M20 cable gland hole size for plastic and aluminum

Use the gland manufacturer's specified mounting-hole diameter, then secure the gland with its sealing washer and locknut. For thin plastic or aluminum walls, this normally means a clearance hole rather than a tapped entry. M20 identifies the thread, not a universal drill size. A controlled cut, adequate thread reach and a flat sealing face matter more than making the hole grip the threads.

What you need

Have the actual gland and its installation drawing available before selecting a cutting tool. Check the complete thread designation, mounting-hole tolerance, sealing arrangement and permitted wall thickness.

Prepare these items:

• A drill press where access permits, or a suitable handheld drill with a securely supported enclosure.
• Clamps and a sacrificial backing piece that support the wall without distorting it.
• A metric step drill suitable for the material and specified hole diameter.
• A plastic-compatible drill, hole saw or other suitable cutter when the wall exceeds the step drill's working depth.
• A deburring tool, measuring equipment and eye protection.
• The gland manufacturer's specified sealing washer, locknut and installation tools.

For aluminum, a compatible cutting lubricant may help control heat and chip buildup. Keep lubricant away from gaskets and clean the sealing face afterward. For plastic, confirm compatibility before applying any cutting fluid.

Do not wear gloves near rotating drilling equipment. Stop the spindle before clearing chips or checking the hole.

Drill the specified clearance hole

1. Confirm the mounting method. A clearance hole lets the threaded gland body pass through the wall. The locknut retains it, while the specified washer or integral seal closes the wall interface. A tapped entry needs sufficient material, correct threads and the manufacturer's specified sealing arrangement. Do not assume a thin enclosure wall can support that arrangement.

2. Read the cutout dimension independently of the thread size. For an M20 × 1.5 entry, the nominal thread diameter is 0.787 in [20 mm]. The pitch is 0.059 in [1.5 mm]. Neither dimension establishes the required clearance-hole tolerance. If the gland drawing specifies a 0.807 in [20.5 mm] mounting hole, use that dimension and its stated tolerance. Do not substitute a generic bolt-clearance size or a tap-drill diameter.

3. Check wall thickness and thread reach. Measure the actual wall, including any molded boss or local reinforcement. Account for the sealing washer and any other specified hardware. Enough thread must remain for the required locknut engagement without bottoming or interference. The article on cable glands with insufficient thread reach explains this stack-up. Choose a suitable gland before cutting, rather than thinning the enclosure wall afterward.

4. Position the hole on a flat area. Leave room for the entire sealing washer, gland shoulder, locknut and tightening tool. Avoid molded ribs, rounded corners, lid joints and bends. Check the inside as carefully as the outside. A hole beside a rib may accept the gland body but prevent the locknut from seating squarely. Allow space for the cable bend without loading the gland sideways.

5. Make the enclosure safe and support the wall. Isolate all energy sources and verify the equipment is safe before working. Preferably drill the empty enclosure away from electrical components. Remove vulnerable parts and protect the door gasket from chips. Clamp the work securely, supporting the wall close to the cut. Support plastic without squeezing it into a bowed shape.

6. Select a cutter that matches the wall. A step drill suits many thin aluminum and plastic walls, provided its cylindrical working land exceeds the wall thickness. Otherwise, the next step can enlarge the entrance before the required diameter passes through. For thicker plastic, use a plastic-compatible cutter capable of producing a straight bore through the full thickness. A suitable sheet-metal punch is another option for aluminum, but only within its material and thickness limits. Do not assume that punch is suitable for plastic.

7. Drill without forcing the cutter. Make the pilot hole required by the selected tool, then enlarge it under control. Follow the tool manufacturer's speed and feed guidance for the material. Ease pressure near breakthrough and stop if plastic melts, cracks or develops white stress marks. Do not force the gland through an unfinished hole. If the specified diameter falls between step sizes, use another suitable cutter or finishing method.

8. Deburr without removing the sealing land. Remove chips and sharp edges from both sides. Keep edge relief minimal; a broad countersink can remove the surface supporting the seal. Measure the finished diameter in two directions to check for obvious ovality. Clean away swarf, cutting fluid and debris before fitting the gland.

Check fit and sealing before service

The gland should pass through the finished clearance hole without being screwed through the wall. It should not need hammering or force that could crack plastic or damage its threads.

Dry-fit the gland and inspect both faces. Confirm that:

• The seal sits completely on a clean, flat surface.
• The shoulder and locknut seat squarely without contacting ribs or nearby hardware.
• Thread engagement meets the gland manufacturer's instructions.
• Tightening does not dish aluminum or visibly deform plastic.
• The cable jacket falls within the gland's stated clamping range.

The cable gland clamping range guide covers cable fit separately. A correct mounting hole cannot compensate for a cable that is too small for the insert.

Tighten the body, locknut and cable compression components as their instructions require. The article on installing a cable gland without losing the rating covers the complete installation beyond hole preparation.

Use an appropriate ingress check for the intended service with electrical equipment safely isolated. A workshop check can reveal workmanship problems, but it does not establish an enclosure rating.

Mistakes that compromise the seal

Tapping a thin wall because the gland has threads. Thread engagement depends on usable wall thickness and thread pitch. For illustration, a 0.079 in [2 mm] wall spans only about 1.3 pitches of an M20 × 1.5 thread. That calculation is not an acceptable-engagement rule. Use the gland manufacturer's requirements when deciding whether a tapped entry is suitable.

Making the hole tight enough to scrape the threads. A clearance mounting arrangement should not depend on thread interference. Scraping can damage the gland, distort plastic and prevent the seal from seating properly.

Flattening a distorted wall with the locknut. More torque does not repair a dished panel. It can worsen distortion, squeeze out the seal or crack plastic. Correct the mounting surface or replace the affected part.

Using a serrated washer as the water seal. Bonding hardware and sealing hardware perform different jobs. Follow the specified hardware stack; do not replace the sealing washer with a toothed washer.

Filling an oversized hole with sealant. Sealant is not a substitute for the specified mounting geometry. If the hole exceeds tolerance, use an approved repair arrangement or replace the panel. Do not assume a larger washer restores the original protection.

What to put on the drawing

Record the complete gland designation, specified cutout diameter and tolerance, wall material and thickness, and hole-center coordinates. Add the required flat sealing area and internal clearance for the locknut and tools. Show the sealing hardware and identify which face receives it.

The cutout designer can lay out openings and check spacing on catalog or custom enclosure sizes. Include the gland drawing with the layout so the mounting requirements remain clear.

For a stainless version, review custom stainless enclosures and factory cutouts. We build custom stainless enclosures, but a new opening does not automatically preserve the original rating. Confirm the seal, component, mounting and rear clearance on the approval drawing.

Enclosures for this job

Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.

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