GB2266769A

GB2266769A - Spirit level

Info

Publication number: GB2266769A
Application number: GB9308015A
Authority: GB
Inventors: Randall John Wright
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-05-06
Filing date: 1993-04-19
Publication date: 1993-11-10
Also published as: DE4315098A1; GB9308015D0; CA2094058A1; JPH0719864A

Abstract

A level instrument having precalibrated angle measuring vials and enhanced tensile strength, is made injecting a hollow extruded aluminum frame 17 placed in an associated mold with moldable structural foam plastic so that the plastic generally fills the frame, forming integral vial retention holders 64, 66, 68 and end caps 72 in the process. The vials are simply press fit into the vial retention holders for secure and precalibrated alignment with a measuring surface along the level instrument. The frame 17 is preferably formed with a cross-section bearing an hourglass-shape in order to increase the strength of the level instrument and reduce the volume of plastic fill required. <IMAGE>

Description

2266769 TUBULAR LEVEL INSTRUMENT AND METHOD OF CONSTRUCTION

BACKGROUND OF THE INVENTION

The present invention relates generally to level instruments and other hand tools having level measuring vials therein of tubular construction, and more specifically to an arrangement for at least partially filling the tube with a core material to provide integral vial mountings and shock-absorbing end caps, the vials being accurately positioned and retained within the vial mountings.

Hand levels formed from a rectangular-shaped, tubular frame are well known in the art, as evidenced by such European models sold in the trade like the Beaver Lecastor, BMI, Fisco Constructor, Fisco Pro, Jumbo, Rabone Artisan 2, and Rabone Craftsman. Clearance holes are generally punched in the frame to accommodate level and plumb vial assemblies, and hang holes are likewise provided in the frame. Mounting brackets for the level or plumb vials may be secured in place in the frame by means of glue (Beaver Lecastor), a press fit (Fisco Constructor), a snap fit in association with flanges protruding from the interior of the frame (BM! and Jumbo), a hot probe weld (BMI), bushings (Fisco Constructor, Fisco Pro-Master, Jumbo and Rabone Craftsman), screws (Fisco Pro-Master and Rabone Artisan 2), or pins (BMI, Rabone Artisan 2, and Rabone Craftsman). The plumb and level vials, themselves, may, in turn, be secured to their respective mountings by means of glue, screws, pins, or a plastic-plastic hot probe weld. Vial cover plates may be added.

Finally, separate plates are press fit into the open ends of the tubular frame.

These hand level designs, however, suffer from a number of disadvantages. First, multiple-step assembly processes are required to secure separate vials and mounting brackets to the frame, and the vials must be calibrated by means of laborious adjustments made to the mountings before the mounting may be permanently secured to the frame. Moreover, the tubular frames are hollow, and therefore are prone to warping or bending when the hand level is dropped, which can markedly affect the accuracy of the instrument. Much thicker tube walls are required to enhance tensile strength of the frame, but this requires greater consumption of materials, which can affect the price and weight of the finished product. Finally, the separate end plates and vial mountings may separate from the frame when the level is dropped or struck by a blow. This shortens the useful life of the instrument.

The Stabila Massgerate 80C2 hand level employs the rectangular metal frame, vial mountings, and hard plastic end plates discussed above. However, BONDOG polystyrene-based filler material is inserted between the mounting and the vial, and thus within a portion of the volume between the mounting and the frame, during the assembly process. Each vial must be separately calibrated before the BONDOO material sets and hardens. In this particular design, though, the BONDOO material appears to be used as an adhesive to secure the vials and mountings inside the frame, and to fill voids therein in a limited fashion. The BONDOO material does not enhance the tensile strength of the level instrument.

SUMMARY OF THE INVE=QX

Accordingly, it is an object of the present invention to provide a level instrument construction employing a tubular, hourglass-shaped profile which reduces the volume of the core material contained therein.

Another object of the present invention is to provide an apparatus in which the tubular frame is at least partially filled with a molded structural care material to provide integral vial mountings and end plates thereto.

Yet other object of the present invention is to provide an apparatus in which level and plumb vials may be snap fit into the integral molded vial mountings without the need to retain or calibrate them in a separate step.

Still another object of the present invention is to provide an apparatus having enhanced tensile strength and cost effective assembly.

3 Other objects of the invention, in addition to those set forth above, will become apparent to those skilled in the art from the following disclosure.

Briefly, the invention is directed to providing a low-cost level instrument having precalibrated angle measuring vials and enhanced tensile strength, comprising injecting a hollow extruded aluminum frame placed in an associated mold with moldable structural foam plastic so that the plastic generally fills the frame, forming integral vial retention holders and end caps in the process. The vials are simply press fit into the vial retention holders for secure and precalibrated alignment with a measuring surface along the level instrument. The frame is preferably formed with a cross-section bearing an hourglass-shape in order to increase the strength of the level instrument and reduce the volume of plastic fill required.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a perspective view of the tubular frame used in the level instrument of the present invention; Fig. 2 shows a perspective and partially cut-away view of part of a mold used during the manufacture of the level instrument; Fig. 3 shows a perspective view of runners and a gate used during the manufacture of the level instrument; Fig. 4 shows a partially cut-away perspective view of the level instrument during assembly; Fig. 5 shows a side view of the assembled level instrument; Fig. 6 is a partial plan view taken along line 6-6 of Fig. 4; Fig. 7 shows an end view of another embodiment of a level instrument of the present invention; and Fig. 8 shows a partial side view of yet another embodiment of a level instrument of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As illustrated in Fig. 1 of the drawings, the hand level 10 includes a tubular frame 12, comprising a perimeter wall 14, at least one cross-web 16, and measuring and support surfaces 17 and 19, respectively. The associated cross-web and perimeter walls create at least two channels 18 and 20 running the length of the tube. Punched near each end of frame 12 are apertures 22 and 24 for accommodating plumb vial assemblies, and a recessed aperture 26 along surface 17 of frame 12 near the middle thereof for a level vial assembly, as will be discussed more fully herein. Also punched into the frame near one end is a hang hole 28. The frame 12 is preferably made from a strong but light-weight material like extruded aluminum.

Using lloutsert" molding, frame 12 may be placed inside a two-piece metal mold 11outsertIl 36 as shown in Fig. 2, having a main body 38 and a mating lid 40 into which plastic resin is injected. Positioned inside body mold 38 are ramped surfaces 42 and 44, as well as protrusions 46, 48, and 50. Ramped surfaces 42 and 44, in turn, terminate along the ends of mold 38 with ramped flanges (not shown). Mating protrusions and ramped surfaces are provided in lid 40.

A runner and gate assembly 52 (see Fig. 3) is inserted into mold 36 with its two terminal ends 54 and 56 thereof entering channel 18 by means of level vial aperture 26. When lid 40 is fastened to mold body 38 and a suitable plastic structural foam resin 58 combined with a blowing agent (either physical or chemical) is injected through runner and gates 52, the material enters channel 18, travels towards each end of frame 12, passes into the region of mold 36 defined by ramps 42, 44, the flanged ramps, and end wall 39, and finally enters and fills channel 20. Moreover, the injected structural foam material cooperates with the interior surfaces of mold body and lid 38 and 40, respectively -- namely, the ramped flanges, and protrusions 46, 48, 50, and 51 (not shown) -- not only to fill the frame with structural foam to enhance tensile strength characteristics of the level instrument, but also simultaneously to mold plumb vial 6 mountings 64 and 66, level vial mounting 68, hang hole 70, and end plates 72 and 74 of the level instrument 10, as shown in greater detail in Fig. 4. The arrows show the flow path of the injected material.

This structural foam injection molding low-pressure process produces a final plastic foam having a closed-cell internal structure sandwiched in a solid skin. This results in thick sections with no sink, low residual stress, and lower density than conventional injection molded parts, thereby reducing materials costs. The low-pressure parameter makes feasible the filling of large outserts like the aluminum extrusion with minimal distortion -- essential for an accurate measuring.tool.

A relatively new technique, called "gas-assisted injection molding11 is ideal for the present invention. In essence, gas generated in the structural foam process described above is replaced by an inert gas, such as nitrogen, which is injected into the resin melt at various locations, depending upon which of the several patented processes is used, after an initial amount of solid-melt resin has first been injected, thereby resulting in a hollow molding much like blow-molded bottles known in the prior art. This technique provides advantages like: (1) lower resin usage for the same volume part, since the internal core is hollow, not cellular as in low-pressure structural foam molding; (2) faster cycle times, because the inherent insulating qualities of the cellular foam structure do not exist in the gas-assisted process; and (3) success with a variety of resin types.

While any thermoplastic resin that is injectable and provides the structural and dimensional characteristics necessary for forming the vial mountings and end plates may be used in the present invention, highimpact polystyrene ('MPS") is preferred because of its favorable cost versus performance characteristics. POLYCAST@ structural foam is one such material. A generous portion of reprocessed HIPS could also be blended with virgin HIPS resin with no noticeable reduction in end characteristics. The plastic fill preferably provides a white reflective surface around the vials for easier reading.

Resin processing temperatures vary from 350OF to over 7000F, depending upon the resin selected. High-impact polystyrene is processed within a 420-450OF range. Mold temperatures lie between 40OF and 100OF for HIPS, and well over 200OF for some of the higher temperature melt resins suitable for producing this product.

Cycle times vary depending upon several factors, but will generally be within the 150-200 second range for conventional low-pressure structural foam molding of HIPS. The hollow parts produced by the gas-assisted injection molding process would reduce this cycle time by 25-40% Fig. 5 illustrates level instrument 10 in its assembled state. Plumb vials 76 and level vial 78 are of the axially-symmetric type, having a uniform cylindrical cross-section of predetermined diameter along their longitudinal axis. cylindrical vials of 3/8 or 7/16-inch diameter are commonly used. Each vial mounting 64, 66 and 68, respectively, features a pair of specially contoured cavities or recesses 80 (see Fig. 4) for the precise alignment and retention of the opposite ends of the cylindrical vials 76 and 78 inserted therein. The construction of these cavities 80 is discussed more fully in U.S. Patent No. 4,571,845 issued to Wright et al., which is incorporated herein by reference.

As shown in greater detail in Fig. 6, each cavity 80 takes the form of a keyway 82 having a cylindrical segment 84 defined on a common cylindrical surface of radius R. This radius is substantially equal to the radius of vials 76 and 78. Cylindrical segment 84 opens into a vial receiving window 86 defined between opposed beveled surfaces 88. Cylindrical segment 84 defines a cylindrical arc 90 in excess of 1800. An arc of approximately 2000 is preferred to permit vial insertion, while simultaneously ensuring vial retention. A slight relief or slope of about 50 may be provided on surface 88 to assist in the insertion of vials 76 and 78 into contoured cavities 80.

More particularly, the arc 90 of cylindrical segment 84 must extend beyond the 1800 semi-cylinder so that ridges 92 defined by the respective limits of the cylindrical segments overlap the vial, thereby grasping and retaining the vial in each cylindrical segment. At the same time, the maximum extent or overlap of ridges 92 must be limited to permit removal of the molding core-pieces 46, 50, and 48 during the injection molding stage of vial mountings 64, 66, and 68, and subsequent insertion of the vials during assembly of the level instrument. The upper angular limit of arc 90 is determined by the vial diameter and plastic material used, although 2700 represents a practical maximum. In the preferred arrangement for the retention of a 3/8-inch vial, the 2000-cavity arc corresponds to a draft 94 an each side of vials 76 and 78 of approximately 0.003 inches for a total overlap of about 0.006 inches.

Accurate alignment of the vial with respect to measuring surface 17 and support surface 19 is achieved by dimensioning contoured cavities 80 for substantially zero clearance fit with the vials 76 and 78, and by properly indexing the core-piece during fabrication. Therefore, once snapped into cavities 80, the vial is accurately retained in a predetermined angular relationship to the level instrument's measuring and support surfaces 17 and 19, respectively.

once frame 12 is injected with structural foam to integrally form plumb and level vial mountings 64, 66, and 68, and end plates 72 and 74, assembly of level instrument 10 may be quickly performed by nonskilled workers simply by placing a vial in each vial receiving window 86 and urging the vial downwardly into the associated cylindrical segment 84. The material forming ridges 92 is momentarily, but elastically deformed as the vial is snapped into the contoured cavity 80. The vial is securely retained without the need for additional securing means such as glue, screws, or retaining tabs. Moreover, the level instrument may be used for accurate plumb and level measuring without recourse to further timeconsuming calibration of the vials.

Although not necessary for retaining the vials 76 and 78, cover plates 100 (see Fig. 5) may be laid over the plumb and level vial apertures 22, 24 and 26 punched into frame 12 and secured by suitable means, like screws 102, to frame 12, screws holes 104 (see Fig. 4) having been provided in the vial mountings during the injection molding of the mountings. Such cover plates should be made from a translucent material like acrylic for unhindered viewing of the vials 76 and 78 retained in the mountings, and may be used to protect the vials from dirt, and to reduce the chances of breakage thereof. Coverplates 100 may also be secured to frame 12 by other suitable means like metal-metal welding.

While a rectangular tube could be used for frame 12, the hourglass-shaped profile disclosed herein provides two principal advantages. First, the tapered profile near cross-web 16 causes channels 18 and 20 to be trapezoidal in cross-section, instead of rectangular. This greatly reduces the amount of structural foam material which must be injected into frame 12 during the molding stage, thereby reducing the manufacturing cost of level instrument 10. Second, the tapered profile near cross-web 16 in association with top and bottom surfaces 17 and 19, respectively, provides a simple and convenient means of gripping the hand level without the need for hand holes punched through the hand level, as is common in the trade.

In order to further reduce the amount of structural foam material required during the injection molding stage, a barrier molding method may be adopted. As illustrated in Fig. 5, a barrier block 106 is inserted into channel 20 prior to injection of the structural foam material, and is positioned generally at the midpoint of the length of frame 12 in order not to interfere with the subsequent molding of plumb vial mountings 64 and 66. The injected plastic material will fill the available regions in channels 18 and 20 of frame 12, but not the region filled by barrier block 106. The barrier block may be made from any suitable material which will not disintegrate when contacted by the injected structural foam material, and is preferably formed from styrofoam or wood.

Injection molding of frame 12 could also be performed by the starvation method in which the pressure and time variables during the molding stage are adjusted so that the structural foam material will not completely fill channel 20 in order to reduce the plastic material volume required. The starvation method could be used alone or in conjunction with the barrier method of injection molding.

Frame 12 could be made from other suitable materials like extruded plastic instead of the extruded aluminum described herein. However, aluminum is the preferred material for the frame, because it is strong, yet light-weight, and may be easily extruded. Moreover, aluminum may conduct heat during the injection molding stage, thereby hastening setting and hardening of the injected plastic fill.

Located on the exterior surface of frame body 12 are longitudinal ribs 108 (see Fig. 5). These ribs are decorative, and also protect the frame body 12 of the level 10 from being scratched.

While particular embodiments of the invention have been shown and described, it should be understood that the invention is not limited thereto, since many modifications may be made. For instance, frame 112 could be provided with more than one cross-web 116 to enhance its strength and to reduce the volume of plastic fill contained therein, as shown by level 110 in Fig. 7 where otherwise corresponding numbers have been retained. Moreover, the vial assemblies could be set into only one face of the level instrument, thereby being read from only one direction. Furthermore, the vial mountings could be positioned with respect to the measuring surface of the level instrument so that the vials retained therein may measure degrees of inclination other than plumb and level, such as a 45-degree angle as shown by bubble vial 120 in Fig. 8. Moreover, the integral vial mounting and/or shock-absorbing end plate surfaces may be machined from the injection molded plastic fill contained by the tubular frame of the level instrument instead of use of a mold to provide these structural and dimensional surfaces during the injection molding process stage, as disclosed, supra. It should be understood that the construction of the present invention may be applied not only to the hand level disclosed herein, but also to any other level instruments like tee-squares, etc. The invention is therefore contemplated to cover by the present application any and all such modifications which fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein.

a -

Claims

What is claimed is:

1. A level determining instrument [or other hand tool] with a cylindrical measuring vial mounted therein and a measuring surface, the level instrument comprising:

a. a hollow tubular frame having open ends and a surface that provides the measuring surface; b. an aperture punched through a side of said frame; and C. injection molded plastic mechanically trapped in at least a portion of the frame, said injected plastic providing structural and dimensional molded surfaces extended into a portion of said aperture to create an integral vial mounting for securely retaining the vial.

2. A level determining instrument as recited in claim 1, wherein said integral vial mounting comprises means integrally formed with said frame for aligning and retaining the vial of predetermined radius and length within said frame in a predetermined angular relationship to the measuring surface thereof, said means including a recess formed in said integral vial mounting adapted to receive the vial, the recess having a longitudinal axis oriented in precise angular relationship to the measuring surface, the length of the recess along its longitudinal axis being equal to or greater than the length of the vial, the recess including a pair of vial alignment and retention surfaces substantially at opposed ends thereof, each retention surface defined by a semi-cylindrical arc having a radius of the vial and an arc greater than 1800 and less than 2700, a-pair of opposed longitudinal ridges defined by the edges of each semi-cylindrical surface, a vial receiving opening defined between the opposed ridges, wherein the vial may be inserted into the recess through the opening, and securely retained by the surfaces and ridges in precise orientation with the measuring surface without additional retention or calibration means.

3. A level determining instrument as recited in claim 2, wherein each arc of each semi-circular recessed surface is between 1900 and 2200.

4. A level determining instrument as recited in claim 3, wherein each arc of each semi-circular recessed surface is approximately 2000.

5. A level determining instrument as recited in claim 2, wherein the ridges define opposed tapered surfaces radiating outwardly from the ridges adapted to guide and enhance vial insertion during assembly.

6. A level determining instrument as recited in claim 5, wherein the opposing tapered surfaces define an angle of approximately 100 therebetween, diverging in the outward direction. 7. A level determining instrument as recited in claim 1, wherein said tubular frame comprises a side surface including a tapered region. 8. A level determining instrument as recited in claim 7, wherein the cross-section of said tapered tubular frame comprises the shape of an hourglass. 9. A level determining instrument as recited in claim I., said hollow tubular frame further comprising a cross-web, thereby forming a first channel and a second channel therein, said injection molded plastic being mechanically trapped in at least a portion of the first and second channels. 10. A level determining instrument as recited in claim 9, wherein said tubular frame further comprises at least one additional cross-web for enhancing the tensile strength thereof, and reducing the available volume therein for the injected plastic fill. 11. A level determining instrument as recited in claim 1, wherein said tubular frame is made from extruded aluminum.

12. A level determining instrument as recited in claim 1, wherein said tubular frame is made from extruded plastic. 13. A level determining instrument as recited in claim 1, wherein said plastic fill is made from injection moldable high-impact polystyrene. 14. A level determining instrument as recited in claim 1, further comprising shock-absorbing end plates to seal the open ends of said hollow tubular frame. is. A level determining instrument as recited in claim 14, wherein said injected plastic extends from each open end of said frame to provide the integral shock-absorbing end plates. 16. A level determining instrument as recited in claim 1, wherein the vial retained in said integral vial mounting is oriented with respect to the measuring surface of said frame to measure plumb. 17. A level determining instrument as recited in claim 1, wherein the vial retained in said integral vial mounting is oriented with respect to the measuring surface of said frame to measure level. 18. A level determining instrument as recited in claim 1, wherein the vial retained in said integral vial mounting is oriented with respect to the measuring surface of said frame to measure a degree of inclination between plumb and level.

19. A level determining instrument as recited in claim 1, further comprising translucent cover plates secured to said integral vial mounting by appropriate means to protect the vial from dirt or breakage. 20. A level determining instrument as recited in claim 1, further comprising at least one additional aperture punched through the frame and an integral vial mounting for providing means for retaining securely at least one additional vial therein. 21. A level determining instrument as recited in claim 1, wherein said aperture passes only partially through said frame. 22. A method for constructing a hand level or other hand tool with a cylindrical measuring vial mounted therein and a measuring surface, comprising:

a. punching an aperture through a side of a hollow tubular frame having open ends and a cross-web forming a first channel and a second channel; b. placing said frame in a mold having interior surfaces specially dimensioned to generally contain the open regions of said frame while providing a structural and dimensional form for a vial mounting and end plates; - 20 C. injecting plastic material into the first channel of said frame through said aperture, said plastic material being transported through at least a portion of the first and second channels to provide a vial mounting in said aperture and end plates extending from the open ends of said frame integral with said frame, said vial mounting having a recess oriented in precise angular relationship to the measuring surface of said frame, the recess including a pair of vial alignment and retention surfaces substantially at opposed ends thereof, and a pair of opposed longitudinal ridges defined by the edges of each retention surface; and d. snap fitting the vial into the recess through an opening defined by the ridges, whereby the ridges are momentarily and elastically deformed by the passing vial, yet securely retain the vial within the retention surfaces in precise orientation with the measuring surface of said frame without additional retention or calibration means.

23. A method as recited in claim 22, further comprising insertion of a barrier block generally in the middle of the second channel of said frame prior to injection of said plastic material whereby available volume in the second channel for said plastic fill is reduced.

24. A method as recited in claim 23, wherein said barrier block is formed from styrofoam.

25. A method as recited in claim 23, wherein said barrier block is formed from wood.

26. A level determining instrument with a mounted measuring vial, comprising a frame portion having at least one concave exterior surface for use as an integral hand grip.

27. A level determining instrument as recited in claim 1, wherein the cross-section of said frame comprises an hourglass shape.