Basic Measuring Tools¶
How to use the holy trinity of measurement: calipers, micrometers, and dial indicators. Plus some tricks I learned from hanging out with old timers.
Digital Calipers: Your Daily Driver¶
What They're Good For¶
- Quick measurements (±0.001")
- Inside, outside, depth, and step measurements
- Checking if you're in the ballpark
- Impressing people who don't know better
What They're NOT Good For¶
- Anything tighter than ±0.0005"
- Final inspection on critical dimensions
- Measuring while the part is still hot
- Convincing quality control that your part is good
How to Not Suck at Using Them¶
-
Zero them properly
- Close the jaws completely
- Hit zero
- Open and close a few times
- Check zero again (if you bought your tool at Harbor Freight, it probably moved)
-
The right amount of pressure
- Just enough to feel resistance
- If you're leaving marks on aluminum, you're squeezing too hard
- If the reading changes when you breathe on it, you're too light
-
Keep them clean
- Wipe the jaws before every measurement
- One chip can throw you off by 0.003"
- Clean & Oil them weekly or they'll get crunchy
Pro Tips Nobody Tells You¶
- The battery will die at the worst possible time. Keep spares, weird little button cell LiIon units
- Temperature matters. Cold calipers + hot part = lies
- Digital doesn't mean accurate. A $20 caliper is still a $20 caliper
- The depth rod sucks on most calipers. Don't trust it for critical stuff
Micrometers: When You Need to Get Serious¶
The Basics¶
Micrometers (aka: The Mic) are like calipers' uptight Grandfather. More accurate (±0.0001") but way more limited in what they can measure.
Reading Them¶
For inch micrometers¶
- Read the whole number on the sleeve (0, 1, 2, etc.)
- Count the lines past the number (each = 0.025")
- Read the thimble (each line = 0.001")
- Add it all up
Example: 2 + 3 lines (0.075") + 17 on thimble (0.017") = 2.092"
For metric: Same idea but in millimeters. Don't mix them up unless you want to make shitty abstract art.
The Feel¶
This is where beginners mess up. You need to develop "the feel":
-
Use the ratchet or friction thimble
- It clicks when you hit the right pressure
- Same pressure every time = consistent measurements
-
The rock test
- Gently rock the mic on the part
- You'll feel when it's perfectly perpendicular
- The reading will be smallest at this point
-
Temperature compensation
- Hold the frame, not the whole mic
- Your hot little hands will expand the frame
- Yes, it matters when we're talking 0.0001"
Taking Care of Them¶
- Never drop a mic. If you do, it's basically ruined
- Check with standards before important measurements
- Store them slightly open (0.025" or so)
- Keep them oiled but wipe before use
Height Gauges: The Forgotten Hero¶
I've notice that nobody talks about height gauges but they're clutch for checking parts on the surface plate.
What They Do¶
- Measure heights (duh)
- Scribe lines at exact heights
- Check parallelism
- Find the center of round things
Using Them Right¶
-
Surface plate must be clean
- One chip under your part = wrong measurement
- Wipe with your palm (you'll feel any debris)
-
The indicator drop method
- Lower until the needle just moves
- That's your zero
- Way more repeatable than "feeling" for contact
-
Checking hole locations
- Find one edge, zero
- Find opposite edge, read
- Half that distance = center
- Write it down because you'll probably forget in 30 seconds
Gauge Blocks: Ultimate Reference Tools¶
These are your "ground truth" - literally ground to millionths of an inch!
How to Wring Them¶
This is like Black Magic that actually yields results:
- Clean both surfaces with gauge block cleaner
- Put them together at a slight angle
- Rotate while applying light pressure
- They'll stick together like magnets (but it's not magnetism)
- To separate, slide sideways, never pull apart
Making Stacks¶
Need 3.7685"? Stack these:
- 3.000"
- 0.700"
- 0.068"
- 0.0005"
Always use the fewest blocks possible. Less surfaces = less error.
Don't Be That Guy Everyone Loves to Hate¶
- Never touch the measuring surfaces with bare fingers
- Never leave them wrung together overnight
- Never use them as parallels in the vise
- Always put them away clean and oiled
Dial Indicators: Mechanize Truth Detectors¶
Already covered in fundamentals, but here's the measurement-specific stuff:
Comparative Measurement¶
The secret to accurate measurement without expensive tools:
- Set up indicator on stand
- Zero on gauge block or known standard
- Replace with your part
- Read the difference
This cancels out indicator errors and is accurate to 0.0001" with a decent indicator.
Checking Flatness¶
- Zero at one corner
- Map the entire surface
- Write down readings in a grid
- High reading minus low reading = flatness error
TIR (Total Indicated Runout)¶
- Always means the full swing of the needle
- 0.002" TIR = 0.001" actual deviation from center
- People mess this up constantly, no idea why?
The Reality Check¶
When to Use What¶
Tape measure: Rough stock, marking cutoff points Calipers: 90% of your measurements are done with this tool, so it pays to buy quality Micrometers: Critical ODs, thickness checks Indicators: Setups, runout, comparative measurements Gauge blocks: Calibrating other tools, critical setups
Accuracy vs Precision¶
- Accuracy: How close to the true value
- Precision: How repeatable
- You need both, but precision is easier to achieve
- Ten measurements averaged beats one "careful" measurement
The Most Important Rule¶
If it really matters, measure it twice with different tools¶
Your calipers say 0.500", your mic says 0.502"? Something's wrong. Figure it out before you waste half a day and make scrap art parts.
Shop Reality¶
Here's what actually happens:
- You'll drop calipers. They'll survive but check zero after.
- Digital readouts lie usually when the battery gets low, so change them often.
- Parallax error is real. Look straight at analog scales.
- Temperature swings in the shop affect everything.
- Chips get everywhere. Clean obsessively.
Your Measurement Kit¶
Start with:
- 6" digital calipers ($50-150 for decent ones)
- 0-1" micrometer ($75-200)
- Dial indicator with magnetic base ($50 Harbor Freight or >$700 Starrett, somewhere in-between is fine)
- 6" steel rule, /w Imperial & Metric ($10)
Add later:
- 1-2" and 2-3" micrometers
- Height gauge
- Small gauge block set
- Depth micrometer
The Bottom Line¶
Good measurement is about consistency and cleanliness more than expensive tools. A careful machinist with Harbor Freight calipers beats a sloppy one with Mitutoyo every time.
But also... buy Mitutoyo when you can afford it. The good stuff really
is better.
Be careful though, there are lots of fake ass clones out there on Uncle
Jeff's website.
If it seems to good to be true, it is!