How to select the right tombstone fixture shape for your application

By: Nick Rupprecht, Business Unit Manager, AMROK

Workholding tombstone fixtures come in a variety of shapes and sizes. How to select the right tombstone fixture shape for your application will depend on:

• The machine envelope
• The size of the parts being machined
• Work volume and productivity capacity
• The use of fixture plates
• Budget
  
  

Basic Tombstone Fixture Shapes

Cube Tombstone

The cube tombstone, or square column tombstone, features faces of equal width. From the top view it looks like a square, which is perhaps one of the most common workholding tombstone fixture shapes. It can be used for a wide variety of applications because each of the four faces can either hold the same operation set-up or a different operation set-up.

Cross Tombstone

The cross tombstone, or plus shape tombstone, also features faces of equal width. It has the same overall column width as the cube tombstone fixture shape, but it has a few key differences when compared to the cube's function. From the top view, the fixture looks like a cross. The faces of the cross are less wide and the corners are cut out. This feature allows for tool or spindle clearance when machining part faces perpendicular to the tombstone face. It also allows you to machine your parts from the sides. The weight is likely to be higher due to the shape of the column walls. The cross tombstone fixture shape is similar in function to the cube tombstone fixture shape.

Double Angle Tombstone

The double angle tombstone, or T column tombstone, features two faces of the same width as the base. The column appears to be a rectangle from the top view. This shape is one of the most common tombstone fixture shapes.It is ideal for machining large parts while remaining within your machine’s work envelope because of its large face width and the distance from the large faces to the edge of the tombstone base. The distance of the wide faces from the edge of the machine's pallet provides full use of the machine work envelope by opening up the space for your parts.

The cube, cross, and double-angle tombstone fixture shapes will typically have the center of their columns cored out with 2” thick walls to reduce weight while retaining rigidity for tough machining.

Single Angle Tombstone

The single angle tombstone, or angle plate tombstone, has one face that is perpendicular to pallet. The face is either square or rectangular and is ideal for very large parts. The face is positioned near or beyond the centerline of your machine pallet to maximize the parts space within the machine’s work envelope.

Distinctive Tombstone Fixture Shapes

Window Tombstone

The window tombstone has two faces that are the same width as the base with the center section of the face cut-out. This tombstone fixture shape is similar to a double angle tombstone fixture shape; however, it also features a window cut-out.

The advantages of the window cut-out include:

• Weight reduction when compared to a double angle tombstone. 
• Machine accessibility to the backside of the workpiece when using only one side of the tombstone.

This tombstone fixture shape is frequently used with removable grid plates containing modular or dedicated fixtures to quickly change out part setups.

Triangle Tombstone

The triangle tombstone features three faces of equal width. The column looks like a triangle from the top view. This tombstone fixture shape has similar benefits to both the cube and the double angle tombstone. The three faces are typically wider than the cube tombstone, but less wide than the tombstone base; whereas the double angle face is the full width of the base.

Y Tombstone

The Y tombstone has three faces of equal width. From the top view, the column appears to be a Y shape. This tombstone fixture shape has a similar use to a cross tombstone. The main difference is tool clearance. The Y tombstone has a greater clearance space for the ability to complete side work on the workpiece. The only downside is the loss of one face as compared to the cross tombstone.

The triangle and Y tombstone fixture shapes will typically have the centers of their columns cored out with 2” thick walls to reduce weight while retaining rigidity for tough machining.

Tombstone Fixture Shapes for Maximum Machining

Hexagon Tombstone

The hexagon tombstone has six faces of equal width. From the top view, the column looks like a hexagon. This shape is ideal for maximizing the number of parts per tombstone, but the faces are less wide than on a cube tombstone fixture shape. It can feature the same overall column width as a cube fixture shape while providing two additional faces.

Octagon Tombstone

The octagon tombstone has eight faces of equal width and it looks like an octagon from the top view. This tombstone fixture shape is ideal for maximizing the number of parts per tombstone. The faces are less wide than on a hexagon tombstone with the same overall column width with two additional faces. The hexagon and octagon tombstone fixture shapes willy typically have the centers of their columns cored out with 2” thick walls to reduce weight while retaining rigidity for tough machining.

After you select the fixture shape that you want, what material for the tombstone would best fit your application? Read: How to Select the Right Material for Your Workholding Tombstone?

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