es, even if the difference in pressure between in front of and behind the throttle portion 23a becomes small by operating the operation portion 30 toa certain extent,the decrease in the flow amount because of the decrease in the pressure difference can be compensated by the in- crease in the flow channel area, so that the relationship between the operation amount of the operation portion 30 and the amount of air jetted out of the nozzle 17b can be made close to a proportional relationship. 

[0028] That is, when the throttle portion 23a is formed not in the trumpet shape but in a simple tapered shape (conicalsurface),asshown in FIG.20,asthe flow amount ofthe airincreases,the pressureinthe accumulatorpro- vided on theupstreamsideofthe release valve decreas- es and the pressure difference between in front of and behind the throttle of the release valve becomes small, so that the change in the flow amount with respect to the change in the operation amountbecomes small. For this reason, after the operation portion 30 is operated to a certain extent, the amount of air jetted out of the nozzle 17b does notlargelychange even ifthe operation portion 30 is operated any furtherand this deteriorates operabil- ity. 

[0029] In this regard, according to the present embod- iment, as shown in FIGS. 9A to 9D, since operati amounts M1 to M3 of the operation portion 30 are not proportionalto the clearance amountsG1 to G3,itispos- sible to make the flow channelareaverysmallatthe start ofthe operation oftheoperation portion 30 and make the flow channel area very large at the end of the operation of the operation portion 30. Specifically, when the oper- ation portion 30 is operated by the predetermined oper- ation amount M1 from an initial position BL of the valve member 31 where the flow channel is closed (see FIG. 9A), the clearance amount is G1 (see FIG. 9B). When the operation portion 30 is operated by twice the prede- termined operation amount M1 (M2) from the initial po- sition BL of the valve member 31, the clearance amount is G2 (see FIG. 9C). When the operation portion 30 is operated by three times the predetermined operation amount M1 (M3) from the initial position BL of the valve member 31, the clearance amount is G3 (see FIG. 9C). In the case ofa simple tapered shape(a conicalsurface), G2 should be twice G1, and G3 should be three times G1;however,inthepresentembodiment,since the clear- ance amount increases at an accelerated rate with re- spect to the operation amount of the operation portion 30, G2 is not less than twice G1, and G3 is not less than threetimes G1.As described above,the presentembod- iment is purposefully structured so that the flow channel area largelyincreasesextremely toward the rear.There- fore,even when the pressuredifference between in front of and behind the throttle portion 23a becomes small by operating the operation portion 30 toa certain extentand the pressure to jet out the air decreases, the decrease in the pressure can be compensated by the increase in the flow channel area, so that as shown in FIG. 10, the relationship between the operation amount of the oper- 

ation portion 30 and the amount of air jetted out of the nozzle 17b can be made close to a proportional relation- ship. 

(Second embodiment) 

5 

[0030] A second embodiment of the present invention will be described with reference to FIGS. 11 to 14. Since the present invention is different from the first embodi- ment only in the structure of the flow amount adjustment mechanism, overlapping descriptions are avoided and only the flow amount adjustment mechanism will be de- scribed. 

10 

[0031] The flow amount adjustment mechanism ac- cording to the present embodiment is for adjusting the amount of air jetted out of the nozzle 17b, and as shown in FIG.11,isprovided with thepipeline formation portion 20 forming the pipe line 23 for circulating air, the valve member31disposed within the pipe line 23 and movable, the operation portion 30 disposed on the tip of the valve member 31 so as to be operable, and the valve member pushing member 36 thatpushes the valve member 31 in the protruding direction.Into thisflow amountadjustment mechanism, the compressed air can be introduced from the upstream side, and when the air supply source such as an air compressor is connected to the tool body 11, the compressed air is supplied to the upstream side at all times . On the downstream side of this flow amount adjustment mechanism, the air outlet 40 to which the dusterpipe arrangement42 (thejointpart41)isconnect- able is open. 

15 

20 

25 on 

30 

[0032] The pipe line formation portion 20 has a hollow portion forming the pipe line 23, and as shown in FIG. 12, is structured so as to take the compressed air into the pipe line 23 through the upstreamopening portion 21 that is open toward the upstream side. On the down- streamside ofthe upstreamopening portion 21,the throt- tle portion 23a is provided that controls the flow amount of the air in the pipe line 23 by gradually changing the amount of clearance between the pipe line 23 and the valve member 31 when the valve member 31 moves. 

35 

40 

This throttle portion 23a is formed,as shown in FIGS.11 and 12, in a stepped shape where tapered portions and cylindricalportions arealternately arranged.Specifically, the following are provided: a firsttapered portion 23c ex- panding toward the downstream side; a firstparallelpor- tion 23f formed in substantially the same diameter as the maximum diameter of the first tapered portion 23c so as to be continuouswith the first tapered portion 23c; asec- ond tapered portion 23d expanding toward the down- stream side so as to be continuous with the first parallel portion 23f; a second parallel portion 23g formed in sub- stantially the same diameter as the maximum diameter ofthe secondtapered portion 23d so as to becontinuous with the second tapered portion 23d; a third tapered por- tion 23e expanding toward the downstream side so as to be continuous with the second parallel portion 23g; and a third parallel portion 23h formed in substantially 

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