Figure 13.6 – The same workpiece with the same prz, but located in each quadrant.

Tooling References—PRZ Not on the Workpiece

Fixtures are custom-made holding devices that

• Hold odd shapes that cannot be held safely or reliably using standard methods.

• Provide quick part-to-part turnaround time.

• Reduce variation in production due to reliable loading and holding.

Machining Center Holding Tooling

Sacrifice plate. A metal plate bolted above mill tables to protect them and provide a utility surface for drilling and tapping threaded holes for clamps and grooves for parallels and pins.

Subplate. Same as sacrifice plates.

Vertical cnc mills, where there are three common holding selections for your plan:

• Precision vise or vises—with hard or soft jaws

• Direct bolt down—over a sacrifice plate or set up on parallels

• Fixture—shop or professionally made

Soft jaw vises are used more often because we need to hold odd-shaped work and also provide a locator for a PRZ. The vise jaws are custom-machined to fi t the part using the CNC control. Often, the finished soft jaw set is given a tooling number and saved for future applications of the program. This fact must then be added to the CNC document under comments—that tooling already exists and need not be made each time the program is run.

Bolting directly to the machine table is less common in CNC work mostly because it’s slow to replace production parts but to also keep fast moving cutters away from the machine table. We use bolt-down holding when the work won’t fit into a vise or to gain a bit more Z axis work envelope for an extra tall workpiece.

For protection during bolt-down holding and for other setup utility reasons, a protective flat plate called a sacrifice plate or sometimes a subplate is usually bolted to the CNC mill table.

Machine Fixtures

Fixtures are a commonly used workholding tool in industry.

References: Gen [1-3].

Assessing questions:

1. Choosing the PRZ location – lathe or mill.

2. PRZ location for turning work.

3. Top surface – lower-left corner.

4. Tooling references – PRZ not on the workpiece.

5. Machining center holding tooling.

Lecture 14. Level – one programming. Code words and program conventions. Programming languages. Level – two programming

Programming Languages

When the control reads commands, transparently it translates each word into a more basic set called machine assembly language (MAL). MAL works at the on/off level to process and send signals to actuate drives and start or stop various machine functions.

Today there are three different methods of compiling a machine-readable program.

1. Graphic Interface. This method generates program commands from screen pictures drawn on the CNC machine’s control.

2. Conversational. This is another programming method whereby the machinist writes the program statement on the CNC control. These command words are based on the spoken language of the country in which it is being used.

3. EIA Codes (ISO). This is the closest we come to a universal language set. The basic vocabulary is standardized by the Electronics Industries Association, EIA – Recommended Standard RS274-D (US Standard). ISO and EIA codes are virtually the same at the beginner level, and there are only a few differences at any level.

CAM software generates line-by-line programs. However, tool and operation notes can be inserted and they can be arranged into outline form by editing.

Code Words and Commands

A code word is a single statement that

• Sets a mode.

• Causes an action.

• Prepares for an action.

• Starts or stops a function (coolant or spindle, for example).

Program words are arranged into sets of command lines (shortened to commands), or command blocks. Whatever you call them, a command is a complete instruction containing words and/or coordinates for one action. If the action includes axis movement, it’s sometimes called a motion statement.

G Codes. These are the prep and motion words. G words with axis coordinates form the heart of the program.

M Code. Words Miscellaneous functions or actions are the second most common words used in the program. They cause utility functions such as tool change, spindle on or off, and coolant.

Control Differences. On older controls, to prevent electrical current surge and CPU data overloading, only one M code is permitted per command line.

T Code Words – Tool Numbers and Time.

S Words-Speeds. Causes the spindle (lathe or mill) to rotate at the specified XXXX RPM. For example, S2000 M03 starts spindle at 2,000 RPM forward rotation.

N Words – Line Number Addresses or Number Entry.

Modal codes or commands work as follows. Upon reading some command words, the control retains their setting.

They stay in effect (turned on) until they are canceled by another word in a future command. The cancel word can be

A program end

Another word from the same group

A specific cancel word—The master cancel words are

G80 (end all canned cycles) and G40 (end tool radius compensation), and some use G49 (with nothing following) to cancel tool length compensation.

The word default has two similar but slightly different meanings in CNC work:

1. Best-Guess Value;

2. Preset Value