Edge Finder vs Probe: Which One Should You Buy?

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Step 1: Understand how mechanical edge finders establish work zero

What is an edge finder tool? An edge finder is a cylindrical tool held in a mill collet or holder to establish part datums. How does an edge finder work? As the rotating tool contacts a stationary edge, friction pulls the contact head concentric with the shank, eliminating initial wobble. Pushing the head past the spindle centerline causes it to kick off sideways, providing clear visual deflection.

Makers produce these tools to tight tolerances. Brown & Sharpe lists an accuracy of ±0.0005″ of nominal size on its standard single-end edge finder with a 3/8″ shank diameter and 0.200″ tip diameter (model 599-792-2). Brown & Sharpe also lists audible models providing an audible click and visible deflection, including model 599-792-20 with a tolerance of ±0.0001″, a 3/8″ shank diameter and 0.200″ contact head, plus model 599-792-24 with a tolerance of ±0.0001″ of nominal size, a 1/2″ shank diameter and 0.500″ tip diameter. Both models feature precision-ground hardened steel construction.

Starrett lists a .200″ contact diameter and .375″ body diameter on its 827A edge finder. Starrett also lists the 827B model with a .200″ contact diameter, .500″ body diameter, +/-0.00012 inches measurement accuracy, hardened tool steel construction, over 140 years of manufacturing heritage, and a 1 Year Limited warranty. Mitutoyo lists model 050103 with a 3/8″ diameter shank and 0.200″ diameter tip, model 050102 with a 1/2 Dia. shank and .200 Dia. tip, and model 050109 with a 1/2″ shank and .500″ tip diameter. Fowler lists an accuracy of 0.0002″ and a diameter of 0.500″ on its combo edge and center finder model 52-575-025-0. Workshop sets like RAHISH TOOLS list an 8 Pcs kit with 1/2″ & 3/8″ shanks.

How to use an edge finder on a milling machine is straightforward. Mount the shank in a collet, such as those in our ER vs TG collets guide. Run the spindle at moderate rotation, jog toward the stock until the contact head kicks off sideways, stop the spindle, and offset the machine coordinate by half the stated tip diameter (such as half of a 0.200″ or 0.500″ tip diameter).

Step 2: Understand how touch probes automate workpiece location

An electronic touch probe replaces visual observation with digital signal detection. Mounted in the spindle, the probe carries a stylus with a ruby or carbide ball. When the stylus touches a surface, an internal mechanism triggers an electrical signal, prompting the machine controller to record the axis coordinate immediately.

Probes connect directly to CNC systems. While manual mills rely on mechanical tools, CNC machines use automated probing cycles, as covered in our CNC vs milling machine guide. Probing routines find corners, pocket centers, and boss locations automatically. For machines described in our what is a CNC router machine article, touch probes transform manual setup into automated software commands.

Kinematic touch probes sense deflection rather than conductivity. They trigger reliably against metals as well as plastics, wax, and hardwoods. The controller software captures the contact point, offsets stylus radius, and updates workpiece datums automatically.

Step 3: Compare setup speed and workflow efficiency

Setup speed divides these tools. An edge finder requires manual spindle engagement, visual observation, and manual coordinate entry for each axis. Repeating this across multiple workpieces consumes workshop time.

Touch probes automate these steps. Running a software macro, the probe touches stock edges and calculates corner coordinates automatically. Probes also find bore centers and detect fixture misalignments when securing parts with methods from our clamping ideas on a CNC router guide.

Probes also measure along the vertical Z axis alongside lateral X and Y axes. Standard mechanical edge finders cannot establish Z zero because their heads only kick off under horizontal loads, requiring feeler gauges or gauge blocks. Touch probes set X, Y, and Z datums in a single routine.

Step 4: Evaluate accuracy and tolerance requirements

Both tools offer solid precision. Mechanical edge finders rely on ground contacts: Brown & Sharpe lists tolerances of ±0.0001″ on audible models and ±0.0005″ on standard units, Starrett lists +/-0.00012 inches on model 827B, and Fowler lists 0.0002″ on model 52-575-025-0. Achieving those numbers depends on eyesight, lighting, feed smoothness, and spindle runout.

Touch probes remove visual estimation by signaling the controller directly on contact. Accuracy depends on calibrating the stylus ball against a ring gauge to account for pre-travel. Machine backlash also affects results; if positioning drifts, see our guide on why is my CNC losing steps.

Step 5: Select the right tool for your machine and projects

Tool choice depends on your machine. For manual knee mills or benchtop machines lacking probing inputs, mechanical edge finders are ideal. They need no wiring, macros, or batteries, working reliably whenever the spindle runs at moderate speed.

For CNC routers and automated mills, production volume decides value. Makers setting up occasional projects on machines from our best desktop CNC for beginners guide can align stock with an edge finder. However, shops running repeated batches, flipping stock, or machining sheet stock gain huge efficiency from probes.

On thin sheet metal, probes excel at surface mapping to maintain consistent milling depth. Edge finders remain dependable for quick manual checks where coolant or rough handling could harm electronics.

Common mistakes to avoid

Avoid these common setup mistakes:

• Running a mechanical edge finder at excessive spindle rotational speeds that stretch internal springs.
• Forgetting to offset the coordinate register by the contact tip radius after kick-off.
• Jogging touch probes at high feed rates without overtravel protection in control software.
• Neglecting spindle and collet cleanliness, which introduces angular runout.
• Attempting to find Z-axis zero with a standard horizontal-deflection edge finder head.
• Using conductive edge finders on non-conductive plastics, or on anodized aluminum without electrical continuity.

Edge finder vs probe: the short version

1. Choose a mechanical edge finder for manual mills and basic setups.
2. Choose an electronic touch probe for CNC routers and automated milling machines.
3. Match tool shank diameter to your collets and holders.
4. Account for stylus or tip radius in coordinate registers.
5. Clean spindles and collets before setting part datums.

Setting accurate datums ensures clean, repeatable machining cycles.

FAQ: lasers and CNC

What is an edge finder tool?

An edge finder is a cylindrical setup tool used in mill collets to locate workpiece boundaries. A spring-loaded tip kicks off sideways upon contact, establishing part zero.

How does an edge finder work?

Under moderate spindle rotation, contact friction pulls the tip concentric with the shank. When pushed past the spindle centerline, the tip kicks off sideways to indicate the edge.

How to use an edge finder on a milling machine?

How to use an edge finder on a mill involves mounting it in a collet, running moderate spindle rotation, jogging until kick-off, and offsetting the axis coordinate by the tip radius.

What is an electric edge finder on a CNC machine?

An electric edge finder completes a circuit between conductive metal and the spindle. Touching the workpiece lights an LED or signals the CNC control without spinning.

Can you use an edge finder on sheet metal?

Yes, provided the sheet metal edge is deburred and tall enough for the contact head. Ensure the tip cannot slip under thin stock.

Can a mechanical edge finder find Z axis zero?

Standard edge finders only detect horizontal X and Y edges. To zero the vertical Z axis, use a touch plate, gauge block, or feeler gauge.

Related: the CNC tooling hub, CNC vs milling machine and ER vs TG collets. Also see our guide to what is a CNC router machine. Also see our best desktop CNC for beginners.