reference will now be made in detail to the present preferred embodiment of the invention , an example of which is illustrated in the accompanying drawings , wherein like numerals indicate the same elements throughout the views . referring now to the drawings , fig1 shows a portable screw driving tool , generally designated by the reference numeral 10 , which has a housing portion 20 , a front end portion 30 where the screws are driven into an object , and a handle portion 40 . a slidable screw feed guide rail is generally designated by the reference numeral 50 , and is slidably mounted to an upper area of the housing portion 20 . the portable tool 10 is often used in a vertical orientation to drive screws downward into a floor surface , such as a patio deck . since the tool 10 is often used to drive vertically downward , it has an adjustable height mechanism , which includes two connecting rods 24 that allow the handle portion 40 to have a variable distance from the housing portion 20 . the handle portion 40 has a pistol - type grip with a trigger 44 , that mechanically actuates an electrical motor ( not shown ) that is inside the housing portion 20 . the top area of the handle portion 40 includes a gripping surface 42 , while a portion of the bottom surface of the rear extension and the rear portion of the handle itself have gripping surfaces , altogether at 48 . the housing portion 20 includes a front housing outer shell structure 22 , which receives the adjustable length connecting rods 24 . a fixed feed tube 26 is located at the front end of the outer shell structure 22 . the front housing 22 comprises a surrounding wall member essentially throughout its length , and there are several ventilation slots 28 in its lower side wall portions . the feed tube 26 houses some of the movable portions of the tool 10 as discussed below . in the illustrated embodiment , feed tube 26 is fixedly attached to the internal mechanical mechanisms contained within housing portion 20 , such as a motor , a gearbox , and a clutch ( not shown ). the portable tool 10 includes an electric motor ( not visible in fig1 ) within the housing 22 as a prime mover device to actuate the fastener drive portions of the tool , and electrical power is provided through an electric cord 46 . the prime mover device is generally designed by the reference numeral 68 . many of the internal “ drive components ” that include the prime mover device 68 of tool 10 are similar to those disclosed in a commonly - assigned companion patent application titled , “ tensioning device apparatus for a bottom feed screw driving tool for use with collated screws ,” filed on sep . 29 , 2004 , ser . no . 10 / 953 , 422 , which is assigned to senco products , inc ., and which is incorporated herein by reference in its entirety . the front end portion 30 includes a movable nose piece 32 , which is attached to a slide body sub - assembly 34 . both the nose piece 32 and slide body sub - assembly 34 are movable in a longitudinal direction of the tool 10 , and when the nose piece 32 is pressed against a solid object such that it displaces linearly toward the rear , the screw driving tool 10 will be actuated to physically drive one of the screws into the solid object ( also sometimes referred to herein as the “ workpiece ”). nose piece 32 has a front surface 36 , which preferably has a rough texture such as sandpaper , so that it will not easily slide while pressed against the surface of the workpiece when the tool is to be utilized . in the illustrated embodiment of fig1 , the nose piece 32 is detachable from the slide body sub - assembly 34 so that nose piece 32 can be re - positioned for different lengths of screws . the nose piece 32 has a plurality of screw length positioning holes 38 ( see fig3 ), which are used to attach nose piece 32 to the slide body sub - assembly 34 at different relative positions to one another . much of the mechanical mechanisms described above for the portable screw driving tool 10 have been available in the past from senco products , inc ., including such tools as the senco model nos . ds200 - d4 and ds300 - d2 . these earlier tools may not have had the precise same construction as described above in reference to fig1 , but there are certainly some similarities . at the upper areas of the front housing 22 is a guide “ rail ” device to direct a strip of collated screws toward the front portion of the tool 10 . this “ guide rail ” structure is referred to herein as a screw feed guide rail 50 , which includes a left rail half 52 and a right rail half 54 . further details of this screw feed guide rail 50 are described below , but in general for the present invention , the screw feed guide rail 50 is movable in the longitudinal direction of the tool 10 , such that it can slide along a set of protrusions or tangs 82 and 84 ( see fig6 and 7 ) at the same time the movable nose piece 32 and the slide body sub - assembly 34 are moved in that longitudinal direction . in this manner , the screw feed guide rail 50 and the slide body sub - assembly 34 are fixedly attached to one another , and this combination is slidably in mechanical communication with the housing portion 20 . the tangs 82 and 84 provide a linear bearing surface against internal portions of the wall member that makes up the housing outer shell structure 22 . it will be understood that the tangs 82 and 84 could instead be included as part of the housing outer shell structure 22 , rather than being included in the screw feed guide rail 50 as described above and illustrated herein , without departing from the principles of the present invention . in this alternative mode of the invention , the screw feed guide rail 50 would then have some type of recessed linear bearing surfaces to receive the tangs that would then protrude from the housing outer shell structure 22 . the main purpose of tool 10 is to drive screws that are provided in the form of a flexible collated strip sub - assembly that is generally designated by the reference numeral 60 . the individual screws 64 are held in place by a flexible plastic strip 62 . as can be seen in fig1 , this collated strip sub - assembly 60 can run along the entire length of the screw feed guide rail 50 from the rear area of the housing portion 20 toward the front end portion 30 of tool 10 . the screws 64 are fed directly along the screw feed guide rail 50 within a guiding slot that is discussed below in greater detail . as the screws traverse through the slot portion of the screw feed guide rail 50 , they are ultimately directed toward the front end of the tool until each of the screws reaches a “ drive ” position at 66 . when viewing the tool 10 at its front - most portion ( i . e ., the left - hand portion as viewed in fig1 ), the left - most screw 64 has been indexed to the drive position at 66 , and thus is now essentially co - linear with the main drive components of the portable tool 10 . as the collated screw sub - assembly 60 is moved through the screw feed guide rail 50 , the plastic strip 62 will eventually make contact with a sprocket ( not visible in fig1 ) that acts as a rotary indexer , and which is located inside the slide body sub - assembly 34 . the sprocket moves each of the portions of the plastic strip 62 into a proper rotary position so that their attached screws 64 eventually end up in the front - most drive position 66 . when the nose piece 32 is actuated by being pressed against a workpiece ( not seen in fig1 ), then a drive bit ( not visible in fig1 ) will move in a linear fashion to push the screw at 66 into the workpiece , and the drive bit will also then be turned in a rotary motion to twist the screw at 66 in the normal manner for driving a screw 64 into a solid object . once the screw at 66 has been successfully driven into the solid object , then the tool 10 is withdrawn from the surface of the solid object , and of course the screw 64 remains behind and has now broken free from the plastic strip 62 . in one mode of the invention , the tool 10 will now be free to allow the sprocket to perform its rotary indexing function and to bring forth the next screw 64 into the front - most drive position at 66 . this type of screw - feed actuation can be referred to as “ indexed on return ,” since the “ lead screw ” is moved into the “ firing position ” at 66 as the nose piece 32 is released ( or “ returned ”) from the surface of the workpiece . the tool 10 can also be configured in an alternative screw - feed actuation mode , in which the lead screw is moved into the firing position at 66 as the nose piece 32 is pressed against the surface of a workpiece ; this type of screw - feed actuation can be referred to as “ indexed on advance .” if tool 10 is configured for indexed on advance , then the lead screw would not yet be in the position at 66 ( as seen on fig1 , 2 , and 4 ) at the moment the nose piece 32 is “ relaxed ” in its non - firing state . instead , the lead screw is not indexed into the firing position at 66 until the nose piece 32 is “ pushed in ” ( or “ advanced ”) toward the main body portion of the tool 10 ( e . g ., toward the handle portion 40 ), which is a state of the tool illustrated in fig5 , and discussed below in greater detail . note that the indexed on advance configuration is a preferred mode of operation for tool 10 . it will be understood that both the indexed on advance and indexed on return screw - feed actuation modes of operation can work equally well with the movable guide 50 of the present invention . other possible modes of screw - feed actuation might be developed in the future that would also work well with the movable guide 50 of the present invention . referring now to fig2 , the portable tool 10 is seen from its side , and the gripping surfaces 42 and 48 are seen as being relatively continuous along the back portion ( to the right in the view of fig2 ) of the tool 10 . a human user will typically use both hands to hold the tool 10 in place while it is being actuated to drive a screw into an object . one of the user &# 39 ; s hands can be placed on the top surface 42 , while the other user &# 39 ; s hand can grasp the handle portion 40 at the lower gripable surface 48 , while also actuating the trigger 44 . referring now to fig3 , the front end portion 30 of the tool is illustrated in greater detail , so that some of the features of the slidable screw feed guide rail 50 can be seen . in fig3 , the collated strip sub - assembly 60 is only partially loaded into the screw feed guide rail 50 , so that the details of the front portions of the guiding rail 50 can be better seen . as can be seen in fig3 , the collated strip sub - assembly 60 has the flexible plastic strip 62 indexed up to a front portion of the slidable screw feed guide rail 50 , just past a bend 56 in the screw pathway of the screw feed guide rail 50 . a wear plate 54 is included within the open area ( e . g ., a slot ) through which the collated strip 60 passes , as the strip 60 approaches the front open area 70 that is the end of the guiding pathway through which the screws pass in the screw feed guide rail 50 . the “ delivery ” area of the screw guide pathway is indicated at 72 , and as the flexible plastic strip 62 exits along the wear plate 58 , it then encounters the slide body sub - assembly 34 , and the plastic strip 62 continues along an open area ( or slot ) 74 in the slide body sub - assembly 34 . the slide body sub - assembly 34 includes a pair of guide surfaces 76 that guide the shank of the individual screws 64 as they pass along the slide body sub - assembly 34 , as they are being indexed toward the front - most “ drive ” position at 66 . in fig4 , the collated strip sub - assembly 60 has now been indexed all the way to the front of the tool 10 , such that its furthest - most screw 64 has been indexed to the drive position 66 . the individual segments of the plastic strip 62 are visible in fig4 , since a portion of the “ left ” rail half 52 is partially cut - away in this view . the screw at 66 will be driven through the opening in the front surface 36 of the movable nose piece 32 . in fig4 , it can be seen that the collated strip sub - assembly 60 will hold the screws 64 in place while the lead screw at 66 will be driven into a solid workpiece , which occurs when the movable nose piece 32 is pushed against the workpiece surface . when that occurs , nose piece 32 will be pushed toward the upper - right corner in fig4 , and the slide body sub - assembly 34 as well as the screw feed guide rail 50 will all move in accordance with this movement of the nose piece 32 . this keeps the collated screw sub - assembly 60 in a relatively intact and non - movable position with respect to the sliding guide rail 50 , and thus the system will tend to keep the screws 64 from becoming misaligned during the firing procedure of the tool 10 . fig5 illustrates the firing position in which the movable nose piece 32 has been depressed with respect to the overall housing portion 20 . the entire guide rail 50 and collated screw sub - assembly 60 have also been moved back ( which is toward the upper - right area in fig5 ). the slide body sub - assembly 34 is not visible in fig5 , because it is now behind the feed tube 26 and front housing outer shell structure 22 . referring now to fig6 , the slidable guide rail 50 is illustrated in a side view , to show some of the construction features thereof . the “ left ” rail half 52 is visible since it is facing the viewer , and the protrusion or tang 82 is now visible . the screw feed guide rail 50 includes a mounting pin 80 that is positioned within an opening of the slide body sub - assembly 34 when the guide rail 50 is attached to the tool 10 . the bend or elbow 56 is readily seen in fig6 . reference numeral 70 refers to the guiding pathway ( or slot ) through which the collated strip of screws moves . referring now to fig7 , some of the other features of the screw feed guide rail 50 are visible , including the “ front ” delivery end of the guiding pathway or slot at 72 , as well as a “ rear ” entry end of the same slot at 78 . the “ left ” tang 82 is visible as it protrudes from the left rail half 52 . a “ right ” tang 84 is visible from the inside of the “ right ” rail half 54 . the wear plate 58 is visible , and is adjacent to a bottom slide surface 86 that is formed as part of the guiding pathway or slot that starts at the rear entry end 78 and exits at the front delivery end 72 of the guide rail 50 . the protrusions or tangs 82 and 84 slide along a pair of interior linear slots ( not shown on fig7 ) within the front housing 22 of the tool 10 . by inspecting the figures hereof , it can be seen that the collated screw sub - assembly 60 will be well retained within the guiding slotted areas ( i . e ., the pathway ) of the slidable screw feed guide rail 50 , and the exit end ( or delivery end ) at 72 will guide the flexible plastic strip 62 and deliver that plastic strip directly to the slide body sub - assembly 34 . this keeps the screws 64 from becoming tangled or bunched , or otherwise becoming misaligned during the operation of the tool 10 . in one configuration of the present invention , a plurality of small arcuate protrusions ( not visible in the figures ) are provided along at least a portion of the guiding pathway ( or slotted area ) 70 of the slidable screw feed guide rail 50 . these protrusions can increase the mechanical resistance against the side edges of the flexible plastic strip 62 , and thereby help prevent this strip 62 from too easily advancing along the slot or pathway 70 . the larger the fasteners ( e . g ., screws ) 64 that are part of the collated strip 60 , the more likely that the increased weight of these larger fasteners will tend to push one of the fasteners of the collated strip 60 past the front indexing position ; if that occurs , then a fastener may not stop at the correct position ( i . e ., at the firing position 66 ), and such fastener might push past the firing position 66 and remain in the flexible plastic strip 62 . of course , that would be an undesirable result . the arcuate protrusions can be made in several different forms ( i . e ., they don &# 39 ; t necessarily need to be arcuate ), and can even be movable , if desired , to act as movable detent guide positioning devices . in one mode of the present invention , the arcuate protrusions are located along both sides of the slot / pathway 70 , and are positioned between the elbow 56 and the entry area at 78 . these protrusions ( or “ bumps ”) can be manufactured as part of a molded plastic guide rail half ( e . g ., one of the halves 52 or 56 ), and they can extend throughout the entire slot / pathway 70 , or for only a portion of this slot / pathway , if desired . it will be understood that the principles of the present invention are applicable to many different types of fastener driving tools , including tools powered by ac electrical power ( e . g ., 120 vac line power from an outlet ), dc electrical power ( e . g ., from a battery or a solar panel ), a pneumatic power source , or a hydraulic power source , for example . in other words , the prime mover device 68 could comprise an electric motor , a pneumatic motor , or a hydraulic motor , for example . in addition , the types of fasteners that can be driven in the manner of the present invention are not limited to screws , but could instead be nails or rivets , for example . some of the components used in the present invention have been disclosed in a commonly - assigned patent , titled “ screw feed and driver for a screw driving tool , u . s . pat . no . 5 , 988 , 026 , which is assigned to senco products , inc ., and which is incorporated herein by reference in its entirety . some portions of the present invention have also been disclosed in another commonly - assigned patent , titled “ screw driving tool ,” u . s . pat . no . des . 462 , 001 , which is assigned to senco products , inc ., and which is incorporated herein by reference in its entirety . it will be understood that the term “ collated screw sub - assembly ” as used herein refers to a strip of screws that are temporarily mounted in a flexible strip of material that exhibits openings and other structures to hold the screws in place until they are needed . in many products , the flexible strip of material comprises plastic , but other materials could be used , if desired . the individual screws are advanced to a driving position in a screw driving tool ( such as portable tool 10 ), and each screw is individually driven from the flexible strip by the tool when the tool is actuated . it will also be understood that the principles of the present invention are applicable to many different types of fastener driving tools , including tools powered by ac electrical power ( e . g ., 120 vac line power from an outlet ), dc electrical power ( e . g ., from a battery or a solar panel ), a pneumatic power source , or a hydraulic power source , for example . in addition , the types of fasteners that can be driven in the manner of the present invention are not limited to screws , but could instead be nails or rivets , for example . all documents cited in the detailed description of the invention are , in relevant part , incorporated herein by reference ; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention . the foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed . any examples described or illustrated herein are intended as non - limiting examples , and many modifications or variations of the examples , or of the preferred embodiment ( s ), are possible in light of the above teachings , without departing from the spirit and scope of the present invention . the embodiment ( s ) was chosen and described in order to illustrate the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to particular uses contemplated . it is intended to cover in the appended claims all such changes and modifications that are within the scope of this invention .