As we have evolved over the centuries, so has the way we use and develop tools. Manufacturing industries now operate using advanced tools and equipment to accomplish more sophisticated work; many of which require autonomous precision. One specific type of equipment, the CNC (Computer Numerical Control) machine, has had quite an evolution since its inception. Today, CNC Machines including, CNC Mills, CNC Lathes, CNC Lasers, CNC Routers, 3D Printers, and more can be found throughout industry. This article discusses the methods of vacuum hold-down in CNC applications, and filtration solutions to protect the vacuum equipment used in vacuum hold-down processes.
Machining of materials such as metal, wood, plastic, and carbon fiber, allow production of precision parts for various industries. Starting in the 1940s, the advent of NC (numerical control) machines began. This servo-controlled piece of machining equipment is programmed to perform a series of machining functions to produce a part. In the early days, this was done using punch cards. Punch cards are rigid paper cards that use a series of holes punched in them to inform or “program” movements of the machine. As the computer age flourished, so did the programming, coding, and control of this machining equipment.
A CNC punch card (or "punched card"). Holes are punched in the card as a way of defining computing instructions or programs.
Fast forward to the present day, now CNC machines are commonly used in production facilities. Along with advances in technology for directing machine activities, other methods have evolved such as the hold-down process for parts being machined. Early on, parts were mechanically clamped to a table while being machined and transformed into a finished part. A challenge with this method is the potential to damage material while physically clamping it in place. Even with the use of protective pads or materials, there is potential to scratch and damage parts as significant pressure is required to keep materials from moving during machining. Slippage and movement of the material being machined can result in high levels of rework or scrap rates and be costly to a manufacturer. Another challenge, particularly with custom part production, is that clamping devices can create obstructions for the machining process. As the tool moves around the table, it can run into a clamping device. This can cause costly damage to the tool or clamping device.
In most modern machining operations, some form of vacuum hold-down exists, which minimizes or eliminates the need for mechanical clamps. Imagine holding the shop vac in your garage to the open palm of your hand, the suction holds the nozzle tight to your hand. The same principle applies with the vacuum hold-down used in CNC machining.
The hold-down table consists of a flat perforated tabletop with a grid of small holes across the entire surface. Using a vacuum pump, vacuum is applied to the underside of the table creating air flow and suction at the holes. With a properly sized vacuum pump system, the suction is strong enough to hold flat materials firmly to the table. This enormous holding force allows a part to be cut or machined without moving. Once the vacuum is broken or removed, the parts can be removed. One disadvantage to this hold-down method is managing air flow through the perforations. If too many holes are open on the tabletop, the system will not reach the vacuum level required to hold materials in place during the machining process. Effective tabletop management requires adjustments before and during production to ensure adequate vacuum is available throughout the machining cycle.
CNC routing machine with large vacuum hold-down table inside a woodworking facility.
Another common vacuum holding method uses a vacuum pod. With this method, the pod is a specific shape to match the profile of the part being held. A gasket material is used around the perimeter of the pod to ensure the machined part is held firmly to the vacuum pod. This method is much more targeted and focuses the vacuum pressure to the specific part or series of parts. The pod usually requires a much lower flow from a smaller vacuum pump as the surface area is much smaller and the area requiring vacuum is very specific.
All vacuum hold-down methods present challenges to the most critical component in this machining process: the vacuum pump. Without an operating vacuum pump, there is no hold-down and no production. With a vacuum pump operating at reduced capacity, operators may experience high scrap rates and frustration with a mid-cycle shutdown. Both scenarios are unwelcome news for operators.
As the materials are cut or machined, particles such as dust or shavings are created and will migrate towards the vacuum pump through any unrestricted openings in the perforated table or pod. Most facilities utilizing vacuum hold-down realize the importance of protecting and maintaining the vacuum pump. Failure of the vacuum pump can shut down an entire production line and result in expensive repairs.
Right to Left: Heavy duty precleaner filter removing large CNC woodworking particulate before a straight-through fine particulate polyester vacuum filter.
How does one protect their vacuum pump and keep production up and running? To answer this question, it is important to understand the process variables. First, what material is being cut or machined? Second, what type of vacuum pump is being used (technology, flow capacity, required vacuum level, etc.)? Third, are there any other additives such as cutting fluids, in the machining process? After all the previous questions are answered, a custom vacuum system with the right filtration solution can be provided.
CNC routing with vacuum hold-down is commonly used in the wood working industry. Just about every piece of furniture has panels or frames cut by a CNC machine from wood or particle-board material. CNC machining and vacuum hold-down is also used for machining other materials such as alloys, foam, carbon fiber, resin, and many others. One common denominator across all materials is that the process debris created during the machining process will get pulled into the vacuum stream. Depending on the material being machined and/or cut, there is a difference in the consistency of the debris. For example, wood creates larger dust particles, while carbon fiber creates much finer particulate. When cutting metal alloys, a cutting fluid is commonly used to aid in the process and can be drawn into the vacuum pump. Given the complexity and multitude of materials used with CNC routing and machining equipment, a vacuum system and filtration solution that meets specific process requirements will help maximize process uptime and operational efficiency.
Contamination in the form of dust and powder particulates is common with wood routing and resin routing applications. When selecting the right filtration for the vacuum pump, it is necessary to understand the size and the volume of particulate carrying over from the CNC process. Depending on the process variables and characteristics of the particulates, a single inline vacuum pump filter may be the correct solution for a light dust load. For heavier dust loads, a sophisticated multi-stage filtration system with specialty filtration media may be required. Wood routing applications typically generate large amounts of dust particulate that carry into the vacuum stream. In this case, a two-stage filtration solution is encouraged. The first stage will remove larger particles and extend the operating life of the primary second stage filter. For example, a Solberg Spinmeister filter can be utilized to spin out the bulk of the material through centrifugal force. The primary filter would be a traditional pleated 5 micron or 1-micron polyester filter element to capture the finer dust that may carry over past the first stage.
Right to Left: A Heavy-Duty Precleaner Filter capturing large particulate before an inline vacuum pump filter in a woodworking CNC application.
Materials that produce finer dust particulate such as carbon fiber require specialty filtration solutions. When machined, carbon fiber produces a fine dust that is similar in consistency to talcum powder. In high dust load applications, this fine dust can quickly clog standard polyester filter media. For this application, reverse pulse technology is an ideal solution to combat heavy loading of fine particulate. The reverse pulse system uses a specialized filter element constructed from a low tact filter media with excellent release characteristics. The filter element is built to withstand periodic pulses of compressed air that send the captured dust to a collection bucket. This self-cleaning filter technology allows for continuous air flow through the vacuum process and extends maintenance intervals. The reverse pulse configuration can be optimized to pulse as needed to keep the process running and clean.
When machining metal alloys, it is common to have cutting fluid in the process to cool the tooling. This vacuum hold-down application sees contaminants in both liquid and particulate forms. A multi-stage knockout system is generally the best approach to address the heavy liquid loads and keep the vacuum equipment safe. Most multistage filtration systems will use baffles, velocity changes and a final stage particulate filter to minimize contaminant migration towards the vacuum pump. When the liquid challenge rate is severe or constant draining is required, automatic drain systems can be added to remove liquid from the knockout tank without disrupting the vacuum process. Automation removes the need for an operator to watch liquid levels and manually drain the knockout tank. Automatic drain systems help minimize the potential for human error and are essential to protecting vacuum pumps in vacuum hold-down applications where liquids are present.
An Inline Vacuum Filter catching fine metal particulate (Middle Left) and a Liquid Separator catching cutting fluid (Top Right) before entering a CNC's vacuum pump.
Vacuum pumps play an important role in CNC production processes. Vacuum hold-down is a highly effective method for keeping parts secure if adequate vacuum is applied. With the high amounts of particulates and liquids given off during the process, protecting the vacuum pump from ingesting harmful contaminants is essential to keeping CNC production equipment running. All vacuum pump technologies are susceptible to diminished performance and failure if enough contaminants are ingested. The cost to rebuild or replace a vacuum pump is far greater than implementing and maintaining a filtration solution.
As outlined above, there are many standard and custom filtration options available to optimize any vacuum hold-down process, no matter how challenging. Choosing the right filtration and separation protection for a vacuum hold-down system can save significant time, money, and frustration for CNC production facility operators. For more information, contact Solberg with the details of your application.