One of the more difficult things to tackle for any vacuum system is holding small parts due to their minimal surface area. The Pierson SmartVac II is a versatile and easy-to-use system geared for all types of machining applications and holding small parts is no exception. In this video, SmartVac customer Jim Abrams takes us through the entire process of making multiple small parts using Pierson’s vacuum chucks:
First let’s look at some definitions that you’ll see when comparing pumps. The first is the term CFM which stands for Cubic Feet per Minute. This is a measurement of the speed of the vacuum flow. This can be equated to a car’s top speed. You’ll see this term most often on electric rotary vane pumps or piston pumps (reverse air compressors). Knowing a pump’s CFM is key to knowing how fast you can remove the air from a tank or refrigeration system.
The next key term is Inches of Mercury or abbreviated as “Hg. This is a measurement of the ultimate power of the vacuum. This can be compared to a car’s horsepower. You’ll see this term on high performance vacuum pumps connected to vacuum chucks where holding power is the most important factor regardless of how long it takes to reach the full vacuum level. Knowing a pump’s maximum Inches of Mercury will tell you how secure of a hold you can achieve.
The majority of all mid-range electric style vacuum pumps will list their CFM and omit their max “Hg. This is because they assume the pump is being purchased as a replacement or the user knows that a high vacuum rating is not an issue. Pumps for vacuum chucks that draw a high CFM are often used in the woodworking industry where porous wood and particle board creates a constant leak. High end pumps that are used in machining plastics and metals require higher holding forces. The maximum Inches of Mercury is key to these pumps.
Sure, a high maximum vacuum coupled with high CFM would be great, but expect to pay for it. Here is a basic price range breakdown of the vacuum pumps seen above:
In a machining environment a vise should always be the first choice in workholding due to the high mechanical clamping force. However, using a vise is not always an option for certain materials and shapes. Some soft or thin materials can be damaged or bowed by a vise. Using a vacuum chuck also allows machining of 5 out of 6 full sides of a part – vises allow limited exposure of 3 sides. If using a vise causes any of these problems, then a vacuum chuck is absolutely an ideal workholding device. The rule of thumb in evaluating if vacuum chuck workholding is right for you is best remembered by the phrase “Short and Fat”.
“Short and Fat” conveys two ideas. First, parts are best held when a large surface area (fat) is presented to the chuck surface. A high vacuum is defined as 26″ – 30″ Hg and produces a holding force of 13 – 15 PSI. For example, a 5″ x 5″ part being held by a vacuum chuck at 28″ Hg (or 14 PSI) has 350 lbs of downward holding force applied to it (5″ x 5″ = 25 sq in. 25 sq in x 14 PSI = 350 PSI). The more surface area a part has, the stronger the hold, the more aggressively material can be removed.
Vises have a theoretically infinite amount of pressure that can be applied to a workpiece whereas vacuum chucks rely on the atmospheric air pressure around us to press a workpiece downward against the chuck surface. Since this force has a limit of 14 pounds per square inch (14 PSI), any forces that might lift the part should be avoided. Rather, material removal should be focused on creating sideways forces, but sideways forces on tall parts can lead to unwanted lifting due to leverage. Short parts do not experience this unwanted leverage.
How tall is too tall? Plugging in a 25:1 ratio into a simple equation offers a good starting point. Divide a part’s surface area by 25 to find a safe part thickness (5″ x 5″ = 25 sq in. 25 sq in / 25 = 1″ tall part). Many other factors play into this equation such as material type, cutter style, feed rate, aggressiveness of cut, etc. so fine tuning to the exact application is encouraged.
“Short and Fat” reminds us of the ideal part shape, but trying to keep a part in place while cutting on it is a discussion for another tutorial.
Atmospheric air pressure exists around 30″ Hg or 15 PSI at sea level (Yes, every square inch of our bodies has 15 lbs of air constantly pressing on us). This is why passengers on planes flying at high altitudes can experience swelling due to lower pressures in the cabin. Cabins are typically pressurized to an equivalent of being at 8,000 ft above sea level. At this altitude there is roughly 11 PSI of air pressure. Translate this into workholding and users at higher altitudes will experience a significant drop in holding force equating to a loss of 0.5 PSI per 1,000 ft above sea level. A high powered vacuum pump operating in outer space would be completely ineffective.
The atmosphere likes to remain at a constant level and it is this principle that causes the wind to blow – air flows from high pressure to low pressure. When objects are tossed around by the wind, they are acting as obstacles to this high-pressure-to-low-pressure flow. When a vacuum chuck is turned on the air pressure decreases below the workpiece which causes higher air pressure above the workpiece to want to fill this low pressure space below it. The workpiece becomes an obstacle which is pressed against the chuck.
The next time someone tells you that a vacuum chuck sucks parts down ask them when was the last time they saw the wind suck over a tree!
A perfect vacuum is a region of space that contains no matter including air or gasses. In practice, there is no such thing as a perfect vacuum. Scientists may speak of vacuums as having no matter, but partial vacuums are the only things that truly exist. A vacuum’s quality refers to its proximity to the characteristics of a perfect vacuum. The closest thing to a perfect vacuum is in outer space. None of the modern technology available can match the vacuum of outer space. Vacuums are most ofter associated with their commercial uses. The most common is the household vacuum cleaner. Light bulbs and car brakes are a few other objects that utilize vacuum power.
When humans are in vacuo, the term for being exposed to a vacuum, they will lose consciousness after a few seconds and die in minutes. At different altitudes in the atmosphere, the pressure differs. The farther from the Earth, the less pressure there is and the surroundings are more like a vacuum. This is the reason humans must wear flight suits above 100,000 ft and space suits outside of the atmosphere. While short-term exposure to a vacuum has no effect, exposure longer than a few minutes will cause death. Also, if a person who has been in a low-pressure atmosphere comes into a high-pressure atmosphere too quickly, the effects can be more damaging than the effects from the vacuum. The alveoli of the lungs can be ruptured or the eardrums shattered.
The measurement of barometric pressure is “inches of mercury”. Its abbreviation is inHg or “Hg. This standard is used almost exclusively in the United States. Weather reports and aircraft use altimeters, which give the pressure in “Hg. The origin of the term “inches of mercury” comes from mercurial barometers, which work like thermometers except that air pressure is measured instead of temperature. The air pressure at sea level fluctuates around 30″ Hg and roughly decreases by 1″ Hg (0.5 PSI) per 1,000 ft of elevation.
In 1650, Otto invented a vacuum pump that was designed to pull air out of whatever vessel to which it was connected. It contained a piston and an air-gun cylinder with one-way flap valves. In many subsequent experiments, he used it to study the effects of vacuum. The most famous involves two copper hemispheres, closed tightly with the air pumped out of them. On May 8, 1654, in front of the Holy Roman Emperor, Guericke harnessed fifteen horses to each hemisphere and showed that they could not be pulled apart. In 1663, he repeated this experiment in front of the Duke of Prussia with twelve horses to each half. If the air were completely sucked out of the sphere, the resulting force would be around 4,500 lbs, enough to lift a small car. The surrounding air pressure would have shut the halves tightly and prevented them from being opened.
These experiments disproved the theory of “horror vacui”, which was the predominant theory for centuries. This theory, which was introduced by Aristotle, stated that “Nature abhors a vacuum” and therefore a vacuum cannot exist. The Roman Catholic Church backed this view and it was therefore rarely challenged. One other experiment Otto performed was to hang the sphere, after the air was removed, and attach weights to it. The hemispheres still did not separate.
The Magdeburg hemispheres are still used in classrooms today to demonstrate the air pressure. Gaspar Schott transcribed this experiment, and his book was read by Robert Boyle. He used those principles to improve the air pump and formulate the beginnings of the ideal gas law. Guericke also pioneered the use of the barometer, a device used for measuring atmospheric pressure, in predicting the weather. This was the forerunner of meteorology. His later research focused on electricity, but it is unfortunately lost to history. There is an Otto von Guericke Society that has repeated the famous experiment in many places around the world. Otto von Guericke died on May 11, 1686 in Hamburg, Germany. The university in Magdeburg is named after him.