Troubleshooting CNC Router Bits

The full productivity of CNC machines will not be realized unless proper tooling is used.

Plastic fabricators purchase CNC routers only after a lot of thought and a reasonably thorough study of what is available in the marketplace. With many options and configurations to choose from, the whole process can be exhausting and intimidating for the first-time buyer. Unfortunately, the process often stops as soon as the machine decision is made, with no consideration given to tooling. Once the CNC router is delivered, the tooling decisions become critical to meet performance expectations. It is far more expedient and productive to plan tooling requirements when the CNC router is ordered.

Putting non-CNC cutting tools on a CNC router is much like buying cut rate gas for a new Porsche. Sure it will run, but it won't run well.

CNC router bits are a must for good CNC router performance. There are a vast array of CNC router bits to choose from depending on the plastic material to be cut. Fine tuning the geometry of the solid carbide tool to find the best tool for the job is often a matter of examining results from various alternatives. Make several cuts with spiral and straight flute bits with 1, 2 or 3 flutes and see which one gives the best finish. Spindle speeds and feed rates, also discussed previously, can be varied to provide the best finish and productivity.

Problems may still occur, particularly when changing to a new set up or upon receipt of a new batch of material. Here are some troubleshooting suggestions that many have used with success.

Proper Collett Maintenance
Heat is the biggest enemy of the tool, and the first place the heat goes from the tool is into the collet. It is also important to note that collets are made of spring steel that will, over a period of time, lose its elasticity and harden, making it increasingly tougher to tighten adequately. As this hardening takes place, the steel does not fatigue evenly and often causes the collet to grip tighter on one side than the other, creating runout in the tool.

It is important to understand that if they are overrun enough, this over tightening will eventually damage the internal spindle taper, resulting in costly repairs. Because it takes place over a period of time it is very hard to notice, but a safe recommendation for collet life is in the range of 400-600 run-time hours. This is about three months in a two-shift operation of normal run-time averages, probably much more frequent than one might expect. This is not the absolute, but it stands to reason if you're running a $75 tool in the machine and you're only getting half the true tool life due to runout caused by bad collets, it doesn't take many tools to justify the added expense of replacing the collet. If collets are not changed, they will eventually become brittle enough to crack or break, potentially causing permanent spindle damage that could have been avoided. Just like changing the oil in your car, this is good preventative maintenance that should be done regularly!

Just as replacement is important, equally as important is cleaning the collets each and every time the tools are changed. Collets are in a brutally dirty environment and are expected to perform a very accurate task while undergoing extremes of heat and dirt.

As material is routed, whether it be plastic, wood, aluminum, or another type of man-made board, the chips carry with them many resins that migrate up the slits in the collet and deposit themselves onto the inside end of the collet ears, usually nearest the mouth of the collet (see illustration below). This minute vibration is often the cause for tool breakage when seen in the actual shank area of the tool instead of down by the cutting edge. The resinous deposit acts as pressure points, gripping the tool tighter at the mouth of the collet. These pressure points often distort the grip on the tool, creating runout. This resin heats up as does the tool, and actually ends up depositing itself onto the shank of the tool, often almost gluing the tool into the collet, leaving brown marks at the mouth of the collet contact on the shank.

These brown marks are a sure sign of collet neglect. To prevent this problem, the resin must be removed from all surfaces that it is prone to build up onto, using a non-abrasive brass tube brush for the inside of the collet and a mild solvent and rag for the external surfaces of the collet and inside spindle taper. It is important to point out that blowing out the collets does not get rid of the resin, nor does soaking them overnight in thinner. A brass brush is the best thing, along with some of the non-flammable cleaners available. Do not use a petroleum-based lubricant for cleaning as it will only act as a magnet for all the dirt and dust by the residue it leaves behind.

Proper Colletting of the Tools
Collets come in two basic types, and their attributes and peculiarities are important in the way in which they secure the tool in the machine. Think of the spindle/collet system as a chain. Just like a chain is only as strong as its weakest link, so too is the collet's relation to the tool. A high-performance tool can only perform if the collet is properly maintained each and every time the tool is changed.

There are a number of things that are important to the system. The first to be discussed are the two basic types of collets themselves. Both types are not always available, depending on the spindle type itself.

1) Half-grip collets (see below) are identified by their slits that run from the bottom or mouth to about 80 percent of the way to the top. This allows them to squeeze the tool with a force primarily directed at the mouth or bottom of the collet. These collets are often counterbored at the top so as not to require the tool to fill the entire length of the collet. This type of collet is the simplest of the two, and is ideal for use in cases where tools sometimes don't have a long-enough shank to fill the entire collet length.

2) Full grip collets (see above) are identified by their slits that run from both ends, almost cutting the collet in pieces. This type of collet tends to have more flexibility and often comes in what is termed as "range collets," which allow gripping a range of shank sizes. For example: 12-13mm is used for 1Ú2-inch shank tools. This full-grip type allows gripping over the entire length of the collet and requires that, in order to be properly used, it should be 75-80 percent full.

The most important portion of the collet is the mouth which is at the bottom. This area is important because all the lateral pressure taken by the tool must be evenly distributed on all ears of the collet for it to cut true or concentric. It is very critical that the 80 percent rule be followed when using a full-grip collet due to the ability of the collet to flare at the back if not full. The collet can then allow tool movement in very minute amounts, oftentimes resulting in tool breakage. There are times that this 80 percent is not possible due to shank lengths available, so it is necessary to fill this void in the back of the collet with a filler plug that is of the same size as the shank to avoid the collapsing problem.

Equally as important as filling the collet properly, it should also be understood that it is possible to overcollet as well. This is when the "flute fadeout" portion of the tool is allowed to extend up inside the collet. This does not allow a firm equal grip by all ears of the collet at the mouth. The tool can then have uneven support at the most critical area and, oftentimes with solid carbide or high-speed steel tools, the tool material is hard enough to actually scar the inside of the collet, causing permanent damage to the collet.

This can also be a common cause for tool breakage. Breakage often results in permanent damage to the collet due to intense pressures exerted, often either "burring" or "mushrooming" the mouth of the collet.

Proper Tool Colletting
The only way to solve problems and make them go away is to understand more fully what the real cause of the problems truly is. Following the above prescribed tips and information on collet maintenance and programming tips can have a dramatic effect on part quality and consistency in any plastic routing application.

The productivity promise of CNC routing can be realized when all parts of the process are taken into consideration. Proper tool selection, fixturing, spindle and collet maintenance are all major links in the CNC routing chain.

Van Niser is director of Plastic Application Engineering at Onsrud Cutter. Based in Libertyville, IL, Onsrud Cutter is a manufacturer of a wide range of cutting tools for the plastics industry. For more information, Niser can be reached at (847) 362-1560.

Plastics Machining & Fabricating
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