Blade tensioning device for scroll saw

A scroll saw is disclosed having upper and lower reciprocable arms terminating in upper and lower ends having a saw blade connected therebetween. A releasable blade tensioning device is mounted on the upper end including a blade support member having a central portion pivotally mounted to the upper end with a first end thereof extending along the upper arm and a second end connectable to the saw blade. The blade support member is pivotable between a first blade tensioning position and a second blade tension releasing position, and a manually rotatable cam is mounted on the first end and a manually operable adjustment screw engageable therewith is mounted on the upper arm. The manually rotatable cam is constructed and arranged to force the first end and the upper arm apart in one rotated position thereof to move the blade support member to the first position, and to permit the blade support member to move to the second position in another rotated position thereof. The manually operable adjustable screw permits adjustment of the tension on the saw blade.

BACKGROUND OF THE INVENTION 
This invention relates to jigsaws or scroll saws which have a saw blade 
connected between the free ends of the arms of a generally C-shaped or 
U-shaped member which is reciprocated about a pivot point at the closed 
end of the member to drive the saw blade. Means are provided to quickly 
connect or disconnect the saw blade between the ends of the arms. 
Typically, at least one end of the saw blade is clamped between a pair of 
blade holders, an example of which is shown in FIG. 6 of the Sutton U.S. 
Pat. No. 4,503,742, issued Mar. 12, 1985. It is difficult to adjust the 
tension on the blade by using such blade holders, but if the blade is not 
correctly tensioned it may tend to break upon encountering a workpiece. 
An example of a releaseable prior art blade tensioning device is disclosed 
in the Dremel U.S. Pat. No. 2,721,587, issued Oct. 25, 1955. The Dremel 
jigsaw has a pivotable blade holder attached to the end of the upper arm 
which can be pivoted between blade tightening and blade releasing 
positions. In the blade tightening position, a lever can be pivoted into 
position to lock the blade carrier in place, with the blade under tension. 
However, no means are provided to readily adjust the amount of tension 
being applied to the blade. 
SUMMARY OF THE INVENTION 
The present invention provides an improved blade support member pivotally 
mounted to the end of the upper arm having an outer end attachable to the 
blade and an inner end disposed generally below the arm. A rotatable, 
offset cam member is mounted on the inner end of the blade support member 
for engagement with the bottom end of an adjustable screw extending 
through and threadedly engaged with an opening in the arm. The cam member 
can be rotated manually in one direction to pivot the blade support member 
so as to tighten the blade and in the other direction to release tension 
on the blade. The adjustment screw can be used in two ways to adjust the 
tension on the blade. First, it may be adjusted prior to tensioning of the 
blade so that the starting position of the blade support member correctly 
corresponds to the length of the blade such that when the cam member is 
rotated to the blade tensioning position the correct tension will be 
obtained. Secondly, the adjustment screw permits fine tuning of the blade 
tension after the cam member has been rotated into the full blade 
tensioning position. A handle is attached to the cam member to permit 
quick release or tensioning of the blade and, as noted above, the actual 
tension on the blade can quickly be adjusted by means of the threaded 
adjustment screw.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, wherein like numerals are used throughout 
the several views to identify like elements of the invention, there is 
disclosed a scroll saw having an elongated base 10 having a table 11 
mounted at one end to support a workpiece. Table 11 is mounted on a 
support frame including a swivel member 12 which permits the table to be 
rotated about a vertical axis, and a trunion assembly 13 which can be 
released by a star handwheel 14 to permit the table to be tilted about an 
axis extending generally along the length of the base 10. 
Base 10 includes oppositely disposed side wall members 10a, 10b defining 
between them an elongated opening or well 15. At the end of base 10 
opposite from table 11 side wall members 10a, 10b define a pair of 
upstanding, spaced bearing support members 16, 17. Mounted between the 
bearing support members 16, 17 for rotation therein is a horizontally 
extending pivot shaft 18. 
A yoke or C-arm member 20 is provided having an upper arm 21 terminating in 
an upper end 22, a lower arm 23 terminating in a lower end 24, and a 
curved bight 25 connecting the upper arm 21 and the lower arm 23. Bight 25 
of C-arm member 20 is mounted on pivot shaft 18. A saw blade 26 is 
connected between upper end 22 and lower end 24 and an electric motor 27 
is provided to reciprocate C-arm member 20 on pivot shaft 18 to effect 
movement of the saw blade 26. 
A support arm 28 connected at its rear end to bearing support member 17 
extends along upper arm 21 and at its free end provides support for a 
blade guard 29, an air nozzle 30 and a holddown member 31. 
A releasable blade tensioning device 33 is pivotally mounted to upper end 
22 of upper arm 21 for pivotal movements about a horizontally extending 
pivot pin 34 between a blade tensioning position and a blade tension 
releasing position. A manually rotatable cam member 35 is mounted on the 
end of blade tensioning member 33 opposite from its connection to saw 
blade 26 for engagement with the bottom end of a manually adjustable pin 
36 which extends through and is threadedly engaged with an opening in 
upper arm 21. 
The releasable blade tensioning device 33 is shown in more detail in FIGS. 
2, 3 and 4. The device includes a blade support member 33a which has a 
bifurcated inner end comprising a pair of flat, coplanar end portions 33b, 
33c extending along opposite sides of the upper end 22. Blade support 
member 33a is pivotally supported on end 22 by means of pivot pin 34 
extending horizontally through openings in the oppositely disposed end 
portions 33b, 33c, the openings being located near a central portion of 
blade support member 33a. End portions 33b, 33c are angled downwardly 
beginning at about the pivot pin 34, with respect to the remaining 
portions of blade support member 33a, so that in the preferred embodiment, 
the inner ends of end portions 33b, 33c extend below upper end 22, as best 
shown in FIGS. 3 and 4. 
The inner ends of end portions 33b, 33c have oppositely disposed openings 
in which manually rotatable pin 37 is mounted to thus extend across and 
below upper arm 22. Pin 37 has bearing portions at opposite ends which 
rotate in the oppositely disposed openings and has an enlarged head at one 
end. A lever 38 extends normal to and is threadedly engaged into an 
opening in the other end of pin 37 to provide for manual rotation thereof. 
Pin 37 has a central portion of smaller diameter offset from a central 
axis of the pin, the offset central portion 37a being engageable with the 
end of manually adjustable pin 36. Offset portion 37 a thus functions as a 
cam member cooperating with the end of pin 36 to pivot blade support 
member 33a between the two positions shown in FIGS. 3 and 4. In FIG. 3, 
lever 38 has been rotated to the blade tension releasing position in which 
the cam member does not exert force against the bottom end of pin 36. To 
increase tension on the blade, lever 38 can be moved to the position shown 
in FIG. 4, so that central portion 37a engages the bottom end of pin 36 
and forces end portions 33b, 33c downwardly to pivot the outer end of 
blade support member 33a upwardly to the blade tensioning position. Pin 36 
can be rotated by means of knurled head 36a to manually adjust the amount 
of tension to be applied to blade 26, either when the blade tension is 
released as shown in FIG. 3 or to provide fine tuning with the blade under 
tension as shown in FIG. 4. 
In the blade tensioning position shown in FIG. 4, lever 38 rests against a 
stop member 21a on upper arm 21, and central portion 37a is slightly over 
center so that the tension tends to hold lever 38 and pin 37 in the 
position shown. 
The free end 22a of upper arm 21 which extends beyond pivot pin 34 is 
formed as a generally flat finger which extends over a portion of blade 
support member 33a. A recess 33d is formed in the portion of blade support 
member 33a which faces free end 22a, and a resilient grommet or pad 40 is 
carried by recess 33d. Pad 40 is preferably made of rubber and is of 
sufficient thickness to substantially fill the space between the bottom of 
recess 33d and the bottom surface of free end 22a, such that it is engaged 
by free end 22a when the blade support member 33a moves to the blade 
tensioning position shown in FIG. 4. Resilient pad 40 serves to compensate 
for small irregularities in the adjustment of the blade support member and 
also functions as a shock absorber. Its most important function, however, 
is to keep tension on the mechanism in the blade tensioning position shown 
in FIG. 4 so that the mechanism 33 will not release if the blade breaks. 
Blade support member 33a has a bifurcated ouer end including end portions 
33e, 33f having a vertical slot between them. A blade holder 41 is 
pivotally suspended in the slot between end portions 33e, 33f by means of 
a pin 42. Blade holder 41 is a generally flat member having an upper 
portion 41a in the slot and a lower portion 41b of reduced thickness 
extending therebelow. Portions 41a and 41b define a generally horizontally 
extending abutment between the portions. A square locking nut 43 is 
threadedly clamped to bottom portions 41b of the blade holder by means of 
a threaded screw 44 extending through an opening in portion 41b. Nut 43a 
is sized so that one edge thereof engages the abutment between portions 
41a and 41b to prevent rotation thereof. The upper end of saw blade 26 is 
clamped between nut 43 and lower portion 41b to hold the blade during 
operation of the saw. As shown in FIG. 2, a similar blade holder 45 is 
secured to a bifurcated end portion of lower arm 23 to attach blade 26 to 
that arm. 
As shown in FIGS. 3 and 4, there is space between the rear edge of blade 
holder 41 and the bottom of the slot between end portions 33e, 33f so that 
the blade holder 41 can pivot rearwardly about the pin 42 when the blade 
26 encounters a workpiece. Permitting blade holder 41 to pivot a small 
amount about pin 42 helps prevent breakage of blade 26 because the tension 
is spread more evenly along the length of the blade. Blade holder 41 has 
an elongated lobe 41c at its upper rear corner adjacent the upper end of 
the slot, engageable with the base of the slot to prevent the blade holder 
from rotating in the opposite direction in the event of blade breakage. 
Thus, if the blade breaks, the end portion of the blade which remains 
clamped in blade holder 41 cannot swing up toward the operator. 
To attach a saw blade 26 to C-arm member 20, adjustment screw 36 is 
initially set so that the distance between blade clamps 41 and 45 is 
approximately correct for the length of the saw blade being used. The ends 
of the saw blade 26 are then clamped into the blade clamps 41 and 45 with 
the blade tensioning mechanism 33 in the blade tension releasing position 
shown in FIG. 3. At that point, further adjustments to pin 36 can be made 
if desired. Lever 38 is then rotated rearwardly to the position shown in 
FIG. 4 to pivot blade support member 33a to the blade tensioning position 
shown in FIG. 4. Further adjustments to the blade adjustment screw 36 can 
be made at different times during the process in order to arrive at the 
correct tension on the blade. When the blade is to be removed or replaced, 
lever 38 is moved back to the blade tension releasing position shown in 
FIG. 3 at which time the blade support member 33a pivots to the release 
position shown in FIG. 3, thus releasing tension on the blade so that it 
can be removed, readjusted, or replaced. 
The blade tensioning mechanism 33 and blade holder 41 are especially 
advantageous when it is desired to cut interior openings in a workpiece. 
Mechanism 33 is released and the end of blade 26 is released from holder 
41. The workpiece is then placed over the blade with the blade extending 
through an opening in the workpiece, and the blade is then re-connected 
and placed under tension. This can be accomplished very quickly, making it 
a simple matter to cut such openings. 
The present invention thus provides a readily adjustable and easily 
releasable mechanism for connecting the blade between the arms of a scroll 
saw.