There has previously been known a portable multi-needle chisel, as disclosed in the Japanese Opened Utility Model Application No.  54371/73, having a similar construction to the conventional surface-chiseling machine for a refractory, a concrete or other materials in different usage.  As shown in FIG.  1, such type of multi-needle chisel includes a main body 14 having a piston 12 contacted with a special cam 10 in association with a motor, which body 14 is provided with a protective cylinder 16.  Within the cylinder 16 are movably arranged a carriage 20 for mounting a number of striking needles 18, as well as a hammer 22, while the cylinder 16 at its front end is provided removably with a stop cylinder 28 having a contact portion 30.  Between the contact portion 30 and the carriage 20 is arranged a spring 32. 
Thus, in such type of chiseling machine the carriage 20 is urged against the hammer 22 by a resilient force of the spring 32, so that the striking action of the hammer 22 for moving the carriage 20 forward may be recovered by the spring 32 for moving the carriage 20 backward, thereby to reciprocate the chiseling needles 18.  Since the chiseling needles 18 are kept in a free state between the carriage 20 and the hammer 22, an efficient chiseling action can not be achieved unless the chiseling needles 18 at their front ends are urged against an object to be chiseled, while at their bases are contacted with the hammer 22.  For example, when the piston 12 produces 2000 strikes per minute, vibration of the protective cylinder 16 generated by the striking action may reduce the striking number to about one third, i.e. about 7000 strikes per minute, which have the striking action on the needles 18.  The remaining 1300 strikes per minute are ineffective.  Thus, the conventional multi-needles 18 at their front ends must be always urged against the object to be chiseled for achieving the efficient operation. 
There has also been proposed a tool utilizing the same type of chiseling needles in combination with an air pressure for reciprocating a piston in a cylinder in order to avoid or reduce wear of a striking mechanism and to reduce noise. 
From this point of view, the inventor has developed a surface-chiseling machine for refractory, concrete or other materials, which may produce a striking action of an air pressure on a plurality of chisels and may control the striking action for achieving its efficient operation and improve its durability. 
A typical machine of such type is illustrated in FIG.  2 of the accompanying drawings.  This surface-chiseling machine includes a hammer body 44 for reciprocating a piston 42 by feeding compressed air alternately before or behind the piston 42 in a cylinder 40, a carriage cylinder 46 extending longitudinally in front of the hammer body 44, a tapet 50 arranged movably in the carriage cylinder 46 and having a rear end contacted with a front end of the piston 42 inserted into an opening 48 at a rear portion of the carriage cylinder 46, a holder 52 attached to a front end of the carriage cylinder 46 and having a plurality of longitudinal chisel-holding holes 54 for allowing the chisels 56 to reciprocate therein in contact with the tapet 50, a first air chamber 60 defined between a step portion 58 protruded from a rear portion of the tapet 50 and an inner circumference of the carriage cylinder 46, a second air chamber 64 defined between a step portion 62 protruded from a rear portion of each chisel 56 and an inner circumference of each chisel-holding hole 54, the first and second air chambers 60 and 62 being fed with the compressed air for urging the tapet 50 and the respective chisel 56 backward so that the chisel 56 may strike forward through a striking movement of the tapet 50 generated by a reciprocating movement of the piston 42, as well as a pipe 66 arranged axially through centers of the piston 42 in the hammer body 44 and of the tapet 50 in the carriage cylinder 46, which pipe at its front end is passed through an opening 68 of the holder 52 for feeding fluid or sucking dusts. 
In the surface-chiseling machine thus constructed, the compressed air to be fed to the hammer body 44 is introduced into an inlet 70 and then divided into a stream on the hammer side and another stream on the carriage cylinder side.  The latter stream is introduced alternately in front of or behind the piston 42 through switching operation of a valve 72 utilizing a cushion pressure, thereby to reciprocate the piston 42 for striking the tapet 50.  On the other hand, the air stream on the hammer side is fed through the cylinder 40 and the carriage cylinder 46 into the first air chamber 60 for urging the tapet 50 constantly against the piston 42 with an adequate force by the step portion 58 of the tapet 50.  Further, the compressed air remains in a peripheral groove 74 and is fed through an air channel 76 of the holder 52 into the second air chamber 64 for urging the chisel 56 constantly against the tapet 50 with an adequate force by the step portion 62 of the chisel 56. 
The tapet 50, which has been subjected to a striking action of the reciprocating piston 42, may transmit the striking action to the chisel 56 which in turn is moved forward against an urging air pressure in the second air chamber 64 to effect a chiseling action on the object, such as a concrete surface.  After the striking action, the chisel 56 is immediately retreated by the urging air pressure in the second air chamber 64 and then contacted with the front end of the tapet 50 for repeating the striking cycle.  In this case, the tapet 50 is normally set at a striking position of the carriage cylinder 46 and urged toward a recess of the carriage cylinder, i.e. the piston 42, with an adequate pressure of the compressed air fed into the first air chamber 60.  The tapet 50 subjected to the striking action may move forward against the air pressure to strike the chisel 56 and then immediately retreated to the initial striking position by the compressed air in the first air chamber 60 for repeating the striking cycle. 
Thus, in such type of surface-chiseling machine, the striking force on the object is highest at the chisel 56 just subjected to the striking action of the tapet 50.  However, the chisel 56 at its step portion 62 is normally subjected to the backwardly urging pressure and becomes weak in its striking force during its forward movement, resulting in the lowest striking force at the chisel 56 with its step portion 62 being close to the front portion of the respective chisel-holding hole 54.  Therefore, the chisel 56 on the way has an intermediate striking force.  If the striking force of the tapet 50 is set much larger than the urging force against the step portion 62 of each chisel 56 in the second air chamber 64, the chisel 56 may move forward vigorously to strike the object and thereafter retreat immediately by the urging air pressure.  Thus, the plurality of chisel 56 may be individually subjected to the striking action of the tapet 50 and scrape the surface of the object, such as a concrete wall, a caster material of a kettle in an ironworks and other solid materials. 
In particular, the surface-chiseling machine thus constructed is advantageous in that more than ten chisels 56 may be normally contacted with the tapet 50 while each chisel may be actuated efficiently with a fewer chisels being idle, and in that each chisel 56 may be retreated by a constant force of the compressed air for achieving its smooth reciprocating movement.  However, in order to impart the required stroke to the chisels 56 in this type of surface-chiseling machine and to generate enough striking force by the compressed air, the larger volume of the cylinder 40 for receiving the piston 42 and of the carriage cylinder 46 for receiving the tapet 50 is necessary, resulting in a totally larger construction which causes a larger mechanical damage during operation and requires replacement of the worn components, leading to troublesome and time-consuming maintenance.  Further, the surface-chiseling machine requires widely variable striking performance of the chisels 56 depending on diversified applications, in which the chisels 56 may be conveniently reciprocated depending on the type of objects with a simple mechanism and a convenient maintenance.  Further such machine should be manufactured in a low cost. 
Accordingly, an object of the invention is to provide a surface-chiseling machine for a refractory, a concrete or other materials, in which the chisels may be reciprocated to produce a required striking force by means of a simple pneumatic mechanism, and which is convenient in maintenance and may be manufactured in a low cost for many purposes.