Grinding, polishing, blending in hard-to-reach areas
Tools, accessories abound for finishing in tight spots
Interviews with several tool and accessory manufacturers provide a broad perspective on capabilities for finishing in hard-to-reach areas.
One of your customers calls to ask you to bid on a product currently in development. You’ve already been doing quite a few programs that involve bending tube and welding it to make subassemblies for his company, so he wants you to take on this new project. You brace yourself. The customer’s designers keep pushing the envelope; each new project is a little more intricate than the previous one. When you finally get the drawing of the new project, you realize that your intuition was right on the money. The bending and welding won’t be too difficult, but many of the parts are so close to each other that grinding and blending the welds and polishing the whole thing will be both challenging and time-consuming.
What to do? You might need to investigate some specialized tools and accessories for getting into those hard-to-reach areas.
Looking for a Few New Tools
Angle grinders are great for heavy stock removal, but they aren’t designed for getting between tightly spaced obstacles, and they aren’t ideal for doing the fine blending and polishing that many projects require. A few specialized tools can help you get into the nooks and crannies of complex assemblies.
Portable Belt Sanders. If you have ever wondered whether a belt sander would be a good tool for grinding down a weld bead in a hard-to-reach area, wonder no more. The hand-held version of this tool, commonly called a finger belt sander or a file belt sander, is portable and equipped with a narrow belt to get into tight areas (see Figure 1). Most manufacturers of these tools provide optional arms that work with abrasive belts in widths from 1⁄8 to 1 in. wide. While each manufacturer’s model is unique, in most cases the arms can pivot and the handle can be oriented in a variety of ways, maximizing the tool’s usefulness.
3M’s pneumatically powered file belt sander develops a maximum speed of 22,000 RPM, capable of driving the belt a maximum of 4,990 surface feet per minute (SFPM). It handles belts from 1⁄8 to ¾ in. in lengths from 18 to 24 in. The tool’s belt housing/guard and handle rotate 360 degrees.
“This versatility lets you really get into restricted spaces,” said Marv Schifsky, technical representative for 3M.
Metabo’s electronic band file, model BFE 9-90, uses belt widths from ¼ to ¾ in. and develops 510 W of output power. The maximum no-load belt speed is 1,770 FPM. Its grinding head rotates 270 degrees.
Fein’s belt file, BF 10-280 E, is an electric tool with an output of 550 W that develops 28,000 RPM. It accommodates belts from 3⁄32 to 3⁄4 in. wide, and the handle rotates 180 degrees.
Dynabrade’s Dynafile® and Dynabelter® pneumatic tools, available in a multitude of variations, develop 5,000 to 25,000 RPM and use belts from 18 to 24 in. long, ¼ to 1 in. wide. The grinding head rotates up to 180 or 360 degrees, depending on the model.
Depending on the arm, the operator can grind with three surfaces—the wheel at the end of the arm, platens that run along the length of the arm, or slack (see Figure 2). Optional arms include various widths of straight arm; curved arms; corner-style arms for grinding curved surfaces; and a vessel arm with a 4-in. wheel for blending.
Operators should be aware that some of these tools are equipped with a knurled steel drive wheel for using coated abrasive belts for the grinding and polishing steps; others have a rubber drive wheel for using woven products for surface conditioning. The rubber wheel doesn’t pose a risk of abrading the the woven belt, if the belt were to catch on something. Some tools come with two drive wheels, and the operator selects the drive wheel that suits the application.
The belts available for this sort of sander from Norton Abrasives vary from 40 grit to 120 grit (woven) and coarse to very fine (nonwoven).
Fillet Weld Grinder. Metabo’s fillet weld grinder, model KNSE 12-150, might look like a right-angle grinder with an extension, but it’s a little different from a right-angle grinder (see Figure 3).
“You cannot mount traditional abrasive discs on this machine,” said Terry Tuerk, senior product manager for Metabo. “It comes with a 1-in. arbor and it uses unitized abrasives, a compressed nylon abrasive material,” he said. “The discs are available in 6 in. diameter, ¼ or 1⁄8 in. thick, and it’s easy to shape the edge of the disc using a file,” he said. This allows the operator to customize the disc to make a gentle radius, tight radius, or a point, depending on the application.
“The discs are available in medium- and fine-grit abrasive,” he said.
Tuerk also pointed out that it’s more sophisticated than many hand tools.
“It has electronic speed stabilization, so it keeps the wheel moving at the optimum speed,” Tuerk said. “It also has electronic power interruption, so if the operator uses the lock to keep the trigger in the on position, the tool won’t restart if power is lost. It’s a safety feature—the operator has to turn it off and turn it on. It also has electronic overload protection, so it will shut down if it begins to overheat to prevent damage to the tool,” he added.
Because its speed is variable, it’s useful for coarse to fine work, Tuerk said. Its speed range is from 1,300 to 4,000 RPM.
“The slower speeds, 1,300 to 2,000 RPM, are used for polishing, and a number of accessories such as compressed felt discs are available for this. The same tool also takes down welds. Let’s say you have a handrail application, and you need to finish a weld to match the handrail itself, you can do that. Then you can use the same tool to take it down to a mirror finish,” he said.
Horizontal-shaft Grinder. Horizontal-shaft tools have been around for some time. The abrasives manufacturers have come up with many consumables for them, effectively making the tool a jack of all trades.
“All sorts of abrasives are available—quick-change discs, flap wheels, cross pads, cartridge rolls, and spiral bands—a whole plethora of shapes and styles for hard-to-reach areas,” said David Long, director of marketing and strategy for Norton Abrasives.
“If you’re familiar with all of the thousands of types of abrasives and tools and attachments made for Dremel® tools, this is the industrial version,” Long said. “Mounted points for horizontal-shaft tools are available in hundreds of shapes,” he said.
Dynabrade’s Dynastraight® series is an example of this sort of tool. Available in models from 950 to 18,000 RPM, these tools have a 5⁄8-11 arbor or ¼ collet. A 6-in. extension is an option for some models; an 8-in. extension is available for the Flapper model, which was designed specifically for finishing tube and pipe IDs.
For getting into really tight areas or small IDs with horizontal-shaft tools, mounted flap wheels as small as ¾ in. dia. are available, as well as unmounted flap wheels down to 3⁄8 in. dia. Fabricators might not be aware that two abrasive types can be used with this tool.
“Coated abrasive products in flap discs and quick-change discs and some of the others now have a nonwoven counterpart,” Long said. “It’s a synthetic nylon substrate impregnated with abrasive, and it can provide a better finish, and is more forgiving, than coated abrasives, which have a cloth substrate.”
The nonwoven abrasive, marketed by 3M under the name Scotch-Brite® and by Norton Abrasives as Bear-Tex®, has a little bit of spring to it. It’s forgiving—it takes quite a bit of pressure to dig in deep and gouge the metal.
3M makes a small mandrel that holds an abrasive cut to length by the operator; this is essentially a flap disc that has a custom diameter (see Figure 4).
Another option is nylon abrasive filament (NAF) brushes.
“They consist of nylon filaments impregnated with abrasive material, and they come in all sorts of shapes and sizes,” said Simon Wainwright, product manager for CS Unitec. “You can put them onto very long shafts, and you can get right inside of tubes and pipes. Because they are flexible, they conform to the ID,” he said.
Die Grinders. As the name implies, these tools are designed for grinding and maintaining dies and molds. Other common uses are flash removal from castings and machining mark blending. The design makes this tool useful for grinding hard-to-reach surfaces on fabricated assemblies.
Operators can’t be too careful in selecting abrasives for these tools. A typical straight-shaft grinding tool has a maximum speed of 1,600 RPM; Fein’s MSh 635 die grinder achieves 48,000 RPM. This is a speed differential of 30-to-1. Any tool intended for a maximum of 1,600 RPM would likely disintegrate at 48,000 RPM, and injuries could result.
Flexible Shaft. Finally, a flexible shaft can expand the usefulness of a bench-mounted grinder and an ordinary die grinder, provided the grinder’s arbor is compatible with the shaft.
“Almost every shop uses angle grinders, whether they are cutting pipe or knocking off burrs or doing some weld removal,” said Wainwright. “The flexible shaft expands the angle grinder’s capabilities—it accepts a large number of tools, such as 2-in.-wide drums, 3-in.-wide drums, sanding pads, even polishing pads.”
Operators need to consider the tool’s speed and the proper abrasive speed for the application.
“A typical 41⁄2 grinder spins at 13,300 RPM. Using a tool half that diameter means you get half the surface speed, which is on the low side, but it’s still within the acceptable range for these small tools,” Wainwright said. In other words, this is another tool that can do initial, rough grinding in addition to fine polishing.
“This allows the operator to grind down a weld, then smooth out the corners, get into tight areas, and even get inside the pipe if he needs to,” Wainwright said.
The flexible shaft also can work with an ordinary bench grinder, as long as it has a standard 5⁄8-11 arbor. Wainwright added that the flexible shaft, combined with the company’s 3-phase, 750-W bench grinder, has drawn the attention of a couple companies involved in manufacturing bicycles, a perfect example of a tubular assembly with hard-to-reach areas. The sheer variety in bicycle designs—styles for men, women, and children for riding on roads or off, for racing or for pleasure—means that bicycle manufacturers need to work with an endless combination of angles, dimensions, and clearances.
“After using the grinder with a sanding disc to deburr the cut tube, they install the flexible shaft to get into tight spots with small tools,” Wainwright said. “Then they use a variety of abrasive drums to get into the corners, and then they use cotton buffing drums to provide a nice finish.”
How Fast Is Fast Enough?
In abrasives, faster isn’t necessarily better. Depending on the workpiece and the abrasive, the optimum speed might be as low as half of the tool’s fastest speed. Operators should remember that when considering the tool’s speed, the primary concern is safety. The performance of the abrasive, while important, isn’t as critical as the operator’s well-being.
“Safety is the most important area,” said Norton’s Long. “You have to make sure the abrasive product’s rating matches the machine, because there are a lot of high-speed machines out there.”
“When you’re using coarse abrasives, 24 grit to 200 grit, speed is of great assistance,” Wainwright said. “They are designed so the abrasive cuts in, they shed when they get to a certain temperature, and the metal fines get cast away before the disc gets glazed over. Higher grits such as 320 are more sensitive to speed, and when you get to 1,200 grit, speed is your enemy. High speeds generate too much heat, causing discoloration,” he said. The telltale sign on carbon steel is a brown color; stainless steels turn blue.
Depending on the material, it might be best to use slower speeds for every step, from coarse grinding to fine polishing.
“Many hard-to-grind materials, such as Stellite® or HASTELLOY®, are best finished at lower speeds than other materials. Faster isn’t always better—you might end up burning the part and creating a metallurgical issue,” Long said.The abrasive’s longevity is another consideration.
“Many customers run the products much faster than they should, and they wear out the product too quickly,” 3M’s Schifsky said. “If the tool is running too fast, some products don’t have the opportunity to do the digging they need to do,” he said. “Excess speed develops excess heat, which can cap the abrasive, or fracture it. The optimum operating speed provides the best life and the best finish. For hard materials such as titanium and stainless steel, you need to run the tool slower than you would for a typical mild steel,” he said.
Overheating the material doesn’t necessarily mean you have to scrap the part. “Removing the discoloration restores the chrome layer, which makes it stainless,” Wainwright said. “Using an abrasive-impregnated nylon brush is one of the best ways to do this, especially when you’re working in difficult-to-reach areas.”
Safety Q & A
Projects that require finishing in hard-to-reach areas invariably lead to a number of questions. These are four common questions from fabricators with answers provided by abrasives manufacturer Norton Abrasives.
Q: Is it acceptable to remove the machine’s wheel guard to grind in tight places?
A: Never. If you are grinding in tight areas where a wheel guard interferes with grinding, use a different tool and wheel. Mounted points, cones, and plugs do not require wheel guards and come in many sizes and shapes. These products are designed for grinding in tight areas.
Q: Is it safe for our customer to mount a Type 27 wheel onto a machine upside down so that the operator can grind the bottom of a workpiece, such as the bottom of a tabletop?
A: No. Type 27 wheels and angle grinders are designed and manufactured so that the depressed center of the wheel faces down (and the wheel’s hub is up). Improper mounting of these wheels can lead to wheel breakage and personal injury. If you must grind under a workpiece, get under the workpiece and turn the grinder over or use a different tool.
Q: Is it OK to use the side of a Type 1 cutoff wheel for grinding?
A: No—cutoff wheels are not designed for use on their sides. Side grinding, twisting, bending, and jamming the wheel are misuses that can cause the wheel to break.
Q: When using small cutoff wheels on angle grinders to cut bolts flush with a concrete floor, is it OK to bend the cutoff wheel to get under the bolt?
A: No. Cutoff wheels do not bend. They break when excessive lateral pressure is applied. Never bend a cutoff wheel. Use a different tool for this application.
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