Stabilizing a CNC Machining Process

The order by which a workpiece is machined is of paramount importance to the success of a process.

Columns From: 5/7/2012 Modern Machine Shop,

Editor's Commentary

From the monthly column: CNC Tech Talk
An appropriate machining order will stabilize the machining process, while an inappropriate order might make it impossible to machine acceptable workpieces. Indeed, machining order is so important that some operators limit the task of process stabilization to the step-by-step order in which workpieces are machined.
 
Someone in your company, typically a process engineer, will determine how a workpiece will be routed through the shop. Based on the capabilities and capacities of the company’s equipment, the process engineer will select machines that are capable of producing acceptable workpieces. 
 
One common process-stabilization problem arises during this initial selection of machine tools. Often, so many workpiece surfaces need to be machined that it is difficult to perform all of the machining operations in one setting. That is, two or more setups may be necessary to completely machine the workpiece. This introduces the possibility of workpiece misalignment from one setup to the next. Any misalignment will result in difficulties holding tolerances among surfaces machined in different setups.
 
Such process mistakes can be costly because all workpieces will be machined in the first setup before the second setup is made. Problems found in the second setup may result in having to scrap the entire lot. 
 
When it comes to an individual setup for a given machine, the most important rule-of-thumb for machining order is to rough everything before you finish anything. By roughing first, you can ensure that workpiece surfaces will not move in the workholding device once the finishing operations begin, because finishing operations do not stress the workpiece or workholding device like roughing operations do.
 
Burr removal can be another machining-order-related problem. Many companies expect CNC operators to remove all burrs after the CNC cycle is completed. Often, certain sharp edges can be eliminated by a simple change in machining order. Reversing the order of machining among finish facing, finish boring and finish turning may prevent the formation of a sharp edge.
 
Unfortunately, you won’t know if changing the machining order will work until you try it. Changes must be made to the CNC program—changes that require the programmer to cut and paste commands from one point in the program to another. While some CNC controls have simple cut-and-paste capabilities, many do not. Also, some that  have this capability are difficult to use. There is an easy way to change machining order without having to make massive changes to the program: Use unconditional branching statements. In custom macro B, these are GOTO statements, but even a simple M99 can be used in main programs to change the program’s execution order. Here’s an example of a process you want to change:
 
O0001 (Main program)

N005 (Rough face and turn)

.

.

N065 (Drill)

.

.

N105 (Rough bore)

.

.

. N150 (Finish bore)

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.

N210 (Finish turn)

.

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N250 M30 (End of program)
 
Maybe you want to make the finish-turning tool run first to see if that will eliminate the burr on the face of the workpiece. Here is the modified program that provides this change:
 
O0001 (Main program)

N005 (Rough face and turn)

.

.

N065 (Drill)

.

.

N105 (Rough bore)

.

.

N148 M99 P210 (Go to line N210)

. N150 (Finish bore)

.

.

N208 M99 P250 (Go to line N250)

N210 (Finish turn)

.

.

N248 M99 P150 (Go to line N150)

N250 M30 (End of program)
 

With three simple commands, we’ve changed the execution order of this program.  

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