Aluminum Extrusion Drilling & Tapping Parameters: Tap Drill Sizes, RPM and Thread Depth

  • By Sam

Published: Friday, August 21, 2026

Aluminum Extrusion Drilling & Tapping Parameters: Tap Drill Sizes, RPM and Thread Depth 

Aluminum extrusion is commonly used in 3D printers, CNC machines, robotics, automation frames, and custom workshop structures. Popular profiles such as 20×20 and 40×40 aluminum extrusion are easy to cut and assemble, but drilling and tapping them successfully depends on more than simply choosing a drill bit.

 

The important variables are the hole diameter, thread size, drilling depth, spindle speed, tap alignment, and the actual geometry of the extrusion.

 

For SPESYN ST3 users, a 3D-printable FGM (front guide head) can also be designed around a specific profile, allowing the extrusion itself to act as a positioning reference while the ST3 handles drilling, depth feedback, and tapping.

 

This article focuses on the numbers behind that process.

1. Start with the Thread Size, Not the Screw Diameter

One of the most common mistakes when preparing a threaded hole is drilling a hole equal to the nominal screw diameter.

 

For example, an M5 screw does not normally use a 5 mm pilot hole when the hole will be tapped.

 

The hole must be smaller so enough material remains for the tap to cut the thread.

For common ISO metric coarse threads, these tap drill sizes are frequently used:

Thread Standard Coarse Pitch Common Tap Drill
M3 0.5 mm 2.5 mm
M4 0.7 mm 3.3 mm
M5 0.8 mm 4.2 mm
M6 1.0 mm 5.0 mm
M8 1.25 mm 6.8 mm

A useful approximation is:

Tap drill diameter ≈ nominal thread diameter − thread pitch

 

For example:

M6 × 1.0 → 6.0 − 1.0 = 5.0 mm

 

This is useful for quick reference, although an actual tap drill chart should still be checked when thread tolerance or engineering requirements matter.

2. Check the Extrusion Before Choosing the Thread

Not every aluminum extrusion with the same outside dimensions has the same internal structure.

 

Two different 40×40 profiles may have different:

  • wall thicknesses;
  • center-hole diameters;
  • slot geometry;
  • internal ribs;
  • aluminum alloy or temper.

This matters particularly when tapping the center bore at the end of an extrusion.

 

Some profiles already have a center opening that is close to the correct tap drill size. Others need to be drilled before tapping.

 

So before selecting M5, M6, or another thread, measure the actual extrusion with calipers instead of relying only on the name “2020” or “4040.”

 

The same principle applies to side drilling: check where internal cavities and ribs are located before deciding the final hole depth.

3. What RPM Should You Use for Drilling Aluminum Extrusion?

There is no single correct RPM for every aluminum drilling operation.

 

The ideal speed changes with:

drill diameter + drill material + aluminum alloy + lubrication + feed rate

 

In general, smaller drill bits can run faster, while larger bits should run more slowly.

 

For a handheld tool such as the ST3, these can be treated as practical starting ranges rather than fixed rules:

Drill Diameter Practical Starting Range for Aluminum
2–3 mm 1,500–2,000 RPM
4–5 mm 1,000–1,800 RPM
6–7 mm 800–1,500 RPM
8–10 mm 500–1,200 RPM

The ST3 Drilling mode operates from 0–2000 RPM, so the user can adjust speed according to drill diameter and cutting behavior.

 

A sharp HSS or suitable metalworking drill bit is usually more important than simply maximizing RPM.

 

If aluminum begins sticking to the cutting edge, reduce heat buildup, improve chip evacuation, and use an appropriate cutting lubricant.

4. Drilling Depth Matters More Than It Looks

A through-hole is straightforward: the drill passes through the material.

 

But aluminum extrusion contains cavities, ribs, and multiple walls, so many operations require controlled depth.

 

For example, you may want to drill only through the first wall rather than through the complete profile.

 

This is where ST3's real-time relative depth feedback with ±0.1 mm resolution becomes useful.

 

Instead of estimating depth based only on the visible portion of the drill bit, the operator can monitor travel as the bit enters the extrusion.

 

A typical process may be:

Set reference → begin drilling → monitor depth → stop before the next wall

 

The display does not automatically stop the drill, but it provides a much clearer reference for the operator.

 

For repeated holes, the same target depth can also be reused across multiple profiles.

5. How Deep Should a Tapped Hole Be?

Thread depth depends on the load, material, screw size, and available material inside the profile.

 

A common engineering starting point for aluminum is approximately:

Thread engagement ≈ 1.5 × screw diameter

 

For an M6 thread, that would mean roughly:

6 × 1.5 = 9 mm of thread engagement

 

In situations where more engagement is required, approximately 2D may be used:

M6 → about 12 mm

 

However, this should not be treated as a universal rule.

 

An aluminum extrusion may not have enough solid material behind the hole, so the internal geometry must always be checked first.

 

It is usually better to use the available structural material intelligently rather than simply making the hole as deep as possible.

6. Drilling Straight Matters Before You Start Tapping

Correct tap drill diameter alone does not guarantee a good thread.

 

If the pilot hole is drilled at an angle, the tap begins with an incorrect axis.

 

This is one reason a profile-specific 3D-printable FGM can be useful.

 

The FGM can be designed so its contact surfaces match the extrusion geometry. Once positioned on the profile, it establishes a physical reference for the ST3.

 

Then the ST3's dual-rail structure controls the forward feed.

 

The relationship is:

Profile geometry → FGM positioning → guided feed → pilot hole

 

For repeated fabrication, a custom FGM can also help maintain the same hole location across multiple pieces.

7. Tapping Parameters Are Different from Drilling Parameters

Tapping should not simply use the same speed as drilling.

 

The ST3 has a separate Tapping mode with a range of 0–400 RPM, specifically separated from its 0–2000 RPM drilling mode.

 

When tapping aluminum, speed should normally remain controlled, especially when starting the thread.

 

The exact setting depends on:

  • thread diameter;
  • tap geometry;
  • cutting vs. forming tap;
  • lubrication;
  • thread depth;
  • extrusion alloy.

More important than chasing a specific RPM is starting the tap straight and allowing chips to clear properly.

 

A crooked M6 thread cut at the “correct” speed is still a crooked thread.

 

This is why the same profile-specific FGM used for drilling can also be valuable during tapping: it preserves the physical reference between the tool and the extrusion.

8. A Practical Parameter Workflow

For a typical aluminum extrusion project, the process can be organized around the parameters rather than around the tool:

1. Identify the required thread
For example, M5 or M6.

 

2. Check the thread pitch
M6 coarse is commonly M6 × 1.0.

 

3. Select the tap drill
M6 × 1.0 commonly uses a 5.0 mm pilot hole.

 

4. Check the available material and target depth
Make sure the extrusion geometry supports the hole.

 

5. Position the 3D-printed FGM
Use the profile itself as the physical reference.

 

6. Drill in ST3 Drilling mode
Select speed based on diameter and cutting conditions.

 

7. Monitor depth
Use the ±0.1 mm relative depth feedback.

 

8. Switch to Tapping mode
0–400 RPM, with controlled feed and appropriate lubrication.

 

This turns drilling and tapping into one connected machining process instead of two unrelated operations.

Parameters Matter—But So Does the Reference

Good aluminum extrusion machining is a combination of correct numbers and correct alignment.

 

The correct tap drill diameter matters. So do RPM, thread depth, drill condition, lubrication, and extrusion geometry.

 

But even the correct parameters cannot fully compensate for a drill or tap that enters the profile at the wrong angle.

 

That is why the combination of SPESYN ST3 and a customizable 3D-printable FGM is particularly useful for extrusion work.

 

The FGM adapts the tool to the profile. The dual rails control the feed direction. The display provides depth feedback. And the dedicated drilling and tapping modes allow the workflow to continue from the pilot hole to the finished thread using the same handheld platform.

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