Guide for Adjusting Hydraulic Pendulum Shear Blades

May 3, 2024 Roberto Magalhães

1. Introduction

In recent years, with the rapid growth of the manufacturing industry, the use of cutting machines as the main sheet metal processing equipment has become more widespread.

One of the most popular options among users is the pendulum hydraulic guillotine, due to its simple structure, low failure rate and excellent cutting quality.

To maximize the cutting quality of the hydraulic pendulum cutting machine, users are encouraged to have a comprehensive knowledge of blade installation and adjustment.

Although the blade adjustment method for hydraulic pendulum cutting machine is described in various literatures, obtaining satisfactory results in practice can be challenging due to factors such as blade length, hardness, and the material and thickness of the plate that is being cut.

This paper, based on an analysis of blade position, size and installation for hydraulic pendulum cutting machine, proposes that blade adjustment involves not only adjusting the blade height, but also adapting the blade propeller.

2. Blade installation requirements in the cutting process of hydraulic pendulum cutting machine

As shown in Figure 1, the oscillating tool holder rotates around point O and cuts the metal sheet under the influence of the hydraulic cylinder.

Fig.1 The principle of shear

To allow continuous cutting from right to left, the blade installed in the tool holder forms an X angle (i.e. cutting angle) with the work table. The main requirements are as follows:

Requirements for front and rear angle:

To guarantee the quality of the cut, the vertical plane between the blade and the work table must always maintain an angle γ. However, as it is not possible to keep the blade on the same rotating cylindrical surface of the tool holder, the front and back angles of the blade may change throughout the cutting process.

At the beginning of cutting, the front angle is large and the rear angle is small due to the small turning radius (OA') of the tool holder. On the other hand, at the end of the cut, the front angle is small and the rear angle is large due to the increase in the turret turning radius (OB').

Avoiding friction:

To avoid significant friction between the front of the blade and the plate being cut, the front face of the blade must always be within the arc of its path of motion throughout the cutting process (from point c to point d).

Release Requirement:

To obtain a better quality of the sheared section, it is crucial to maintain, as much as possible, a constant gap △ between the blade and the plate to be cut (see Figure 2).

Fig. 2 Shear clearance

Blade clearance must remain consistent along the entire length of the blade. Improper adjustment can increase wear and damage the blade, and may even cause the blade to collide with the table or the metal sheet topple over.

To meet the above requirements, it is essential to adjust the front of the blade as close as possible to a spatial spiral surface, to ensure that the front and back angles remain constant during the cutting process.

3. General adjustment method is of hydraulic pendulum cutting machine blade

Since it is impractical to fit the front face of the blade to a spatial spiral surface, the requirement for a spatially curved surface is typically met by adjusting the thickness of the adjustment joint between the blade and the tool holder, as illustrated in Figure 3.

Fig. 3 Blade adjustment

Despite its simplicity, the joint thickness adjustment method still has some limitations. To meet the ideal blade installation requirements on spatial parallel curves, the blade must meet the following criteria:

x=R cosθ
y=R sinθ (1)
z=Rθ·ctg

Where:

θ – angle of rotation of the tool holder around the axis
Ф – shear angle

The blade must be a space propeller, with its front surface being a cylindrical propeller. However, using the simple joint adjustment method may result in the following two problems:

Gap Problem:

A simple method for adjusting the thickness of the joint is to align it along the length of the blade with a straight line, resulting in a blade that is a straight line, as illustrated in Figure 4.

Fig. 4 The blade edge is a straight line.

This results in an actual gap between the blade and the board being cut of △+. As λ varies with the oscillation angle θ of the tool holder, the gap between the blade and the plate being cut becomes variable. The range of change for λ is as follows:

λ=R（1- cosβ） (2)

Where:

β – tool holder oscillation angle from the beginning to the end of the cut
β = arcsine( btgФ /R)

For example, if we calculate using QC12Y-6×200 (with R=469mm, Ф =1.5° and b=1600mm), the variation range is approximately 1.8mm. If a 1100mm blade is used to adjust the joint thickness, the variation range is 0.88mm, which exceeds the recommended clearance of 0.5mm when cutting a 6mm steel plate.

It is evident that although the joint thickness adjustment method is simple, it cannot guarantee a constant gap between the blade and the metal sheet throughout the shearing process, which negatively impacts the shearing quality.

Front angle problem:

The joint thickness adjustment method ignores the requirement that the front of the blade be a spiral surface and instead replaces it with a plane perpendicular to the bench, which cannot guarantee the desired front angle (generally between 1, 5° to 2° to ensure cutting quality and blade strength) during shearing.

For a blade width W, the distance between the top and bottom edges and the ideal helical surface can be calculated as follows:

X'=R{1- cos(arcsin(y /R)} (3)

By substituting the relevant parameters of QC12Y-6×200 into equation (3), a value of X' = 6.87mm is obtained and the maximum frontal angle is -arctan (x'/y') = 4.91°. A negative value indicates a negative frontal angle.

Clearly, such a large range of variation in the front angle cannot guarantee the desired cutting quality.

4 . Solutions

(1) Clearance problem

The reason for the excessive shear gap in the previous analysis and calculation is that only two straight lines were used to approximate the spiral line segment of the blade during the entire shearing process. Using the multi-line segment approximation, the maximum gap can be reduced.

The blade of QC12Y-6×200 is 1100mm and the spacing between locating holes is 200mm. If gaskets are used in each positioning hole for adjustment, the shear clearance variation, λ, can be calculated as 0.03 mm using formula (2) and would meet the requirements. The thickness of the adjustment joint can be determined by calculating the height of each straight line segment approaching the curved arc.

To meet the frontal angle requirement (γ = 1.5° to 2.0°), it is necessary to increase the distance, y, between the turret rotation axis and the work table. Y depends on the center of rotation of the tower and the thickness of the plate. The smaller the turning radius of the tower, the thicker the sheet metal and the higher the Y value. These factors must be taken into consideration during shear design.

In practice, a gap adjusting device is often used to increase the shear gap, but this sacrifices the shear quality. Figure 5 shows the chamfered joint, and the chamfered angle, n, is ground in the direction perpendicular to the work table (an angle of 1.5° is selected in the design, and the turning radius of the tool holder can be slightly increased if small ) to compensate for the error caused by using a plane perpendicular to the work table to approximate the spiral surface.

Fig. 5 Chamfered joint

To better meet the requirements of the helical surface, when the tool holder is long, the surface that contacts the blade can also be ground with a 1° inclined plane along the blade length direction, as indicated by the line dotted in Figure 5. The longer the tower, the more pronounced the effect becomes.

5. Practice results

The methods described above were applied to adjusting the blade clearance of the QC12Y-6×3200 and Q12Y-12×2500 shears, as shown in Table 1. The data demonstrates that by using the angled gasket and trimming the gasket at each mounting hole, blade clearance can be reduced without cutting during the actual cutting process, resulting in better cutting quality.

It is important to note that the experimental data in Table 1 includes the impact of blade shape errors on the minimum shear clearance.

Table 1 The contrast of minimum shear clearance between two blade adjustment methods/mm