CNC Machine Operation

A CNC (computer numerical control) machine follows a written program to move a cutting tool or a workpiece along exact paths. It repeats those moves part after part, which is how a shop holds a few thousandths of an inch across a thousand pieces. The machine does the cutting, but it has no idea whether the cut is going well. You supply the eyes, ears and judgment. A good operator catches a dull tool, a loose part or a wrong offset before it turns into a scrapped batch, a damaged spindle or an injury.

What You Will Be Able to Do

• Name the common CNC machine types and what each one is good at.
• Explain machine zero and work zero, and why the difference matters.
• Use cycle start, feed hold, the overrides and the program stops correctly.
• Respond to an alarm in the right order, including a spindle overload.
• Run a job and check it as it runs, without putting your hands where they can be hurt.

What a CNC Operator Does

Job titles vary by shop, but the work usually splits into three roles. In a small shop, one person may do all of them.

A normal shift as an operator is a loop. You load a part, close the door, press cycle start, watch and listen, unload, deburr, measure at the set interval and write the results down. While the machine cuts, you are often deburring the last part or checking the next tool. The job looks simple until something changes, and noticing that change is the skill.

Types of CNC Machines

Machining Centers

A machining center is a CNC mill with an automatic tool changer. The workpiece is held on a table or in a fixture, and a rotating tool removes material.

• Vertical machining center (VMC): The spindle points down. This is the most common machine for general work, mold work and production parts. On a typical 3-axis VMC, the table moves in X (left and right) and Y (front and back), and the spindle moves in Z (up and down).
• Horizontal machining center (HMC): The spindle points sideways. Chips fall away from the cut instead of piling up in pockets, which helps on deep pockets and heavy cuts. An HMC usually has a rotary table (the B axis) so it can reach four sides of a part, often on a tall fixture called a tombstone that holds many parts for high-volume work.

3-Axis Versus 5-Axis

A 3-axis machine reaches the part from one direction. To cut another side, you stop, unclamp, turn the part and clamp it again. Every one of those moves takes time and adds a little position error.

A 5-axis machine adds two rotary axes that tilt or turn the part or the spindle. It can work on several faces of a part while the part stays clamped in one setup. That means fewer fixtures, less handling and better accuracy between features on different faces. It also means the tool can approach from angles that are harder to picture, so a crash is harder to predict and the machine's motion deserves extra attention. A 4-axis machine sits in between, with one rotary axis.

Lathes and Turning Centers

On a lathe, the workpiece spins in a chuck or collet and the tool stays still until it is fed in. Lathes make round parts such as shafts, bushings, fittings and pins.

• 2-axis lathe: X moves the tool in and out from the centerline. Z moves it along the length of the part. This covers most turning.
• Live tooling lathe: Adds powered tool stations and a C axis that indexes the spindle. It can mill flats, drill cross holes and tap while the part stays in the chuck, which removes a trip to a mill.
• Swiss-type lathe: The headstock slides the bar forward through a guide bushing, so the cut always happens right next to the support. It suits small, long, slender parts such as medical devices, connectors and watch components.

Routers and EDM

• CNC router: Built like a large, lighter mill. It cuts wood, plastics, composites, foam and aluminum for signs, furniture, packaging and aerospace panels.
• Wire EDM: EDM stands for electrical discharge machining. Sparks erode the metal instead of a tool cutting it. A thin wire, usually brass, cuts profiles through the part with very tight accuracy.
• Sinker EDM: A shaped electrode burns a cavity into the part, which is common in mold and die making. There is no cutting force, so hardened steel and carbide can be machined.

How the Machine Knows Where It Is

Every CNC machine uses a coordinate system. On a mill, X and Y are the table moves and Z is the spindle direction. On a lathe, Z runs along the spindle centerline and X runs across it.

There are two zero points, and mixing them up is how tools get broken.

• Machine zero is a fixed point built into the machine by the manufacturer, found by home switches or the machine's position system. You cannot move it. Every time the machine is homed, it knows where it is relative to this point.
• Work zero (also called program zero) is a point on the workpiece that the setup person chooses, such as a corner of the top face on a mill or the finished face on a lathe. The program's numbers are all measured from here.

The control connects the two by storing the distance between them in a work offset, usually G54. Think of machine zero as the front door of a building and work zero as the desk where you work. If someone says "two steps left of the desk," you need to know where the desk is first. If the stored offset is wrong, the machine runs a perfect program in the wrong place.

Reading the Control Panel

Panels look different from one builder to the next, but the same controls appear on nearly all of them.

The control also has modes. If a button seems to do nothing, you are often in the wrong one.

Starting Up the Machine

1. Check air pressure, way lube and coolant level, and look for leaks or anything left on the table.
2. Power up the control and clear the startup messages.
3. Home the axes if the machine needs it. Watch for clearance first if a tool or part was left in the work area.
4. Run the spindle warm-up if the machine has sat idle. Builders publish a warm-up routine for this because a cold spindle can be damaged by high speed.
5. Confirm the program number, revision and tools against the job paperwork.

Running a Job

These steps assume the job has already been set up and proven.

1. Clear chips from the vise, fixture or chuck jaws. A chip under a part tilts it and throws off every dimension.
2. Load the part so it seats fully against its stops or locators, and clamp it. Check that it really is seated.
3. Close the door. Set the overrides where the job calls for them.
4. Confirm the control is in Memory or Auto mode and at the start of the program.
5. Press cycle start with your other hand near feed hold, and watch the first moves.
6. Stay alert to each tool change and the first contact with the part.
7. Wait for the spindle and all axes to stop fully before you open the door.
8. Unload, clean off coolant and chips, and deburr as the job calls for.
9. Measure at the interval on the inspection plan and record the results.

Feed Hold and the Overrides

Feed hold is the normal way to pause. Axis motion stops, but on most machines the spindle keeps turning. It is not a safe state for reaching in. Wait for all motion to stop before the door opens. In a tapping cycle, feed hold may not take effect until the tap is back out, because stopping a tap partway through would break it.

Feed override slows or speeds the programmed cutting feed. It is the tool you use on a first pass or when a cut sounds wrong. Feed override does not control rapid moves. If you are worried about a rapid move, turn down the rapid override as well. Remember that changing spindle override without changing feed also changes how much each tooth cuts.

Program Stops

Two codes in the program can pause it.

• M00 (program stop) halts the machine every time it reaches that line. You press cycle start to go on. Programmers use it where something must always happen, such as flipping a part or adding a clamp.
• M01 (optional stop) halts only if the Optional Stop switch on the panel is on. If the switch is off, the program runs straight through. It is used for spots where you may want to look, such as after a roughing tool, while running the first few parts.

Read the setup sheet to see whether Optional Stop belongs on or off for your job. Turning it off when a check was meant to happen means the check does not happen.

Alarms and What to Do

An alarm is the control telling you it has stopped for a reason. Treat every alarm in this order.

1. Stop and look. Is anything moving, smoking, leaking or broken?
2. Read the whole alarm message and write down its number.
3. Check the part, tool and workholding before you touch reset.
4. Fix only what you are trained to fix. Tell your lead or setup person about the rest.
5. Restart from the place your shop's procedure says. Do not just press cycle start and hope, because the program may resume with a different tool or offset than you expect.

Spindle Overload

A spindle overload alarm means the spindle motor has been drawing more power than it should. Most of the time the reason is cutting force. The cut is too heavy for the setup, or the tool has gone dull and now rubs and pushes instead of cutting. Check the tool first, and look at the load meter trend on the parts before this one. A meter that has been climbing for an hour points at wear.

The cause can also be hardware. On a lathe, a chuck that has not fully closed on the part, or a part that is jammed or bound in the jaws, can load the spindle the instant a heavy cut begins, even with a sharp tool and a proven program. If the alarm comes at the very start of a cut, check the workholding before you blame the tool. A tailstock pressing too hard, a slipping drive belt or a failing spindle bearing can also cause it. Those last ones are for maintenance.

After a Crash

If a tool hits the part or fixture, press the emergency stop or feed hold, depending on which you can reach faster. When a tool dives into the work at rapid speed right after cycle start, the program is rarely the first suspect. A wrong tool length offset or a wrong work offset is the usual cause, because the control is moving to the right number from the wrong starting point. Do not restart, and do not back the axes out until you have looked at what touched. Tell your lead right away, even if nothing looks broken. A hit can knock the spindle, tool holder, fixture or work offset out of line, and that error will show up on later parts. Telling someone takes a minute. Hiding a crash can cost a batch of parts or a spindle.

What to Watch During a Run

Parts usually drift out of tolerance gradually, and the machine gives you warnings first.

• Sound: Learn what a good cut sounds like on each material. A new squeal, a chatter or a change in pitch means something changed.
• Chips: Watch for a change in color, shape or size. Long strings wrapping around the tool or part are a problem.
• Load meter: A slow climb over many parts usually means wear. A sudden jump means something broke or the stock changed.
• Finish: If a surface suddenly turns rough, look at the insert first. A worn or chipped cutting edge is the usual cause.
• Size: Plot or write down your measurements. Numbers creeping toward one limit tell you a correction is coming.
• Coolant: Check that it is flowing onto the cut and has not shifted away.
• Chip conveyor and tank: A clogged conveyor can back chips up into the work area.
• Tool life: Change inserts at the count on the setup sheet, not after the part is ruined.

Write down what you change and why. The next shift needs to know, and a record of your offsets and tool changes helps the next setup go faster.

CNC-Specific Safety

Keep every guard, door and interlock working, and never bypass one. If an interlock misbehaves, report it to maintenance. The points below are the hazards particular to CNC work.

• Stay out of the cutting area while anything moves. Do not adjust a clamp, touch chips or measure while the spindle is turning or axes are moving. Press feed hold and wait for everything to stop.
• Wear eye protection everywhere in the shop. Chips travel a long way.
• Keep anything loose away from rotation. Tie back long hair and take off rings, watches and loose sleeves. Do not wear gloves near a turning spindle or chuck. A rotating part can catch a glove and pull your hand in. Gloves are for handling stock and sharp tools with the machine stopped.
• Respect a spinning lathe part. Do not run a chuck above its rated speed. Never let bar stock stick out the back of the spindle without a bar feeder or support tube, because it can whip and bend. Check that a heavy or unbalanced part is held well enough, since it can be thrown with great force.
• Keep the door window in good condition. The window is designed to stop a thrown part or broken tool. Coolant can weaken it over time. Report any cracks or clouding.
• Move the axes slowly near the part. Use a small jog step or the handwheel close to the work. One wrong button at rapid speed can drive the tool into the part.
• Watch for fire. Fine titanium or magnesium chips can burn, oil-based coolant and its mist can ignite, and the oil in a sinker EDM tank must stay above the spark. Know which extinguisher your shop uses for metal fires. Water on burning magnesium makes the fire worse.
• Take care with coolant. It can irritate skin and lungs and makes floors slick. Use barrier cream or gloves as your shop directs, wipe up spills, and use the mist collector. Report coolant that smells foul, because that usually means bacteria.
• Lock out the machine for maintenance. Follow your plant's procedure for work inside the machine beyond normal operation.

Warning: Chips are sharp and often hot. Never clear them with your hands, and never reach in while the spindle turns. Clear them with the machine stopped, using a brush, a chip rake or hook, or the chip conveyor.

Use a brush or rake first. Blowing chips with compressed air throws them at people nearby and drives them under way covers and into seals. If your plant permits air at all, OSHA 1910.242(b) requires the pressure at the nozzle to be under 30 psi, with effective chip guarding and personal protective equipment. Never blow chips off your own clothes or skin.

Common Mistakes

Check Your Understanding

1. Your lathe alarms on spindle overload the moment the first heavy cut starts. The insert is new, and the program ran fine on the last 40 parts. What do you check?

Check the workholding. A new insert and a proven program point away from the cut itself. See whether the chuck closed fully on this part and whether the part is jammed or bound in the jaws. If that is fine, call your lead, because a hardware fault is possible.

2. You press feed hold because a cut sounds wrong. The axes stop. Can you open the door and feel the part?

No. Feed hold pauses axis motion, and on most machines the spindle is still turning. Wait until all motion has stopped, and stop the spindle as your procedure directs, before you open the door.

3. The setup sheet says to check the part after the roughing tool, but your machine runs straight past that spot. The program has an M01 there. Why did it not stop?

M01 only stops when the Optional Stop switch is on. Turn it on as the setup sheet directs.

4. A coworker hands you the air gun to clear chips off a fixture. What do you do?

Use a brush or chip rake with the machine stopped. Air throws chips toward people and into the machine. Only if your plant permits air would you use it, under 30 psi at the nozzle, with chip guarding and PPE, and never on yourself.

Key Takeaways

• Machine zero is fixed by the builder. Work zero is set on the part. A wrong work offset runs a good program in the wrong place.
• Feed hold pauses motion but is not a safe state. Wait for everything to stop.
• M00 always stops. M01 stops only if Optional Stop is on.
• Read every alarm before you reset it, and report every crash.
• A spindle overload can come from the cut or the tool, or from a chuck or part that is not held right.
• Clear chips with a brush or rake, not your hands, and not with air unless your plant allows it under OSHA's limits.