Dial Indicator Mastery: Stop Guessing, Start Measuring¶

Author: Johannes A. Nilsson

The Tool That Tells No Lies¶

A dial indicator is like that brutally honest friend - it'll tell you exactly how fucked up your setup is, whether you want to hear it or not. Master this tool and you'll go from eyeballing it and saying "yeah, that looks about right" to actually knowing what the fuck you're doing.

What's a Tenth?¶

In the US, machinists think in inches, for some dumb reason. And they have lingo. So first things first - when machinists say "tenth," they mean 0.0001" (one ten-thousandth of an inch). Not 0.1" or 0.01". I once got this wrong and I was the shop punching-bag for weeks.

0.001" = One thousandth = "One thou"
0.0001" = One ten-thousandth = "One tenth"
0.00001" = Nobody cares, you're not making rocket parts at SpaceX

Translate that to metric:

0.01mm = About 4 machinist tenths
0.001mm = One micron (now we're just showing off)

Reading Runout¶

Runout is how much your part wobbles when it spins. It's the difference between a smooth running part and a vibrating piece of shit.

How to Check Runout¶

Mount the indicator so it touches the surface you're checking
Zero the dial (rotate the bezel until the needle points to zero)
Rotate the part slowly by hand
Watch the needle - it'll swing between high and low points
Total runout = Maximum reading minus minimum reading

Pro Tips¶

Always take multiple passes - sometimes crap gets under the indicator tip
Check in multiple spots along the length
If runout suddenly changes, you probably have a chip or ding
Clean everything first. A chip of swarf will fuck-up your readings

Tramming a Mill Head¶

If your mill head isn't perpendicular to the table, every feature you cut will be crooked. Here's how to fix that.

The Setup¶

Clean the table - I mean surgical clean
Mount indicator in spindle - use a mag base or indicator holder, you can use a collet or a drill chuck, but it's more likely to introduce error so be warned
Position indicator tip about 4-6" from spindle centerline
Lower until it touches table with about 0.020" preload

The Process¶

Rotate spindle by hand to sweep a circle
Find high and low spots - mark them with a Sharpie
Calculate the difference
Adjust the head until readings are equal

The Math Nobody Explained To Me¶

If you're 0.002" off over an 8" sweep:

That's 0.002" ÷ 8" = 0.00025" per inch
On a 3" deep hole, that's 0.00075" off at the bottom
Might not matter for clearance holes
Will definitely matter for reamed holes

Which Way to Adjust?¶

The indicator reads HIGH when it's closer to the work. So:

High reading at the front = Head tilted forward
High reading at the back = Head tilted back
High reading on the left = Head tilted left
You get the idea

Centering in a 4-Jaw Lathe Chuck¶

This is where the boys become men. Anyone can chuck up in a 3-jaw and hope. Real machinists use a 4-jaw and dial that shit in.

Basic Method¶

Chuck the part snug but not tight!
Mount indicator on a mag base on the carriage
Position tip on the OD (Outside Diameter) of your part
Spin the chuck by hand and find the high spot
Loosen the high jaw, tighten the opposite jaw
Repeat until you hate your life (or get under 0.001" runout)

The Fast Method¶

Get it close by eye first
Use the "half the difference" rule:
- If you're 0.010" out
- Move 0.005" (half)
- Check again
- Repeat with smaller moves

Secret Tricks¶

Tap don't crank - Small taps with a dead blow mallet move the part predictably
Two indicators - One on X, one on Y if you're fancy
The wiggle test - Grab the part and wiggle. If it moves, it's not tight enough
Start at the chuck - Get it running true near the jaws first, then check the end

Common Fuck-Ups¶

Reading the Wrong Scale¶

Most indicators have two scales. The small dial counts revolutions. Miss this and you'll be off by 0.100" instead of 0.001".

Not Preloading¶

The indicator needs some preload (compression) to work both ways. Start with 0.020-0.030" compression.

Forgetting About Cosine Error¶

If your indicator isn't perpendicular to the surface, you're not reading true. At 30° off, you're reading 13% low.

Magnetic Base Creep¶

Cheap mag bases slip. Always double-check your zero after adjusting anything. Buy quality tools. My suggestion is buy Starrett measurement tooling.

Indicator Types and When to Use Them¶

Dial Test Indicator (DTI)¶

The one with the lever arm
Best for tight spaces
Good for tramming
Reads perpendicular to the arm

Plunger Dial Indicator¶

Straight push type
Better for depth measurements
Good for checking runout
More accurate for linear moves

Digital Indicators¶

Great for people who can't read dials
Battery always dead when you need it
Nice for relative measurements
Usually have better resolution

Taking Care of Your Indicators¶

Don't drop them - They're not hammers
Keep them in the case - Not rolling around in your toolbox
Clean the tip - Crud builds up and affects readings, better, clean the surfaces before you measure.
Check calibration - Use gauge blocks occasionally
Oil the stem - A drop of way oil keeps them smooth

Practice Exercises¶

Check a precision ground pin - Should have near zero runout
Indicate a vise jaw - Is it actually parallel to your table travel?
Check your spindle - Mount a test bar and check the runout
Find the center of a hole - Use an indicator to find exact center

Final Wisdom¶

An indicator doesn't make parts accurate - it just tells you how inaccurate they are. The skill is in knowing:

When "good enough" is actually good enough
How to adjust things to fix the problems
When to walk away and get a beer

Remember: The indicator is always right. If the reading doesn't make sense, you're either measuring wrong or your setup is more fucked than you thought. Usually both.