US 3255540 A
Description (OCR text may contain errors)
D. M. GILMAN SHEET STRETCHER June 14, 1966 Filed May 6, 1964 INVENTOR. DAVID M GIL/MAN BY W $344M ATTO R N EYS thereof; and
United States Patent 3,255,540 SHEET STRETCHER David M. Gilman, Brookline, Mass, assignor to Graphic Equipment of Boston, Inc., Boston, Mass., 2 corporation of Massachusetts Filed May 6, 1964, Ser. No. 365,313 6 Claims. (Cl. 38-102.91)
The present invention relates to sheet stretching and more particularly to an improved means for rapidly and efliciently stretching or flattening material in sheet form.
Stretching of materials is particularly important in the silk screen art. Commonly silk sheets or screens are stretched on a frame, nonprinting areas are blocked off, and ink pressed through the portions of the screens corresponding to printing areas while they are held in the stretched position. In prior art procedures for stretching silk screens, various clamps or key pieces are used which often require wooden constructions. Such constructions are diflicult to assemble and disassemble thus increasing the time required to change screens and consequently the cost of such methods.
Accordingly, it is an important object of this invention to provide a sheet stretcher which rapidly and eficiently stretches a sheet and maintains it in a stretched condition for any desired period of time.
It is another object of this invention to provide a sheet stretcher in accordance with the preceding object which has relatively few parts and can be operated by hand with case.
It is still another object of this invention to provide a sheet stretcher in accordance with the preceding objects which can be constructed entirely of metal and consequently has a long trouble-free working life.
According to the invention the sheet stretcher has an enclosing frame having an upper sheet bearing edge providing opposing edge portions. Opposing clamps are provided on or adjacent opposing edge portions of the frame and are mounted for movement toward and away from the edge portions so that a sheet can be mounted in the clamps and stretched over the bearing edge when the clamps are moved away from the edge. Preferably each of the clamps are mounted on the frame by upwardly extending threaded spindles. The clamps each preferably comprise a lower clamp member defining an elongated channel opening on an upper surface and extending inwardly into the lower clamp member toward the frame. An upper spring clamp member has an overturned lip which is designed to fit within the longitudinally extending channel and clamp the sheet between walls of the channel.
Other features, objects, and advantages of the invention will be better understood and appreciated from the following detailed description of one embodiment thereof selected for purposes of illustration and shown in the accompanying drawings, in which:
FIG. 1'is a top plan view of a preferred embodiment of this invention;
FIG. 2 is a perspective view of a portion of one side FIG. 3 is a top plan view of a sheet to be stretched by the device of FIG. 1.
As best shown in FIG. 1, the sheet stretcher 14) of this invention has a generally polygonal stretching frame 11 with four clamping means 12 mounted on the sides of the frame. The frame 11 is preferably constructed of metal angles joined at their corners by conventional brazing,
welding or bolting techniques. Upstanding walls or flanges 13, 14, 15 and 16 provide an upper rectangular or square bearing edge 21 extending around the frame and preferably lying a plane. Depending or outwardly flared base flanges 17, 18, 19 and 20 extend outwardly of walls 13, 14, 15 and 16, respectively.
In the preferred embodiment of the invention four identical stretcher clamps 12 are each mounted insliding relationship with the upstanding walls of the frame by threaded spindles 23 as best shown in FIG. 2. Preferably, each of the clamps is individually moveable toward ledge 29. The lower clamp members are preferably aluminum extrusions although other metals can be used. An elongated channel or recess 30 is provided in the lower clamp member 24 along its length with an end 31 spaced closer to the side wall of the frame than the opening 32 on the surface 25. T he recess 30 is defined by side walls 33 and 34 and an outer rabbeted recess 35 adjacent the opening 32.
An upper clamp or spring member 36 is provided for each lower stretcher clamp member 24. The upper clamp member 36 is preferably formed of a metallic strip and has an elongated overturned lip 37 having an outer configuration designed to fit within the recess 30. A substantially flat strip portion 38 of the upper clamp member 36 is adapted to lie on surface 25 of the lower clamp member 24. The locking and clamping action ofeach stretcher clamp 12 will be further described with reference to the operation of the sheet stretcher.
The threaded spindle 23 is threadably engaged with a vertically extending threaded bore in stretcher clamp 12. Preferably three identical threaded spindles are used for each stretcher clamp 12 although one, two or more threaded spindles can be used. The structure and operation of only one spindle will be described since all of the spindles are identical. The lower end of the threaded spindle 23 passes through the base flange 17 as best shown in FIG. 2 and has a threaded locking collar 54 and an end knob 56 on the lower side of flange 17. Preferably the portion of the spindle between collar 54 and knob 56 is unthreaded and adapted to rotate freely in a hole in the flange 17 of slightly greater diameter than the diameter of the spindle. Thus, rotation of the knob 54 rotates the spindle and consequently causes movement of stretcher clamp 12 along the axis of the spindle 23 toward or away from the plane of the upper bearing edge 21.
The particular threaded spindle arrangement may vary considerably. For example, in some cases the spindle may be fixed to the clamping member and a collar arrange ment may be provided for moving the spindle along its axis with relation to the lower flange 17. During movement of the stretcher clamp 12 the projection 27 preferably engages the side wall 13 tending to stabilize the clamp 3 12 and leg 28 acts as a positive stop or ultimate limit for stretching of a sheet in the stretcher frame.
A sheet of silk screen to be stretched in the sheet stretcher of this invention is shown at 40 in FIG. 3. Preferably the silk screen 40 is generally rectangular in shape having cut-away corner portions with points 41, 42, 43 and 44 corresponding to the corners of the frame 11. Flaps 45, 46, 47 and 48 are adapted to overly the upper bearing edge 21 and be clamped to the stretcher clamps 12. Although it is preferred to stretch sheets having the configuration shown in FIG. 3 in a rectangular frame, square, rectangular or irregular shaped sheets may also be stretched over'the bearing edge 21.
Turning now to the operation of the sheet stretcher of this invention, stretcher clamping members 12 are raised to their uppermost position preferably at or slightly below the plane defined by the bearing edge 21. Upper clamp members 36 are removed and a silk screen 40 placed over the frame 11 with portions 41, 42, 43 and 44 thereof overlying the corners of the frame. The flap portions 45, 46, 47 and 48 overlie the clamping members with ends of the silk screen 40 extending to or slightly beyond the outermost edges of the stretcher clamps. Upper clamp members 36 are then placed in position with each strip 38 lying substantially parallel to each upper surface and each lip 37 entering recess and having silk screen separating it from the recess and lying in the position shown in dotted lines at 49. In this mounting position, the threaded spindles 23 have their upper ends substantially at the lower end of threaded bores 52. Preferably after all of the clamping upper members 36 are positioned as described, knobs 56 of each stretcher clamp member are rotated and the stretcher clamps 12 move downwardly over the threaded spindles 23. As the upper ends of the threaded spindles 23 gradually approach the upper surfaces 25, the sheet 40 is stretched over the bearing edge 21 with each of the flaps being stretched at portions 50 and directing an upward force against inner edge 26 of upper clamp members 36 pivoting them into preselected positions with respect to the lower clamping members as best shown in FIG. 2. The upward force exerted on the inner edges 26 of the upper clamping members 36 cause portions 38 thereof to act as levers with the inner ends of lips 37 pressing portions of the silk screen against walls 33 of the recess and other portions 39 of the lips 37 acting as fulcrums and pressing second portions of the screen against the locking groove walls 35 as seen in FIG. 2. This action effectively locks the screen to the stretcher clamp. Each stretcher clamp 12 can be individually movedtoward the base flanges stretching the silk screen 40 the desired amount and holding it for indefinite periods of time.
In some cases collar 54 can comprise a threaded collar which may 'be moved upwardly on the spindle 23 to lock the stretcher clamp member 24 in positive position when the correct degree of stretch is attained. However, even without the use of a threaded collar the tension in the silk screen tends to positively stabilize the position of the clamping members.
When a screen is to be changed, the threaded spindles 23 are rotated moving the stretcher clamp members 12 to their uppermost position reducing the upward force on the edge 26 of upper clamping members 36 and allowing ease of removal of both the upper clamping members and the silk screen. It is a feature of this invention that each of the clamping members is individually adjustable assuring ease of adjustment to stretch along various lines of stretch in the sheet.
Although specific embodiments of this invention have been shown and described, many variations thereof are possible. For example, although the recess 30 extends inwardly at an acute angle to upper surface 25 and the planeof the sheet 40 at an acute angle, this angle may be varied at least up to 90. The particular means for moving the stretcher clamps toward and away from the planes of the bearing edge 21 may vary considerably, as may the cross section of the clamping members and metal angle braces. Although it is preferred to use metal such as aluminum for all of the elements of the inventive structure, other material may be substituted in certain cases. The specific sheet material being stretched may be silk, cotton, wool, textiles, paper and others.
Consequently, the breadth of this invention is to be construed as limited only by the spirit and scope of the appended claims.
What is claimed is:
1. A sheet stretcher comprising a generally polygonal frame,
said frame having a sheet bearing edge lying substantially in a plane and defining opposing edge portions, first and second opposing stretcher clamping means for clamping opposing ends of a sheet to be stretched, said first and second stretcher clamping means each lying on opposing edge portions and means mounting said first clamping means for movement toward and away from said plane whereby a sheet may be mounted on said first and second clamping means and stretched over said bearing edge when said first clamping means is moved away from said plane, said first and second stretcher clamping means each comprising a lower clamping member and a pivotable upper clamping member,
said upper clamping member having a lip dimensioned to be received in a recess defined by said lower clamping member and maintained therein without the need for additional fastening means when pivoted into preselected position with respect to saidlower clamping member by a clamped sheet during said movement away from said plane.
2. A sheet stretcher in accordance with claim 1 wherein said polygonal frame comprises an angle member defining an upstanding wall and an outwardly flared bottom flange with an upper portion of said upstanding wall defining said sheet bearing edge.
3. A sheet stretcher in accordance with claim 1 wherein said lower clamping member is mounted for said movement by a threaded spindle having one end passing into said lower member and a second end mounted on said frame whereby rotation of said spindle causes said movement,
said lower clamping member having a surface defining and elongated recess extending into said clamping member at an angle to said plane.
4. In combination with a sheet stretcher comprising a substantially polygonal frame having an upstanding substantially polygonal wall defining an upper sheet bearing edge lying substantially in a plane,
the improvement comprising a plurality of opposing clamping means mounted on said frame for movement toward and away from said plane,
said clamping means comprising an elongated lower clamping member having an upper surface defining a longitudinally extending recess therein,
said recess opening on said surface and extending inwardly into said member toward said wall to a recess end closer to said wall than said recess opening,
and an upper clamping member comprising a strip having a longitudinally extending over-turned lip dimensioned to fit within said recess with a layer of sheet material separating said lower member from said upper member whereby when a force is applied to said strip upon movement of said clamping means away from said plane, said lip clamps said sheet in said recess.
5.'The improvement of claim 4 wherein said clamping means are mounted on said frame by a plurality of threaded spindles threadably engaged with said clamping means so that rotation of said spindles in a first direction causes movement away from said plane and rotation of said spindles in a second direction causes movement of said clamping means towards said plane.
6. The improvement of claim 4 wherein said clamping means are composed of a metal and said polygonal frame is square.
References Cited by the Examiner UNITED STATES PATENTS 6 FOREIGN PATENTS 4/1953 Belgium. 6/1933 Switzerland.
FRANK J. COHEN, Primary Examiner.