A Blade Micrometer measures the outside diameter of workpieces which have narrow grooves. "T" slots and other features. 

Features:

• Measuring Range: 3-4"
• Graduation: 0.0001"
• Accuracy: 0.0002"
• Anvil & spindle hardened & precision ground with microlap finish on ends
• Non-rotating spindle
• Ratchet thimble
• Blade thickness - 0.030": blade width = 0.256"; blade penetration = 0/315"
• sizes over 1" include a standard