what size clearance hole for M3 — Sharp AI

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what size clearance hole for M3

You should use a 3.2 mm clearance hole for a standard M3 screw joint, but increase this to 3.4 mm if the screw must pass through a stack of multiple parts. According to Telemark's documentation on standard hole sizes, the 3.2 mm dimension provides a close fit that accurately locates the part, whereas the 3.4 mm dimension offers a free fit that prevents alignment issues across multiple stacked components .

This recommendation is based on the mechanical requirement that a fastener must have sufficient space to slide freely so the joint can clamp tightly. Telemark's lesson on clearance rules states that a screw requires at least 0.15 mm of clearance per side (0.3 mm on the diameter) to absorb manufacturing tolerances, drill variations, and minor misalignments; without this room, the screw binds in the hole, loading it in shear rather than tension and preventing the faces from clamping together .

Your choice of fabrication method will also affect the final hole size you should design. Telemark's guide on printed and laser-cut holes notes that FDM 3D-printed holes typically shrink by 0.1 mm to 0.3 mm on the diameter, meaning you must design them larger or ream them out after printing, while drilled aluminum holes naturally tend to run slightly oversized .

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1

The standard sizes

Telemark·Team owned

Memorise the M3 row. It is most of an FTC robot.

| Thread | Tap drill | Close clearance | Free clearance | | --- | --- | --- | --- | | M3 x 0.5 | 2.5 mm | 3.2 mm | 3.4 mm | | M4 x 0.7 | 3.3 mm | 4.3 mm | 4.5 mm | | M5 x 0.8 | 4.2 mm | 5.3 mm | 5.5 mm | | M6 x 1.0 | 5.0 mm | 6.4 mm | 6.6 mm |

Two rules generate the whole table:

tap drill      = major diameter - pitch
clearance hole = major diameter + 0.2 mm  (close)
                 major diameter + 0.4 mm  (free)

Close fit where the part must locate accurately and you can afford to fight it slightly during assembly. Free fit where the part is one of several in a stack, or where a slot would have been better anyway.

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2

The one rule behind all of it

Telemark·Team owned

A screw needs room to pass. The minimum clearance is 0.15 mm per side, so 0.3 mm on the diameter. Under that, the joint stops behaving like a clamped joint and starts behaving like a pin in a hole.

That number is not arbitrary. It has to absorb four things at once:

| What it absorbs | Typical size | | --- | --- | | Drill cutting oversize or undersize | 0.05 to 0.1 mm | | Position error between two parts | 0.1 mm or worse across a stack | | Thread and head runout on the fastener | small, but real | | Paint, anodising, burrs, and swarf | small, and always there |

Add those and 0.3 mm on the diameter is the honest floor, not a generous allowance.

An M3 screw through a 3.0 mm hole binds. The parts cannot pull together, so tightening loads the screw in shear instead of clamping the faces, and the joint develops slop the first time it is hit. It also cannot be assembled without persuasion, which is how holes get reamed out with a drill bit at a competition.

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3

Fill-in-the-Blank Practice

Telemark·Team owned

The minimum clearance for a screw is __________ mm per side.
The M3 clearance hole is __________ mm and the M3 tap drill is 2.5 mm.
When three or more holes form a pattern, make one hole locate and __________ the rest.

Show answers

0.15 (0.3 mm on the diameter)
3.2 (close fit; 3.4 mm free)
slot

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4

Exercise

Telemark·Team owned

Measure the last three holes your team drilled for M3 screws. Record each diameter. Anything under 3.2 mm is a joint that cannot clamp properly, and anything over about 3.6 mm is a joint that will not locate.

5

Hole

Telemark·Team owned

The hole feature places a properly specified hole rather than cutting a circle and hoping.

This matters because it carries the specification with it: clearance or tap size, counterbore or countersink, depth, and thread. That connects directly to Lesson 4.1, and it is why a hole feature beats an extruded circle.

The mistake that costs a build day: a 3 mm hole for an M3 screw. An M3 screw is 3 mm across, so it will not fit a 3 mm hole. A clearance hole for M3 is 3.2 to 3.4 mm; a tapped hole is 2.5 mm before threading.

| Screw | Clearance hole | Tap drill | | --- | --- | --- | | M3 | 3.2–3.4 mm | 2.5 mm | | M4 | 4.3–4.5 mm | 3.3 mm | | M5 | 5.3–5.5 mm | 4.2 mm |

An extruded circle records only a diameter. A hole feature records the intent, shows in the drawing correctly, and can be changed from clearance to tapped in one edit. Every hole on a part someone else has to manufacture should be a hole feature.

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6

The Two Holes, Again

Telemark·Team owned

This was covered in Lesson 2.4 and is repeated here because it is the most common single mistake in FTC fabrication.

The part the screw passes through gets a clearance hole, larger than the screw.
The part the screw threads into gets a tapped hole, drilled at the tap drill size, which is smaller than the screw.

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7

Two Holes That Look Similar and Are Not

Telemark·Team owned

Every screwed joint has two holes, and they are different sizes.

The clearance hole is in the part the screw passes through. It is larger than the screw so the screw slides freely. If it is too tight, the parts cannot pull together and the joint depends on the screw shank instead of the clamping force.

The tapped hole is in the part the screw threads into. It is smaller than the screw, because the tap needs material to cut threads out of.

For M3, the clearance hole is 3.2 mm and the tapped hole is drilled at 2.5 mm. Those numbers are 0.7 mm apart and they are not interchangeable. Drilling 3.2 where you needed 2.5 means the tap has nothing to cut and the hole is scrap.

On a sketch or CAD drawing, annotate every hole as either "CL" for clearance or "TAP" with the thread size. This single habit eliminates most ruined parts.

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8

Overview

Telemark·Team owned

Lesson 4.1: Metric Fasteners, Thread Callouts, and Hole Sizing

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An FTC robot is held together by a few hundred screws, nearly all of them M3 or M4. Choosing them correctly is not glamorous, and choosing them incorrectly is the reason a subsystem falls off in the second qualification match.

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9

II. Review the Drawing

Telemark·Team owned

A student submits this note with a part. Identify the 3 standards violated.

Bracket, 1/8 in aluminium.

- Four M3 holes at 3.0 mm, all located to nominal, no slots.
- Mounts to the chassis with one M3 screw through the middle hole.
- The M3 holes on the far face are tapped directly into the 1/8 in plate.

Show answers

1. The clearance holes are 3.0 mm. That is the screw's own diameter, leaving zero clearance against the 0.15 mm per side standard. The screws bind, the joint cannot clamp, and the part will need reaming to assemble.

Fix: 3.2 mm close fit, or 3.4 mm free fit in a stack.

2. Four holes located to nominal with no slots. Position error accumulates across a four hole pattern, so all four cannot be held simultaneously. One hole should locate and the rest should be slotted.

Fix: one close-fit locating hole, three slots with a few
millimetres of adjustment.

3. One screw to the chassis, and tapped threads in 1/8 in plate. A single fastener is a pivot rather than a joint, and 3.175 mm of engagement is below the 4.5 mm standard for M3 in aluminium.

Fix: two fasteners spaced apart. Replace the tapped holes with
through holes and nuts, or tap into a thicker section.

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10

Reading a Fastener Callout

Telemark·Team owned

A complete callout looks like this:

M3 x 0.5 x 12 SHCS
 |    |     |   |
 |    |     |   +--- head style: socket head cap screw
 |    |     +------- length in mm, under the head
 |    +------------- thread pitch in mm
 +------------------ nominal major diameter in mm

Most common metric sizes have one standard coarse pitch, so it is usually written "M3 x 12" with the pitch implied. The pitch matters when you are tapping, which is why Module 2 covered it.

Length is measured under the head for most heads, and overall for flat countersunk heads, because a countersunk head sits inside the material. Ordering 12 mm flat head when you wanted 12 mm of shank gets you a screw that is short.

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11

Printed and laser cut holes

Telemark·Team owned

The 0.15 mm rule is about the fastener. The process adds its own error on top.

FDM printed holes come out undersize, typically 0.1 to 0.3 mm on the diameter. Design them oversize deliberately, or drill after printing.
Laser cut polycarbonate moves with the kerf and the heat affected edge. Cut a test coupon before committing a plate.
Drilled aluminium comes out oversize, so a drilled hole is already closer to clearance than the drill size suggests.

Never assume the nominal. Measure one, then design against what your shop actually produces.

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12

Fill-in-the-Blank Practice

Telemark·Team owned

Negative clearance, where the hole is smaller than the shaft, is called __________.
A drilled hole typically comes out __________ its nominal size, which is why a precision bore should be reamed.
FDM printed holes typically come out __________, so they should be designed oversize or drilled after printing.

Show answers

interference
oversize (larger than)
undersize (smaller than)

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13

Fill-in-the-Blank Practice

Telemark·Team owned

In the callout M4 x 0.7 x 16, the number 0.7 is the thread __________.
A bolted joint carries load primarily through the friction generated by __________ force, not through the screw itself.
When a bracket keeps rotating out of position, the correct fix is usually a second __________ rather than more torque.

Show answers

pitch
clamping
fastener (screw)

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14

Overview

Telemark·Team owned

Lesson 12.1: Clearance, Tap Drills, and the 0.15 mm Rule

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Everything else in this track asks you to decide something. This module is the part that is already decided.

A team that re-derives hole sizes every time gets them wrong eventually, usually at 11pm in week seven. These numbers go on the wall.

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15

Thin Material and Thread Engagement

Telemark·Team owned

Thread strength comes from the length of thread engaged, and a general rule is that you want at least 1.5 times the screw diameter of engaged thread in aluminum. An M3 screw wants at least 4.5 mm of tapped depth.

That means 1/8 in aluminum plate, which is 3.175 mm, gives less engagement than the rule wants. Options:

Use a nut or a nutplate on the back instead of tapping
Tap into the thicker web of an extrusion rather than the sheet face
Use a threaded insert
Accept the reduced strength deliberately for a low-load joint, and write down that you did

Plastic threads strip under load and creep over time. Use a through hole with a nut and a washer.

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16

Exercise

Telemark·Team owned

In a piece of scrap aluminum at least 6 mm thick, drill and tap two M3 holes: one at the correct 2.5 mm tap drill, and one at 2.7 mm. Thread a screw into each and compare how they feel and how much torque they take before stripping. Record the difference.