Published in the February/March 2018 issue of Performance Plastics.
This is the second article in a four-part series on how to avoid bad cut edges when using CNC routers. The focus of the first article was on machine fundamentals, if you missed out, take a look. This article addresses poor quality cut edges issues related to vacuum decks and machine hold down.
A crucial component for achieving a proper, clean finish to a cut is holding the material steady and flat during the cutting process. If the material is not held in place properly, there may be inaccurate cuts, which means the part will not be the correct size or shape. A vacuum pump or a blower is typically used to power the vacuum deck. Each of these has its own strengths and weaknesses; in general, when cutting non-porous materials, the pump is a good general purpose solution. If you are processing mainly larger parts or porous materials, then a blower may be more appropriate. If in doubt, always check with your machine supplier who will be able to provide specific advice for your type of work.
Holding steady
Here are a few suggestions to minimize the movement of parts during the cutting process.
- Engage in good housekeeping. Before cutting any part, ensure the router deck is free of chips and dust which may prevent the part from lying flat on the sacrificial bed. If the part is not flat, the vacuum hold is reduced.
- Use a sacrificial bed/spoilboard. A sacrificial bed, also known as a spoilboard, is a disposable surface placed on top of the router to protect it from damage. Typically, medium density fiberboard (MDF) is used. Because it is a porous material, it allows air to flow through it to the pump or blower.
- Examine the sacrificial bed. Visually inspect the sacrificial bed to determine if there is an overabundance of cutter marks which can cause vacuum bleed. If so, it may be time to either replace the bed (which should be done approximately every six months) or skim the bed’s surface.
- Regularly skim the sacrificial bed. A clean, smooth surface is vital for effective material hold down. To maintain this surface, it is necessary to regularly skim the bed using a skimming tool; much like a Zamboni skims an ice rink. A general rule of thumb is to skim the bed at the beginning of each week.
- Check for plumbing leaks. If everything else appears okay (that is, there is a clean bed with no obstacles) and there is still vacuum loss, check the pipework connecting the pump or blower to the router. Leaks will reduce the effectiveness of the vacuum hold.
Some CNC routers include aluminum vacuum decks and an auto zone management option, which allow zoned areas of the deck to be opened or closed to vacuum hold down as needed. The zones are opened or closed either manually or this can be fully automatic with zones being activated by the machine controller, depending on where the cutting tool is operating.
This is an efficient method of distributing vacuum hold down throughout the router and increasing the hold in the area where it is most needed. By ensuring parts are held steady throughout the cutting process using a vacuum pump or a blower, operators will avoid inferior cut edges on their products.
Part three of this series will focus on how the cutter selection affects cut quality.