MACH 122 Abrasive Machining Work Holding OBJECTIVES After

MACH 122: Abrasive Machining Work Holding

OBJECTIVES After completing this unit, you should be able to… �Describe the basic operating principle of common grinder chucks. �Explain care of grinder chucks. �Describe methods of holding odd-shaped, nonmagnetic, and thin workpieces.

WORK HOLDING ON THE SURFACE GRINDER Magnetic Chucks � The most common work-holding device for the surface grinder is the magnetic chuck, of which there are three types: The permanent-magnet. The electro-permanent-magnet chuck.

WORK HOLDING ON THE SURFACE GRINDER Magnetic Chucks - Permanent Magnet �Permanent-magnet chucks are composed of a series of alternating plates of powerful magnets. Pole pieces inside can be moved to increase magnetic flux for firm work holding or to diminish magnetic flux to permit releasing the part. Fig. L-79 Permanent-magnet chuck, the most widely used surface grinding chuck. (Mitutoyo America Corp. )

WORK HOLDING ON THE SURFACE GRINDER Magnetic Chucks - Electromagnetic Chucks � Magnetized when electrically energized. Heat is a byproduct of power being continuously applied. � Generally, grinding is done on electromagnetic chucks with the use of grinding fluids, which carries away the generated heat. Some electromagnetic chucks are constructed with internal fluid passages for internal cooling.

WORK HOLDING ON THE SURFACE GRINDER Magnetic Chucks �After the current is removed, some residual magnetism can remain in the workpiece, making removal from the chuck difficult. An electrical demagnetization process (degaussing) is used to remove any residual magnetism from the part. Common type of magnetic chuck for reciprocating surface grinder. The stop strips at the back and left side are adjustable for height and can be used to prevent work from sliding off the chuck. (Courtesy of Walker Magnetics)

WORK HOLDING ON THE SURFACE GRINDER Magnetic Chucks - Electro-permanent-magnet � This third type of magnetic chuck has distinct advantages in two important areas. The electro- permanent-magnet chuck retains holding ability with or without electrical power. Once the magnetism is established it is not necessary to apply additional energy to the chuck. � Alnico is used in these chucks because it is easier to magnetize & demagnetize than ceramic (ferrite) commonly used in permanent-magnet chucks. � Al. Ni. Co= Aluminum (Al), Nickel (Ni), Cobalt (Co)

WORK HOLDING ON THE SURFACE GRINDER Magnetic Chucks - Electro-permanent-magnet � Magnetic chucks for surface grinders are usually rectangular. Those for vertical spindle rotary grinders are circular. Workpieces can be held on magnetic sine chucks for grinding angles or compound angles.

WORK HOLDING ON THE SURFACE GRINDER Vacuum Chucks � Air under the workpiece is evacuated, so the force of atmospheric pressure holds the part. Vacuum chucks are useful for holding thin workpieces. tab

HOW TO DO IT… l Care of Grinder Chucks � Step 1 Carefully clean the chucking surface before mounting a workpiece. � Step 2 Be sure the workpiece is free from burrs. � Step 3 On a magnetic chuck, be sure small parts span as many magnetic poles as possible to ensure maximum holding power. � Step 4 Deburr the chucking BURRS surface from time to time. Periodic deburring of the chuck with a granite deburring stone, or with a fine-grit oilstone, is a good practice.

WORK HOLDING ON THE SURFACE GRINDER Odd-Shaped Workpieces tab � If large enough, a magnetic chuck will exert the most reliable holding power on an odd-shaped workpiece. If the area of contact with the chuck is large enough. � Sometimes it is necessary to block a small or thin part with additional material. The blocking needs to be thinner than the workpiece.

WORK HOLDING ON THE SURFACE GRINDER Odd-Shaped Workpieces � Sometimes it is necessary to chuck on an area that is smaller than desirable. Chuck setup for a workpiece with a projection on chucking side. The work is supported on laminated magnetic parallels. In these cases, the work must be supported with magnetic parallels or vee-blocks, specially designed for grinder work. A set of magnetic parallels & vee blocks can be useful. tab

WORK HOLDING ON THE SURFACE GRINDER Odd-Shaped Workpieces �Laminated accessories are made with tab nonmagnetic & soft steel inserts so lines of magnetic force will be conducted through to the workpiece. They must be treated with great care because the magnetically permeable materials are soft. �Odd-shaped workpieces may also be held by traditional precision milling-type machine tool work-holding methods such as vises or clamps.

WORK HOLDING ON THE SURFACE GRINDER Nonmagnetic Work tab � Nonmagnetic materials may be held on a magnetic chuck by blocking the parts with steel (magnetic) tooth clamps Comblike teeth will keep a nonmagnetic workpiece from sliding off the chuck. Tooth clamps in use. Note that the toothed clamps are lower than the surface to be ground.

WORK HOLDING ON THE SURFACE GRINDER Thin Workpieces tab � Thin material already warped or twisted will be pulled flat on either a magnetic or vacuum chuck. These workpieces may assume their original distorted shape after grinding and removal from the chuck. � Thin material can be held and ground by using a minimum amount of holding force, for light cuts � It is also possible to shim under a warped thin workpiece until one side has been ground true. The part may then be turned over and the true side placed on the chucking surface.

WORK HOLDING ON THE SURFACE GRINDER Thin Workpieces tab � Grinding of thin cold-rolled steel stock that has not been annealed or normalized can be frustrating. The outside of the material is in much greater compression than the core. � It is important to grind uniform amounts from both sides of such materials. And to be prepared to grind opposite sides multiple times. � It is best to use hot-rolled steels or workpieces that have been annealed, or otherwise appropriately heattreated, for grinding.

Review Questions � What is the most commonly used work-holding device used with surface grinders? � What are three different types of magnetic chucks? � Which type of magnetic chuck creates heat? � What is alnico and how is it used? � Vacuum chucks are best suited for what type of workpiece? � What must be done before placing workpiece on a magnetic chuck? � What can be done to secure a small part on the chuck. � Why is grinding thin cold rolled steel sometimes a problem? � To limit warpage of material, what procedure must be followed?