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Microchip Deflection Tool
Every machinist must be aware of tool deflection, as too much deflection can lead to catastrophic failure in the tool or workpiece. Deflection is the displacement of an object under a load causing curvature and/or fracture.
Another way to minimize deflection is having a full grasp on the differences between a long flute and a long reach tool. The reason for such a difference in rigidity between the two is the core diameter of the tool. The more material, the more rigid the tool; the shorter the length of flute, the more rigid the tool and the longer the tool life. While each tooling option has its benefits and necessary uses, using the right option for an operation is important.
The below charts illustrate the relationship between force on the tip and length of flute showing how much the tool will deflect if only the tip is engaged while cutting. One of the key ways to get the longest life out of your tool is by increasing rigidity by selecting the smallest reach and length of cut on the largest diameter tool.
Reached tools are typically used to remove material where there is a gap that the shank would not fit in, but a noncutting extension of the cutter diameter would. This length of reach behind the cutting edge is also slightly reduced from the cutter diameter to prevent heeling (rubbing of noncutting surface against the part). Reached tools are one of the best tools to add to a tool crib because of their versatility and tool life.
Long Flute tools have longer lengths of cut and are typically used for either maintaining a seamless wall on the side of a part, or within a slot for finishing applications. The core diameter is the same size throughout the cutting length, leading to more potential for deflection within a part. This possibly can lead to a tapered edge if too little of the cutting edge is engaged with a high feed rate. When cutting in deep slots, these tools are very effective. When using HEM, they are also very beneficial due to their chip evacuation capabilities that reached tools do not have.
Diameter is an important factor when calculating deflection. Machinists oftentimes use the cutter diameter in the calculation of long flute tools, when in actuality the core diameter (shown below) is the necessary dimension. This is because the fluted portion of a tool has an absence of material in the flute valleys. For a reached tool, the core diameter would be used in the calculation until its reached portion, at which point it transitions to the neck diameter. When changing these values, it can lower deflection to a point where it is not noticeable for the reached tool but could affect critical dimensions in a long flute tool.
Tool deflection can cause damage to your tool and scrap your part if not properly accounted for prior to beginning a job. Be sure to minimize the distance from the tool holder to the tip of the tool to keep deflection to a minimum. For more information on ways to reduce tool deflection in your machining, view Diving into Depth of Cut.
I like that this post shared with us it is important to have our tools inspected by professionals in the event we encounter a defect. It makes sense to have it properly inspected because they are knowledgeable in the area. The other day my brother mentioned his power drill is faulty after he ended a live tooling demonstration, so I will ask him to have it checked.
Chip control issues often lead to other problems such as shortened tool life, conveyor stoppages and poor surface finishes, while also creating safety hazards. These concerns cost shops countless hours of production time. But certain strategies can be implemented to help shops recognize potential pitfalls, allowing them to take control of their chips, resulting in longer tool life and better productivity.
While chips may sometimes be viewed in a negative light, they do bring advantages to the cutting operation when properly handled. In almost every metalcutting process, excess heat is generated. The excess heat only has a few paths of escape: the environment, the workpiece, the cutting tool and the chip. For steels using the optimum cutting speed, dry machining will result in about 75 percent of the heat leaving with the chip, 10 percent in the material and 15 percent transmitted to the cutting edge. Heat-resistant alloys will realize less benefit, but even a modest 25 percent leaving with the chips will help with tool life.
The appearance of the chips can also provide a second line of sight (of sorts) for the operator when the windows to the workzone are obscured by coolant, scratches or tool turrets. If chip control is good at the start of a run, but then the chip bin begins to fill up with long, stringy chips, the cutting edge could be damaged. Other changes during the operation may indicate built-up edge (BUE) of the top rake. The machinist would then know to increase the speed or select a coating with better lubricity or less chemical affinity.
As with chip shape, there is no correct size of the chip, but small is almost always better. The chip should fit correctly into the tool flutes, slot, groove or bore. This consideration is especially critical when dealing with small part boring. Another argument for small chips is their influence on floor space: Can work cells be placed closer together with smaller chip bins? Could more space be available for additional production equipment? How much time is dedicated to removal and recycling?
Long, stringy chips are undesirable, so the goal in any tool design is to break the chip up by changing its path/curl. While in broad terms there are three options for chip control, the most effective way is typically by incorporating a chipbreaker. Modestly priced and offering high returns, chipbreakers can either be built into the cutting face of the tool or brazed/clamped onto the cutting tool. In some cases, the holder of the cutting tool can be used as a chipbreaker.
Clearance angles and rake angles are important to chip formation and control. Clearance angles are used to decrease the amount of rubbing of the tool against the workpiece and are always positive or zero, never negative. Rake angles control the sharpness or bluntness of the tools and can also be used as part of a chipbreaker, as in the case of a positive rake face.
High-pressure coolant can serve as a highly effective chip control solution. It has an intermediate cost because of additional equipment purchase and maintenance effort, but it provides a lot of benefits with improved tool life and stability.
Finally, process and programming changes can be applied, usually at a moderate cost because of the involvement of multiple aspects of components processing. These strategies also bring benefits through improved tool life and stability.
Tool alignment is a good place to start. Center height is critical for chip control because if the cutting tool edge is not positioned correctly, the chipbreaker may not provide optimal results and excessive tool wear will become an issue. Above center causes friction and vibration, and below center causes vibration and increased shearing zone. Machine alignment should be inspected regularly and frequently because machine crashes and maintenance issues that lead to misalignment often go unreported.
Tooling with high stability is important, making use of a highly rigid setup. If the insert rolls when indexing, the cutting height, and thus the chip control, can be affected. There should be as much material as possible under the cutting edge. If the insert is pushing off at each revolution, chip control will be difficult. Modular tooling can reduce deflection by shortening the overall length from the tool turret to the cutting edge.
For almost all ISO turning inserts, depth of cut (DOC) is critical to chip control. As a common rule of thumb, the depth should be at least 2/3 of the radius size. For the best results, the depth should be 1 to 2 mm (0.039 inch to 0.079 inch) past the radius. One common misconception is that the larger the depth, the poorer the tool life. The fact is that when adjusting feed, cutting speed or DOC, it is DOC that has the least effect on tool life. For this reason, increasing the depth of cut should be the first action to take when poor chip control is present.
Feed is also very critical to correct chip control. If the feed is too low, the chip will form on the hone and primary rake and not make use of the chip control geometry. Low feed rates can also cause the workpiece material to build up on the cutting edge and change the chip behavior. On the other hand, if the feed is too high, the chip can become too compressed, which can result in high pressure and tool breakage.
When other options have failed, programming changes can still aid in chip control. For grooving, speed pecking can be incorporated. This process involves the deliberate division of passes into smaller plunges. The grooving tool is plunged into the workpiece material at about 0.015 inch, for example. At each plunge, the cutting tool is retracted half the feed rate and moves to the next depth. Pecking in rapid succession does not significantly increase the cycle time. 2ff7e9595c
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