CNC CNC Mill Setup & Operation 3 — Questions and Answers
Question 1: A part drawing calls for a slot tolerance of ±0.001". After milling, measurement shows the slot is 0.003" too narrow. Which corrective action is most appropriate?
- Increase spindle speed and re-run
- Adjust the cutter diameter offset (wear offset) to remove additional stock (Correct answer)
- Change the G41 code to G42
- Increase the depth of cut and re-run the program
Correct answer: Adjust the cutter diameter offset (wear offset) to remove additional stock
Increasing the wear offset value by half the total error on each side allows the control to reposition the cutter path outward to widen the slot.
Question 2: What does the M08 miscellaneous function command do?
- Stops the spindle
- Activates flood coolant (Correct answer)
- Initiates a tool change
- Enables the chip conveyor only
Correct answer: Activates flood coolant
M08 turns on flood coolant; M09 turns it off, and M07 is typically mist coolant.
Question 3: When tramming a CNC mill head, what condition are you correcting?
- Backlash in the lead screws
- Spindle misalignment (tilt) relative to the machine table surface (Correct answer)
- Incorrect work zero offset
- Excessive tool runout in the spindle taper
Correct answer: Spindle misalignment (tilt) relative to the machine table surface
Tramming aligns the spindle axis perpendicular to the table by sweeping a dial indicator in an arc and adjusting head tilt until the reading is consistent.
Question 4: What is the purpose of a G28 command in a CNC milling program?
- Sets the work coordinate origin
- Returns all axes to the machine home (reference) position (Correct answer)
- Switches from absolute to incremental positioning
- Activates the fourth-axis rotary table
Correct answer: Returns all axes to the machine home (reference) position
G28 commands the machine to travel through an intermediate point and then return to the machine reference (home) position.
Question 5: Which cutting parameter most directly controls surface finish quality in CNC milling?
- Depth of cut (DOC)
- Feed per tooth (chip load) (Correct answer)
- Number of tool offsets registered
- Work coordinate system number
Correct answer: Feed per tooth (chip load)
Feed per tooth determines the theoretical scallop height; reducing chip load generally improves surface finish, especially on finishing passes.
Question 6: In CNC milling, what does SFM stand for and how is it used?
- Surface Feet per Minute — used to calculate proper spindle RPM for a given cutter diameter and material (Correct answer)
- Servo Feed Multiplier — a scaling factor for axis feed rates
- Standard Feed Measure — the ISO unit for chip load
- Spindle Frequency Monitor — a diagnostic display
Correct answer: Surface Feet per Minute — used to calculate proper spindle RPM for a given cutter diameter and material
SFM (Surface Feet per Minute) is the recommended cutting speed for a material; RPM = (SFM × 3.82) / cutter diameter in inches.
Question 7: A program block reads: G91 G01 X1.5 Y-0.75 F10.0. What positioning mode is active?
- Absolute positioning from the work zero
- Incremental positioning from the current tool position (Correct answer)
- Polar coordinate positioning
- Machine coordinate positioning
Correct answer: Incremental positioning from the current tool position
G91 activates incremental mode, so X1.5 Y-0.75 moves the tool 1.5" in X and 0.75" in the negative Y direction from wherever the tool currently is.
CNC CNC Mill Setup & Operation 3
A part drawing calls for a slot tolerance of ±0.001".
After milling, measurement shows the slot is 0.003" too narrow.
Which corrective action is most appropriate?