Milling says : Avoid thin-walled sections. Thin walled castings are difficult to deal with. They are often hard to hold in chucks and fixtures. During machining, the thin walls tend to produce high frequency vibration. The result of this vibration can be seen as “chatter marks” on the workpiece and as chipped or broken tools.
Grinding says : Of course, everybody should try to avoid thin-walled sections, but there are times when design characteristics will not allow it. The rigidity of the grinder, using the minimum feed available, can help adapt grinding technique to the part. It is unusual to have to consider vibration of the thin sections. If vibrations do occur, in roughing, it is usually possible to remove any resulting traces of chatter in the sparkout passes.
A grinder has a table that is constantly rotating, or reciprocating, under a continuous downfeed of the grinding wheel.
Milling says : Remember a casting has a draft. This makes the depth of the cut vary when cutting with the draft. Make sure that the machine has enough horsepower and the work holders sufficient strength to handle the greatest depth of cut which will be encountered. This is particularly important when machining to a corner or shoulder. Stalling of the machine could result in considerable damage to the workpiece, the cutting tool, and the machine tool itself.
Grinding says : Draft is not a problem in abrasive machining. A grinder has a table that is constantly rotating, or reciprocating, under a continuous downfeed of the grinding wheel. The “high” point of the work is contacted first with gradual feed into the full surface area. Remember that the grinding wheel diameter is at least 1⁄2 the diameter of a rotary table and cover completes the width of the table and a reciprocating grinding machine.
The coolant absolutely carries away all the chips and swarf.
Use of Coolant
Milling says : Consider proper application of coolant. Although many cast iron machining applications can be performed satisfactorily in the dry condition, properly applied cutting fluids can extend tool life. Cutting fluids are best applied under pressure directly at the tool-chip interface. The use of coolant is particularly dangerous, however, in interrupted cut applications. Because if may promote the formation of heat checks in the edge of the cemented carbide tool.
Grinding says : Too bad that the cemented carbide tool is so sensitive to coolant. Coolants are a must for abrasive machining for many reasons; one of the principal ones being to keep the work cool. Coolants on high-powered vertical spindle surface grinders provide better cutting action and longer wheel life. This is documented. Finally, the coolant absolutely carries away all the chips and swarf.
Make sure that the machine has enough horsepower and the work holders sufficient strength to handle the greatest depth of cut which will be encountered.
Entry and Exit Angles
Milling says : Have the tool enter and leave the work gradually. This is a machining technique that can aid in avoiding heat checks and chipping out of the casting. The use of rounded corners and gradual sections changes in the design of the casting can help in promoting this technique. Tool geometry can help in achieving this objective. Use of a high lead angle helps to avoid abrupt entry and exit.
Grinding says : There is total wheel contact in abrasive machining on a vertical spindle grinder, and line contact on any type of grinder. There is nothing like cutter vibration or tooth-load. The direction of wheel pressure is down on the part, toward the magnetic chuck which tends to hold the part tight in relation to the cutting action of the wheel. Entry and exit angles are not critical.