6 Factors to Consider for the Use of Fixture Base Plates

When used on machine tools, base plates can be set up and installed more easily through threaded holes, hinge holes, and slots. These are generally made according to the shape of the machining center’s workbench.

Their functions include adjusting the height of the workpiece, protecting the machine tool workbench, and adjusting the orientation of the workpiece as needed, allowing for multi-face simultaneous processing.

I. Types of Base Plates and Bases

Vertical machining centers mainly use base plates. Horizontal machining centers, in addition to using basic base plates, also utilize single-face bases, double-face bases, and four-face bases.

Types of Base Plates and Bases

In order to accommodate various workpiece sizes in the horizontal machining center, six-sided and eight-sided bases are also utilized. In Figure 2, the areas marked in darker orange indicate the surfaces that can be machined.

II. Types of Base Plate and Base Mounting Surfaces

The base plate and base fixture mounting surfaces can be installed in various ways such as T-slot, bolt hole, and datum hole formats. Furthermore, to achieve the best possible alignment between the machining center’s precision and the fixture mounting surface’s accuracy during actual processing, there also exist custom-made methods.

Types of Base Plate and Base Mounting Surfaces
Figure 3: Pros and Cons of Typical T-Slots, Bolt Holes, and Datum Holes

Each machine tool possesses individuality due to factors such as the slope of the factory floor where it is located or the tilt of the main shaft after years of use.

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This can be accommodated by machining the excess material held by the fixture mounting surface, conforming to the individuality of the machine tool, and thereby enhancing the accuracy of the fixture mounting surface. Additionally, fixture and workpiece disassembly and assembly can take place outside the machine tool in a quick-change fixture mode.

III. Determining Size and Positioning Methods

When selecting a base plate or pedestal, it’s essential to confirm the dimensions and positioning methods in advance. The basic dimensions for horizontal machining centers are 400×400, 500×500, 630×630, and 800×800.

Depending on the machine tool manufacturer, there may also be dimensions such as 300×300, 1000×1000, and special sizes like 550×550. The sizing for vertical machining centers varies between manufacturers and is not regulated by any specific rules. Each machine tool manufacturer has their designated worktable dimensions, which can be referred to in the machine tool manual.

Figure 4

The base plate and the pedestal of a horizontal machining center are typically positioned using worktable blocks installed in the X and Y directions. The base plate and pedestal are aligned by fitting against these blocks. The location of the worktable blocks is usually set at the 0° and 270° positions of the machine tool worktable, although there can be variations depending on the manufacturer of the machine tool.

The center of the worktable on a vertical machining center typically has a key slot. The X-direction of the base plate and pedestal is positioned through this key slot, and the Y-direction is positioned using a T-slot.

However, not all T-slots can achieve precise positioning. Depending on the machine tool manufacturer, a machine tool usually only has one or two T-slots with positioning capabilities.

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IV. Confirming the Maximum Load Capacity of the Machine Tool

First, confirm the maximum load capacity of the machine tool, then verify the total weight of the workpiece, fixture, base plate, and pedestal. The total weight must absolutely not exceed the maximum load capacity of the machine tool. Some designers may calculate the weight of the workpiece and the base plate and pedestal, but neglect to calculate the weight of the fixture. This is something that absolutely should not be overlooked.

Figure 5

If the total weight exceeds the machine tool’s maximum load capacity, methods such as reducing the thickness of the baseplate, decreasing the thickness of the base mounting surface, or designing a hollow base can be considered to decrease the overall weight.

V. Confirming Base Height and Maximum Turning Diameter

When choosing a baseplate or base, the maximum height and turning diameter of the machine tool must be pre-determined. Especially in horizontal machining centers, the maximum turning diameter is a necessary factor to confirm. Typically, when the base size is 400X400, the maximum turning diameter is Φ630.

When the base size is 500X500, the maximum turning diameter is Φ800. The aforementioned sizes are common, but there is no explicit rule to use these particular base sizes. When the fixture clamps the workpiece from top to bottom, the top of the fixture (the top of the bolt in the following figure) should be within the maximum turning diameter.

Figure 6

VI. Confirmation of Machining Scope and Travel

Ensure in advance that the machine tool’s machining scope (the travel range of the main spindle) covers all the areas required to be processed on the workpiece. Especially when using a milling cutter for surface processing, consider whether the spindle’s travel allows the cutter to fully retract from the workpiece during the retraction phase.

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This consideration must be taken into account during design. If retraction is not considered, ensuring surface precision can be challenging. When the milling cutter exits the workpiece, it must maintain a minimum distance of 10mm from the workpiece, which means the cutter needs to completely depart from the workpiece’s range.

Figure 7

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