why is it 3.2 and not 3.0 — Sharp AI

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why is it 3.2 and not 3.0

You use a 3.2 mm hole instead of 3.0 mm because a screw needs a minimum clearance of 0.15 mm per side (0.3 mm on the diameter) to prevent binding and allow the joint to clamp properly. As shown in this lesson's standard sizes table, this 3.2 mm close clearance hole gives the 3.0 mm major diameter of an M3 screw the room it needs to pass through freely .

This extra space is the absolute floor required to absorb real-world tolerances. This lesson on hole standards notes that this clearance absorbs drill cutting errors, position errors when stacking parts, fastener runout, and surface obstructions like paint, anodizing, burrs, or swarf .

If you use a 3.0 mm hole, the screw will bind in the hole instead of passing through it. This lesson on tapping and threads explains that when a clearance hole is too tight, the parts cannot pull together, which loads the screw in shear rather than clamping the faces together and causes the joint to develop slop the first time it takes an impact .

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1

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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2

Overview

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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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3

The standard sizes

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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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4

Exercise

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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

Fill-in-the-Blank Practice

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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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6

When 0.3 mm is not enough

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The floor assumes one hole through one part. Add parts and the position errors stack, as in [Lesson 4.4](/mechanical/module-04/spacers-and-stackups).

Two plates drilled separately: use free clearance, or clamp and drill them together.
Three or more holes in a pattern: make one hole close fit to locate, and slot the rest. Trying to hold position on four holes at once is how a part ends up unusable.
A part you cannot re-make: slot it. A slot costs nothing at design time and saves the part.

Designing every hole round and then filing one out is the amateur version of the same decision. Deciding in CAD which hole locates and which one adjusts takes thirty seconds and produces a part that assembles.

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7

Printed and laser cut holes

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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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8

Two Holes That Look Similar and Are Not

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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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9

The Tap Drill Rule

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The tap drill for a standard thread is approximately:

tap drill diameter = major diameter - pitch

For M3 x 0.5: 3 - 0.5 = 2.5 mm. For M4 x 0.7: 4 - 0.7 = 3.3 mm. For M5 x 0.8: 5 - 0.8 = 4.2 mm.

That produces roughly 75% thread engagement, which is the standard target. Going to 100% engagement gains almost no strength and dramatically increases tapping torque, which is how taps break.

Use the reference below to get both holes for the thread you are using.

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10

Hole

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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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11

Overview

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Lesson 2.4: Tapping Holes and Choosing the Correct Tap Drill

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Tapping cuts threads into a hole so a screw can bite directly into the part, with no nut on the far side. It is how a motor mounts to a plate you cannot reach behind, and it is how most bracket-to-extrusion joints work.

It is also the operation where a two second mistake ruins a part that took an hour to make, because a tapped hole drilled at clearance size cannot be recovered.

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12

Thin Material and Thread Engagement

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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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13

The Two Holes, Again

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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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14

Fill-in-the-Blank Practice

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The tap drill diameter is approximately the major diameter minus the __________.
Reversing the tap a quarter turn periodically serves to break the __________.
A general guideline for thread engagement in aluminum is at least __________ times the screw diameter.

Show answers

pitch
chip
1.5

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15

Exercise

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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.

16

II. Review the Drawing

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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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