Programming of CNC Machines. Ken Evans
of cut is determined by the amount of material to be removed from the workpiece, cutting tool flute length or insert size and the power available from the machine spindle. Always use the largest depth of cut possible to ensure the least affect on the tool life.
Cutting Speed, Spindle Speed, Feedrate and Depth of cut are all important factors in the metal-cutting process. When properly calculated, the optimum metal-cutting conditions will result. Refer to the Machinery’s Handbook and the tool and insert ordering catalogs from the tool and insert manufacturers for more information on recommended depths of cut for particular tooling.
Certain steps must be followed in order to produce a machined part that meets specifications given on a blueprint. These steps need to be organized in a logical sequence to produce the finished part in the most efficient manner. Before machining begins, it is essential to go through the procedure called Process Planning. The following are the steps in the process:
1.Study the working drawing or blueprint.
2.Select the proper raw material or rough stock as described on the blueprint.
3.Study the blueprint and determine the best sequence of individual operations needed to machine the required geometry.
4.Transfer the information onto planning charts.
5.Use in-process inspection to check dimensional values as they are completed while the part is still mounted on the machine.
6.Make necessary corrections and deburr.
7.Perform a 100% dimensional inspection when the part is finished and log the results of the first article inspection on the quality control check sheet.
8.Take corrective action if any problems are identified.
9.Begin production.
Planning Documents
A blueprint may be thought of as a map that defines the destination. This destination is the end product. The roads available to get to this destination may be numerous. We do not start the trip without first determining what the destination is and how we are going to get there.
Planning sheets resemble the required path to the destination. They are written descriptions of how to get there (to the end product). The following are descriptions of sample planning documents.
The Blueprint
The information given on the blueprint will include the material, overall shape and the dimensions for part features. The geometry determines the type of machine (mill or lathe) to be used to produce the part. By studying the blueprint, material and operations (drilling, milling, boring, etc.) can be identified. The tools and work holding method can also be determined. Occasionally, the geometry will require multiple machines to manufacture the part, and thus additional operations will be necessary.
Figure 5 Machined Part Blueprint
Operation Sheet
The purpose of this planning document is to identify the correct order for operations to be performed and the machine to be used. For example, suppose you are required to produce the part shown in Figure 5 above. Saw cutting the rough stock into blanks would come first. Then the part must be turned on a lathe to create the five-inch diameter and rough turn the diameter for the hexagon. Next a milling machine is used to cut the hexagon and drill the bolthole circle. Before any inspection can be carried out, the part must be deburred. Finally the part can be inspected for accuracy.
The operation sheet is particularly useful when many identical parts are machined (production run). The operation sheet is similar to directions or a how-to approach. The process needed to manufacture the finished part has been decided in advance and is documented for future use.
When small batches of parts are to be made, there may not be an operation sheet. It is the machinist’s responsibility to study the blueprint and decide the necessary steps to machine the part. The operation sheet can aid in this decision making process.
With CNC machining, multiple part geometry features can be performed in one setup. In some cases, when using a CNC Mill Turn Center, a part might be machined to its completed status without ever using another machine. This is very efficient and another advantage of the use of CNC equipment.
To complete an operation sheet, study the blueprint; then decide on the steps necessary to machine the part. Document the machining process and refine any problems the process has, then list the operations in the correct sequence in which they will be performed.
The top section of the operation sheet is for reference information and includes:
The date the document is prepared or revised
The name of the person preparing it
The part name and the part number (from the blueprint)
The quantity of parts to be manufactured
Since some parts may require a large number of operations, it is possible that more than one operation sheet will be needed to document the whole process. The top section also includes a sheet numbering system (Sheet _____ of _____). This information must be included. Other information included on the operation sheet header is the material, the raw stock size for the part, followed by the operations list.
CNC Setup Sheet
The CNC Setup Sheet is the document that tells the machinist what tools are to be used and any specific information related to tools. For instance, it may be necessary to have a certain amount of extension for a drill to be able to completely machine through a part. This document is where the Operator/CNC Machinist finds this information. In Part 5 or 6 of this text you will be introduced to Computer Aided Design and Computer Aided Manufacturing (CAD/CAM) and how you can develop CNC Setup Sheets within the CAD/CAM programs. Many companies today are going to electronic “paperless” factories. The CNC Setup Sheet has two sections. The top section is for reference information and includes:
The date the document is prepared or revised
The name of the person preparing it
The part name and part number (from the blueprint)
The machine being used
Note: If more than one machine is to be used to manufacture a single part, separate setup sheets are completed for each machine.
The CNC part program used in the manufacturing process
Workpiece Zero reference points for the part (program zero)
Work holding devices
Note: If more than one device is needed, the operation number(s) and process are also included.
The lower half of this form lists the tool(s) by number, description and offset. There is a column for comments, remarks or explanations, if needed. Specific tool requirements, like minimum tool length extension, can be entered in the comments section
Quality Control Check Sheet
This planning document is used for the final inspection stage of the machining process. Once the part is completed, it is necessary to check all of the dimensions listed on the blueprint to verify they are within the specified tolerance. The Quality Control Check Sheet is an excellent method to document the results of this inspection and a valuable tracking tool.