1. Scope--Blast cleaning may be defined as a secondary manufacturingprocess in which a suitable stream of solid particles is propelled withsufficient velocity against a work surface to cause a cleaning orabrading action when it comes in contact with the workpiece.

As indicated in the definition, blast cleaning may be employed for avariety of purposes. Ordinarily, it is considered as a method forremoving sand from castings, burrs or scale from forgings, millproducts, or heat treated parts; to promote machinability, and tominimize the possibility of interference in actual operation. Inaddition to this use, blast cleaning also produces an excellent surfacefor industrial coatings. All these objectives are often accomplished inthe one operation.

1.1 History--The cleaning problem of removing sand and scale has alwaysbeen associated with the casting, forging, and heat treating of metal.As recently as the beginning of the twentieth century, foundrymenconsidered the chisel, hammer, dull file, and wire brush the chiefweapons for attacking this problem. Hand tools were gradually augmentedby "rattling" or tumbling methods.

Pressure blasting was first introduced in 1870 by Gen. Benjamin ChewTilghman. He discovered that metals, stone, and glass could be shadedor etched by jets of sand. He took out patents covering pressureblasting with sand driven by compressed air, steam, and water; withsand struck by a paddle wheel, thrown centrifugally, or dropped from aheight through a tube. Thus, General Tilghman advanced the principlesupon which modern blast cleaning is based.

Tilghman's first commercial machine used a steam blast as a method ofpropelling the sand. This proved to have several disadvantages. Thesteam moistened the sand, necessitating a drying operation. It alsotended to hide the work, break glass objects, and rust metals. As aresult, Tilghman changed to a tank-type compressed air machine, whichproved more successful.

Other men and companies entered the blast cleaning equipment field. Anatural development was the blasting machine in which sand could berefilled during operation. This was accomplished by pouring sandthrough a standpipe of sufficient height to overcome tank pressure.

Cabinets and barrels featuring pressure air blasting nozzles wereadvertised in magazines in 1895. That same year W. W. Sly introducedthe exhaust tumbling barrel, which also increased the dust removalproblem. This was partially solved by the first cloth screen dustarrester patented in 1897.

Many industries recognized the superior surface quality of castings,forgings, heat treated parts, etc., cleaned by blast cleaning. However,the high operational cost and low productivity of the process at thattime limited its use to the cleaning of large castings or productswhere high quality was necessary regardless of cost.

An old leanto in back of the plant comprised the up-to-date blastcleaning department of 1915. A blower fan provided the ventilation.Wooden walls which splintered were soon replaced by steel sheets. Thesand blasting distorted the sheets and cut the nails. Brick was usednext and proved more durable. However, the silica dust produced washarmful to the operator.

In 1917 humane sand blast rooms were developed in which the operatorstayed out of the blast zone. Downdraft ventilation, rubber-lined steelwalls, and better lighting all served to improve the sand blastingoperation.

A new phase of development took place with the introduction of metallicabrasives in the 1920's. Acceptance was slow, due to the availabilityand low first cost of sand. Industry soon recognized the improvement inthe quality of the finish and the lower cost, through increaseddurability of the shot. Improvements in reclaiming the metal abrasiveeventually won over most of industry. One of the chiefbenefits--savings in storage space and handling--was not appreciateduntil some years after the adoption of metal abrasives.

The development of metallic abrasives began with the use of chilledcast iron shot. The comparatively short shot life of chilled cast ironlead to the development of malleabilized and annealed cast iron shot.Next to be introduced into the metallic abrasive field was cast steelshot and more recently cut wire shot. Although these latest shot have ahigher initial cost than the iron shot, their life is much greater. Inmany instances, they have proved to be more economical than the ironshot. These newer shot demand that the blasting equipment be operatedto minimize shot losses.

As more companies produced metallic abrasives, more and more name andclassification systems were used. This resulted in the need forstandardization of abrasive classification for the entire blastingindustry. A big step in this direction was taken in 1943 when a groupof shot producers, users, and equipment manufacturers met in Detroitand formed the Shotpeening Committee of the SAE. This committeeestablished size and nomenclature standards for shot and grit (SAEHandbook). Also, the committee has worked, and is still working, on astandard testing procedure for the endurance and wear of metallicabrasives.

With the introduction and acceptance of metallic abrasives, the blastcleaning suppliers were able to offer industry their next bigimprovement--the use of centrifugal force for blast cleaning. Here theabrasive is thrown by a revolving wheel, which propels the abrasivemechanically. The idea was first proposed in the 19th century butabandoned. The large volume of sand that even the smallest machinesrequired, plus the excessive wear of sand on metals, made its useprohibitive. In 1933 the American Foundry Equipment Co. demonstrated atBenton Harbor, Mich., the use of a barrel-type machine using a wheel topropel the abrasive. Later, the Pangborn Corp. introduced a similarunit at Detroit, and another was introduced by the W. W. Sly Mfg. Co.in Cleveland. Among later improvements was the construction by theCargill Detroit Co. of operatorless single-purpose equipment to provideprocess control on certain high production parts.

Mechanization of blasting equipment, metallic abrasives, and the use ofcentrifugal force led to the development of more high productivecleaning equipment to meet growing production demands. The improvedblasting equipment developed includes the tumble type batch cabinets,continuous monorail cabinets, rotating tables, and the latestdevelopment--the continuous barrel type.

In the field of general cleaning the centrifugal or airless type is byfar the most popular means of metallic abrasive propulsion. However, inthe case of specialized or precision cleaning, air propulsion ofmetallic abrasives is more adaptable because of its ease of control andgreat flexibility. As parts become more complex in size and shape,specialized cleaning becomes more and more necessary.

Work in the development of abrasives, blasting equipment, processes,and standization goes on. The blast cleaning history has closelyparalleled that of mass production. The blast cleaning industry hasmade many valuable contributions to the industrial growth of America,and will continue to do so.

1.2 Present Status--The present trend of blast cleaning, with fewexceptions, is to use the continuous-barrel type of equipment withcentrifugal blast wheels for all small castings that can be handled asbulk material. The use of the continuous-monorail type of blast cabinetequipment with centrifugal abrasive propulsion for large castings andforgings is very extensive. These two types of equipment are fastreplacing the old pressure blast equipment. The centrifugal type ofblast equipment has proved more economical per ton of cleaned castingsthan the previously used pressure blast equipment, with few exceptions.An exception to the preceding statement is the specific specializedtype of pressure blast equipment on a part that has internal pocketsand on which it is necessary to direct the blast stream at a small areathat cannot be reached with the widely used centrifugal-type equipment.

The present trend in ferrous metal blasting abrasives is towardmaterial having superior breakdown resistance. The result of thisprogressive change is a lower cost per ton of cleaned castings withreduced abrasive material breakdown provided adequate auxiliaryequipment can be installed adjacent to the blast cleaning machine tosalvage the blast material that otherwise would be carried out inpockets of the cleaned work and lost.

1.3 Secondary Effects

1.3.1 Combined Cleaning And Peening--Combined cleaning and peening isapplicable to parts where it is necessary to remove scale, providedthere is no subsequent heat treatment. Parts that are being treated inthis manner include automotive connecting rods, axleshafts, andsteering knuckles. However, on parts such as axleshafts and steeringknuckles, which require machining in critical areas, some of theeffectiveness of the peening is lost.

Parts with small surface imperfections may be improved by lessening theeffect of the stress-raisers during the cleaning operation.

Some controversy exists on the question of the inspection of blastcleaned parts. It is thought by some that small defects, ordinarilybrought out by pickling, will be obscured by the blasting operation.Others claim that these small defects are made less detrimental by thepeening action of the cleaning operation. Some surface defects may beconcealed to the extent that an inspection other than visual may benecessary.

1.3.2 USE AS AN INSPECTION TOOL--Blasting is applied to facilitateinspection of selectively hardened parts; this blasting often serves asa cleaning operation as well. The inspection of chilled iron parts anddecarburized areas on hardened parts is made easier by blasting. It isalso possible by blasting to show leaks in masked areas aftercarburizing and hardening.