A clear view on vision panels

The cutting zone in a machine tool is dangerous. Hot chips and large quantities of coolant are often safety threats, but a tool rotating at a high speed is an even greater one. In addition, the workpiece in a turning center often has a large mass. Tool breakage, clamp failures, crashes and other accidents may generate high-energy projectiles, requiring enclosures to protect shop personnel.

When turning, clamps and workpieces are the main concern, so enclosure designers target masses of at least 2.5 kg. When milling, pieces of the cutting tool may be ejected, so projectiles with a mass below 100 grams pose a concern. In both cases, the projectile energy can be quite high.

The peripheral velocity of a rotating tool is given by r × ω, where r is the tool radius, and ω is the rotational speed in radians per second. For a 100mm-dia. milling tool rotating at 15,000 rpm, the peripheral velocity (same as the cutting speed) is:

That is about a quarter to half the muzzle velocity of a handgun. The energy contained in a projectile increases with the square of the velocity, E = ½ × m × v2. The energy contained in a 100-gram projectile traveling at 105 m/sec. is:

When turning a 300mm-dia. workpiece at a spindle speed of 1,500 rpm, the peripheral velocity is about 25 m/sec., and the energy contained in a 2.5-kg projectile is about 780 Nm. Needless to say, it is important to keep the guard doors closed when a workpiece is rotating.

Yet at the same time, the operator often needs to see the machining operation. Most machine tool enclosures incorporate transparent, impact-resistant panels for vision purposes. A material like polycarbonate, which is used in safety glasses, would seem like an effective choice, given that a single transparent polycarbonate sheet only has to be about 2.7 times thicker than a steel sheet to provide the same impact resistance.

However, a study by the German Machine Tool Builders’ Association (VDW) found that polycarbonate panels can become brittle. Polycarbonate material is somewhat porous and becomes more so when the surface is scratched. Coolant and cleaning fluids attack and penetrate the polycarbonate material, causing it to become brittle. The panels sometimes become difficult to see through as well, leading the operator to open the enclosure.