in-school training I would have them do projects using this technique. It will take several practice tries for an apprentice to understand the principles and see opportunities for their application.

Understanding this math can lay a foundation for future skills growth.

Table 1. Calculations for 0.493 in. radius (per the example at left)

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