November 2011 Archives

When locating work-pieces that receive external forces, location datum surfaces must be provided in locations to take account for such forces. Errors in datum location provisions will result is the following problems.

Therefore, locating of work-pieces with applicable external forces must take in consideration the sizes and the directions of the allocable forces.

Example

Case of drilling fixtures

In case of [Fig.1], there are two forces that apply.

1. Rotation force of the drill bit (Torque)
2. Plunging force of the drill bit (Thrust)

[Fig.1-a shows a datum structure that can withstand these two forces. [Fig.1-b] shows an example with different locating pin layout, and it can be seen that the work-piece cannot be securely located against the torque.

Burrs and other foreign objects on datum surfaces induce fixture wear and locating inaccuracies. Here, locating pin basics regarding how to avoid the negative effects of burrs and foreign matter.

Countermeasures against burrs and foreign matter

There are following three methods of avoiding inaccuracies induced by burrs, chips, and foreign matter.

1. Use of datum feature designs easy to clean.
2. Self cleaning design where location action cleans automatically.
3. Employing designs that avoid negative effects by burrs and foreign matter.

In the following, a) will firstly be explained.

a) Use of datum feature designs easy to clean. (see [Fig.1])

Explanation

Use the following viewpoints to design for easy to clean datum features.

• Select small wear resistant locating pins to make for easy to clean designs.
• Avoid hard to see fixture designs. Make the designs so the trouble can easily be seen.
• Locate the datum features high above the fixture top surface so the chips can would drop below for easy removal.

There are various shapes and dimensions devised for locating datum features in order to correspond to many types of work-piece shapes. The datum basics are explained here.

Locating datum basics

(1) Integrated locating datum method ([Fig.1])

(2) Assembled datum surface method

(3) Locating datum pin method

Due to the advancements in mechanization and automation, many types of product work-pieces are now being processed on machinery devices. Objects as large as the 9th generation LCD panels to chip mounted components as small as several micrometers, automated processing are performed while the work-pieces are being held. The work holding principles and technology will be explained in the following.

(1) The basics of work holding

• Work holding technology is needed for all fixturing, automated assembly, and processing.
• Mechanisms to hold work-pieces by some means (retaining, gripping, chucking) are called "Work Holders".
• There are mechanical, electrical, air, hydraulic means to hold the work-pieces.
• The operational relationship between the work holders and the process tools (manipulators) is the same whether the process is for cutting or assembly.
• The shaping and assembly process are performed by applying desired movements of the tooling on the work-pieces held by work holders.

(2) Classification of work-piece external shapes

• Work-piece shapes can be classified into the following three types.
  1. Flat surface
  2. Cylindrical surface
  3. Compound shape surface
• The flat surfaces include: edges, flanges, steps, surfaces, shoulders, slopes.
• Cylindrical surfaces can be inner circumference and outer circumference.
• Compound surfaces are the ones not belonging to a) or b), and often occurring with formed parts (cast, forged, injection molded parts)

(3) Classification of Locating

Locating can be classified into the following four types (see [Fig.2])

1. Flat surface locating
2. Coaxial locating
3. Radial locating
4. Compound locating