Edge Finding: Because Eyeballing It Is For Amateurs¶

Author: Johannes A. Nilsson

Why Edge Finding Matters¶

You can't machine what you can't find. Every dimension on your part is measured from somewhere, and if you don't know where that somewhere is, you're just making expensive scrap. Edge finding is how you tell your machine exactly where your part is, not where you think it is.

The Paper Method (Old School)¶

This is what I learned first because I was too broke for real tools. Still works great.

What You Need¶

Zig-Zag or similar Cigarette paper (0.001-0.002" thick)
Regular paper (0.003-0.004" thick)
Your spindle (NOT spinning)
Patience

Paper Method Process¶

Put paper between tool and work
Move table until paper drags when you pull it
You're now 1 paper thickness away from the edge
Zero your DRO and account for paper thickness + tool radius

Pro Tips¶

Use the same paper every time (consistency matters)
Pull straight out, not at an angle
Feel for the "grab" not the "tear"
Works great for finding tops of parts too

When to Use It¶

Setting Z-height on tools
Finding edges when you forgot your edge finder
When you need dead quiet operation
Working with non-conductive materials (for you lucky EDM guys)

Mechanical Edge Finders (The Wobbler)¶

The classic tool that every machinist has dropped at least once.

Mechanical Operation¶

These have a spring-loaded tip that wobbles around until it touches the work, then it kicks off to the side.

Mechanical Edge Finder Technique¶

Chuck it up in a collet (not a drill chuck, you savage bastard)
Spin at 500-1000 RPM (too fast and it won't work)
Approach slowly from the side
Watch for the kick - the tip will suddenly jump sideways
You're now 0.100" away (for a 0.200" tip) from the edge

Mechanical Edge Finder Gotchas¶

Different tip sizes exist - 0.200" is common, but check yours
Runout matters - A bent edge finder is useless
Clean the tip - Oil and chips make it stick, keep it clean and oiled
Height matters - Find the edge at the same Z-height you'll be cutting

Types of Mechanical Edge Finders¶

Standard Wobbler¶

Most common
Usually 0.200" diameter tip
Good for ±0.0002" if you're careful

Double-End Edge Finder¶

Has different diameter ends
Handy for inside corners
One end usually has a point for finding centers

Wiggler Sets¶

Comes with various attachments
Ball ends for finding holes
Point for scribing
The one attachment you need is always missing

Electronic Edge Finders¶

Welcome to the 21st century. These beep when you touch the work.

Electronic Operation¶

Basic, it completes a circuit through the machine when the tip touches work. Simple but effective.

Electronic Edge Finder Advantages¶

Audible feedback - Hard to miss the beep
Works at any speed - Even stationary
Very repeatable - ±0.0001" is achievable
No "kick" to watch for

Electronic Edge Finder Disadvantages¶

Needs conductive material - Won't work on plastics or ceramics
Batteries die and they do so always at the worst possible time, keep a backup stocked!
More expensive than mechanical
Can be fooled by coolant or chips

Electronic Edge Finder Technique¶

Clean everything - Oil is an insulator
Touch off multiple times - Average the readings
Listen for the tone change - Not just the first beep
Check your ground - Bad ground = bad readings

Finding the Center of Holes¶

Because sometimes you need to put a hole in a hole.

Dial Indicator Method¶

Mount indicator in spindle
Lower into hole
Adjust table until indicator reads zero all around
You're centered (if your spindle is true)

Ball End Method¶

Use wiggler with ball attachment
Let it find its own center in the hole
When it stops wobbling, you're close
Fine tune with indicator

The Coaxial Indicator¶

Expensive but awesome. Two indicators in one body find center fast.

Advanced Tricks¶

Finding Angles¶

Use edge finder on two points along an edge, do the math for the angle.

Finding Round Stock Centers¶

Touch off four sides (90° apart), average X and Y positions.

The Safety Touch¶

Always do a "safety touch" - back off and approach again to verify your first reading.

Using Gauge Pins¶

For critical work, use a known diameter gauge pin in the spindle instead of an edge finder.

Common Mistakes¶

Spindle Deflection¶

Pushing too hard bends the spindle and gives false readings. Let the tool do the work.

Forgetting Tool Diameter¶

Edge finder says you're at X=0.100" but you forgot to account for the 0.100" radius. Now your part is 0.200" off.

Dirty Surfaces¶

Chips, oil, and rust all conduct electricity differently. Clean your shit, and clean it often.

Wrong RPM¶

Too fast and mechanical finders won't kick. Too slow and you can't see the wobble. RTFM!

Accuracy Comparison¶

From worst to best:

Eyeballing: ±0.030" (Just don't you schmuck)
Paper method: ±0.002"
Mechanical finder: ±0.0005"
Electronic finder: ±0.0002"
Dial indicator: ±0.0001" (but slow)

When to Use What¶

By Method Type¶

Paper Method¶

Quick and dirty setups
Non-conductive materials
When you're in a hurry
Quick Z-height settings
Leveling a 3D printer bed

Mechanical Edge Finder¶

Most general work
When you need to see the edge
No batteries required
Works in coolant

Electronic Edge Finder¶

Critical dimensions
Production work
When repeatability matters
Clean, dry setups

Dial Indicator¶

Finding hole centers
Maximum accuracy needed
Checking your other methods
When time doesn't matter

The Golden Rules¶

Find edges at cutting height - Parts aren't always square
Find multiple points - Verify your edges are actually straight
Account for tool deflection - Especially with small tools
Write down your zeros - Memory is unreliable
Verify with a test cut - Trust but verify

Final Thoughts¶

Edge finding is like foreplay - rush it and you'll regret it later. Take the time to do it right and your parts will actually match the print. And remember, the most expensive edge finder in the world won't help if your part moves after you find the edge. Clamp your shit down properly.

The difference between a hack and a professional machinist is about 0.002" - and most of that comes from proper edge finding. So learn this set of skills and get well paid.