Whether soft or hard or anywhere in between, metal materials present unique cutting issues. And that’s why it’s important to understand a material’s characteristics long before the first cut is made. By doing so, fabricators can create the best opportunity to achieve the longest bandsaw blade life and the best production results from their sawing operations.
Fortunately, it’s not necessary to have a degree in metallurgy to understand a material’s characteristics. There are plenty of rules of thumb to follow, as illustrated below.
Soft materials typically include carbon steels, aluminum and copper. And despite the descriptor, soft materials, in some cases, can be just as difficult to cut as hard materials. Soft materials tend to load the tooth gullets on =the blade, resulting in premature wear and, in a worst-case scenario, can even lead to tooth strippage.
When the tooth gullets are filled, only the very tip of the tooth is doing any cutting. This can cause chip loading in the gullets and excessive forces on each tooth. The basic rule of thumb for tooth selection on any given material is to have no fewer than three teeth and no more than 10 to 14 teeth in the material at all times.
Typically, this allows the teeth to grab a chip and release it before the gullets are full. Because soft materials tend to load quicker, it’s imperative to stay away from a blade with a fine tooth pitch. In most cases, and dependent upon the specific soft material, it’s better to select a blade with a coarser tooth pitch than a finer one.
Hard materials, such as Inconel, Monel and Hastelloy, present a different set of problems. Based on the cutting characteristics of these materials, it is very unlikely that the gullets will fill, so blade selection should reflect that. The teeth have a much more difficult time engaging the material and typically pull a much thinner chip. This means selecting a blade that follows the higher end of the rule of thumb because more teeth will be engaged in the cut than would be possible when cutting soft materials.
For example, cutting 3-in. aluminum using a 1.4-2 tooth pitch would stay within the criteria of having a minimum of three teeth in the material at all times. Based on the cutting characteristics of aluminum, this would typically work well. However, using the same blade to cut a similar-size piece of Inconel 718 would cause premature blade failure due to the teeth being too coarse to engage and pull the necessary chip load. In this case, choosing a 3-
4 tooth pitch would likely be better, as there would be more teeth engaged without concern of loading the gullets, resulting in optimal cutting productivity and maximum blade life.
The Starrett Advanz MC5 and MC7 are good examples of blades that have been specifically designed to cut hard alloys such as Inconel. They are comprised of a multi-chip carbide tooth and a special backing material to make them not only last longer, but also cut faster than standard carbide blades.
It’s always best to select a blade specifically for the material being cut, but bi-metal blades are available that can handle hard and soft materials. Typically, a high-speed M42 edge provides excellent blade life and the ability to maintain production rates.
However, in many cases, tooth pitch is the deciding factor more so than the type of blade (bi-metal, carbide, etc.). As indicated earlier, tooth pitch is critical because too many teeth engaged in the material at one time can cause loading and poor blade life. Too few teeth typically causes strippage to the teeth.
As a rule of thumb, soft materials require faster blade speeds while hard materials require slower blade speeds. Soft materials allow for higher feed rates while using less feed pressure and hard materials require more feed pressure and lower feed rate.
For greater understanding, it makes sense to ask an operator to think of blades in one-tooth increments when deciding where to adjust the feeds and speeds. For example, when cutting aluminum, that one tooth doesn’t need a lot of pressure to engage the material because aluminum is soft and can be cut fairly quickly. On the other hand, when cutting Inconel, that one tooth needs a lot of pressure to penetrate the material. While feed pressures increase with the hardness of the material, feed rates decrease.
Sizing it up
The size of the material determines which tooth pitch to use. In general, the smaller the material or part, the finer the tooth pitch should be. Alternatively, a coarser tooth pitch is desirable for larger parts.
This is also true of materials that are bundled. While taking note of the individual size of the material bundled, also consider the size of the bundle as a whole. Bundles can present problems when determining tooth pitch and it is likely there will be no perfect solution. That said, arriving at a tooth pitch by understanding what true material size the blade will be cutting helps determine the proper pitch.
In one example, when bundle cutting angle iron, a customer based tooth selection on the size of each angle and used a blade with a 10-14 pitch to cut a bundle that was
approximately 10 in. by 10 in. The tooth pitch was much too fine to cut what was essentially a solid in the middle of the bundle, so a Starrett saw specialist recommended a 3-4 tooth pitch and production and blade life increased dramatically.
In another example, a customer had been successfully using Starrett blades for quite a while. However, they encountered a situation where blade life had decreased and they weren’t sure why. They contacted Starrett and it was revealed they had been cutting carbon steels with a bi-metal blade, but had switched to Inconel when they had taken on a new job.
Not realizing the big difference it made when cutting the much harder material, they used the same blade and same speeds and feeds. By switching to a different blade, such as a carbide-tipped blade, and re-adjusting the speeds and feeds, they are now cutting the Inconel successfully.
There are many unique sawing applications, but when in doubt, it is best to consult with the bandsaw blade supplier to determine the optimal blade to do the job at hand.