The quality of joining metal sheets in body repair or when assembling attachments directly depends on how accurately the seats are prepared. Holes for blind rivets are a critical element that determines the strength of the assembly and its durability under conditions of vibration and loads. Incorrect selection of drill diameter often results in fasteners either not being installed or, worse, falling out during vehicle operation.
Many craftsmen make the mistake of believing that the diameter of the hole must strictly correspond to the diameter of the rivet body indicated on the packaging. However, in reality, the technological process requires taking into account tolerances, the thickness of the metal coating and the characteristics of the tool itself. Blind rivets Different manufacturers may have slight differences in the geometry of the head, which dictates the need for a careful approach to the selection of cutting tools.
In this article we will look in detail at how to calculate the ideal hole size, what standards exist, and why it is sometimes worth deviating from generally accepted standards for the sake of connection reliability. Understanding these nuances will allow you to avoid defects and re-assembly of components.
Standard diameters and tolerances according to GOST
The basis for selecting a drill is the nominal diameter of the rivet, which is usually indicated by an integer (for example, 3.2 mm, 4.0 mm, 4.8 mm). According to specifications and generally accepted practice, hole diameter should be slightly larger than the diameter of the rivet body to ensure free entry of the shank without jamming, but still ensure a tight fit of the head.
For standard aluminum and steel rivets, the following calculation principle is adopted: an allowance of 0.1 to 0.3 mm is added to the nominal diameter. For example, for fasteners with a diameter of 4.8 mm (often labeled M5), a drill with a diameter of 5.0 mm or 5.1 mm would be optimal. Using a smaller diameter drill bit will distort the hole during installation, and using a drill bit that is too large will create a gap that cannot be compensated for.
⚠️ Attention: Never use drills with a worn cutting edge to prepare holes for critical connections. A dull drill “crushes” the metal, creating a hardening and changing the actual diameter of the hole, which can lead to jamming of the rivet at the insertion stage.
It is also important to consider the material of the parts being connected. If you are working with thin sheet steel or aluminum, the tolerances may be tight. In case of use stainless steel rivets, which have a higher hardness, the gap can be made as small as possible, since the risk of deformation of the fastener body during installation is lower.
Rules for choosing a drill for different types of rivets
Choosing the right drill is not only a matter of diameter, but also of the geometry of the cutting part. For holes under blind rivets High-speed steel (HSS) drills with cobalt additives are best, especially when drilling into hardened steel or stainless steel. Ordinary drills for wood or concrete are absolutely not suitable, as they produce jagged edges of the hole.
When working with composite materials or sandwich panels, which are often found in tuning and special equipment, the situation becomes more complicated. It is important here not to delaminate the material. In such cases, it is recommended to use special drills with a sharp point that prevents scoring, or pre-drill with a small diameter pilot drill.
- 🛠️ Drills with a polished groove and a sharpening angle of 130-140 degrees are ideal for aluminum alloys, which reduces metal sticking.
- 🔩 For high-strength steel, use drills with titanium nitride coating (golden color), they last longer and remove heat better.
- 📏 Always check the diameter of the drill with a caliper before starting work, as the markings on the shank may be erased or not correspond to reality due to a manufacturing defect.
Particular attention should be paid to multilayer bags. If you are fastening several sheets of metal, the total thickness of which is large, it is important that the walls of the hole are even throughout the entire depth. The taper of the hole is unacceptable, since spacer The rivets may not expand evenly, weakening the connection.
When drilling holes for rivets in a painted body, be sure to use a center punch to prevent the drill bit from slipping and damaging the paintwork outside the joint area.
Correspondence table for rivet and drill diameters
To quickly navigate the dimensions, craftsmen use special tables. Below is data that is relevant for most standard situations during body repair and assembly of components. Please note that these values are guidelines and specific conditions (such as when using rivets with a larger head) may require adjustments.
| Nominal diameter of rivet (mm) | Designation (DIN/ISO) | Recommended drill diameter (mm) | Allowable range (mm) |
|---|---|---|---|
| 3.2 | M3 | 3.3 - 3.4 | 3.25 - 3.5 |
| 4.0 | M4 | 4.1 - 4.2 | 4.05 - 4.3 |
| 4.8 | M5 | 4.9 - 5.1 | 4.85 - 5.2 |
| 6.4 | M6 | 6.5 - 6.6 | 6.45 - 6.8 |
| 8.0 | M8 | 8.2 - 8.4 | 8.1 - 8.5 |
Using a drill from the "Acceptable Range" range is possible, but extreme values require increased care during installation. For example, a 3.5 mm hole for a 3.2 mm rivet may require more force to flare, which can result in thread stripping (in the case of threaded inserts) or premature breakage of the shank in conventional rivets.
Exact matching of the diameter of the drill and the body of the rivet (taking into account a tolerance of +0.1...0.3 mm) is the main factor ensuring a tight fit and no backlash in the connection.
Drilling technology and edge processing
The process of creating a hole begins with marking. The accuracy of the drilling points determines the geometry of the entire assembly. After punching, it is necessary to select the correct rotation speed of the cartridge. For small diameters (up to 4 mm), the speed should be high, and for diameters 6 mm or more, the speed should be reduced to avoid overheating of the drill and metal.
An extremely important step is deburring. After the drill passes through the metal, sharp protrusions remain at the edges of the hole. If they are not removed, rivet head will not fit tightly to the surface, forming a gap. Moisture will enter this gap, causing corrosion, and the connection itself will have less shear strength.
⚠️ Attention: When removing hangnails, do not overdo it. Chamfering at an angle of 90 degrees (countersinking) is only permissible for blind rivets. For standard ones with a semicircular head, countersinking will weaken the edge of the metal and can lead to cracks when riveting.
To process edges, it is convenient to use a tapered countersink drill or even a larger drill, turning it by hand. Mechanical processing should be carried out at low speeds so as not to remove excess metal and not to increase the diameter of the hole beyond the permissible limit.
☑️ Preparing the hole for the rivet
Common errors and ways to resolve them
One of the most common problems is a “walking” drill, which makes an oval instead of a round hole. This occurs when the workpiece is not firmly fixed. Vibration during drilling causes hole diameter It is different in its different sections, and the rivet fits in skewed.
Another mistake is trying to fix a hole that is too large by driving a larger rivet into it or using caulk as a filler. This is a gross violation of technology. Sealant does not provide shear strength, and oversized fasteners can split the bridge between the edge of the hole and the edge of the part.
- 🚫 Drilling without cooling when working with stainless steel leads to the release of the metal of the drill, it stops taking metal and begins to slide, enlarging the hole.
- 🚫 The use of rusty or oxidized rivets that are tight even in the correct hole often ends in the stem breaking before the head begins to form.
- 🚫 Ignoring the thickness of the package: if the hole is drilled at an angle (not 90 degrees), the rivet will experience bending loads and quickly fail.
If the hole still turns out to be larger than necessary, the only correct solution would be to drill it out for the next standard size of the rivet or install a repair sleeve. Trying to “conquer” a big gap by force means guaranteeing marriage.
What to do if the hole is too small?
If the rivet does not fit, do not try to hammer it in under any circumstances - this will deform the body and break the notches. It is necessary to carefully go through the hole with a caliber drill of the same diameter, but always manually or at very low speeds of the drill, controlling the process.
Specifics of working with hidden and reinforced rivets
Blind rivets require special precision. For them, it is not enough to simply drill a through hole - it is necessary to perform countersinking at an angle of 90 or 120 degrees (depending on the standard). The depth of countersinking must be strictly calculated: if you remove little metal, the head will stick out; if there is a lot, the cap will fail and the expansion will not form correctly.
Reinforced rivets (for example, type Huck or with an increased shoulder) are often used in areas subject to high vibrations, such as mounting bumpers, sills or frame elements. The holes for them must be made with a minimum tolerance; it is often recommended to use drills with an h9 tolerance or even do a pass calibration after drilling.
When working with blind rivets that have rubber seals (for sealed bodies), the diameter of the hole plays a role in the pressure of the seal. A hole that is too large will not allow the rubber cuff to be evenly distributed around the perimeter, and the seal will be compromised.
⚠️ Warning: When installing blind rivets into thin sheets (less than 1mm), countersinking may completely destroy the material around the hole. In such cases, use rivets with a flat countersunk head that do not require deep countersinking, or reinforce the fastening point with a washer.
Frequently asked questions (FAQ)
Can a wood drill bit be used for metal?
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, they quickly become dull, overheat and make a hole with torn, uncalibrated edges, which is unacceptable for high-quality riveting.
What to do if the rivet spins in the hole?
This is a sign that the hole diameter is too large or the rivet is defective (small collar). If the gap is small, you can try using a rivet with a larger head. If the hole is badly broken, it must be drilled to the next size or welded/sealed and drilled again.
Do I need to lubricate the rivet before installation?
Under normal conditions no lubrication is required. However, if you are using steel rivets in aluminum parts, it is recommended to use an insulating paste or lubricant to prevent galvanic corrosion. Lubrication may also be necessary if the hole is made to a minimum tolerance and the rivet is tight.
Which tool is better: hand or pneumatic?
For one-time jobs and hard-to-reach places, a manual riveter is suitable. For larger volumes and to ensure the consistent force required to properly form the head in a calibrated hole, it is better to use a pneumatic or cordless tool.