Cnc machining nm09/2


Practical exercise 11 — section 6



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Practical exercise 11 — section 6


Note: This exercise should only be carried out under the supervision of your teacher.

Instructions



  1. Take the program tape (or computer disc) to the machine along with a print out and a part drawing ofNM09 -Exercise 8 as well as the operators setup sheet

  2. Read the tape into the machine (or send the program direct from the computer).

  3. Perform a verification check on the program through the graphic simulation function, if fitted.

  4. Prepare the work holding device according to the operators setup sheet

  5. Mount the tools in the tool changer and verify that they match the tools and tool numbers in the program

  6. Set the tool offsets and register in the appropriate part of the memory

  7. Select a suitable tool change position and enter this into the program

  8. Establish the work piece datum and enter this in the appropriate part of the machine memory

  9. Dry run the program and edit errors that may occur.

Complete the ‘Operators Set up Sheet’ for job NM09 -exercise 8

Figure 6.1
Exercise 11 — Operators setting up sheet (NM09 exercise 11)


Section 7: CNC machine operation


This section covers the operation of a CNC machine in manual, jog and automatic mode to machine a work piece to drawing, including verification and editing of programs on machine.

Objectives


At the end of this section, you will be able to:

  • operate the CNC machine using manual data input (MDI) mode

  • safely verify safe operation of program

  • edit a program using the CNC control panel

  • produce a pre determined component using automatic mode

  • compare completed component to required specifications and complete a report on the comparison

  • maintain work place safety standards at all times.

Safety reminders


  • In the workshop, always wear safety glasses, safety boots, hair protection and suitable clothing.

  • Avoid back injuries -lift the correct way.

  • Do not use a machine fitted with a Danger Tag.

  • Know where the first aid station is.

  • Don't run or play in the workshop.

  • Use ear muffs or plugs to protect your hearing.

Special rules for this section


  • take care when handling sharp cutters and tooling

  • do not work on the machine unless your teacher has cleared you to do so

  • ensure all tools will clear other machine parts when mounted in magazine

  • do not make any adjustments to the machine unless your teacher is present.

Do not work on the machine unless your teacher has cleared you to do so. Dry run program first with tool clear of work piece by a minimum 50 mm. Do not run the program unless your teacher has given approval to do so. Ensure all guards and safety devices are in position before operating the machine.

7.1 Quality Assurance

7.1.1 Surface finish


There are two types of surface finish obtained from milling operations:

  1. A circular pattern, produced by the cutter teeth when the surface is at 90 0 to the spindle axis, such as flat faces that have been face milled.

  2. A linear pattern, produced by the teeth during peripheral milling, such as the sides of a slot that have been end milled.

Peripheral surfaces generally are easier to obtain a good surface finish on because of the sliding effect as the tooth engages or disengages the act at the thin end of the chip (depending on whether up cut or down cut milling is used.)

Axial surfaces are produced by teeth with sharp comers (HSS endmills, etc), or radiused comers (inserts), or by milling inserts with parallel lands -in which case the finish may be stepped because of uneven insert height.

In this case, one wiper insert can be fitted to the cutter and set to be the lowest insert, thereby acting as a finishing tool built-in. The length of the flat (wiper) is usually about 10 mm for most insert shapes, enough to cover the feed distance for one revolution of most cutters except perhaps large diameter fine pitch face mills.

Surface finish is also dependent on:



  • machine condition

  • cutter design

  • cutter geometry

  • cutting data (speed, feed, depth)

  • workpiece shape

  • workpiece clamping.

7.1.2 Size


The sizes produced by milling are a result of the correct positioning of the cutter, which in tum is under the direct influence of machine offsets.

The height of faces is controlled by tool length offsets (Z-axis), increasing the offset value results in a lower machined surface.

The size of peripherally milled surfaced is controlled by cutter radius offset in conjunction with cutter radius compensation. Increasing the radius offset will increase the external size of the part for a given tool and decrease the internal size of the part for the same tool.

You will be required to measure the first part from your program and report on the measurement obtained. You will then make any necessary adjustments to offsets to compensate for inaccuracies found.


7.2 Machine operations


Note: Operation of the machine tool is only to be done under the teacher's supervision.

7.3 Control Panel


Most controls offer similar functions and uses, even though the panels differ in appearance.

For you to operate the machine correctly and confidently, it is important that you understand the functions of, and uses for all the buttons and switches on the control panel.

The most frequently used controls are:


  • The mode selector switch. This selects

    • auto operation

    • jog (or manual) operation

    • MDI

    • program edit facility

    • zero set

    • tool offsets

    • etc.

  • The cycle start button. This initiates program operation, either complete by one press or a block at a time depending on the setting of the single block switch.

  • The feed hold button. A red or an orange coloured button immediately adjacent to the cycle start button (usually green). Pressing the feed hold button will halt slide motion only, pressing cycle start will resume the operation of the program.

  • Spindle and feed override switches. Multi-position switches used to vary the programmed rates. The feed override usually overrides rapid traverse motion, but some machines have a separate switch for this.

  • Emergency stop. A large red button placed conspicuously on the control panel, it may also be duplicated elsewhere on the machine. Pressing this cuts power to the machine (and on some machines to the control as well), thereby effecting a halt to all machine motions. This button should only be pressed in an emergency situation where personal injury or machine or tool damage is likely, and not for clearing alarms or other minor problems.

  • Other switches or buttons. Dry run, block delete, optional stop, Z-axis inhibit, axis jog, axis zero, homing (zero return), keyboard, handwheel and 'soft' key which are buttons placed under the VDU (screen) and have a variety of functions as indicated on the bottom of the screen above them. They are so named because their function is controlled by software, they are not 'wired-in' for just one function.



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