GB2166673A - Abrasive blasting - Google Patents

Abstract

A simple and reliable abrasive blasting device comprises an upright cylindrical metal container 1 with a removable top plate 2, and a jet block or T piece 6 attached to its bottom plate, including a jet nozzle 20 for feeding the abrasive medium into the side of an air passage extending from a compressed air source to a blasting nozzle 8. The air pressure is also applied to the top space of the container. The blast nozzle and metering jet have identical diameters. <IMAGE>

Description

SPECIFICATION Abrasive blasting This invention relates to blasting equipment of the kind using an abrasive powder such as sand or grit, carried in a stream of gas, usually air.

Apparatus of this kind is well known, in particular for the cleaning and surface finishing of metal articles, for example for removing rust from vehicle bodies and the like. Numerous forms of air-jet abrasive blasting equipment are known, but existing equipment is expensive, requires relatively high air pressures, and is inefficient with regard both to the abrasive effect and the amount of abrasive powder used. Because excessive amounts of abrasive are used, it is difficult or impossible to use existing air-jet abrasive blasting equipment in the open air and blasting enclosures are normally required. The excessive use of abrasive also increases the operating cost appreciably. The relatively high capital and operating costs of existing air-jet abrasive blasting equipment reduce the field of use of such equipment.

An object of the present invention is to provide abrasive blasting equipment which on the one hand is simple and inexpensive and on the other hand is of high efficiency with regard to the utilisation of abrasive material and with regard to the effectiveness of the abrasive effect.

The abrasive blasting equipment according to the present invention, of which a particular embodiment wili be described hereinafter, has several inventive aspects including the following.

The abrasive powder is held in an upright reservoir and is emitted through a small jet at the lower end of the reservoir into a stream of air under pressure, carrying the abrasive to a blasting nozzle, and the air pressure is also applied to a space above the abrasive powder. It has been found that this provides smooth and efficient feeding of the powder into the air stream, at pressures as low as 20 p.s.i.

The container for the abrasive is of substantially uniform cross-section throughout its height, or may taper gently, and its cross-sectional area is selected to provide optimum feeding of the abrasive powder under the internal air pressure.

The blast nozzle, and the jet aperture through which the abrasive powder emerges into the air stream, are of substantially equal diameter. The nozzle and jet can be plain cylindrical bores.

According to another aspect of the present invention, ceramic articles, in particular fired clay pipes, are cut by means of a gas-borne jet of abrasive powder, preferably but not necessarily using abrasive blasting equipment as set forth above.

At present, clay pipes are cut using annular diamond cutters, to form holes for receiving branch pipes, when making pipe junctions. Diamond cutters have disadvantages in this application, in particular when cutting a hole obliquely to make a Y junction : the localised contact of cutter and pipe and consequent uneven pressure make it difficult or impossible to guide the cutter accurately into the pipe being cut.

It has been found that an air-borne abrasive jet can provide rapid and accurate cutting of holes in clay pipes at any angle. The abrasive blast nozzle is rotated in a circle, manually or automatically, for example using a trammel attached to the pipe being cut, and cuts through the pipe wall in one or more circular passes. Because there is no direct mechanical contact between the blast nozzle and the pipe, there are no problems associated with localised contact and uneven contact pressures as in the case of a diamond cutter. Cutting accuracy is therefore enhanced.In the case of cutting to form an oblique joint, it may be desirable to provide for movement of the blast nozzle towards and away from the surface being cut, so as to maintain a suitable cutting distance throughout the circuit described by the nozzle during cutting, since the abrasive jet spreads with increasing distance from the nozzle and therefore its cutting speed, accuracy, and fineness of cut may be impaired if the distance between the nozzle and the pipe surface is excessive.

The accompanying drawings show abrasive cutting equipment embodying the present invention.

In the drawings: Figure 1 is a side view of air blast equipment embodying the invention, and Figure 2 is a section of a jet block.

The illustrated cutting equipment comprises an upright tubular container 1 for example of metal pipe, sealed with a top plate 2 and welded bottom plate 3. The top plate has a handle 4 and the bottom plate has three or four legs 5. The illustrated container is intended to be portable. Alternatively, it can be mounted on a fixed support, for example associated with an air compressor or reservoir.

Welded or otherwise fitted to the bottom plate, there is an inverted T-shaped jet block 6. This has a horizontal air passage of uniform cylindrical bore, which communicates with the interior of the container through a metering jet 20 of controlled size and a valve 21.

The downstream end of the jet block 6 is connected, for example by a hose 7, to a blast nozzle 8. The latter is preferably a simple tubular nozzle with a plain cylindrical bore.

At the upper end of the container is an inlet piece 9, which communicates with the interior of the container through the upper wall of the latter.

The inlet piece is connected to a T piece in the air supply hose 11 on the inlet side of the jet block 6 for example by a hose 10. The inlet side of the Tpiece is connected to an air supply, for example a pump, air line or air reservoir, for example by a hose 11. Valves 12, 13 are provided at or adjacent to the T-piece and inlet 9, to stop and start air flow, and to connect or disconnect the interior of the container. An air vent 14 with a valve 15 are mounted on the top plate 2, or in the upper side wall of the container.

In an alternative design the inlet 9 and T piece are combined.

The top plate and handle are detachable from the upper end of the container and are provided with suitable seals and clamping nuts 19 so that in operation the container is hermetically sealed.

In use, the container is filled with sand, grit, or other particulate blasting medium, leaving a space at the top of the container. The top plate is then fastened in place and air under pressure is applied to the hose 11. The valve 12 is opened to establish an air flow through the hoses, jet block and blast nozzle, and then the valve 13 is opened so that the air pressure is transmitted to the space above the abrasive medium, in the upper end of the container.

Under the air pressure in the container, the column of abrasive powder in the container acts as a ram or piston, causing the abrasive powder to be fed evenly through the metering aperture of the jet block 6, into the air stream passing along the cross-piece of the jet block. The abrasive particles are entrained in the air flow and are carried to and expelled from the nozzle 8, emerging as a fine jet of abrasive particles which can be used for cutting or cleaning the surface of an article.

It will be seen that the described abrasive blasting equipment is of extremely simple and accordingly extremely cheap construction. It has nevertheless been found that this equipment can provide extremely efficient abrasive blasting, providing efficient cutting or cleaning at relatively low air pressures and relatively low air flow rates, using only minimal amounts of abrasive powder The described equipment emits just enough abrasive powder to produce the desired effects, with little or no waste, and therefore in many circumstances can be used in the open, without a blasting enclosure.

By control of the air pressure and flow rate, a variety of blasting effects can be obtained, ranging for example from fine cutting or fine marking of a surface, to sand-cleaning of wood to leave a smoothly finished clean surface. The distance between the blast nozzle and the surface being treated also affects the effect of the abrasive jet.

For convenience and safety, the nozzle may be provided with a long handle 22.

It has been found that the performance of the equipment is best, if the blast nozzle and metering jet are of the same size. If the sizes differ appreciably, the efficiency of blasting and also the smoothness of flow of abrasive powder out of the container are impaired.

It has also been found that particularly satisfactory performance is obtained if the container has a cross-sectional area of 125 to 160. preferably 140 cm2. Appreciably larger or smailer container crosssections lead to irregularity in the outflow of abrasive medium.

Equipment as described has been found to give excellent results, with the following parameters.

Diameter of blast nozzle and metering jet : 1.5 to 3 mm.

(nozzle and jet of plain cylindrical bore) Reservoir, metal tubing 13.3 cm. (52-") diameter Abrasive medium : 60 gauge sand.

Air pressure : 20 p.s.i. to 70 p.s.i.

Air flow rate : 5 cfm.

It has also been found to be important, that the air pressure applied to the top space of the container should be substantially equal to the pressure of the air which conveys the abrasive medium to the nozzle.

For cutting holes in clay pipes, the nozzle 8 is mounted on a trammel so as to move in a circular cutting path, which can be perpendicular to or oblique relative to the axis of the pipe being cut.

Claims (9)

1. Abrasive blasting equipment comprising an upright reservoir, an exit jet at the lower end of the reservoir, a passage for flow of air under pressure, into which the jet opens for feeding the abrasive medium from the reservoir into the air stream, and means providing communication between the upper region of the reservoir and the source of air under pressure.

2. Abrasive blasting equipment according to claim 1 in which the reservoir is of subtantially uniform cross section throughout its height, or tapers gently, its cross sectional area being selected to provide optimum feeding of the abrasive medium under the air pressure in the reservoir.

3. Abrasive blasting equipment as claimed in claim 1 or 2 in which the air passage communicates with a blast nozzle, and in which the blast nozzle and the jet aperture have substantially equal diameters.

4. Abrasive blasting equipment as claimed in claim 3 in which the jet and nozzle have plain cylindrical bores.

5. Abrasive blasting equipment as claimed in any preceding claim in which a T piece attached to the bottom end of the reservoir, constitutes the said jet and a portion of the air stream passage into which the jet opens.

6. Abrasive blasting equipment as claimed in any preceding claim in which the reservoir is closed at its upper end by a top plate which is detachable for filling the reservoir.

7. Abrasive blasting equipment as claimed in any preceding claim in which the reservoir has a cross sectional area in the range 125 to 160 sq.

cms.

8. Abrasive blasting equipment as claimed in any preceding claim in which the said jet has a diameter in the range 1.5 to 3.0 mm.

9. Abrasive blasting equipment, substantially as herein described with reference to the accompanying drawings.