How to design the fixture base plate
When used on machine tools, base plates, with their threaded holes, reaming holes, and slots, facilitate the setup and installation of fixtures. They are typically manufactured according to the shape of the machining center’s worktable, serving functions such as adjusting workpiece height, protecting the machine table, and adjusting workpiece orientation for simultaneous multi-faceted machining.
I. Types of Base Plates and Bases
Vertical machining centers primarily use base plates. Horizontal machining centers, in addition to basic base plates, also use single-sided, double-sided, and four-sided bases.
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Horizontal machining centers, to accommodate various workpiece sizes, also use six-sided and eight-sided bases. In Figure 2, the darker orange markings on the workpiece indicate the machined surfaces.
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II. Types of Mounting Surfaces for Base Plates and Bases
The mounting methods for base plate and base fixture mounting surfaces include T-slots, bolt holes, and reference holes. Furthermore, in actual machining applications, to ensure the machining center’s accuracy matches the fixture mounting surface’s accuracy as perfectly as possible, custom-machined mounting surfaces are also available.
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Advantages and disadvantages of typical T-slots, bolt holes, and datum holes
Each machine tool has its own unique characteristics, such as the slope of the ground in the factory where the machine tool is located and the tilt of the spindle after years of use. This can be addressed by machining the excess material held on the fixture mounting surface to fit the individual machine tool, thus improving the accuracy of the fixture mounting surface. Another option is to allow for tooling and workpiece assembly/disassembly outside the machine tool: quick tooling change.
III.
Determining Dimensions and Positioning Methods
When selecting the base plate and base, the dimensions and positioning method must first be confirmed in advance. The basic dimensions of horizontal machining centers are 400X400, 500X500, 630X630, and 800X800. Depending on the machine tool manufacturer, there are also dimensions such as 300X300 and 1000X1000, and some special sizes such as 550X550 are also produced by machine tool manufacturers. The dimensions of vertical machining centers vary from machine tool manufacturer to manufacturer, and there are no specific rules specifying the dimensions. Each machine tool manufacturer has its own specified dimensions for the worktable; please refer to the machine tool’s instruction manual.
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The base plate and base of a horizontal machining center are typically positioned using worktable blocks installed in the X and Y directions. The base plate and base are positioned by aligning with these worktable blocks. The worktable blocks are generally located at the 0° and 270° positions of the machine tool’s worktable, but the exact positions may vary depending on the machine tool manufacturer. Vertical machining centers typically have a keyway in the center of the worktable. The X-direction of the base plate and base is positioned via the keyway, and the Y-direction is positioned via a T-slot.
However, not all T-slots provide precise positioning. Depending on the machine tool manufacturer, a machine tool usually only has one or two T-slots with positioning functionality.
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Confirming the Machine Tool’s Maximum Load Capacity
First, confirm the machine tool’s maximum load capacity. Second, confirm the total weight of the workpiece, fixture, base plate, and base. The total weight must absolutely not exceed the machine tool’s maximum load capacity. Some designers calculate the weight of the workpiece, base plate, and base, but forget to calculate the weight of the fixture, which absolutely cannot be ignored.
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If the total weight exceeds the machine tool’s maximum load capacity, methods such as reducing the thickness of the base plate, reducing the thickness of the base mounting surface, or making the base hollow can be used to reduce the overall weight.
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Confirming Base Height and Maximum Slewing Diameter
The maximum height and maximum swing diameter of the machine tool must be confirmed in advance when selecting the base plate and base. This is especially important for horizontal machining centers, where the maximum swing diameter is crucial. Generally, for a base size of 400x400mm, the maximum swing diameter is Φ630mm. For a base size of 500x500mm, the maximum swing diameter is Φ800mm. These two dimensions are common, and there is no explicit requirement to use this size base. When the fixture clamps the workpiece from top to bottom, the top of the fixture (the top of the bolt in the image below) must be within the maximum swing diameter.
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VI. Confirmation of Machining Range and Stroke
Confirm in advance whether the machine tool’s machinable range (spindle travel range) covers all the parts of the workpiece that need to be machined. Especially when using a milling cutter to machine surfaces, the spindle stroke must be sufficient to completely remove the cutter from the workpiece’s range during retraction. This must be considered in advance during the design phase. Without considering retraction, ensuring surface accuracy is quite difficult. When the milling cutter retracts from the workpiece, a minimum distance of 10mm must be maintained between the cutter and the workpiece; the cutter must be completely out of the workpiece’s range.
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