An incorrectly selected drill diameter for rivets leads to the fastener turning in the hole or deformation of the material during installation. If the hole diameter exceeds the nominal value by more than 0.2 mm, the rivet head may fall inward without creating the necessary pressure. Otherwise, if the fit is too tight, the rivet shank breaks before the head is formed, leaving a defective fastener in the metal. The accuracy of the selection of the cutting tool here is a critical factor in the reliability of the connection.
The main difficulty is that standard drills are produced in millimeter or inch gradations, and the sizes of rivets often have their own tolerances. To obtain a high-quality result, it is necessary to take into account not only the nominal diameter, but also the material into which the drilling is performed. Aluminum, steel and plastic require different approaches to the choice of drill geometry and drill chuck rotation speed.
It is a mistake to use a dull tool that “burns” the metal instead of cutting, increasing the actual diameter of the hole due to cold hardening. Correctly selected drill should enter the material with moderate force, creating a clean, burr-free edge. Only in this case can you count on the strength of the connection declared by the fastener manufacturer.
Principles for selecting hole diameter
The key rule when preparing a hole is to ensure a minimum gap between the body of the rivet and the walls of the drilled cavity. The standard technological gap is usually from 0.1 to 0.2 mm for diameters up to 5 mm inclusive. For larger fasteners, it is permissible to increase the gap, but it should not exceed 0.3–0.4 mm, otherwise the use of special washers or sealants will be required.
When choosing a tool, it is important to pay attention to the accuracy class of the drill itself. Drills marked h8 or h9 have a smaller diameter variation and provide a more predictable result. Using cheap high-speed steel drills without cobalt additives can lead to runout and expansion of the hole during operation.
⚠️ Attention: Never use drills with visible wear or chips on the cutting edge to prepare holes for blind rivets. This is guaranteed to lead to installation defects.
There is a direct relationship between the length of the working part of the rivet and the quality requirements of the hole. If the body of the rivet is short, even minimal axial displacement or ovality of the hole will lead to misalignment. In such cases, it is recommended to use guide bushings or drilling jigs.
Use a center punch to make precise marks before drilling. This will prevent the drill from drifting when starting work and ensure that the hole is perpendicular.
Correspondence table for rivet and drill sizes
To simplify the task of selecting a tool, there are standardized tables that regulate the ratio of the diameter of the fastener and the required size of the cutting edge. Below are data for the most common types of blind rivets used in the automotive and construction industries.
| Nominal diameter of rivet (mm) | Recommended drill diameter (mm) | Allowable tolerance (+ mm) | Rod material |
|---|---|---|---|
| 2.4 | 2.5 | 0.1 | Aluminium/Steel |
| 3.0 | 3.1 | 0.1 | Aluminium/Steel |
| 3.2 | 3.3 | 0.1 | Aluminium/Steel |
| 4.0 | 4.1 | 0.2 | Steel/Stainless steel |
| 4.8 | 4.9 / 5.0 | 0.2 | Steel/Stainless steel |
When working with inch dimensions, which are often found in imported fasteners, conversion to the metric system must be as accurate as possible. For example, a 1/8" rivet is 3.175mm and a 3.2mm or 3.3mm drill bit is ideal for it. Rounding up plays a positive role here, making installation easier.
It is important to consider that for rivets with an enlarged head or countersunk head, the diameter requirements may vary slightly. In the case of a countersunk fastening, the shape of the countersink is also critical, which must exactly match the angle of the cone of the head.
Influence of material on tool selection
The material of the parts to be joined dictates the choice of not only the diameter, but also the type of drill. For aluminum and soft alloys of non-ferrous metals, drills with a large sharpening angle and polished grooves, which prevent chips from sticking, are optimal. Using steel drills on aluminum without proper geometry often leads to biting and torn hole edges.
When working with high-strength steels or stainless steel, it is necessary to use cobalt drills (marking Co5 or Co8). A conventional tool quickly overheats and loses hardness, which changes its geometry and, as a consequence, the diameter of the resulting hole. The hole becomes oval or cone-shaped.
- 🛠️ For plastic and composites, use drills with sharp sharpening and a minimum angle at the tip to avoid delamination of the material.
- 🔩 When drilling multilayer structures (sandwich panels), the diameter of the drill is selected according to the hardest layer, but taking into account the risk of deformation of the soft layers.
- 💧 The use of cutting fluid (coolant) is mandatory when drilling deep holes in steel to maintain size.
Particular attention should be paid to composite materials, often used in body repair. Here, the diameter of the drill for rivets must be selected taking into account the prevention of fiber spalling. Two-stage drilling is often recommended: first with a pilot drill, then with the main diameter.
⚠️ Attention: When drilling thin sheet metal (less than 1 mm), there is a high risk of deformation of the edges of the hole. Use step drills or place a block of wood on the back side.
Features of drilling for blind rivets
Installation of countersunk rivets requires not only the exact diameter, but also the correct preparation of the countersink. The diameter of the rivet body should correspond to the hole, and the conical part should fit perfectly into the chamfer. If the angle of the countersink does not match the angle of the head, the rivet will either not sit flush with the surface or will only be held in place by the edge.
To obtain a high-quality result, combined countersink drills are often used, which simultaneously make a hole of the required diameter and form a cone. However, such tools are sensitive to distortions and require rigid fixation in the chuck. An alternative is to drill the hole separately and then countersink.
The depth of the hole also matters. If you drill a through hole with a drill that is too large, the head may fall through. If the material is thick, it is important to ensure that the shank of the rivet does not rest against the bottom of the blind hole ahead of time.
Countersinking technology
The angle of a standard countersunk head is usually 120 degrees. Use a countersink with the appropriate angle. For aluminum the angle can be 90 or 100 degrees, check the fastener manufacturer's specifications.
Common mistakes and their consequences
One of the most common mistakes is ignoring the runout of the drill or screwdriver chuck. If the drill “walks”, the hole turns out to be larger than the nominal value and has torn edges. This leads to the fact that during installation the rivet is deformed unevenly, and the connection loses its tightness and strength.
Another problem is using larger diameter drills “by eye” when you don’t have the right size at hand. A difference of even 0.3 mm can be critical for dynamic loads. In such cases, it is better to drill the hole to the next standard size and use a larger diameter rivet if the design allows.
- 📉 Overheating the material changes its properties and can lead to melting of the edges of the hole in the plastic, reducing the effective diameter.
- 🔨 Impact methods for installing rivets (hammering) with a tight hole destroy the internal structure of the fastener.
- 🌪️ Failure to remove chips from the hole before installing the rivet creates a false feeling of a tight fit.
Poor sharpening of the drill leads to the formation of a so-called “entry cone”, when the entry hole is wider than the exit hole (or vice versa, depending on the angle). This creates a step that interferes with the passage of the rivet body. Visual inspection of the quality of the hole before installation is mandatory.
The main principle: the hole should be as close as possible to the rivet rating, but ensure free manual entry of the fastener without effort.
Tools and quality control
For professional work, it is not enough just to have a set of drills. Regular monitoring of their condition and size is necessary. A caliper with an accuracy of at least 0.05 mm should be a mandatory attribute of a riveting specialist. Periodically checking the diameter of the drill allows you to promptly eliminate a worn tool.
In addition, it is important to properly organize the drilling process. Using stationary drilling machines or guide stands for drills gives a much more accurate result than working with a handheld tool. The verticality of the drill entry directly affects the geometry of the hole.
☑️ Check before installation
Compliance with hole preparation technology guarantees the durability of the connection. A rivet installed in a properly prepared hole works as a monolithic part, distributing loads evenly over the entire contact area. Neglecting the accuracy of the drill diameter negates all the advantages of this type of fastener.
Can I use a wood drill bit for metal when installing rivets?
Strongly not recommended. Wood drills have a different sharpening geometry (often a central tip) and a different spiral angle. When you try to drill metal, such a drill will quickly become dull, heat up and, most likely, break or make a hole of the wrong shape.
What if the hole is already drilled and it is too big?
If the gap exceeds the permissible 0.2-0.3 mm, there are two ways: use a rivet of the next larger size (requiring drilling) or use a rivet with an enlarged head, which will cover the excess space. At critical nodes it is better to weld or re-rivet the connection.
Do I need to lubricate the drill when working with aluminum?
Yes, using a lubricant (special coolant, kerosene or even soap solution) when drilling aluminum prevents metal from sticking to the cutting edge. This keeps the drill diameter stable and extends drill life by ensuring clean hole edges.
How to choose a drill bit for non-standard size rivets?
For non-standard sizes, it is necessary to measure the diameter of the rivet body with a caliper in several places and select a drill that is closest in value to the larger side, but with a minimal difference. It is acceptable to use reamers to calibrate a hole after a drill of a smaller diameter.