Читать книгу Farm and Workshop Welding, Third Revised Edition - Andrew Pearce - Страница 10

Whenever you set out to weld, braze, or solder, cleaning the work is number one priority. And with steel, the easiest, fastest, and most effective route to get there is via one or more abrasives. There’s a wide choice out there, in form (cloths, discs, flap wheels, stones) and in method of using them, from manual work to power tools. Once the latter swing into play they bring a risk element, too — so for starters, here’s something to remember: angle grinders and their bench-mounted brethren can do you serious harm if not treated right.

Given that encouragement, we’ll sort out safety first. Mostly we use grinders on steel and on stone without a second thought, but ponder this. Spinning without load, the outer edge of a typical angle grinder disc flies by at 80m/s. That doesn’t sound like much. Yet 80m/s is 290km/hr, or a scorching 180mph. And if you’re in any doubt over what can happen when a disc bursts, just search the web for “angle grinder accident.” So here’s how to avoid that.

A good-quality abrasive wheel or disc is made and tested to very strict standards and is extremely unlikely to give the user any grief — but only when the wheel is chosen, mounted, and used properly. Humans being humans, accidents tend to start in the last part of that.

We’re dealing here with bonded wheels. That is, types where the abrasive is held in a solid matrix or binder. The nature of the binder and the type of construction then splits bonded wheels into two broad classes — resinoid grinding/cutting wheels (or discs) used mainly in handheld tools, and vitrified wheels used in bench grinders. The two classes have very different properties and need very different treatment. There are also flap wheels, made from overlapping abrasive strips — more on those later.

A word on quality and its impact on safety and service life. In Europe, all bonded wheels must conform to EN12413. Among many other things, this requires each wheel to show the relevant standard and to carry vital information on its maximum permitted peripheral (outer edge) speed and rpm. Good-quality wheels always carry this info, along with pictograms laying out the wheel’s application(s), recommended safety equipment, and markings that allow the wheels’ origin to be traced. Look for these, plus the oSa (Organisation for Safety of Abrasives) symbol — see “Wheel markings” for more. Lower-quality wheels are usually cheaper and may not have all of the above. They can also be much more variable in quality and will probably have a shorter service life. Abrasive flap wheels must conform to a different standard — EN13743.

For material removal and cutting metal or stone, thin resinoid-bonded wheels — usually angle grinder discs on the farm — are the thing. Ranging in thickness from ¹/₃₂" to ⅜" (1mm to 10mm), they hold abrasive grit in a resin matrix, and always feature fiberglass mat reinforcement. The resin and mat bring toughness, allow high-rpm operation and in thinner discs, allow significant flexibility. Resinoid wheels are built up from layers of bond and mesh, pressed in a mold and cured at relatively low temperatures; a metal center disc usually provides an accurate mounting surface. For typical farmsized 4", 4½", 5", 7", and 9" (100mm, 115mm, 125mm, 180mm, and 230mm) angle grinders, the maximum peripheral wheel speed is 80m/s, regardless of wheel diameter.

Vitrified bonded wheels are quite different. Designed for use on bench grinders, they use a mineral or synthetic material to hold the abrasive. Fired at very high temperature, this turns into a hard, brittle, and glass-like bond. As there is no separate reinforcement, vitrified wheels are more prone to shock damage than resinoid wheels and can’t stand high rpm. So their maximum peripheral speed is lower at 35m/sec-50m/s.

Wheel care and machine use comes down to six golden rules. Stick by these, add a healthy dose of respect for the equipment and you’ll be fine.

The rules:

• Match the wheel to the work.

• Be sure that the machine can’t physically spin the wheel faster than its maximum allowable peripheral speed or rpm (see “Stumped for speed”).

• Mount the wheel properly.

• See that machine guards are in place and secure.

• Keep the wheel’s design in mind while using the tool — for example, don’t grind with a cutting disc.

• Wear the right protection for eyes, hands, ears, and lungs.

A full safety code can be found at www.fepa-abrasives.org.

Types Explored 1: Resinoid Cutting and Grinding Wheels

For safety and good service life, it’s essential that discs for grinding and cutting are intact, spin at the right speed, and be mounted securely. Beyond that, how you present the wheel to the work has a big impact on material removal speed as well as safety.

And of course the machine’s guard must be fitted. Some of the nastiest accidents happen after a guard has been taken off, usually for better access to a corner or to fit the wrong size of wheel. Such daftness aside, changing wheels and using a handheld grinder comes down to common sense, though not everything is obvious, as the pictures below show.

2.1a. and b. See that the machine you use can’t spin the disc faster than its maximum allowed speed. Find the disc’s max rpm (2.1a) or calculate it if it’s not visible — the box “Stumped for speed” shows how. Here it’s 6,650rpm. Then find the grinder’s max spindle rpm (2.1b); in this case 6,600rpm. As that’s 50rpm lower the wheel’s limit, the pairing is safe to use. Last, check out the disc for cracks and/or missing bits before you mount it, and junk it if you find either.

2.2a. and b. Not everyone notices that the upper spindle flange on angle grinders is generally reversible. The side with a raised inner rim (2.2a) is for use with grinding discs. Fit it so the raised section locates and centers the disc. Use the flat and/or recessed side (2.2b) with cutting discs. Get the flange the wrong way round with a cutting disc and the slim raised rim limits clamping force; the disc can then slip and may stall in the cut, with a risk of damage to you and it if you just yank it out with the tool under power.

2.3. Discs with a depressed center must be fitted with the low section toward the machine body, as the depression adds mechanical strength. Don’t fit a disc the wrong way up even if the tool allows — it’s not designed to run that way.

2.4. Switch off the grinder at the mains and unplug it before changing wheels. Go easy when tightening the flange; just nip it up enough to grip the wheel, no more, then spindle rotation will build on that. Over-tightening means struggle when you come to change discs.

Wheel Markings

A good-quality disc carries all the info you need to suit it to the job, match it to the machine, and use it safely. Pictured is a 9” (230mm) angle grinder cutting disc that, like all good-quality products, has info permanently etched in during manufacture rather than added as a sticky label. First thing to look for is the EN12143 mark, which on this wheel is shared with a unique identifying barcode (A). Next find the wheel’s max permissible speed, here given in m/s and rpm (B). Also here are the wheel’s dimensions in mm and inches, plus a code — A46T-BF41. Use Table 2.1 to crack these codes: this one breaks down as follows:

• A = abrasive type (aluminum oxide)

• 46 = grit size (46 per linear inch)

• T = grit hardness (hard)

• BF = bond type (Bakelite/glass fiber reinforced)

• 41 = shape specification (flat cutting-off wheel)

Item C in the picture is the range of applications the disc is designed for. Also obvious should be a selection of symbols (D). These point to the safety equipment the user should wear and show specific details, for example that the wheel contains no iron, sulfur, or chlorine (so won’t contaminate stainless steel) and that it must not be used for grinding. Item E is the oSa symbol, confirming that the makers belongs to the Organisation for Safety of Abrasives; this guarantees that the product meets at least EN 12413.

Finally, look at the center ring. This should show the wheel’s validity date. The organic bonding in resinoid wheels degrades over time, so a three-year shelf life is usually recommended. The ring should be stamped with the use-by year, along with a two-digit code showing which quarter it was made in. In this case, the expiration date was the third quarter of 2012.

Wheel markings

2.5a. and b. An angle grinder’s guard can — and should — be rotated to suit the job. In 2.5a it’s set for cutting: the lower edges of the guard line up with the grinder’s body. Picture 2.5b shows the guard turned through 90° for grinding. The important thing is that the guard always sits fully between the wheel and you.

2.6. Run grinding discs at 30°–45° to the work. A relatively small contact patch then removes material fast. Shallower angles spread the wheel’s pressure and slow down material removal. Low angles also thin the edge of the wheel, which often then loses small chunks and becomes ragged — usually as wheel diameter decreases.

2.7. Flap discs need to run at a shallower 10°–15° angle to put maximum abrasive in contact with the work.

2.8. Cutting wheels are thin, partly to concentrate pressure on the working edge and partly to minimize cut width. Although reinforced, these wheels are definitely not designed to take side loads — it’s very easy to snap a 4½" x ¹/₃₂" (115mm x 1mm) item with just finger pressure. Bring this picture to mind every time you’re tempted to grind with a cutting disc.

2.9. It follows from 7 that a cutting disc should be used vertical to the work, as shown. Load then passes safely up through the disc. Side loads can fracture a disc wheel or tear out the center, so trying to cut in anything other than straight lines is likely to bring grief.

Flap Discs

Resinoid discs are the roughnecks of the angle-grinding world. We’ve seen that the grinding variety is best used at a relatively steep 30°–45° angle to the work, which concentrates your downward pressure to make the abrasive cut. Resinoid discs can handle any material apart from aluminum and other soft metals that clog the abrasive, but they do leave an uneven finish. On heavy jobs like removing old weld metal, scouring out rust pits, or cleaning up gas-cut edges, that’s not a problem.

Resinoid discs cover most bases. Goodquality ones aren’t expensive and offer sensible service life. On top of that most farms work predominantly with steel, and getting a newly welded fabrication or repair out of the door usually takes priority over the job’s appearance — but sometimes you need something a little more genteel. That’s where flap wheels step in.

Pictures show items from Norton’s range, but other brands have comparable products. Expect to pay a premium for high-end items, but expect the reward of better performance and longer life.

Where can you use flap discs? At the three stages stage of repair and fabrication — surface prep, metal removal, and cosmetic finishing.

1. Surface preparation. Here the broad class of non-woven discs is good at getting shot of paint, mill scale and light rust, along with platings like zinc galvanizing and other corrosion-preventers. Rather than holding an abrasive in a rigid and reinforced resin, non-woven discs put it in a web of synthetic fibers and hold it with a dry smear-resistant adhesive. Various types and grades of abrasive can be held in the web, which is supported by a thin, consumable backing plate. By adjusting the type of abrasive and its grit size, makers generate a family of non-woven discs able to grind at one extreme and polish at the other. Non-woven discs do a fine job removing light rust, paint, scale, and other weld contaminants from metal, plus they can strip unwanted crud or paint from cement or even wood. Some types can handle relatively heavy grinding work as well, depending on the pressure applied, and are can even work on rough, snaggy material. But naturally around such surfaces, the softer the disc, the more care that you need to take to avoid damaging it.

Stumped for Speed?

How do you find the safe maximum rpm that a wheel can be spun at, if all you can see is its max peripheral speed? A little recreational math is called for:

So for a wheel of 230mm (9in) diameter and a max peripheral speed of 80 m/sec:

Which rearranges to

So in this example, safe maximum rpm = 6,642.

A non-woven disc is typically ½" (12mm) deep, that is, front face-to-back face. When the front face is used at a shallow 15°–20° angle, its generous area lets you cover a lot of ground quickly. Or where access allows, you can use the outer rim in the angle of a joint for a final clean before welding. Negatives compared to resinoid discs are more-orless sensitivity to sharp edges, a shorter service life, and higher cost/life ratio.

2. Metal removal. Usually this involves tidying-up joint edges before welding and if necessary, beveling them. The ability to grind a decent bevel also comes in handy when sharpening hedge cutter flails and mower blades. Resinoid discs will do the job, but producing a flat, uniform bevel is a bit of a lottery. Instead, a flap disc produces a much better finish and is more controllable. Flap discs are made from overlapped strips of abrasive cloth anchored to (and supported by) an angled fiberglass backing plate. The plate’s angle is a shallow 15°, which automatically suggests the disc should be used at a similar low angle; this in turn produces a big footprint on the work and lets the flap disc deliver a flat, uniform finish. Beyond grinding bevels, flap discs find plenty of use in blending or tidying metal, de-burring edges and holes, or on any other work where their forgiving, contour-following nature can be exploited. The downsides? You can’t grind with a flap disc’s edge, service life is shorter than for resinoid types, the per-item cost is higher, and the flat footprint makes cleaning out rust pits more difficult. Horses for courses, as they say.

3. Finishing. Where time allows it’s always satisfying to bless a job with a good cosmetic finish. Not a mirror polish (though that’s possible) but knocking off weld spatter and blending-in bead edges, then perhaps stripping off all old coatings on repaired parts before repainting. Here a flap wheel and nonwoven discs can work as a team: a flap wheel for initial metal removal and a non-woven one(s) for final blending and stripping. This suggests each has its own exclusive territory, though in reality their applications overlap. In common their big contact area means fast work, and as ever with abrasives, smaller grits deliver a finer finish — see “Graded Grains.”

Graded Grains

By juggling the abrasive and bond, resinoid and vitrified wheels are produced. Within these the bond strength, the abrasive type and the grit size suits the disc or wheel to different work.

Resinoid cutting and grinding wheels use several different abrasives. Two are very common and suited to farming applications:

• Aluminum oxide (alumina). A general-purpose abrasive for grinding and cutting metals only. Will not cut stone. Naturally occurring or synthesized.

• Silicon carbide. Recommend for stone but will also cut most steels. Sharper grains than aluminum oxide.

With either, grit size choice is narrow and not too important in general farm work.

Good-quality resinoid wheels carry a hardness grade letter — see Table 2.1. Grades show the relative holding power of the bond: soft grades have earlier letters in the alphabet and release abrasive earlier, so exposing fresh sharp grains.

Vitrified wheels give a wider choice of abrasive. The most common are:

• Aluminum oxide. Color white or brown; general-purpose use.

• Silicone carbide. Color green. For harder materials including high-speed steels and tungsten carbide tooling, nonferrous metals and cast iron.

• Ceramic. Color blue. For hardto-grind materials. A long-lasting high-performance material.

• Alumina-zirconia. Comes from the fusion of aluminum dioxide and zirconium dioxide, toughening the alumina. Extremely hard grains with high resistance to dulling.

Grit size choice is also wider in vitrified wheels, running from 24 (coarse) to 120 (fine). Which to buy depends on the application: coarse grits give faster material removal, finer grits give a finer finish. One option is to mount coarseand fine-grit wheels on one bench grinder, say a 46 grit on one side and a 60 grit on the other. Alternatively, fit an aluminum oxide wheel and a silicon carbine one so the one grinder can handle a range of materials.

In general, softer-bond wheels are best for hard materials like drills, lathe tools, chisels, and so on, along with large contact areas and rapid material removal. Harder grades are more general-purpose and are better for softer materials, on small contact areas and where longer service life is important. Medium-hardness wheels suit most farming applications.

The Safe Angle

Flap- and non-woven discs are slightly less threatening than the resinoid variety. Although spun at the same rpm by the grinder, fragments from a non-woven disc generally have less mass and so lower impact energy. That’s not to say that the basic rules of angle grinder use can be ignored, though, so . . .

• Mount the disc securely using the right flanges.

• Use the grinder’s guard, set so it’s completely between the disc and you.

• Wear eye protection and a mask that protects against abrasive/matrix/material dust.

Practicalities

For best performance, 4½” (115mm) flap or non-woven discs with ceramicbased abrasive should be used with a 1,000W–1,500W angle grinder. Why? Lower-powered machines won’t necessarily put enough energy into the abrasive to fracture its grains (a process necessary to create fresh cutting edges) so the disc can’t perform as intended. Other considerations:

• As non-woven discs are substantially thicker than a conventional dished item, you may need to add a cranked section to the grinder’s pin spanner so it can reach the spindle flange.

• Disc depth means that a quick-release guard can make it easier to change non-woven discs

• Non-woven and flap discs have less dense, more flexible base material than resinoid ones. So they follow contours better, feel less harsh to use and generate less vibration — but they do shed many tiny flecks of matrix as they wear, so post-work cleanup needs to be thorough.

How Does an Abrasive Wheel Cut?

The way discs and wheels cut or grind is pretty nifty. Abrasive grains — which today are often synthetic — are held in the wheel’s bonding matrix. Each grain is like a small chisel, able to slice off slivers of material. Like any cutting tool, a grain loses its edge after a while so starts to rub more than it cuts, so friction goes up. The grain’s temperature climbs until it melts the holding matrix, at which point the spent grit is flung free and fresh, sharp abrasive is exposed. Clever, eh?

Can I . . .

. . . grind on the side of a vitrified wheel? Tempting, especially if the face of the wheel is not flat and you need to sharpen a drill or a lathe tool on it. But it’s risky: conventional flat vitrified wheels are not designed to stand side loads and can shatter. Instead, dress the wheel’s proper working face flat and use that (see photos 2.16a, 2.16b, 2.17).

. . . use a worn-down 9” (230mm) disc in a smaller angle grinder? Another temptation to avoid! Spindle speeds on smaller grinders must be relatively high to produce the right peripheral speed on a small-diameter wheel. So there is a real risk of over-speeding a worn larger 9” (230mm) wheel, even though its diameter is much smaller than when it was new.

Vitrified Wheels

To round off this section we’ll gallop through the third major type of abrasive wheel — those used with bench grinders and low-low-powered craft tools.

As we said earlier, vitrified wheels hold their abrasive in a mineral or synthetic material. During manufacture the wheel is fired in a kiln at very high temperature, which turns the material into a hard, brittle and glass-like bond. This leaves vitrified wheels more prone to shock damage than resinoid discs discussed last time, and more rpm-sensitive. Their relative fragility means you must take particular care in mounting them and in use, and key points are covered in pictures.

It’s essential to make sure that the grinder can’t spin the wheel(s) too fast. Both the wheel and grinder should have info on their respective speeds (peripheral/rpm on the wheel, spindle max rpm on the grinder), and as before the grinder must spin more slowly than the wheel’s allowable maximum. If you can’t be sure that the wheel will be working at a safe speed, don’t mount it!

And it’s doubly important to check a vitrified wheel’s physical integrity before you fit it. Look for cracks or damage, and do a ring test. The highest risk of bursting or fragmentation comes each time a wheel accelerates from rest, so once a new wheel is mounted, stand clear on start-up and run it for a minute or two to make sure all is OK. You should treat a vitrified wheel with particular care when grinding; make contact with it gently and let the wheel do the work. Don’t force material in, and keep the work down on the grinder’s tool rest.

Again stick by the six rules:

1. Match the wheel to the job.

2. Be sure that the machine can’t spin the wheel faster than its maximum allowable peripheral speed or rpm.

3. Mount the wheel properly.

4. See that machine guards are in place and secure.

5. Respect the wheel’s design capability in use.

6. Wear personal protection for eyes, hands, ears, and lungs.

2.10a. and b. A bench machine’s info plate should show spindle rpm (2.10a); measure it using a tacho if not. Likewise the wheel should carry its maximum peripheral speed and/or rpm (2.10b). If the wheel won’t be spun faster than its design speed and is physically in good shape (see 2.11), then go ahead. If you can’t find the wheel’s maximum speed, DON’T USE IT!

2.11. Vitrified wheels can stress-crack if not stored and handled with care. So keep them dry, laid flat or stood on edge. Surprisingly they are porous and will take up water, which can unbalance the wheel and increase the risk of bursting. So before fitting any vitrified wheel, check it visually for water stains, cracks and chipping. Then support it on a finger and tap it with a length of hardwood, not a metal tool. A good wheel will ring, a damaged one will produce a dull clunk. The ringing won’t be like a bell but you’ll recognize it.

2.12. Bench grinder wheels commonly have a center bore of 1¼" (32mm or 31.75mm). This is much bigger than the spindle of smaller machines, so bushes are needed to match one to the other. Usually plastic, these are slightly tapered, come in pairs and are readily available. It’s essential that the bushes are a snug fit in the wheel. Don’t use Imperial ones in metric wheels as they are slightly too small; the resulting slack lets the wheel run eccentrically, greatly increasing the risk of bursting. Metric bushes are OK in Imperial wheels as their taper accommodates the size difference. Also make sure that the wheel’s two soft paper blotters are in place. These cushion the wheel against surface roughness, which will otherwise act as stress raisers (see photo 2.13).

2.13. Vitrified wheels are sensitive to point pressure. This can be the source of a dangerous stress fracture in use. Before mounting a wheel, check the spindle and both flanges for grit or metal fragments that, if trapped against the wheel, will concentrate stress (arrows).

2.14. Unplug the grinder before working on it. Tighten the spindle nut just enough to secure the wheel snugly: it will selftighten in use.

2.15a. and b. Plenty of accidents come from tool rests that are not set close enough to the wheel. The gap in pic 2.15a is too big — material can be pulled into it, risking damage to the work, wheel, and operator. Set a minimum clearance gap to a new wheel, then as the wheel wears down keep adjusting the tool rest to maintain the gap at less than ⅛" (3mm) (2.15b). If you can, set the tool rest on (or a little below) the spindle centerline to minimize the tendency of the work to pull into the clearance gap. When using a wire brush in place of a wheel, it can be safer to remove the tool rest.

2.16. Should a wheel burst, it will cause minimum havoc if the guards enclose as much of the flying material as possible (A). Use the grinder’s own eye shield (B). If that’s too scratched, switch to safety glasses or goggles. Grinding without eye protection is only for fools.

2.17a. and b. Vitrified wheels can clog with soft material or can glaze over (2.17a). Then the only way to expose fresh grains is to use a dressing tool: the cheapest and easiest option is a dressing stick. Available from engineering suppliers, the stick must be sharper-grained than the wheel if it’s to work. So for example, choose silicon carbide for an aluminum oxide wheel. Use the stick as shown in 2.17b. Dressing is also an informal way to bring the face of a wheel back to flatness before sharpening a drill. The softer bond of resinoid angle grinder discs means they self-clean, so don’t need dressing.

2.18. A star wheel dresser exposes fresh grains, takes out minor eccentricity from the wheel to cure vibration, and/or reshapes the grinding surface for specific jobs. Available in different sizes, they use replaceable hardened, serrated, free-running wheels to dislodge old abrasive. Dressing is a dusty job, so be sure to wear protective gear and cover anything close by that matters. Be sure to re-adjust the grinder’s tool rest when you’re done!