Background

When using a saw frame where a strip-shaped hacksaw blade is mounted, it is important that the saw blade is tensioned with sufficient force, since the torsional stiffness of the blade depends on that force. Too low tension force allows twisting of the blade and makes a crooked cut, especially if the teeth of the blade are unevenly worn. The tension force must not be too large, since that could cause fracturing of the saw blade or the saw frame due to excessive stress.

It is known from the patent US 4,256,156 that the tension force in a hacksaw frame can be indicated by measuring the angular movement of a lever which is part of the tensioning mechanism. In this case, the lever moves already when the saw frame is adjusted to the actual length of the saw blade, which may vary, and this design requires that the indication be reset to zero each time a blade has been mounted, to measure only the extra movement during tensioning. It is, however, difficult to know exactly when to reset, since the sawblade may move at its clamps, or there may be paint or particles in the clamping holes of the saw blade, making it difficult to tell when tensioning begins. The need for an extra zeroing operation each time is also a disadvantage.

The present invention concerns a hacksaw frame with a tension force measuring device which measures primarily the force, not the motion in the tensioning mechanism. It does not need any zeroing operation, and it will also indicate whether the tension force changes during use of the saw.

Description

Figure 1 shows a hacksaw frame according to the invention.

A saw blade (1 1) is attached to the frame which comprises a spine (12), a front bracket

(13) and a handle (14). The saw blade (1 1) has holes in each end, penetrated by pegs located on the front bracket and the handle, and the saw blade is tensioned by moving one peg (15), shown as the peg on the handle, in a direction away from the other peg (16).

Several tensioning mechanisms are known including screws and levers, but the invention can be used with all tension mechanisms.

The bending moment in the spine (12) is directly proportional to the tension force in the saw blade, and is measured with an indicator arm (17) parallel to and attached to the spine (12) near one end of it. The indicator (17) has a point (18), and one can observe its position relative to an index (19) near the other end of the spine. The index may consist of a linear scale or openings or otherwise. Since the indicator (17) can be long, the resolution is much better than with a device like US 4,256,156. As an alternative, the spine can be made more rigid with the same resolution, with improved safety and durability.

Since saw blades are made with different material and cross-section, the optimal tension force differs, and it is preferable to have an index with markings for carbon steel, high speed steel and bi-metal blades. Since hacksaw frames may be slightly deformed when carelessly used, the index should preferably also have a zero point and the indicator (17) should be attached in an adjustable way. The adjustment is not necessary in normal use, and not when changeing blades.

The spine (12) can have an I- shaped or U-shaped cross-section, or be a tube. If it is a tube, the indicator (17) can be located inside the tube as shown in figure 2 and is then well protected against mechanical damage, but use of the tube to store spare blades is then difficult.