This lesson on metric fasteners and hole sizing is all about making sure your robot does not fall apart on the field. As Game Manual 0 points out, most of our robot is held together by M3 or M4 screws, and if you get these details wrong, your subsystems will fall off or go crooked during a match 3, 8. The core takeaway from this lesson is that a bolted joint does not hold things together by having the screw block movement directly; instead, tightening the screw stretches it to clamp the parts together, and the friction from that clamping force is what actually carries the load 2, 7. To make this work, you have to get the holes right. This lesson emphasizes that every joint needs two different holes: a clearance hole in the first part so the screw slides through freely, and a smaller tapped hole in the second part for the threads to bite into 1, 9. If you drill a clearance hole where you meant to tap, the tap will not have any metal to cut and you will ruin the part 9. You also need to make sure you use at least two fasteners spaced apart for any bracket, because a single screw just acts as a pivot and will let the bracket rotate out of alignment no matter how hard you torque it 4, 7. Finally, this lesson teaches you how to read a fastener callout like M3 x 0.5 x 12 so you can grab the right hardware 5. You will learn to choose the right head style—like using strong socket head cap screws for structure and button heads only when clearance is tight—and how to pick the right length 6. If your screw is too short, it will not engage enough threads and will strip; if it is too long, it will bottom out in the hole, making the joint feel tight when it is not actually clamping anything at all 6.

explain this lesson

This lesson on metric fasteners and hole sizing is all about making sure your robot does not fall apart on the field. As Game Manual 0 points out, most of our robot is held together by M3 or M4 screws, and if you get these details wrong, your subsystems will fall off or go crooked during a match . The core takeaway from this lesson is that a bolted joint does not hold things together by having the screw block movement directly; instead, tightening the screw stretches it to clamp the parts together, and the friction from that clamping force is what actually carries the load .

To make this work, you have to get the holes right. This lesson emphasizes that every joint needs two different holes: a clearance hole in the first part so the screw slides through freely, and a smaller tapped hole in the second part for the threads to bite into . If you drill a clearance hole where you meant to tap, the tap will not have any metal to cut and you will ruin the part . You also need to make sure you use at least two fasteners spaced apart for any bracket, because a single screw just acts as a pivot and will let the bracket rotate out of alignment no matter how hard you torque it .

Finally, this lesson teaches you how to read a fastener callout like M3 x 0.5 x 12 so you can grab the right hardware . You will learn to choose the right head style—like using strong socket head cap screws for structure and button heads only when clearance is tight—and how to pick the right length . If your screw is too short, it will not engage enough threads and will strip; if it is too long, it will bottom out in the hole, making the joint feel tight when it is not actually clamping anything at all .

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Sources16

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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Fill-in-the-Blank Practice

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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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Overview

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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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Exercise

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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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Reading a Fastener Callout

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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

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| 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.

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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How a Bolted Joint Actually Carries Load

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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.

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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Fastener Guide > Threaded Fasteners > Thread Standards and Naming > Metric (ISO)

Game Manual 0·CC BY-NC 4.0

The ISO metric thread standard is commonly used worldwide, and for many products sold by FTC vendors.

A metric thread is designated by the letter M followed by the outer diameter in millimeters, for example M4 for a 4 mm major diameter bolt. This is then followed by a "x" and the pitch, also in millimeters. For example, "M4X0.7" means a M4 thread with a 0.7 mm pitch.

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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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Fastener Guide > Threaded Fasteners > Thread Standards and Naming

Game Manual 0·CC BY-NC 4.0

There are two main systems of naming and sizing threads that you will encounter in FTC: UTS imperial sizing and ISO metric sizing.

The universal thread standard (UTS) is based on the imperial system and commonly used to size threads in the US and Canada. A UTS thread is named by a number indicating the major diameter of the thread, followed by the thread's pitch measured in threads per inch.

For major diameters of " or greater, first number is simply the major diameter in inches. For those smaller than ", the following code is used:

===== ===== no. Major Diameter (inches) ===== ===== #0 0.0600 #1 0.0730 #2 0.0860 #3 0.0990 #4 0.1120 #5 0.1250 #6 0.1380 #8 0.1640 #10 0.1900 #12 0.2160 ===== =====

Some common imperial thread sizes in FTC include -20 (one quarter inch major diameter, 20 threads per inch) and #6-32 (0.138 inch major diameter, 32 threads per inch)

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Reading Metric and Imperial Without Mixing Them

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FTC build systems mix units freely: goBILDA and REV use metric fasteners on hole patterns that may be spaced in millimeters, while much stock is sold in imperial sizes, and field dimensions are imperial.

Two habits prevent almost all unit errors:

Write the unit every time. "3.2" in a notebook is meaningless. "3.2 mm" is a measurement.
Do not convert unless you must. Convert once, at the boundary, and write both. Repeated conversion is where rounding error accumulates.

The conversions you will use constantly:

1 in = 25.4 mm exactly
1 mm = 0.03937 in
1/8 in = 3.175 mm
M3 clearance = 3.2 mm = 0.126 in

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Fill-in-the-Blank Practice

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Minimum thread engagement in aluminium is __________ times the screw diameter.
A bracket needs at least __________ fasteners, spaced apart.
A set screw on a round shaft should bear on a machined __________.

Show answers

1.5
two
flat

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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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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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Overview

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Lesson 12.2: Thread Engagement, Fastener Count, and Retention

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A joint either clamps or it does not. These are the four numbers that decide which, and none of them are worth re-arguing in week seven.

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| Standard | Value | Why | | --- | --- | --- | | Thread engagement in aluminium | at least 1.5 x diameter | Below it the threads strip before the screw yields | | Thread engagement in steel | 1.0 x diameter | Steel threads carry more per turn | | Fasteners per bracket | two, spaced apart | One fastener is a pivot, not a joint | | Screw protrusion past a nut | at least 1.5 threads | Proves full nut engagement | | Washer under a head on plastic | always | Spreads load so the head cannot sink |

For M3 that means 4.5 mm of tapped depth minimum. An M3 tapped into 1/8 in plate, which is 3.175 mm, is already below standard: use a nut, an insert, or a thicker section.

A bracket held by a single screw rotates about that screw. Every load cycle works it a little further out of position, and no amount of torque fixes it because torque was never the problem. Two fasteners spaced as far apart as the bracket allows is the standard, and it costs one screw.

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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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