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

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Technical Context

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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Reading a Fastener Callout

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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Head Styles and When Each Is Right

Head	Recognize it by	Use when
Socket head cap (SHCS)	Cylindrical head, hex socket	Default for structure. Strong, high torque capacity
Button head	Domed low head, hex socket	Where clearance is tight or a snag hazard exists
Flat head (countersunk)	Conical underside	Where the head must sit flush with the surface
Set screw	No head at all	Locking a hub or collar onto a shaft

Prefer socket head cap screws for anything structural. Button heads have a shallower socket and strip more easily under torque, and flat heads rely on the countersink being cut accurately to seat correctly.

Screw length matters more than teams expect

Too short, and there is not enough thread engagement to carry load. Too long, and the screw bottoms out in a blind hole and stops pulling the joint tight, so the joint feels tight while carrying no clamping force at all. That failure is invisible during assembly and shows up as a joint that loosens in one match.

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The Two Holes, Again

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.

1BringThe thread size the joint uses.

2ChangeThe thread, and compare the four diameters drawn to scale.

3ReadHow close the tap drill and clearance sizes are. Mark every hole before drilling.

Tap Drill and Clearance Reference

Two different holes. Pick the wrong one and the joint is ruined.

ThreadM3 x 0.5M4 x 0.7M5 x 0.8M6 x 1.0#6-32 UNC#8-32 UNC#10-32 UNF1/4-20 UNC

M3 x 0.5 to scale

Tapped hole (smaller than the screw)Clearance hole (larger)

The tap drill and the clearance hole differ by only 0.70 mm, which is why they are so easily swapped. Mark every hole on the drawing as TAP or CL before anyone picks up a drill.

2.50 mmTap drill0.098 in, for a 75% thread

3.2 mmClose clearanceTight location, less adjustment

3.4 mmFree clearanceEasier assembly, some slop

2.50 mmMajor minus pitchThe rule of thumb, for comparison

Thread	Pitch	Tap drill	Close fit	Free fit
M3 x 0.5	0.50 mm	2.50 mm	3.2 mm	3.4 mm
M4 x 0.7	0.70 mm	3.30 mm	4.3 mm	4.5 mm
M5 x 0.8	0.80 mm	4.20 mm	5.3 mm	5.5 mm
M6 x 1.0	1.00 mm	5.00 mm	6.4 mm	6.6 mm
#6-32 UNC	0.79 mm	2.71 mm	3.8 mm	4.0 mm
#8-32 UNC	0.79 mm	3.45 mm	4.4 mm	4.6 mm
#10-32 UNF	0.79 mm	4.04 mm	5.1 mm	5.3 mm
1/4-20 UNC	1.27 mm	5.11 mm	6.7 mm	7.0 mm

The default FTC fastener. Used across REV and goBILDA hole patterns.

A tapped hole receives the threads and must be drilled small. A clearance hole lets the screw pass through freely and must be drilled large. Drilling a clearance hole where you needed a tapped hole cannot be undone, so mark which is which on the drawing before anyone picks up a drill.

Values are not saved. Nothing is sent anywhere.Open in the workbench

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How a Bolted Joint Actually Carries Load

A properly made bolted joint does not carry load on the screw. Tightening the screw stretches it slightly, and that stretch clamps the two parts together. The friction between the clamped faces carries the load.

Three consequences:

Clamping force is the point. A screw that is snug but not tight provides little clamping force, so the joint relies on the screw in shear, and the parts shift within the clearance hole under load. That shifting is what you feel as slop.

Two fasteners resist rotation, one does not. A single screw through a bracket lets the bracket pivot around it. Two screws spaced as far apart as the bracket allows resist rotation directly. When a bracket keeps going crooked, the answer is a second screw, not a tighter one.

Washers spread the load. Under a screw head on soft material, a washer prevents the head from digging in and losing clamping force as the material deforms. On polycarbonate or printed plastic, a washer is not optional.

Grade and material

Most FTC hardware is stainless or alloy steel. Stainless is corrosion resistant and softer, so it strips more easily; alloy steel socket head screws are stronger and better for high-load joints. Aluminum screws exist and are almost always a mistake for structural use.

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Photograph neededA stripped M3 tapped hole in thin aluminum, photographed beside a good one in the same plate for comparison.Framing: Macro, both holes in one frame, angled light so the thread crests catch the light.Add the file to static/ and pass its path as the src prop.

A stripped tapped hole next to an intact one. The stripped threads have been flattened into the bore, which is why the screw spins freely but pulls out under load.

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Fill-in-the-Blank Practice

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

Show answers

1. pitch
2. clamping
3. fastener (screw)

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Exercise

Find every single-fastener bracket on your robot. For each one, determine whether it can rotate about that fastener under load. Every one that can is a future alignment problem.

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