A coaxial indicator (also called a Co-Ax indicator or coaxial centering indicator) is a precision setup tool used to quickly locate the center of a bore, boss, or cylindrical workpiece feature while the machine spindle rotates. It enables machinists to accurately align a workpiece with the spindle axis, reducing setup time and improving workpiece accuracy.
Whether you're setting up a milling machine, jig borer, or lathe, learning how to use a coaxial indicator properly ensures precise centering and repeatable results.
WHAT IS A COAXIAL INDICATOR USED FOR?
A coaxial indicator is primarily used for:
- Bore centering and hole location
- Precision milling machine setup
- Fixture, spindle, or shaft alignment
- Lathe setup operations
- Locating inside and outside diameters
- Aligning cylindrical workpieces with machine spindle centerlines
Unlike traditional dial test indicators, a coaxial indicator allows the spindle to rotate while continuously displaying the amount and direction of deviation or misalignment.
Choosing the Correct Feeler
Coaxial indicators use interchangeable feelers to contact the workpiece surface.
- Straight Feeler: Used for indicating inside diameters up to approximately 4-1/4 inches.
- Curved Feeler: Used for outside diameters up to approximately 4-1/4 inches and inside diameters larger than approximately 1 inch.
HOW TO USE A COAXIAL INDICATOR
Step 1: Pre-Position the Workpiece
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Move the machine table until the spindle is approximately centered over the bore, boss, or outside diameter being indicated. The initial position should place the indicator within its operating travel range.
Step 2: Install the Indicator
- Insert the coaxial indicator into the machine spindle, attach the restraining arm and let it rest against a suitable stop.
- The restraining arm prevents the indicator body from rotating, allowing the dial face to remain stationary and easy to read during operation.
- A magnetic indicator base is typically used as a stop for the restraining arm.
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Select the appropriate feeler based on the application and secure it using the thumbscrew.
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Adjust the rocker bracket so the indicator hand remains near the center of its scale while the feeler contacts the workpiece.
Step 3: Verify Contact and Clearance
- Ensure the feeler maintains smooth, continuous contact with the workpiece surface without excessive pressure and without contacting the floor of a pocket or step.
- Turn the spindle by hand:
- Confirm there is adequate clearance all the way around the workpiece by checking for any potential interference.
- Be sure the feeler is touching the workpiece all the way around its circumference.
- Verify that the indicator hand moves freely.
Step 4: Rotate the Spindle
Step 5: Adjust the Machine Table
- Move the machine table along one axis until the indicator hand travel is minimized.
- Then adjust the second axis until the indicator hand movement is further reduced.
- Repeat these adjustments as necessary until the indicator hand shows zero movement throughout a full spindle revolution, confirming true center alignment.
WHY COAXIAL INDICATORS IMPROVE SETUP EFFICIENCY
Professionals trust Travers Tool Co. for quality coaxial indicators because they offer higher accuracy than manual edge finders and reduced setup time compared to test indicator sweeping methods.
Key advantages include:
- Faster centering of workpieces
- Reduced setup error on manual mills
- Improved repeatability between operators
- Increased shop productivity
Pro Tips and Related Resources
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Indicate twice before machining when working with critical tolerances.
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Pair a coaxial indicator with a quality work stop system for repeat production runs.
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Use minimal indicator feeler length to reduce deflection and improve stability.
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Understanding how to read a dial indicator is an essential first step in mastering the use of a coaxial indicator.
PARTS OF A COAXIAL INDICATOR AND THEIR FUNCTIONS
Understanding the function of each component helps ensure accurate setup and proper operation.
Shank
The shank is the mounting stem of the coaxial indicator. It fits into the machine spindle, collet, or tool holder and supports the entire instrument during operation.
Purpose:
- Secures the indicator to the machine’s spindle
- Allows the indicator to rotate with the spindle
Restraining Arm
The restraining arm prevents the indicator body and dial face from rotating while the spindle turns. One end attaches to the indicator, while the other rests against a fixed stop on the machine.
Purpose:
- Keeps the dial face stationary and readable
- Prevents the indicator body from spinning
- Allows the operator to observe dial movement during spindle rotation
Indicator Body
The indicator body houses the internal mechanism that converts feeler movement into dial movement. It serves as the main structural component of the instrument.
Purpose:
- Protects the precision internal components
- Supports the dial face and rocker assembly
- Transfers movement from the feeler to the indicator mechanism
Dial Face
The dial face displays the amount of deviation from center through the movement of the indicator hand.
Purpose:
- Shows the magnitude of spindle-to-workpiece misalignment
- Allows the operator to monitor progress during centering
- Indicates when alignment has been achieved by showing little or no needle movement
Rocker Bracket (Butterfly)
Often called the rocker bracket or butterfly, this pivoting assembly holds the feeler and transfers its movement to the indicator mechanism.
Purpose:
- Holds the feeler securely in place
- Allows the feeler to pivot and maintain contact with the workpiece
- Transmits motion from the feeler to the dial mechanism
Straight Feelers
Straight feelers are designed primarily for contacting internal diameters.
Purpose:
- Indicate bores and inside diameters
- Reach into holes and recesses
- Provide accurate centering of internal features
Typical Applications:
- Locating the center of drilled holes
- Aligning bored features
- Setting up fixtures with internal locating diameters
Curved Feelers
Curved feelers are used primarily on outside diameters and larger internal diameters.
Purpose:
- Contact external cylindrical surfaces
- Reach around bosses and shafts
Typical Applications:
- Centering on shafts and bosses
- Locating outside diameters
- Indicating large internal features
Center Point Feeler
The center point feeler is a specialized attachment used to locate a previously established center point.
Purpose:
- Locate punch marks and center-drilled holes
- Find the exact center of a marked location
- Position the spindle directly over a known center reference
Typical Applications:
- Toolroom work
- Prototype machining
- Picking up layout points
- Positioning from center-drilled locations
Indicator Hand
The indicator hand moves around the dial face in response to feeler movement.
Purpose:
- Displays the amount of offset from true center
- Indicates changes as machine adjustments are made
- Shows when the workpiece is aligned with the spindle axis
Thumbscrew
The thumbscrew secures the selected feeler in the rocker bracket.
Purpose:
- Holds feelers firmly during operation
- Allows quick feeler changes
HOW THE COMPONENTS WORK TOGETHER
When the shank is mounted in the machine spindle, the restraining arm keeps the indicator body stationary while the spindle rotates. The selected feeler contacts the workpiece surface and is held by the rocker bracket. As the feeler moves in response to any misalignment, the internal mechanism transfers that movement to the dial face. The operator adjusts the machine table until the indicator hand shows minimal or zero movement, confirming that the spindle and workpiece centers are aligned.
Understanding each component of a coaxial indicator allows machinists to use the instrument more effectively, reduce setup time, and achieve greater accuracy in milling, EDM, lathe, and precision machining operations. Using a coaxial indicator on a manual milling machine is one of the most effective ways to achieve fast, accurate centering.
With proper technique and quality equipment from Travers Tool Co., machinists can significantly improve setup speed, reduce scrap, and maintain consistent part accuracy across all manual milling operations.
KEY TAKEAWAYS
What is a coaxial indicator used for?
A coaxial indicator locates the center of a bore, boss, or other cylindrical feature while the spindle rotates without rotating the indicator face. It aligns the feature with the spindle axis for accurate machine setup.
What are the parts of a coaxial indicator?
The main parts of a coaxial indicator include the shank, restraining arm, indicator body, dial face, rocker bracket (butterfly), straight feelers, curved feelers, center point feeler, indicator hand, and thumbscrew.
What’s the difference between straight and curved feelers?
Straight feelers are used for inside diameters up to approximately 4-1/4 inches. Curved feelers are used for outside diameters up to approximately 4-1/4 inches and inside diameters greater than approximately 1 inch.