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[machinist] Maintaining good cut quality when using an abrasive saw - The Fabricator
- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] Maintaining good cut quality when using an abrasive saw - The Fabricator
- Date: Sun, 20 Aug 2017 00:43:37 -0400
Maintaining good cut quality when using an abrasive saw - The Fabricator
http://www.thefabricator.com/article/sawing/maintaining-good-cut-quality-when-using-an-abrasive-saw
Maintaining good cut quality when using an abrasive saw
Understanding why quality can change as the abrasive wheel wears down can help to avoid bad cuts
A fabricator that can find the right abrasive wheel for the job, can figure out the right cutting speed and downfeed rate, and can find the right saw with enough power likely doesn’t have to worry about poor-quality cuts while abrasive cutting.
Change in cut quality is a common problem, even when the abrasive saw has been optimized for the type of cut desired. Changes might not appear when you are cutting small-diameter stock, but put in a larger piece of the same material and soon you will see scorch marks and a ragged edge.
Is the wheel getting dull? The short answer is a qualified no.
Abrasive wheels are self-sharpening. Unlike metal saw blades, the abrasive wheel cuts by abrasion. As the wheel spins, grit is released, exposing fresh and sharp grit. This is why the wheel diameter shrinks.
To produce the desired cut, the wheel composition must be correct for the material being cut, and the wheel must be rotating at the optimal speed. Also, the downfeed rate must be optimal, and there must be sufficient horsepower for the size of the stock being cut. Vary any one of those conditions, and the quality of the cut can change.
What Is the Wheel Grade?
When the cut quality has degraded, the first place to look is at the abrasive wheel. A common mistake is not using the correct wheel for the type of material being cut.
You can choose from a number of basic wheel types, but they cannot and should not be substituted. Wheels are made in a range of grades from A, which is the softest, to Z, which is the hardest. The abrasive materials come in grain sizes from 8, which is the coarsest, to 600, which is superfine.
The most common abrasive materials are aluminum oxide for cutting mild steel; silicon carbide for glass, ceramics, and similar materials; and zirconia (diamond) for alloy steels. Cutting superalloys such as INCONEL®, duplex steels, and Hastelloy® requires wheels with proprietary formulations for the specific application. Wheel manufacturers have developed specific types of abrasive material, with grit and the bonding agent tailored to cut certain material types and stock sizes.
Wheel hardness is commonly misunderstood as well. While it’s logical to assume you need a hard wheel to cut through hard stock, actually it’s the opposite. The harder the material, the softer the wheel and vice versa. Harder materials require a softer abrasive wheel that breaks down faster. Softer materials are easier to cut, so a harder wheel can do the work with less wear and longer life.
Another area to look at, especially when cutting superalloys, is the wheel formula. You may have been cutting, for example, INCONEL 188 without a problem, but trying to cut INCONEL 718 suddenly results in a poor-quality cut. This is where a saw manufacturer or distributor can help you get the right wheel for the material being cut.
Do You Understand the Wheel Motions?
Understanding wheel speed is absolutely critical to making high-quality cuts. Abrasive wheels have two basic motions: axial and linear. Axial motion is the rotational speed of the spindle measured as revolutions per minute (RPM). It remains the same regardless of the wheel diameter. Linear motion is the speed at which the edge of the wheel travels. This is expressed as surface feet per minute (SFPM).
Figure 1
These different sized discs reveal that they both can have the same axial motion yet much different linear motion.
To visualize this, imagine two discs. One is half the diameter of the other.
Place the discs upright and mark the point where the discs touch the surface (see Figure 1). Then roll each disc until it has made one complete revolution. Notice that both discs rotate at the same RPM, but the edge speeds (SFPM) of the discs are different. The smaller disc travels half the distance of the larger-diameter disc.
Applying this thinking to the abrasive wheel, you understand that the SFPM for a new wheel gradually reduces as the wheel diameter shrinks. As the speed slows, the wheel can effectively “bog down,” producing scorching and ragged edges. This effect might not be evident when you are cutting small-diameter, soft stock, but it appears all too frequently in larger-diameter stock and harder alloys.
Can You Determine the Correct Speed?
The rule of thumb for dry cutting speed is in the range of 12,000 SFPM and in the range of 8,000 SFPM for wet cutting.
If the quality of the cut is important, dry sawing should be used only on very small-diameter, soft stock, because without coolant the heat buildup contributes to scorching and rough edges. The slower wet cutting speed allows the coolant to flush out the grit and to act as a heat transfer agent to prevent scorching.
Assuming the saw was built and optimized for the material you are cutting and the wheel type and diameter, you need to establish the relationship between the saw’s RPM and the wheel diameter at various points of wear. For example, a wet-cutting abrasive saw with a 14-inch wheel should have a spindle speed of about 2,200 RPM to produce about 8,000 SFPM. (These numbers do not have to be exact. Consider them as guidelines.)
To help you calculate SFPM and RPM, use these simplified formulas to make the math easier:
0.262 x D x RPM = SFPM
SFPM / 0.262 x D = RPM
In this case, a 14-in. wheel rotating at 2,200 RPM would have an edge speed of 8,070 SFPM (0.262 x 14 x 2,200 = 8,070). But look what happens as the wheel wears down:
Figure 2
The part on the left shows a cut on which RPM was adjusted to compensate
for wheel wear. The part on the right shows a cut on which the downfeed
rate was adjusted until the cut began to exhibit scorching.
0.262 x 13 in. x 2,200 = 7,493
0.262 x 12 in. x 2,200 = 6,917
0.262 x 10 in. x 2,200 = 5,764
0.262 x 8 in. x 2,200 = 4,611
How critical is that reduction in SFPM? It all depends on the material you are cutting and its size and shape. Mild steel and soft alloys under 1 in. diameter shouldn’t be a problem. If your stock is thin-wall tubing or a structural, you may be able to produce a good-quality cut in slightly larger-diameter materials. Alternatively, if you are cutting solid larger-diameter superalloys, such as 2 to 3 in. diameter, you may begin to see evidence of scorching at 4,600 to 5,700 SFPM.
Saws are available with a variable-speed drive that allows you to adjust the RPMs to compensate for the wheel wear, but in most cases that type of drive is not really necessary. Only when the diameter of the stock gets close to 20 to 30 percent of the wheel diameter does a variable-speed drive make a difference.
What If the Spindle Speed Is Fixed?
Adjusting the downfeed rate can compensate for a slower SFPM. This enables the wheel to keep breaking down, providing the abrasion needed to cut the material. This is especially important when cutting larger-diameter, harder alloy stock.
Even though you may be using a wet saw, you still see considerable friction along the sides of the wheel. Lowering the downfeed rate reduces that friction.
Because the downfeed rate depends on the material type, the stock size, and the machine horsepower, saw distributors or manufacturers can’t provide a precise formula for adjusting downfeed. In general, as the SFPM decreases, reduce the downfeed rate to prevent scorching (see Figure 2). If the saw has a manual downfeed, you should be able to feel the effect of the SFPM change and compensate accordingly.
Figure 3 shows what happens when you do not take into account RPM and downfeed rate when abrasive cutting.
Figure 3 and 4
(right)This occurs when you make no RPM and downfeed adjustments in an abrasive cut.
(left)This cut was made on a machine with enough horsepower for the job and without a need to adjust RPM or downfeed rate.
Does the Machine Have Sufficient Horsepower?
If you can produce high-quality cuts in small stock but have quality issues in larger stock, your machine may not have enough horsepower to maintain the optimal cutting action of the wheel. Even if the saw has a variable-speed drive, cutting larger, higher-alloy stock can require more horsepower than cutting the equivalent-size softer stock.
In general, when abrasive cutting large-diameter and hard material, you need more horsepower (HP) (see Figure 4). For example, if you have a 20-in. wheel and are cutting relatively soft, small-diameter stock, a 20-HP motor is sufficient. Cutting a larger piece of superalloy (with the appropriate wheel) could require a 40- or even a 50-HP motor.
To ensure repeatable good-quality cuts regardless of the wheel diameter, you may want to establish your own database using material scrap of various sizes and composition. Note the RPMs required at various points and type of stock. If you cut bundles and are experiencing problems, you may have to reduce the size of the bundle as the wheel wears down.
Following these guidelines can help you understand how to achieve the desired quality in whatever stock you are cutting. Again, these are guidelines, not rules.
Given all the different machines, different wheel compositions, the range of materials, and operator expertise, you should consult your saw distributor or saw manufacturer for advice.
Scott Allen
Kalamazoo Industries
Kalamazoo, MI 49048
Published In...
The FABRICATOR
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.
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- [machinist] Maintaining good cut quality when using an abrasive saw - The Fabricator, Lawrence London, 08/20/2017
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