January 2026

No. 5 How to Thread the End of 4040 T-Slot Extrusion: Hand Tapping vs. Using a Drill and Tap Jig

If you want to join 4040 aluminium profile with end fasteners, the end of the extrusion needs an M8 thread. It's one of the most common jobs in T-slot framing, and one of the easiest to get wrong. A crooked thread means a joint that won't sit flush, and a snapped tap can ruin the profile.

This guide covers how to tap the end of a 4040 profile by hand, the risks involved (especially broken taps), and why a dedicated drill and tap jig is the better option.

Why thread the end of a profile?

4040 extrusion has a 6.8 mm core hole running through the centre of its length, which is the correct tap drill size for an M8 thread. Tapping this hole at the end of the profile lets you bolt the member to another profile using an M8 button head bolt, with no brackets. It's also the thread used by T-slot end fasteners, which clamp the end of one profile to the side of another with a steel plate and two anti-rotation tabs. For how that joint works, see our guide: [no-6-t-slot-end-fasteners-how-to-join-aluminium-profiles-cleanly-and-securely].

Because the hole is already there, you usually don't need to drill. You only need to cut the thread.

The biggest risk: a broken tap in the hole

Before you pick up a tap, understand what can go wrong. Taps are made from hardened steel, which makes them strong in a straight line but brittle when twisted sideways or overloaded. Aluminium is soft, so it's easy to assume tapping is forgiving. It isn't. Aluminium chips are long, soft, and sticky, and they can pack into the tap's flutes and jam it.

A tap that snaps inside the core hole is a serious problem:

It's very hard to remove. The hardened steel is tougher than a standard drill bit, so you can't simply drill it out. Specialist extractors can sometimes work, but they often fail in a narrow hole and can damage the surrounding aluminium.
It usually means scrapping the profile. The broken piece blocks the thread you were cutting, and the profile you spent time cutting to length is often written off.
It's frustrating and costly. You lose the tap, the profile, and the time, and you still have to start again.

What causes taps to break

Nearly every broken tap comes down to a lack of care and attention at one of these points:

Starting off-axis. A tap that enters at an angle is being bent as it cuts. The sideways load builds with every turn until the tap snaps.
Forcing it. Pushing harder when the tap feels tight is the most common mistake. Tightness is a warning that chips are packing or the tap is binding, so stop and back off.
Not breaking the chip. Turning the tap continuously forward lets the long aluminium swarf pack the flutes. Back the tap off regularly to snap and clear the chips.
No lubricant, or too little. Without cutting fluid, aluminium galls and welds itself to the tap, which raises the torque sharply.
Rushing. Speed generates heat, makes the aluminium stick, and leaves no time to feel the tap loading up.
A worn or damaged tap. A dull tap needs more force to cut, so it loads up faster and is more likely to break.
Uneven pressure on the wrench. Pushing down on one side of a tap wrench side-loads the tap. A T-handle used with even, two-handed pressure is safer.
Losing concentration partway through. Many taps break on the last turn or two, when the job feels finished and attention drops.

Option 1: Tapping the end by hand

Tools and materials

4040 extrusion, cut square and deburred
M8 x 1.25 tap. A spiral flute tap is a good choice for aluminium, as it helps carry chips back out of the hole.
Tap wrench or T-handle
Cutting fluid or tapping lubricant (a dedicated aluminium cutting fluid works best)
Clamps or a vise
Try square
Safety goggles

Step by step

Prepare the extrusion. The end must be cut clean and square, because the tap will follow whatever the end face and hole give it. Deburr the end and clear any swarf out of the 6.8 mm hole.
Secure the profile. Clamp it firmly so it can't move. Any movement while tapping will pull the thread off-axis and load the tap sideways.
Lubricate generously. Apply cutting fluid to the hole and the tap, and top it up as you go.
Start the tap straight. Insert the tap and begin turning slowly with light, even pressure. The first few turns decide the whole thread, so check alignment from two sides with a try square. If it's crooked, back the tap out and restart rather than hoping it corrects itself.
Cut in short strokes. Turn the tap forward about a half to one full turn, then back it off a quarter to half turn to break the chip. Repeat until you reach full depth.
Stop if it feels tight. If the tap resists, don't push through. Back it out, clear the flutes, re-lubricate, and restart gently.
Clear the chips. On a deeper thread, back the tap fully out part-way through, clean the flutes, and re-lubricate.
Clean and test. Back the tap out, blow out the swarf, and run an M8 button head bolt in by hand. It should turn smoothly and pull up flush.

How deep should the thread be? Aim for at least 1.5 times the bolt diameter (about 12 mm) of full thread engagement, and more if the length allows. Check your end fastener's datasheet for the recommended depth.

The problems with tapping by hand

Hand tapping works, but it depends heavily on feel, steady hands, and constant attention. The common problems are:

Broken taps. One moment of forcing it or a crooked start can snap the tap and scrap the profile, as described above.
Crooked threads. If the tap starts at an angle, the bolt pulls the profile out of square.
Stripped or cross-threaded holes. Aluminium is soft, and a poor start is hard to recover from.
Inconsistent results. Doing ten ends by hand will give you ten slightly different threads, which shows up as gaps across a frame.

Option 2: Use a drill and tap jig (the better option)

A T-slot end drill and tap jig removes the biggest cause of broken taps: the tap being started or turned off-axis. It clamps to the end of the profile and holds a locator sleeve in line with the centre hole, so the drill or tap enters exactly perpendicular (90°) to the end face and stays there for the whole cut.

What you gain:

Far less risk of a broken tap. With the tap guided straight, the sideways loading that snaps taps is largely removed, which protects your tap and your profile.
Straight, repeatable threads. Every end is tapped square to the face, so joints sit flush.
Faster work. Clamp, tap, release, and repeat. This is a big time saver on frames with many joints.
Confidence for beginners. You don't need a machine shop or years of tapping experience.
Drilling guidance too. If a core hole needs cleaning up, or a profile doesn't have one, the jig guides the drill as well as the tap.

A jig doesn't remove the need for care. You still need lubricant, a slow pace, and regular chip clearing, but it takes away the alignment problem that catches most people out.

What the kit includes

Multi-Profile Drill Jig: Precision-compatible with 2020, 3030, and 4040 T-slot aluminum profiles.
M6 & M8 Thread Taps: High-grade taps for standard T-slot end-tapping and joint assembly.
Ratcheting Tap Wrench: Reversible ratchet mechanism for smooth, controlled tapping in tight spaces.
Countersink / Deburring Bit: Prepares drilled hole edges before tapping to ensure clean thread entry.
M6 & M8 Tap Adapters: Quick-connect drive adapters for standard power drill chucks.

⚠️ Special note: fitting the M8 tap through the locator sleeve

Our kit includes two taps (M6 and M8) that both pass through the one threaded bit locator sleeve. Although the thread sizes differ, both taps have a 6.8 mm shank, so the same sleeve guides either one.

If your M8 tap won't fit through the sleeve, you're not doing anything wrong. The M8 thread is wider than the 6.8 mm hole, so the threaded end can't be pushed through. Don't force it.

The fix: feed the tap through the sleeve the other way round, shank end first. Pass the plain 6.8 mm shank through the hole from the profile-facing side until the threaded end seats against the sleeve and the shank comes out the other side. Then attach your tap wrench or T-handle and tap as normal.

The M6 tap's thread is narrower than the hole, so it passes through easily, but the same shank-first method works for either tap.

Using the jig

Secure the profile to your bench with clamps or a vise.
Mount the jig. Fit it onto the end of the extrusion so it sits flush against the cut face and locks into the T-slots. The locator sleeve should line up with the centre hole.
Lubricate the sleeve and the tap.
Load the tap, then tap through the guide. Feed the tap through the sleeve shank end first (see the special note above), attach your wrench, and turn slowly by hand. Let the jig keep it straight and use the same short-stroke method as above: forward, then back to break the chip.
Remove the jig, clean and test. Blow out the shavings and check the thread with an M8 button head bolt.

Pro tips for a clean thread (and an unbroken tap)

Don't rush. Forcing a tap generates heat, makes the aluminium bind, and can snap the tap.
Never force a tight tap. Resistance means stop, back off, and clear the chips.
Clear chips often, especially on deeper threads.
Use plenty of lubricant and reapply during the cut.
Replace worn taps. A dull tap is a breakage waiting to happen.
Match your hardware. Your tap, bolt, and fastener should all be M8 x 1.25.
Choose the right bolt length. For end fasteners, an M8 button head bolt around 40 to 60 mm is typical. Choose the shortest bolt that gives good thread engagement.
Test the thread before assembly, so you find problems at the bench, not on the finished frame.

Final thoughts

Tapping 4040 by hand is possible, and for a one-off joint it may be all you need, provided you give it your full attention. But one lapse can leave a broken tap stuck in the profile. If you're building a frame with multiple joints, a drill and tap jig turns a fiddly, high-risk job into a repeatable one, and it pays for itself in saved profiles, taps, and time.

Ready to thread like a pro? Get our T-Slot End Drill and Tap Jig today. Then see how to put your threaded ends to work in our guide to [no-6-t-slot-end-fasteners-how-to-join-aluminium-profiles-cleanly-and-securely].