M6.5 and M8 Proximity Sensors: Sensing 1 mm and 2 mm in Tight Spaces

Below M12 you are into subminiature territory: M8 and M6.5 bodies sensing at 1 mm or 2 mm. They exist because sometimes there genuinely is no space, and they are unforgiving because there is no margin.

The numbers

Our D30C range covers both small bodies at two sensing distances each:

• M6.5: 1 mm and 2 mm
• M8: 1 mm and 2 mm

Both are available PNP or NPN, normally open, three wire, 200 mA. Note the 200 mA rating rather than the 300 mA of the larger bodies, which matters if you are driving anything other than a PLC input.

The tolerance problem

Design to 70 to 80 percent of rated and a 1 mm sensor gives you a working gap of 0.7 to 0.8 mm. That is less than the thickness of a business card. Everything now matters:

• A bracket that flexes under acceleration moves the gap by more than the gap.
• Thermal growth across a 100 mm arm is comparable to the whole sensing distance.
• A target in stainless rather than mild steel can halve what you have.
• Paint or plating on the target is a meaningful fraction of the gap.
• Swarf or a film of coolant on the face reads as a target.

So specify the 2 mm version wherever it will fit. The 1 mm parts are for cases where the mechanical envelope genuinely forbids anything else, and they want a rigid bracket, a mild steel target and a clean environment.

Where they earn their place

• Grippers and end effectors. Confirming jaw open and jaw closed, where the sensor travels with the arm and mass matters.
• Tool changers and turrets. Position confirmation inside a housing that was designed around the tooling, not around the sensor.
• Small pneumatic actuators. Though if the actuator has a magnet in the piston, a magnetic proximity switch on the barrel is usually the better answer. See sensing cylinder position.
• Textile, packaging and assembly machinery where the mechanism is dense and the sensor has to live inside it.

Mounting

The clearance rules scale down with the body, which sounds convenient and is not, because manufacturing tolerances do not scale down with them.

Use the shielded variant and mount it flush in the bracket. On a small body a flush mount in a well machined pocket is far more repeatable than a sensor standing proud on a locknut, because the pocket controls the position and the locknut does not. Where you can, design a shoulder the sensor seats against rather than relying on the nuts to hold a dimension.

Protect the cable at the back. On a moving arm the cable is the thing that fails, not the sensor, and on a subminiature body the cable exit is the weakest point. Specify a cable length that reaches the first fixed point without a joint, and support it within 50 mm of the sensor so the flexing happens in the cable and not at the gland.

When to go bigger instead

Before committing to M8, check honestly whether the bracket could be redesigned for M12 at 4 mm. The difference in reliability over a few years is large, and a bracket is cheaper to change at the drawing stage than a sensor is to replace quarterly once the machine is in production.

If the answer is genuinely no, then fit the 2 mm M8, use a mild steel target, make the bracket stiff, and accept that this sensor will need checking at every planned shutdown.

Ordering

D30C-0802-PM-2 is M8, 2 mm, PNP, normally open, three wire at 200 mA. D30C-0601-NM-2 is the M6.5 at 1 mm in NPN. The ordering code guide decodes every position, and the sensing distance chart shows how the small bodies sit against the rest of the range.