- Created: 2018-04-04
When it comes to waterjet cutting, all abrasives are not created equal
by Glenn Osborne, president and owner, G & W Machinery Sales Inc.
Whether large, small or in between, all fabrication shops are fighting a common battle to boost productivity and control costs. Though not a lot can be done about unplanned machine maintenance or material prices, there are areas where expenses can be reduced. Most are quick to jump on these cost-saving opportunities.
So while foregoing gourmet coffee in the breakroom is a no brainer, just remember that when it comes to waterjet abrasives, sometimes cheap (quite literally) just won’t cut it.
In shops throughout the country, the single biggest component of operating an abrasive waterjet machine is the abrasive itself. It’s little wonder that most shop owners view it as an opportunity to reduce costs. However, ignoring the key role that abrasives play in the cutting process and basing selection on price alone can be a costly mistake.
Waterjet metal processing has its advantages over other alternatives. Waterjets don’t emit the high temperatures associated with lasers or plasma machines, for example. This low-temperature cutting minimizes the risk of damaging part edges. This makes waterjets ideal for processing complex parts, such as those with intricate detail, acute angles and tight radii. Additionally, because of their reliable edge quality, these machines are often the best choice when processing thicker materials.
A waterjet uses a high-pressure water stream to accelerate abrasive particles and shoot them out of the waterjet’s nozzle. Those particles impact the workpiece and grind material away to cut a slot. A motion system then guides the nozzle around the material to cut out the desired shapes. In this regard, the abrasive is the “cutting tool” component of the waterjet.
The speed at which the nozzle can be moved around the shape is determined by several factors, including the type and volume of abrasive being used. Because the abrasive is doing the cutting, it pays to understand how it influences the cutting process and the bottom line.
Here, an operator is using a waterjet to cut relatively thin material. When cutting material 1 in. in thick or less, a 5 percent increase in speed is gained using crushed versus alluvial garnet.
Types of garnet
Garnet is the industry’s most common abrasive. It’s plentiful, nontoxic and hard enough to cut an array of materials. At the same time, garnet is not so hard as to wear out the waterjet’s nozzle components too quickly. Garnet for waterjet cutting generally comes in two forms: crushed and alluvial.
Crushed garnet, often referred to as “hard rock,” is mined from deposits in mountain ranges primarily throughout a range of areas such as New York, Montana and China. The multi-step process involves extracting and pulverizing large rocks. At this point, the garnet particles are isolated from the other minerals. These particles have been broken along natural fissures creating very sharp edges. It is these hard, sharp edges that allow for the fastest cutting. However, the extensive process to mine and produce garnet makes it a costly abrasive.
Alluvial garnet is sedimentary garnet, often referred to as “beach garnet.” Vast deposits of alluvial garnet exist around the world, including in India, Australia and Africa. These garnet particles have been naturally “tumbled” over time, causing their edges to be more rounded and less sharp – much like river rocks. The mining and processing of this garnet is relatively easy compared to crushed garnet. As a result, alluvial garnet is the lower cost and more widely used abrasive.
Unfortunately, there’s no governing body to keep a watchful eye over standards for garnet. Consequently, purchasing garnet can be somewhat of a gamble as most abrasive suppliers simply don’t guarantee the quality of their garnet. It is not like the gasoline industry with EPA octane ratings where 87 octane is the same amount of energy no matter who produces it, and they disclose that it is mixed with 10 percent ethanol. For the abrasives industry, an 80-mesh garnet is merely a rough designation and not an exact standard.
Therefore, every abrasive waterjet owner and user should know a few specific facts about garnet:
- Size distribution of the garnet particles can vary greatly. They can come in the form of useless dust that clogs the feed system, producing inconsistent cuts or as large pieces that will plug the nozzle.
- Hardness of the garnet varies depending on the mining location. Garnet is rated between 7.5 and 8.5 on the Mohs scale. (Used in classifying minerals, the scale runs from 1 to 10. The position on the scale depends on the ability to scratch lower rated minerals.) That’s a 13 percent spread in the hardness. A softer garnet requires a slower cut than a harder garnet.
- Garnet purity. Poor cleaning and separation techniques at the mine can leave contaminates mixed in with the garnet as well as lots of dust. Also, to extend their supply of garnet, some producers are now blending it with other minerals, such as staurolite. However, this mixture does not cut well in a waterjet as staurolite is approximately one third the hardness of garnet and has a lower mass. Because of this, it breaks up much easier and cannot grind away as much material per grain.
And again, crushed garnet cuts faster than alluvial. In order to achieve comparable cut quality of crushed garnet with alluvial garnet, cutting speed should be reduced by at least 5 percent. Furthermore, on thicker, harder materials, the cut rate should be slowed by 20 percent.
So, you are probably wondering which garnet is right for you. The answer of course depends on several factors, including your shop rate, your consumption rate of garnet, the cost of crushed and alluvial garnet, and the thickness and type of material being cut.
As an example, say a company has a shop rate of $150 per hour. They can buy crushed garnet for $0.35 per lb. and alluvial for $0.27 per lb. Their garnet consumption rate is 0.75 lbs. per min. and they cut a variety of thicknesses.
If the company is cutting materials 1 in. or thinner using crushed garnet:
- A 5 percent increase in speed is gained: 1.05 x $150 per hour = $7.50 per hour more revenue
- Additional cost of crushed garnet versus alluvial results in: $0.35 - $0.27 = $0.08 per lb. X 0.75 lb. per min. X 60 min. per hour = $3.60 per hour
- Net increases in revenues $7.50 - $3.60 = $3.90 per hour
- Therefore, if they run 2,000 hours per year, they can bill an additional $7,800
- When cutting thicker materials, it would be as much as $31,200 per year in additional revenues, based on a 20 percent increase in speed.
As is true with various thicknesses in material, softer materials, such as aluminum, require adjustments in cutting speeds, as well. For example, for 1.5-in.-thick aluminum, the speed would only need to be slowed down about 5 percent whereas for stainless steel, it could be as much as 25 percent. The same effect on aluminum may not be seen until it is more than 2 in. thick.
Overall, using a less expensive garnet may be costing you money in the long run. By spending $0.08 more per pound for better garnet, you can produce $78,000 more income over the 10-year life of a machine. In many cases, those who choose the least expensive abrasive tend to experience the most problems cutting and have much more downtime as a result.
Waterjet abrasives are one of the few costs that shop owners can control. While you may be tempted to lean toward lower cost abrasives without giving it much thought, don’t be too quick to dismiss the need for higher quality, more costly abrasives.
Look at buying low-cost abrasives for your high-cost waterjet as similar to buying a $250,000 racecar and then equipping it with cheap retread tires. You can certainly get around the track, but you’ll have to go slower to complete a lap. That approach won’t win you many races – or new business.
So why not maximize the full capabilities of that waterjet with the best abrasive? Just like with racing, to be in the running you have to run the right equipment. Don’t be lured into the price game; stay focused on the efficiency of your system. It will pay off in the long run.