Читать книгу Automotive Machining - Mike Mavrigian - Страница 7
ОглавлениеPrior to test fitting and/or final engine assembly, cleanliness is absolutely critical. This includes every component involved (block, crankshaft, connecting rods, pistons, camshaft, timing system, oil pump and pickup, oil pan, valvecovers, intake manifold, cylinder heads, rockers, pushrods, lifters, timing cover, etc.). There is no such thing as “too clean.”
A variety of cleaning methods are available, depending on the application. These include hot tanks, spray cabinets, ovens, airless shot blasting, cabinet media blasting, tumbling, and manual cleaning involving hand or power tools.
An airless shot blaster slings steel (or stainless steel) shot onto the block or cylinder heads as the part is rotated while secured in an adjustable cage. Compressed air is not used. Following shot blasting, the part must be tumbled in a rubber-lined drum to shake any remaining steel shot from the part.
An airless shot blaster cabinet uses a high-speed impeller that blasts steel shot at the parts as the parts slowly rotate in a cage. This machine does not use compressed air. The steel (or stainless steel) shot is about .030 inch in diameter. After the part has been blasted, the part must then be tumbled in a large tumbling drum to remove any remaining shot. Airless blasting with steel shot is intended for cast-iron parts only and should not be used on softer aluminum parts. Components may only be airless shot blasted after they have been degreased and are thoroughly dry.
Solvents heated at approximately 170 degrees F break down contaminants faster than room-temperature solvents. However, solvents must be chosen carefully to be compatible with ferrous (steel or iron) or nonferrous (alloys) components. After a part has been cleaned in a hot tank, it must be rinsed in hot water and then blow-dried. At this point, steel or cast-iron surfaces tend to surface rust very quickly, so application of a rust inhibitor is required as soon as possible. Hot tanks are available in various types, including hot-soak and hot-soak with agitation and spray jets.
A “jet” spray machine uses heated solvent and a series of high-pressure spray nozzles. The parts being cleaned are secured in a cage or appropriate mount, on a turntable. The part is rotated during the wash and rinse cycle. Liquids, depending on the application, can include hot water and/or solvents appropriate for the material being cleaned. High-caustic solvents may be appropriate for steel or cast-iron; a detergent-based cleaner is best for aluminum parts.
Threaded Holes
Regardless of the cleaning method, all threaded holes in the engine block should be inspected and cleaned. Be sure to clean all threaded bolt holes and give special attention to the cylinder head deck and main cap threaded holes. Using a rifle-style bristle brush, scrub all female threaded holes. To make sure that threads are in good condition, and to remove any contaminants/debris, it’s a good idea to run a chasing tap through all threaded holes.
Do not confuse this with a cutting tap. Common cutting taps tend to remove metal, which is to be avoided. Chasing taps are specially designed to both clean and re-form existing threads without removing excess material. Especially for main cap bolt holes and the block’s cylinder head deck bolt holes, using a chasing tap helps to ensure that the threads are clean while retaining the necessary strength and integrity.
Scrub the block exterior with a clean, soft brush soaked in hot water and Dawn and rinse. I’m not trying to promote the brand of detergent, but for some reason, Dawn seems to do the best job of removing oils and grease. When all internal and exterior surfaces have been thoroughly rinsed, blow clean compressed air through all bolt holes, passages, and exterior surfaces. At this point, when the block is dry, immediately apply a thin coating of clean engine oil to main saddles, lifter bores, and cylinder bores to prevent surface rusting. Cast-iron blocks tend to oxidize (surface rust) very quickly when clean and dry, so this is a good time to mask and paint the block exterior. If the part is not to be painted, apply a thin coat of a rust inhibitor to all surfaces, especially if the part is stored for a while prior to assembly.
All threaded holes in a block should be chased to ensure cleanliness and thread condition. Here, a block’s cylinder head deck hole is being chased. Drive the tap with a hand driver only. A variety of sizes is needed to cover all holes. The most critical threaded holes include the block’s head deck holes and the main cap holes.
Commonly available parts washers that use a cold (unheated) solvent are useful for degreasing smaller parts such as connecting rods, pistons, camshafts, etc., but this requires allowing the parts to soak and then be hand-brushed or scraped to remove grease and other solvent-soluble contaminants. However, since no pressure is involved, this may or may not remove contaminants from blind holes or passages. Also, this process is time-consuming and is not generally used in an engine builder’s shop.
A hot jet spray wash is commonly used to clean blocks, cylinder heads, crankshafts, and more. The table rotates as high-pressure heated solvent or detergent is sprayed.
This block has been oven cleaned, tumbled, and washed in a jet spray cabinet to a like-new finish.
Often referred to as thermal cleaning, a dedicated oven can be used to degrease an engine block or cylinder heads. A dedicated cleaning oven “cooks” the component, turning grease, oils, paint, and other contaminants to ash. The component is initially heated to about 375 degrees F to cook off surface vapors, which are then oxidized in a separate chamber at about 1,300 degrees F. The oven’s primary chamber then rises to a higher preselected temperature of about 600 to 700 degrees F, for steel or cast-iron components. When the cleaning cycle is complete, which usually takes about three hours or so, all sludge contaminants have been reduced to easily disposable ash. After the component has cooled, it is then washed/rinsed in a hot tank to remove remaining particles.
Aluminum components require more care because extreme temperatures can result in distortion. Typically, aluminum castings cook at about 500 degrees F. This lower temperature also reduces the chance of valveseats and guides loosening in cylinder heads. Regardless of material (steel, iron, or aluminum), the oven temperature must drop very slowly after the cooking stage until reaching room temperature, to prevent metal distortion.
Although a hot tank accumulates sludge and requires periodic cleaning and proper disposal, the use of a cleaning oven eliminates the need to dispose of hazardous waste (sludge), making ovens a bit more environmentally acceptable.
Although most cleaning ovens are designed to rotate the part during the heat cycles to evenly heat the part, some engine builders like to preheat some parts to cook off any residual oils, prior to glass bead blasting.
This block has been degreased, but surface rust and some contaminants remain.
The same block has now been shot blasted and tumbled. The block is now ready for machining.
There are instances where washing/scrubbing and rinsing by hand are applicable, such as performing a final wash and rinse of an already-machined engine block prior to assembly. Using very hot water (as hot as your hands can tolerate) and Dawn dishwashing liquid can produce excellent results. With the block on a stand that allows you to rotate the block, shoot hot water into all oil and coolant passages, and then pour Dawn into the same passages. Using dedicated (and clean) bristle brushes that are designed for block cleaning, scrub passages as access permits; follow with multiple hot water rinses until all traces of soap are eliminated.
This Chevy big-block has been stripped down and placed into a cleaning oven. The block is secured within a tubular cage. With the cage mounted in the oven, the machine rotates the block during the thermal cleaning process to distribute heat evenly, cooking off all grease, paint, rust, and sludge.
Here, the airless shot blasting process has been completed, resulting in a clean, down-to-bare-metal block that’s ready for tumbling. After the block has cooled down in the oven, the block, still mounted in its cage, is moved to an airless steel shot blasting cabinet, which cleans off the ash created by the oven.
An enclosed blast cabinet can be ideal for cleaning certain parts, such as pistons, connecting rods, mounting brackets, valvecovers, etc., but the media must be selected carefully. A variety of blasting media is available, including but not limited to “glass bead,” crushed walnut shells, crushed corncobs, crushed pecan shells, plastic bead, and soda. Depending on the media, you can accomplish not only cleaning to bare metal, but also lightly deburring razor-sharp edges where desired. Never use a high-abrasive media such as sand, which is simply far too aggressive. If an abrasive media is used, such as glass bead, the component must be thoroughly and carefully washed and rinsed afterward to remove all bead particles. For that reason, it is not wise to glass bead blast an item such as an intake manifold, because the possibility of particles being trapped inside runners is too great.
The cage is then disassembled and removed from the block. Using the shop’s overhead crane, the caged block is removed from the shot blaster.
Finally, the block is placed into a rubber-lined tumbler drum. The drum rotates, allowing the block to tumble randomly. As the block tumbles, any remaining steel shot is knocked loose and removed from the block.
Block Cleaning Brushes
Regardless of the method of degreasing used, always clean oil and water passages with appropriate-sized bristle brushes. A long-bristle rifle brush can be used to clean oil galley passages from the front to rear of a block. Small-diameter rifle brushes can be used to clean oil passages in crankshaft main and rod journals. A dedicated-diameter, long-handled rifle brush can be used to clean camshaft bores. Very small-diameter specialty rifle brushes are also available specifically for cleaning the inside of oil-through pushrods. After brushes have been run through any passage, the passage must then be rinsed with pressurized hot water followed by blowing dry with compressed air. After you’re finished with brush work, and before you store the brushes, wash all brushes with hot water and detergent and rinse with hot water to remove any contaminants from the bristles.
Cabinet Tips
Blasting cabinets are wonderful pieces of equipment to have in any shop for cleaning, deburring, and/or achieving a soft “footprint” for planned coatings. However, long-term use creates some wear issues. The viewing window (usually glass) can eventually become etched, even if you’re not blasting directly into the window. Although this doesn’t affect function, it makes viewing more difficult. Consider installing peel-off clear window film on the inside surface of the glass. Just like peel-off clear film that’s used on many race helmet visors, as the exposed layer of film becomes etched/foggy, you simply peel off to expose the next layer, or peel off the existing layer and install a fresh layer. As an example, Goodson Tools & Shop Supplies offers these films (you can cut to fit your window) under P/N GB-FILM. The use of this protective film saves the expense and time of replacing the viewing glass.
Keep an eye on the cabinet door gasket. Especially if the cabinet is designed to open from the top (with the weight of the door continually compressing the gasket), the seal can eventually become compressed, leading to a loss of cabinet vacuum (assuming your cabinet features a vacuum pump) and external media leakage. With the cabinet door closed and matched, when you turn on the cabinet’s vacuum pump, your glove arms should quickly be drawn into an extended position (as though they were being inflated). If not, you have a vacuum leak.
Blasting cabinet gloves are always an issue, especially when you’re blasting smaller handheld items, where you end up blasting into the glove fingertips. Buy the highest quality gloves available to extend fingertip life (avoid the bargain imported stuff). A variety of glove styles is available, some made as one-piece (glove and arm) and some with replaceable gloves (attaching to the arm sleeve with a clamp). If you’re replacing the arm sleeves (whether the arms feature integrated gloves or separate gloves), you need to order them with the appropriate port hole size so that the entry of the sleeves fits your cabinet’s ports. Also, make sure that the sleeves are long enough to enable you to reach all the way to the rear of the cabinet. Short arms are annoying, forcing you to stop work and open the cabinet to retrieve an item that’s out of reach.
Pay close attention to the cabinet’s interior lights. Use only the type of bulb recommended by the cabinet manufacturer. The bulbs must be heavy duty, with outdoor-grade glass that’s thick and withstands accidental blasting.
If you expect to rely on your blasting cabinet, keep spare consumables at the ready to avoid downtime. This includes spare light bulbs, gloves, a viewing glass, blast gun nozzle (ceramic or carbide), feed hose, hose clamps, and door seals. Preplanning avoids the need to place an emergency order from an out-of-town supplier while the cabinet sits idle.
Depending on the location of your blasting cabinet, you might consider a freestanding welding curtain that surrounds the cabinet area. Unless the cabinet is located in a separate “dirty” room, this minimizes airborne media dust that can easily scatter throughout the shop.
Prior to glass beading any part, the part should already be thoroughly degreased, removing all grease, oil, and soft sludge. Otherwise, bead sticks to these contaminants, potentially trapping bead in crevices and holes. If a cleaning oven is available, the part may be cooked for a few minutes to remove any oils prior to glass beading.
Glass bead blasting cabinets feature rubber gloves that allow the operator to handle and manipulate parts in the cabinet. These gloves are prone to wear and should be inspected on a routine basis. Gloves are available in various sleeve diameters and lengths to accommodate all blasting cabinet models. A blasting cabinet features a glass viewing window. The inside surface of the glass eventually becomes abraded, making it difficult to see through. Depending on how much use the cabinet sees, plan to replace the glass about once each year.
Blasting cabinet guns feature a ceramic nozzle that provides the appropriate media spray pattern. The ceramic nozzles, although very hard, eventually wear, lowering media blasting efficiency. Nozzles are easily replaced. It’s wise to keep a few new spares handy.
A view of a hopper in a glass bead cabinet. Depending on the cabinet’s hopper design, media may start to build up on the tapered walls. In conjunction with airborne moisture absorption, the media is less able to accumulate over the media feed tube. Keeping the hopper full with the required level of fresh media helps to avoid this.
Maintaining a blast cabinet’s clear viewing glass and adequate dust vacuum improves worker visibility.
If the part to be cleaned features hidden passages that cannot be positively cleaned, avoid blasting with an abrasive media. Also, avoid bead blasting into female threaded holes, since beads can be trapped, and the abrasive action can degrade the thread integrity. After bead blasting, use compressed air to flush all traces of the bead particles. Do not try to initially remove bead particles with a wet rinse, as this can easily cause beads to be stuck to surfaces, possibly in small clumps inside any passages. Also, never use an abrasive media on a part that is wet or features oil or grease. The part must be degreased, rinsed, and dried prior to blasting.
Concerns about potentially contaminating threaded holes with media can be avoided by plugging these holes with temporary bolts. After bead blasting, remove the bolts and manually clean female threaded holes with a chaser tap and solvent. I discuss the use of chaser taps later in this chapter.
Soda (sodium bicarbonate, essentially a form of baking soda) is a nice choice, since it’s water soluble and allows easier removal via rinsing. Soda’s benefits include being able to clean even surfaces that are still contaminated with oil or grease. Soda does a great job of cleaning, but it does not allow you to soften sharp edges because it’s not as aggressive as other media such as glass bead. Although soda might be usable in a conventional blast cabinet, it’s best to use a blast system that is designed for soda for highest efficiency and dust reclamation.
There are distinct advantages to the use of soda, instead of traditional methods using glass bead or plastic. Unlike these other medias, soda easily removes surface grease and oil residue, as well as paint, eliminating the need for a precleaning step. One distinct benefit relates to cleaning an intake manifold (especially a dual-plane-style manifold). The danger when using glass bead (or other abrasive media) lies in the concern for particulates becoming trapped inside hidden/hard-to-reach runner areas, which could lead to disastrous results in a running engine. Because soda is water soluble, entrapment is avoided with a simple water rinse. Any particles that might remain don’t pose an abrasion danger (because they’re not hard abrasive particles) and effectively break down during engine operation.
Benefits of Soda Blasting
• Soda is granular, suitable for use in pressure pots or in cabinet systems.
• Soda crystals are sharp and provide outstanding cleaning/stripping performance when delivered under relatively low pressure but with high velocity.
• Soda is “friable,” meaning that it fractures into smaller particles. This increases cleaning performance while softening blasting impact. Unlike other blast media, soda is relatively soft and doesn’t damage metal surfaces.
• Soda is nontoxic and nonhazardous, with a nearly benign pH of 8.2.
• Soda is water soluble and easy to rinse clean.
• Soda leaves no abrasive “grit” that can damage moving parts.
A wide range of blasting media is available, each with its own characteristics. Depending on the material to be blasted, including cast-iron, cast-aluminum, steel, etc., and type of component including blocks, heads, manifolds, pistons, connecting rods, etc., and the desired surface finish, attention must be paid to selecting the appropriate type of media for the specific application.
Sand
Commonly available silica sand is far too aggressive and should never be used on any engine-related surfaces. This is often used on car components, body panels, frame rails, and suspension parts. But sand is so abrasive that it literally eats away at soft aluminum, pits cast-iron, and does irreparable harm to other steel parts.
Glass Bead
Depending on the specific grade (grit), glass bead media is produced as preformed tiny balls, which, depending on the grade of glass, produce a smoother and “brighter” finish than angular abrasives (results in a matte satin finish). Glass bead, like most blasting media (other than soda), does not act as a desiccant, so the component must be dry and grease-free prior to blasting. Grease, oil, etc., are eventually removed during blasting, but over the long haul, you end up contaminating the glass bead in the hopper. The same applies to layers of paint that flake off easily. Large flakes (larger than, say, your little fingernail) collect in the hopper and eventually clog the feed tube. When/if this occurs, a quick fix is to place the tip of a bolt into the gun nozzle and hit the foot pedal. This causes the bead to “back-flush,” temporarily clearing the system for continued use. Regardless, glass bead or any blast media should be routinely changed to remove foreign contaminants and to renew the capabilities of the media.
What most people don’t realize is that a huge range of glass bead is available, from very, very aggressive down to ultra-fine grades. All glass bead is not created equal, so pay attention to what you’re buying. A fine-grade glass bead produces a matte satin finish.
Crushed Glass Grit
Similar to glass bead in initial appearance, but the particles feature random shapes and sharper edges. This cuts faster and is more aggressive than glass bead, and produces a medium texture.
Soda
Unlike hard and aggressive media such as sand, baking soda (sodium bicarbonate) is able to strip to bare metal without abrading the parent surface. Soda media is “softer” than other types of media and is “friable,” which means that the soda crystals break down upon impact (akin to throwing a snowball against a brick wall as opposed to throwing a rock against the same wall). Essentially, soda does the job without damaging the metal. Because soda is “softer,” and because it is water soluble, it leaves no abrasive grit that can damage moving parts (such as hood or door hinges).
Soda is nontoxic and nonhazardous, making its use far safer than other abrasive or chemical stripping methods. However, dry blasting generates a great deal of dust. For this reason, it is recommended that a dust mask be used, as well as eye and hearing protection.
In addition to blasting sheet metal material for body refinishing/restoration, soda is also an outstanding choice for blast-cleaning components such as frames, suspension parts, brake parts, engine components, etc. Because no hard abrasive grit is present, components such as engine blocks, cylinder heads, intake manifolds, etc., can be cleaned and then rinsed without concern for trapping damaging grit particles.
Aluminum Oxide
Depending on blast pressure, this can be very aggressive and leaves a coarse, textured finish. It’s economical, but be aware that it cuts very quickly and removes more metal. Used properly (level of blast pressure), you can avoid a too-rough finish.
Speed-Beed
This is a hybrid mix of 50- to 80-grit glass bead and 80-grit aluminum oxide. This provides faster cutting than glass bead but is not as aggressive as aluminum oxide.
Plastic Bead
Generally available in 30- to 40-grit size, plastic bead is good for removal of paint and surface rust. Plastic bead produces less blasting heat. This may be a good choice for cleaning gears, tooling, etc., because it doesn’t alter dimensions, and it doesn’t etch the surface.
A Special Note Regarding Intake Manifolds
Obviously, intake manifolds (single plane or dual plane) feature intake runners. Even after careful degreasing, blasting with an abrasive media can be dangerous, especially when dealing with a dual-plane intake manifold where you don’t have easy reach and access to all port surfaces. Fine media particles can embed into the material (or can be stuck in a remote piece of sludge or carbon), increasing the chance of that media eventually being sucked into the engine. If you do decide to blast an intake manifold with, for example, glass bead, you need to make absolutely certain that you eliminate any possibility of trapped particles inside the runners. If you feel the need to blast and you’re worried about leaving harmful particulates behind, soda is the best choice because it’s water soluble when rinsed. Even though the soda doesn’t pose a threat to the engine, any rust scale, dirt, etc., that’s held captive poses a serious risk. Regardless of the type of media you use, you still need to flush the runners surgically. If you do media-blast an intake manifold where you are uncertain about hidden debris, a follow-up cleaning in an ultrasonic unit along with a hot, soapy rinse/flush should remove any remaining contaminants.
Walnut Shell
Crushed walnut shell (aside from being biodegradable) is relatively gentle and is ideal for preserving surface micro finish while cleaning. Under controlled conditions, it’s also useful for those instances where you want to remove paint topcoats but retain the primer layer. Like most media, walnut shell is available in a variety of grit, from extra coarse to extra fine.
Corncob
Corncob is even softer, good for finalizing a nice satin finish. Either walnut shell or corncob can be very effective in removing paint and rust, without eating away at the base metal. And these organic types of media are relatively cheap.
A fine grade of walnut shell or corncob doesn’t remove any of the metal, it doesn’t etch or scratch softer metals, and the material doesn’t imbed into the metal surface. Walnut or corncob are great choices where a finer finish and retention of detail is important.
Steel Grit
A fine grade of steel grit leaves a relatively smooth finish. Steel cuts very fast and lasts longer (because it doesn’t fracture, as opposed to aluminum oxide). However, steel grit will likely create more wear and tear on your blasting equipment (nozzle, pickup tube, power head, hose, etc.). It is not for delicate parts or surfaces.
Silicon Carbide
This media stays sharper and lasts longer than other types, but is aggressive and creates even faster wear of blasting equipment.
Tumbling and Vibratory Cleaning
This method involves “tumbling” immersed parts in a soup of dry or wet media, where tumbling or vibration treats the parts. The media can be steel, ceramic, or other material that is available in a wide variety of shapes and sizes. The type of media depends on the desired results, which can include cleaning, deburring, or polishing. Tumblers come in a range of sizes, from small to industrial-large. The parts are randomly rotated in the media mix, with contact among the media particles stripping the parts down to bare metal. Small tumblers are excellent choices for cleaning small items such as valves, valvesprings, retainers, rockers, etc. Unlike blast cleaning, there’s no particle or dust cleanup required and no nasty solvents to deal with. To protect valvestems, a piece of runner tubing can be slipped over the valvestem.
Vibratory tumblers may use either a dry or wet slurry of stones. A wet slurry is commonly used for polishing as opposed to cleaning.
An ultrasonic cleaning system is another option for cleaning delicate parts where you want to avoid abrasives or strong chemicals. The parts are placed into a water tank, and frequencies are induced into the water to essentially vibrate particles loose. For example, this type of cleaning system is popular among shops that restore high-pressure fuel injectors for diesel engines.
Cleaning with Hand and Power Tools
In certain instances, surfaces may be cleaned of heavy buildup of paint, sludge, or carbon deposits if you don’t have access to the proper shop equipment. A handheld pneumatic scaler or scraper may be used to remove heavy sludge from a block exterior. A handheld scraper or pneumatic scraper may be used to remove stubborn gasket material. A flared/flat-face wire brush may be used on a drill or pneumatic die grinder on a block exterior, on cylinder head combustion chambers. Again, this is only to remove heavy deposits.
A Roloc bristle pad powered by a drill or die grinder may also be used for these same purposes. However, never attempt to clean or prepare any critical surfaces by hand, such as cylinder head or block decks, intake manifold decks, etc. You can easily damage the surfaces by creating waviness on the decks, which then requires resurfacing to regain proper flatness. You simply cannot obtain a flat surface, because you cannot control your hand and tool pressure adequately enough. Also, do not attempt to hone cylinder bores or lifter bores with the use of a handheld tool. You aren’t able to achieve a precise diameter or cylindrical uniformity.
By the way, Scotch-Brite pads, although wonderful for scuffing or cleaning some metal surfaces, have no place in an engine preparation room. The tiny fibers left over from use can easily contaminate not only the engine that you’re working on, but any other engines or equipment in the same room. Whether or not you use Scotch-Brite pads during initial cleaning, never have them anywhere near the engine during assembly.
A Roloc bristle head operated with a pneumatic die grinder can be used to remove deposits from various external surfaces.
