by Sekhar Rakurty, product development engineer, The M. K. Morse Co.
As the name suggests, a hole saw is a power tool accessory used to make annular cuts. To make the cuts, hole saws have a sawtooth on one end of a cylindrical, cup-shaped body and are generally made out of a bandsaw blade to cut a variety of materials. They’re also distinguishable based on what tooth edge material is used, including carbon, bimetal (featuring a high-speed steel cutting edge with low-alloy steel as the backer), carbide-tipped (tungsten carbide), diamond grit and carbide grit.
AMPLE APPLICATIONS
Combined with an innovative side slot shape, the M.K. Morse tooth design offers easy slug removal.
Determining the right tooth edge material for the application is half the battle when it comes to overall hole saw performance. In fact, one of the leading causes for lower tool life is choosing incorrect tooth edge material.
In general, carbon hole saws are used in wood applications whereas bimetal hole saws are used to cut general-purpose metals, wood and plastics. Carbide-tipped, carbide grit and diamond grit hole saws are well- suited for handling difficult-to-cut materials, such as fiberglass, ceramics and stainless steel. The most commonly used holesaws, however, have a bimetal cutting edge because of the variety of materials they can cut at a lower cost.
Typically, bimetal hole saws are made by welding a tool steel edge wire to a low-alloy steel, which serves as a backer. However, when the cutting edge wire is improperly welded to the steel backer, it can cause the holesaw to fail. In fact, a substandard weld zone is the leading cause of premature tooth failure. If the weld zone failure is recurring, using a different tooth edge material, such as a carbide-tipped hole saw, might be a good option.
Other considerations to keep in mind are the accessories that can be coupled with a holesaw. Generally, when hole saws are used with a pilot drill, operators can benefit from ease of use and longer tool life. A hole saw without a pilot drill can move laterally (creating larger kerf, a condition called “walking”) before engaging into the cut.
When a holesaw is attached to an arbor (via a pilot drill bit), it can be used on a drill press or on a cordless or corded drill. In a holesaw/arbor combination, a lower total indicated runout (TIR) is essential for long tool life and ease of cut. For most hole saw manufacturers, TIRs are determined using the ASME (B94.54-1999) standard. Regarding the standard, it’s important to note that it does not recommend operators consider a hole saw as a precision tool.
TOOTH DESIGN
Along with the saw tooth edge material, the tooth design, such as the tooth spacing, tooth angles and gullet design, can affect the cutting performance. In a typical hole saw design, side slots are used to remove slugs from the holesaw with relative success. Recent research conducted at the M. K. Morse R&D lab has shown that the tooth design plays a critical role in slug removal.
To improve the cutting process, slug removal should be closely observed. In some applications, operators may notice that the cut time might be shorter than the slug removal time. In such situations, an operator should consider shifting to a holesaw with better slug removal system, such as one with M. K. Morse’s new patent- pending tooth design.
Combined with an innovative side slot shape, the M.K. Morse tooth design offers the easiest slug removal feature in the market. In fact, tooth design has a significant effect not just on the slug removal, but also on the ease of cut, tool life and cut surface finish.
Another option to make slug removal easier is by not using a pilot drill. As mentioned earlier, the pilot drill is used to prevent the holesaw from walking. However, by not using the pilot drill, the large kerf (caused by walking) can be increased, which, thereby, reduces the slug diameter. A smaller or reduced slug diameter will make the slug removal process easier and quicker.
A good holesaw should be able to make an annular cut with ease while also delivering on quick slug removal.
Cutting conditions and work holding can have a significant effect on everything from vibrations and harmonics to tool life and cut finish.
In general, hole saw performance is evaluated based on the cut time, tool life, ease of slug removal and cut finish. Although holesaws shouldn’t be considered precision tools, often times, operators have high expectations. They expect a good cut finish and easy slug removal, which are influenced just as much by the type of material being cut as the tooth design.
Typically, to meet cut finish expectations, reducing the feed (plunge) rate is a good option. Remember, however, that there are a few exceptions to this rule, such as cutting thin metal sheets. As many operators know, achieving a good cut finish while cutting thin sheets of metal is a challenge. Therefore, using a rigid workholding device, proper drill extension, and a relatively high feed and RPM are recommended.
Selecting the correct cutting conditions (RPM and feed rate) is another essential approach for achieving better holesaw performance. If an operator experiences higher vibrations or harmonics while using a holesaw, it can be an indication of incorrect RPM and feed (plunge) rate. However, higher vibrations can also be due to improper tooth design.
One way to reduce the vibration or harmonics is to reduce either the RPM or the feed rate. Also, the combination of the tooth design (angles and pitch) and the kerf variation will reduce the harmonics. The new M. K. Morse holesaw has been designed to optimize tool life and harmonics. Also, the patent-pending tooth design has been rigorously tested to prove that it has the best slug removal feature in the global marketplace.
In summary, a good hole saw should be able to make an annular cut with ease while also delivering on quick slug removal. Hole saw accessories, such as the pilot drill bit and arbor, play an essential role in hole saw performance. Furthermore, cutting conditions and work holding can have a significant effect on everything from vibrations and harmonics to tool life and cut finish. Selecting the correct cutting conditions (RPM and feed rate) is another essential approach for achieving better hole saw performance.