Self-Centering Three-Point Internal Micrometer 6-50mm – Carbide Tips, Ratchet Thimble & Calibration Ring

Description

I. In-Depth Product Analysis

Measuring internal diameters accurately in blind holes, through holes, and deep bores requires a tool that not only provides fine resolution but also eliminates the positioning errors inherent in manual two-point Calipers . Our H-315 Series Self-Centering Three-Point Internal Micrometer is engineered specifically to address these challenges. The three-point contact system utilizes three precisely machined, synchronized expanding wedges. When the thimble is rotated, these contacts extend outward at identical rates, forcing the gauge to automatically align with the absolute centerline of the bore. This ensures that every reading reflects the true geometric diameter, whether you are checking a simple through-hole, a deep recess, or a blind hole with a flat bottom.

The series is designed with great versatility in mind. For diameters ranging from 6mm to 12mm, the measuring heads are crafted from specially hardened, high-grade steel to maintain the compact form factor required for tiny bores, offering a remarkable 0.001mm resolution. For diameters above 12mm up to 50mm, the measuring contacts are equipped with wear-resistant carbide tips . This upgrade provides exceptional durability against the abrasive wear of cast iron, hardened steel, and rough-machined surfaces, ensuring the instrument maintains its specified 0.004mm to 0.005mm accuracy over years of continuous heavy-duty industrial use.

The ratchet thimble mechanism is integrated across all models to guarantee that a consistent, non-varying measuring force is applied to the workpiece every single time. This eliminates the variable of human hand pressure, which is critical for ensuring perfect repeatability across different operators and measurement sessions. An extension rod is included for deep-hole inspection, and every unit comes with a dedicated precision calibration ring , allowing operators to instantly establish a traceable absolute zero baseline right in the workshop. Both silver and black finish options are available to suit different workshop aesthetics.

II. Technical Specifications

The H-315 series is available in both Silver (H-315-XX-050) and Black (H-315-XX-150) finishes, covering the 6-50mm range with matched calibration rings and extension rods.

(Note: The table below lists models for the 12-50mm range, which feature carbide measuring faces. The 6-12mm range features specially hardened steel measuring faces and provides 0.001mm resolution.)

Model Number (Silver)	Model Number (Black)	Measuring Range	Accuracy	Setting Ring	L1 Dimension	A Dimension	Extension Rod
H-315-04-050	H-315-04-150	12 - 16mm	0.004mm	Φ16mm	80mm	0.5mm	150mm
H-315-05-050	H-315-05-150	16 - 20mm	0.004mm	Φ16mm	80mm	0.5mm	150mm
H-315-06-050	H-315-06-150	20 - 25mm	0.004mm	Φ25mm	90mm	0.5mm	150mm
H-315-07-050	H-315-07-150	25 - 30mm	0.004mm	Φ25mm	90mm	0.5mm	150mm
H-315-08-050	H-315-08-150	30 - 40mm	0.004mm	Φ40mm	97mm	0.5mm	150mm
H-315-09-050	H-315-09-150	40 - 50mm	0.005mm	Φ40mm	97mm	0.5mm	150mm

Core Specifications:

• Resolution: 0.001mm (for 6-12mm models) / 0.005mm (for 12-50mm models).

• Measuring Faces: Hardened steel (6-12mm) / Hardened carbide tips (12-50mm).

• Operation: Ratchet thimble for consistent measuring force.

• Design: Self-centering three-point contact for automatic alignment.

• Included: Matched precision calibration ring and 150mm extension rod.

III. FAQ (Frequently Asked Questions)

Q1: What is the primary advantage of the three-point self-centering design over a standard two-point caliper?
A: Standard two-point Calipers are highly susceptible to being tilted slightly inside the bore. This off-angle error results in a false reading of the diameter. The three-point design mechanically eliminates this issue. As the three contacts expand outward synchronously, they automatically force the gauge into perfect alignment with the absolute centerline of the hole, ensuring that every reading reflects the true dimension, regardless of operator positioning.

Q2: Why do models under 12mm use steel measuring faces instead of carbide?
A: At the miniature scale of 6-12mm, there is extremely limited physical space to house three internal wedges and contacts. To maintain the compact form factor required for such small bores, these heads are manufactured from specially hardened, ultra-high-grade steel. For applications requiring carbide at this size, please consult our H-375 series (2-6mm models), which utilize a specialized two-point expanding probe. For the 12-50mm range in this series, standard carbide measuring faces are provided for exceptional wear resistance.

Q3: Can this micrometer measure deep holes?
A: Yes. Each unit is supplied with a standard 150mm extension rod. This allows you to extend the measuring head deep into long workpieces, such as hydraulic cylinders, deep engine bores, and large tubes, while keeping the adjustable thimble and reading scale easily accessible outside the workpiece.

Q4: How do I calibrate this micrometer for accurate measurements?
A: Each unit comes with a matched precision calibration ring . Before measuring your workpieces, clean the measuring contacts and insert the head into the calibration ring. Gently tighten the thimble using the ratchet mechanism until it clicks. Use the zero adjusting wrench (supplied) to align the zero mark on the sleeve with the thimble's mark, establishing a perfect traceable zero baseline.

Q5: What is the difference between the silver and black finish models?
A: The H-315-XX-050 series features a classic silver finish, while the H-315-XX-150 series offers a sleek black finish. The performance, accuracy, and measurement specifications are completely identical between the two versions; the choice simply allows you to match the tool to your workshop's aesthetic or personal preference.

IV. Applications & Industry Uses

• Precision Machining & CNC Turning: Accurate verification of bored, reamed, and honed holes in precision parts.

• Automotive Engine Manufacturing: Measuring cylinder bores, valve guide diameters, and transmission housing internal dimensions.

• Aerospace Component Inspection: Verifying the internal diameters of hydraulic fittings, structural tubing, and precision-machined castings.

• Mold & Die Manufacturing: Precision checking of core pin holes, ejector bores, and injection mold cavities.

• Hydraulic Cylinder & Pneumatic Tube Inspection: Measuring deep internal diameters using the 150mm extension rod.

• Quality Control & Inspection Labs: A reliable standard for verifying the internal dimensions of master gauges and fixtures.

• Industrial Maintenance: Assessing internal wear and ovality of pumps and heavy equipment bores during scheduled overhaul.

V. Accuracy & Working Principle (Accuracy & Mechanics)

The H-315 series achieves its precise 0.001mm/0.005mm resolution through a precision mechanical wedge system. Inside the measuring head, a high-accuracy threaded spindle drives three precision-ground wedges forward as the thimble is rotated. Because these wedges are machined to exact equal angular tolerances, they force the three measuring contacts to expand outward radially at an identical, uniform speed. This synchronized expansion guarantees flawless self-centering alignment. The ratchet thimble ensures that the measuring force applied is absolutely consistent, preventing the distortion of delicate internal surfaces and ensuring perfectly repeatable measurements. The final reading is derived from the precisely marked graduations on the sleeve and thimble, translating the spindle's linear displacement into a clear, readable dimension.

VI. Calibration & Maintenance

1. Daily Zero Check with Calibration Ring: Before starting a shift, clean the three measuring contacts. Insert the head into its matched calibration ring , gently tighten using the ratchet thimble, and use the zero adjusting wrench to align the zero line if necessary.

2. Clean the Contacts: After every use, gently wipe the measuring contacts with a soft, lint-free cloth. Any metal chips, oil, or grit left on these precision surfaces will directly impact measurement accuracy.

3. Protect the Small Heads: The 6-12mm measuring heads are highly delicate. Handle them with care. Always insert them gently into bores and do not force them, as the internal wedges are miniature and precise.

4. Annual Professional Calibration: For strict compliance with ISO 9001 or AS9100 quality standards, the instrument should undergo a full, traceable calibration by an accredited metrology laboratory every 12 months .