A CNC (computer numerical control) machinist is a very important manufacturing job. CNC machinists use computer-controlled machines to shape metal, plastic, and other materials into precision parts. They are like advanced manual machinists, but instead of cranks and levers, they use specialized computer software and advanced machinery to create automotive parts, medical equipment, aerospace components, and many other important products we use every day.
CNC machinist positions are growing as more manufacturers automate production. It’s a great career for those with mechanical skills and comfort working with advanced shop software and equipment. However, competition can be fierce for the best CNC machinist jobs at top companies. This makes interview preparation very important.
The Top 20 CNC Machinist Interview Questions and Answers
Here are 20 of the most common CNC machinist interview questions, with example answers:
1. What experience do you have with CNC machining?
Example answer: Although I’m a recent graduate, I have over 2 years of hands-on experience with computer numeric control (CNC) mills and lathes. As part of my vocational program, I spent 6 months operating Haas and Fadal mills and lathes to produce parts to +/- 0.005 inch tolerances. Additionally, I’m familiar with Mastercam and SOLIDWORKS CAM software to program toolpaths.
I also have experience performing test cuts, adjusting offsets to hit tolerances, editing programs at the machine, and measuring finished parts with micrometers, calipers, and CMMs. My skills enable me to quickly produce precision parts from drawings with minimal scrap loss.
2. What important skills should a CNC machinist have?
Example answer: There are several critical skills a CNC machinist must have:
- Blueprint and CAD/CAM program reading skills – A machinist must read and interpret technical drawings and CAM toolpaths to understand job specifications
- Measuring and inspection skills – They must properly utilize precision measuring tools like calipers and micrometers to verify part dimensions
- Attention to detail – Machinists must notice tiny inconsistencies that can affect part function
- Troubleshooting ability – They must debug programs, investigate machine issues, and solve problems
- Manual machining skills – Even with CNC, some manual fabrication work is required
- Machine operation/control skills – Appropriately power up, home out, and control the machines
- Math and geometry knowledge – CNC work requires strong spatial reasoning and math calculation abilities
Machinists lacking these core skills will struggle to work efficiently and produce accurate parts meeting design specifications. I have experience with all these areas from my past roles and vocational coursework.
3. What CNC machines are you familiar with operating and programming?
Example answer: So far in my training, I’ve learned to effectively operate and program Haas and Fadal CNC milling machines and lathes. These included 3-axis vertical mills like the Haas VF-2, 5-axis trunnion mills like the Haas UMC-750, and twin-spindle twin-turret lathes with live tooling like the ST-35. I can load production programs and offsets, adjust workholding devices, tweak programs at the machine, and produce parts matching the print specifications. Additionally, I have hobby-level familiarity with Tormach CNC mills, which I’ve used to create custom aluminum parts for car restoration projects.
4. How do you ensure tight dimensional tolerances when machining?
Example answer: Making precise parts within “tight tolerance” specifications takes careful planning and exacting work. Following my training, here is my process to ensure accurate CNC machined components:
First, I review prints carefully to see all dimensions and the maximum allowable tolerances – then I determine optimal fixturing and workholding to avoid part movement under cutting forces. I adjust my programs or modify CAM-generated toolpaths to leave extra material for finishing passes.
During production, I routinely verify dimensions with digital readouts, gauges, and micrometers. After initial test cuts, I tweak work offsets, spindle speeds, and feed rates to optimize surface finish and accuracy. I carefully measure and document test part dimensions at all stages to confirm they fall within specifications. Finally, I use nourishing cutting fluids liberally to prevent tool and part thermal expansion that can affect precision. Following these steps methodically helps me maintain very tight tolerances below 0.005 inch.
5. How do you verify part accuracy during a production run?
Example answer: To verify part accuracy during a longer production run, there are three best practice checks I follow:
First, I periodically stop the machine and manually inspect the last completed part using calibrated gauges and measurement tools. I check all dimensions on print to catch any possible deviations.
Secondly, I monitor tool wear and machine performance very closely over time. As tools begin to wear, tolerances can drift outside specifications. I replace worn tools proactively to prevent scrap.
Finally, I randomly pull parts and do spot measurements at regular intervals using the CMM (coordinate measuring machine). The CMM checks every dimension precisely so I can determine if any dimensions are trending out of position. It provides an early warning to update offsets and keep tolerance stack-up minimal.
Following this progressive inspection procedure ensures consistent accuracy and quality over longer production runs.
6. How do you minimize EDM tap breakage when programming a CNC machining center?
Example answer: Tap breakage is a common issue when tapping holes with a “peck tapping” electrical discharge machining (EDM) cycle on a machining center. However, following best practices can minimize tooling damage and scrap loss.
First, I select optimal taps ensuring correct diameters and appropriate styles for the machine’s capabilities. Then I program appropriate peck depths, retract distances, and RPM to match the tooling specifications and material hardness. Conservative “pecks” of 0.100-0.250” and high enough RPM is critical to evacuate chips while reducing loading/breaking stresses on the delicate tap flutes.
Additionally, I always program a block to reverse tap direction once full depth is reached to break the chip and relieve loading forces during the dwell. Finally, I use cutting fluid or vacuum extraction liberally to keep holes very clean when tapping to prevent chip welding or clogging.
7. How can you reduce machining cycle times during production?
Example answer: There are several proven methods machinists can use to reduce cycle times and increase production efficiency on CNC equipment:
First, I analyze toolpaths to ensure optimal feed rates and spindle speeds are used for the given material. Unnecessarily light cuts increase cycle times. I tweak parameters to the machine’s capabilities.
Next, when production quantities permit, I suggest using indexable carbide tooling instead of HSS. Carbide allows much heavier chip loads per pass while maintaining finish requirements. This saves major cut time.
I also suggest overlapping operations by using the machine turret, tailstock or sub-spindle, and available live tooling stations together in one cycle. Taking as many concurrent cuts as possible in one operation maximizes usage.
Finally, I analyze setup and job staging to save time. This includes pre-staging blanks, off-machine deburring/measuring when possible, and standardizing workholding setups. Less setup adjustments directly equate to time savings. Following this approach, I’ve reduced cycle times by 15-30%+ on previous production jobs.
8. Describe 5 key predictive and preventative maintenance checks CNC machinists should perform.
Example answer: Five of the most critical maintenance checks CNC machinists should perform regularly to prevent machine issues include:
- Spindle runout and alignment checks – Indicates alignment/bearing issues over 0.0005” runout
- Ballscrew/motor backlash tests – Assesses ballscrew or servo motor wear if backlash exceeds spec
- Way surface lubrication and inspection – Ensures smooth slipper/gib movement under loads
- Axis motor winding and encoder checks – Verifies phases wired correctly with no dropped counts
- Coolant filtration and concentration testing – Prevents pumps/lines from getting plugged and degrading cutting performance.
Doing these quick checks during maintenance intervals or operator shift changes is essential for catching major mechanical issues early before they cause machine breakdowns, poor quality output, scrapped parts, or safety issues. Proper maintenance and awareness of these factors also allows me to adjust programs to compensate if a parameter drifts out of spec and threatens tolerances.
9. How can you verify correct work offsets to run a job you’ve never operated before?
Example answer: When prepping to run a new CNC job with unfamiliar offsets and workholding, I always go through this simple verification process:
I begin by checking for any existing designated tooling “reference tools” meant for that job and validating their offsets. If there are no special reference tools, I choose trusted end mills with accurate offsets to use as references.
Next, I manually jog to multiple points above the centerlines of the workpiece, near the far corners/extents of the fixture area and well clear of the stock. At each location, with the spindle turned off, I zero out my machine’s current work coordinate readout for X/Y/Z and save each point as a referenced work offset, with clear names indicating their positions.
Finally, with a known reference tool loaded, I jog near my saved points and use touch-off handwheel motions to precisely re-establish them. Comparing my current DRO coordinate positions with the previous offsets provides reliable confirmation my offsets/references are properly calibrated for the new setup/program. This gives me very high confidence to perform test cuts without issue.
10. When machining deep pockets, how can you prevent tool breakage and poor surface finish results?
Example answer: Cutting very deep cavities or pockets brings unique challenges like tool load pressure, chip evacuation issues, vibration, and heat buildup. Following some best practices can improve results:
First, I’ll pick long reach tooling if possible to maximize flute engagement and chip carrying capability. For general milling, variable helix end mills also aid chip ejection.
Critical parameters I focus on optimizing are chip thinning, tool stepovers/overlaps, and cut widths. Keeping chiploads thin, using rest/dwell motions, and using 1/3 – 1/2 cutter diameters for passes prevents tool overload, deflection, and breakage risk. This also reduces vibration from interrupted cuts that causes poor finish.
For problematic materials, I’ll suggest programmer or machine dwells between passes so heat dissipates before continuing. Finally, high pressure coolant directed at the tip helps enormously with chip evacuation. This prevents chip welding or re-cutting issues that accelerate tool wear and finish problems.
11. How can you reduce chatter or harmonics when machining?
Example answer: Several methods exist to reduce harmonics, workpiece vibration, and chatter during CNC machining operations to improve finish quality, accuracy, tool life and prevent potential scratches or even breakage.
Optimizing feed/speeds is always the first step – but beyond that:
- Changing axial and radial depths of cut, width of cut parameters or stepovers until harmonics dissipate
- Improving workholding method – vices, high friction or damping materials in fixtures
- Use well maintained end mills with sharp edges – re-sharpening regularly
- Increase number of flutes for more even cutting forces or use variable flute/pitch end mills
- Machine in opposite directions when possible to negate harmonics.
Lastly, in stubborn cases, I’ve had good success applying high viscosity greases or oils like plastics diffuse very lightly at resonating locations. This tuning/damping approach eliminates chatter very well when traditional methods fall short. It takes slight trial and error tweaking but enables hitting critical tolerances.
12. How can you identify the best feeds and speeds for machining a new material?
Example answer: When machining a never before run exotic aerospace alloy, cast iron, or other unusual material for the first time, identifying optimal feeds/speeds when no proven parameters exist takes trial and error testing. Here is my process:
I consult machinability databases and references like Machinery’s Handbook to locate any available starting recommendations for that alloy. Then if possible, I’ll obtain a sample material scrap piece to do benchtop testing.
I prepare the sample mounting it rigidly in a vise and use a manual machine or basic CNC program to make conservative step-over passes at speeds from 100% down to 40%, noting finish, power, vibration, tool wear, etc as I go. I’ll also sometimes intentionally push speeds too high to purposely induce failure and understand the upper limits.
From this testing, the optimal combination of surface finish, tool life, and efficient metal removal rate derives the best speed and feeds I can then program for initial test cuts in the CNC machine. I further fine tune parameters during production as needed until I learn the ideal ranges. Taking the time for methodical testing ensures I don’t ruin expensive workpieces and efficiently determine the sweet spot for that material.
13. You get bad finish and tolerances on a job. How do you investigate, identify and correct the issue?
Example answer: When a CNC job unexpectedly produces bad finish quality or goes out of print tolerance specifications, my general process for diagnosing it includes:
First, I rule out any workholding instability issues allowing vibration or workpiece movement by verifying my setup. Then I methodically analyze all tooling – ensuring each tool called in my program matches what’s loaded in the machine, has proven trustworthy offsets, and has freshly sharpened edges. Worn tools often create finish problems.
If tools check out accurately, I’ll study the toolpaths and CAM program details. Verify proper speeds/feeds for the material, look for missing/conflicting cut regions causing redundant work, and confirm stepover distances aren’t too wide creating ridges. I may adjust stepover distance gradually until finish improves. Troubleshooting the CAD/CAM programming side often reveals the culprit.
Finally, if parameters seem correctly optimized but finish or tolerance issues persist, I’ll tweak some machine parameters accordingly. I increase rigidity where possible, tweak accel/decel rates to prevent tool jumping into cuts, adjust jerk control, verify calibration, and tweak encoder compensation. Compensating for mechanical wear influencing results can solve stubborn problems.
14. What key things would you check if a CNC mill is cutting inaccurately or out of tolerance?
Example answer: Cutting inaccuracies often result when crucial machine parameters fall out of proper specification for demanding applications requiring tight tolerances or certain materials prone to work hardening or thermal changes. My general process would be:
I first methodically eliminate obvious causes – worn tools, poor fixturing, underpowered roughing passes bending long tools, etc. Then I check key mechanical, control, and calibration aspects:
- Encoder settings matched to motor poles/counts and tested for possible dropped or doubled counts
- Backlash, torque or load meter results on X/Y/Z confirming ballscrew/motor specs are in tolerance
- Machine geometry – squareness of tool turret, spindle, and axis movements
- Axis acceleration/deceleration and max rate settings possibly causing overshoot
- Encoder compensation/backlash compensation parameters possibly inadequate
- Servo tuning and stability during demanding cuts
- Possible vibration or harmonics during heavy cuts influencing dynamic accuracy
I also consider workpiece material factors – any unpredictable work hardening, inconsistent hardness, or thermal expansion during cuts throwing results off in production after good first-offs check out. Adjusting key parameters to compensate usually restores cutting accuracy per established procedures.
15. How can you reduce impact marks and peck marks on parts when drilling/boring holes?
Example answer: Peck marks and impact indentations around any CNC drilled or bored hole are a common problem affecting appearance and precision. However, optimizing the drilling cycle and parameters can minimize this issue.
The best methods I employ involve:
- Adding a slight dwell or pause at the end of each peck retract to allow time for the material stresses equalize rather than retracting the moment the drill disengages fully.
- Extending the dwell duration at hole bottom allows elastic material recovery before retracting fully out of the hole.
- Additionally, I adjust boring bar speeds, feeds and depth cuts to ensure adequate chip carrying capability based on material and hole size. Proper chip evacuation prevents notching at transitions when retracting.
- Finally, when able drilling through full holes, I add a final reaming pass at full drill depth to evenly smooth out any remaining peck marks to high surface finish spec.
Following these best practices for my hole drilling, even highly stringent aerospace surface finish requirements are attainable.
16. How can you detect excessive machine tool chatter or harmonics?
Example answer: Detecting problematic harmonics and machine vibration requires close attention because it can develop intermittently from factors like tool wear, cut depths, problematic materials/parameters, etc. But, clear symptoms alert me to chatter including:
- Audible oscillating noise from machine changes in pitch sounded like a “machine gun”
- Visual pulsing of coolant stream or visible shaking/oscillation of tool carousel
- Resultant poor finish with clear evenly-spaced lines/ridges representing the harmonic waves
- Finish deterioration getting progressively worse pass to pass.
- Overly conservative programmed feeds/speeds NOT causing it.
I mitigate harmonics using several modifications – altering cutting direction when possible, adjusting width of cuts and depths, ensuring properly clamped shortest overhangs on tools, checking machine lubrication, and verifying optimal sharp tooling. When very stubborn, I may lightly apply damping greases to resonating surfaces as a last resort tuning fix.
17. What steps would you take to skim delicate warped or bowed stock flat and straight again on a CNC mill?
Example answer: Resurfacing badly warped plates or bow-twisted bar stock flat again accurately requires careful technique and planning to prevent gouging or scrap loss. Here is my best practice approach:
- First, identify if the stock is bowed corner-to-corner or side-to-side and mount it very securely to prevent lifting/vibration during surfacing cuts
- Then, I’d touch off my shortest possible fly cutter or face mill at multiple points across the peak and valleys, mapping the total indicated runout and deviation amounts precisely
18. How can you reduce burrs when machining aluminum parts while still maintaining speed?
Example answer: Aluminum is prone to burr formation due to its soft, gummy nature. Deburring each part adds significant time. Here are effective methods I use to minimize burrs during production while maintaining reasonable cycle times:
Firstly, climb milling aluminum eliminates back-cutting that lifts nasty burrs. So whenever possible, I program toolpaths for climb directions.
For holes, I suggest extractor style drill bits. The specially designed flutes curl chips internally preventing exit burrs even at high RPMs. Peck drilling cycles also help fractures burrs.
Slowing cutting speed 30-40% while increasing chip load and number of flutes maintains overall rate but allows more controlled shearing. Combine this with reduced stepover distances, and burr formation reduces significantly.
Finally, I advocate for use of rigid solid carbide tooling over high speed steel. The precise cutting edges, specialized coatings, and stiffness of solid carbide provides a sharper shearing action with less tearing that creates burrs. Carbide does cost more but prevents tedious manual deburring in most cases.
19. How can you identify and remedy resonance issues on a CNC lathe producing bad surface finish results?
Example answer: When turning longer bar stock and getting unexpected poor finish quality from resonant harmonic vibration, I take this systematic approach to remedy it:
First, I verify tooling integrity, ensure rigid mounting, and rule out loose gibs/joints allowing workpiece movement. Once confident in the machine’s current state, I analyze the harmonic patterns in the finish marks, noting their frequency and amplitude.
If feasible for the part, I alter cut depths and feed rates attempting to detune from the resonating frequency. Changing insert geometries/signatures also alters cutting forces.
Additionally, I may slightly offset the alignment of centers/chucks to purposely avoiding exciting vibrations of the rotating assembly’s natural frequency.
Finally, if available, I utilize vibration sensors to precisely measure harmonic levels during cuts. Then fine adjustments to the previous parameters enables “tuning out” the resonance for smoother running. I follow this methodology to eliminate chatter problems and restore good surface finish.
20. What are some steps CNC machinists can take to minimize scrap rates in production?
Example answer: Minimizing scrap is crucial productivity and cost-saving measure for any machine shop. As a CNC machinist, I focus on several proactive things to reduce rejects or rework:
First, meticulously validating and proving out new programs via simulation when possible or conservative test cuts to confirm inputs. Verifying coordinates, tools, posting processing before attempting full production prevents programming errors from ruining raw materials.
Secondly, consistent review of machine condition via maintenance logs and daily checks. Worn components directly increase chances of tool crashes, dimensional errors, poor performance and rejects over time. Being proactive replaces parts before failure.
Additionally, continuously refined efficiencies in staging blanks, standardizing setups/fixturing, and off-machine parts handling streamlines workflow and reduces handling accidents that damage components.
Finally, real-time statistical process control analysis highlights production trends. Addressing output variability immediately maintains standards. This holistic overview of the entire machining process minimizes reasons for scrapping materials.