Shop Floor Inspection¶
How to check your parts efficiently without turning into the QC department.
Philosophy¶
You're a machinist, not a metrologist. But if you don't check your work:
- Scrap happens at the worst time
- Rework kills your efficiency
- Trust in your capability disappears
- Your reputation goes to shit
The trick is checking smart, not checking everything.
First Article Inspection¶
What to Check on Part #1¶
Always check:
- Critical dimensions (noted on print)
- Anything with tight tolerance (±0.001" or less)
- Features that reference datums
- Mating surfaces
- Anything you had trouble with
Skip (usually):
- Reference dimensions
- Stock thickness (if you didn't machine it)
- Non-functional features
- Anything marked "TYP" after you check one
The Order Matters¶
Check in this sequence:
- Datums first - Everything references these
- Critical features - The stuff that'll scrap the part
- Tight tolerances - Before tool wear matters
- Everything else - If you have time
Documentation¶
Even if nobody asked, write it down:
Part: Widget-123 Rev B
Date: 3/14/24
Machine: Haas VF2
| Dim | Nominal | Tol | Actual | Tool |
| ---- | ------- | ----- | ------ | ---- |
| A | 2.500 | ±.005 | 2.4985 | T3 |
| B | 0.750 | ±.001 | 0.7495 | T7 |
| Hole | Ø.500 | +.003 | 0.5015 | T12 |
Why? Cover Your Ass (CYA), plus you'll refer back to this.
In-Process Inspection¶
When to Check During the Run¶
Check every part:
- First 5 parts (tools warming up)
- After any adjustment
- After tool changes
- If something sounds different
Check periodically:
- Every 10th part on long runs
- Every 5th on aluminum (tool wear)
- Every 20th on cast iron (stable)
- When you're bored (find problems early)
What to Check¶
The smart way:
- Pick 2-3 critical dimensions
- Rotate what you check
- Focus on what changes (tool wear dimensions)
- Skip what can't change (fixture located features)
Trend Tracking¶
The secret to catching problems before scrap:
eg:
| Part # | Dimension | Direction |
|---|---|---|
| 1 | 0.7500 | -- |
| 10 | 0.7498 | ↓ |
| 20 | 0.7495 | ↓ |
| 30 | 0.7491 | ↓ <- Adjust now! |
Tool wear is predictable. Use it.
Quick Checks That Save Your Ass¶
The Finger Test¶
For de-burred edges:
- Run finger along edge
- Smooth = good
- Catches = needs work
- Cuts you = you forgot to de-burr, dingus
The Sharpie Test¶
For checking cutter contact:
- Color surface with Sharpie
- Take light pass
- Check coverage
- Full width = good, spots missed = tram issue
The One-Two-Three Block Test¶
For perpendicularity:
- Part against 1-2-3 block
- Should sit flat
- Light test - no gaps
- Faster than indicating
The Drop Test¶
For tapped holes (aluminum only):
- Thread in appropriate screw
- Hold part vertical
- Screw should spin out freely
- If it binds, tap is dull
Catching Problems Early¶
Listen to Your Machine¶
Sound changes mean something:
- Higher pitch = tool dulling
- Chatter = something's loose
- Squealing = needs coolant
- Hammering = broken insert
Check the part when sounds change.
Watch Your Chips¶
Chips tell the truth:
- Color change = heat = size change
- Shape change = tool wear
- No chips = rubbing = about to break
- Stringy = adjust speed/feed
Feel the Part¶
After roughing (when cool):
- Hot spots = tool rubbing
- Rough areas = chatter you didn't hear
- Steps = tool deflection
- Raised edges = excessive tool pressure
Statistical Process Control (SPC) Lite¶
You don't need software. Just paper.
The Poor Man's Control Chart¶
Nominal: 1.000" ±0.002"
1.002 | • •
1.001 | • •
1.000 | • •
0.999 | •
0.998 | • <- Trending bad
Time →
When 3 points trend the same direction, adjust.
The 10% Rule¶
Adjust when you've used 10% of tolerance:
- ±0.005" tolerance
- Adjust at 0.001" from nominal
- Gives you room for error
- Prevents scrap
Capability Quick Check¶
Make 5 parts, measure same dimension:
- Range under 1/3 tolerance = good
- Range 1/3 to 2/3 tolerance = marginal
- Range over 2/3 tolerance = fix something
Tools for Efficient Inspection¶
Go/No-Go Gauges¶
Best investment for production:
- Instant pass/fail
- No math
- No misreading
- Operators can self-check
When to use:
- Holes (plug gauges)
- Shafts (ring gauges)
- Threads (thread gauges)
- Any repeated feature
Gauge Pins¶
Poor man's bore gauge:
- Plus pins (should fit)
- Minus pins (should not fit)
- Faster than measuring
- Good for checking while part's in machine
Air Gauging¶
For high-volume precision:
- Measures to 0.000050"
- No contact (no scratches)
- Fast
- Expensive but worth it
Vision Systems¶
The future is now:
- Optical comparators for profiles
- Video measuring for small parts
- No touch = no deflection
- Great for flexible parts
Common Inspection Mistakes¶
Measuring Hot Parts¶
Aluminum grows 0.0001" per inch per 10°C. That's 0.001" on a 1" part at 100°C. Wait for it to cool or do the math.
Wrong Tool for the Job¶
- Calipers for depth = bad
- Micrometer for ID = worse
- Tape measure for ±0.010" = you're fired
Dirty Tools/Parts¶
One chip = wrong measurement!
Clean:
- Measuring surfaces
- Part surfaces
- Your hands
- The air (blow off before measuring)
Parallax Error¶
Reading at an angle = reading wrong:
- Digital tools fix this
- Analog needs perpendicular viewing
- Mirrors on good tools show proper angle
Assuming Instead of Checking¶
"The fixture locates it perfectly" - Until it doesn't "The CNC is always accurate" - Until it isn't "I checked the first one" - Tool wear is real
The Economics of Inspection¶
Time vs Risk¶
High risk (check everything):
- Critical parts (High Temp, High pressure)
- One-off customs parts
- Your first attempt
Low risk (spot check):
- Non-critical features
- Proven programs
- Repeat work orders
- Large tolerances
The Hidden Cost¶
Bad parts cost more than inspection time:
- Scrap material
- Machine time
- Your time (twice)
- Loss of your consumers confidence
The 10-100-1000 Rule¶
- $10 to prevent
- $100 to catch in-house
- $1000 to fix in field
Building Good Habits¶
Start of Shift¶
- Check measuring tools (zero, damage)
- Review prints for critical dimensions
- Plan inspection points
- Set up documentation
During Production¶
- First article complete check
- Periodic critical dimensions
- Track trends
- Adjust before out of tolerance
End of Shift¶
- Final part inspection
- Document any adjustments
- Note for next shift
- Store those gauges properly
Your Inspection Setup¶
At Your Machine¶
- 6" calipers
- 0-1" micrometer
- Go/no-go for repeat jobs
- De-burring tools
- Calculator
At the Bench¶
- Surface plate
- Height gauge
- Indicators
- Gauge blocks
- Good lighting
In Your Head¶
- Which dimensions kill parts
- How features relate
- What changes with wear
- When to stop and check
The Bottom Line¶
You can't inspect quality into parts, but you can inspect scrap out of production. Check smart:
- Know what matters
- Check it efficiently
- Track trends
- Adjust early
- Document everything
The best machinists catch their mistakes before they become problems. The worst ones let QC find them. Which one do you wanna be?