What is this lesson about

Lesson 4.1 from Telemark focuses on selecting and using metric fasteners, specifically M3 and M4 screws, to build reliable structural joints on an FTC robot . The lesson teaches students how to interpret a standard metric fastener callout—which defines the screw's major diameter, thread pitch, length, and head style—and how to choose the correct head style, such as socket head cap screws for structural strength or button heads for tight clearances . It also reinforces the fundamental distinction between clearance holes that a screw passes through and tapped holes that the screw threads into, while warning against using screws that are too short to engage or too long to tighten fully .

Furthermore, the lesson explains that bolted joints carry loads through clamping force and friction rather than the screw itself, meaning that loose screws lead to joint slop . To prevent brackets from rotating out of alignment, the lesson advises using at least two spaced-apart fasteners rather than over-tightening a single screw, and recommends using washers to distribute the clamping force on softer materials like plastic . Students apply these concepts through exercises designed to find and eliminate single-fastener pivot points on their robots .

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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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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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Exercise

Telemark·Team owned

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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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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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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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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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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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

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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The standard that is not a number

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Every fastener that matters gets a witness mark. A paint pen line across the head and onto the part, so a pit crew can check thirty of them visually in under a minute instead of feeling each one with a driver.

If a marked fastener keeps moving, the fix is structural rather than chemical: the joint is flexing, and threadlocker only delays it. That is [Lesson 4.2](/mechanical/module-04/thread-locking).

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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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Overview

Telemark·Team owned

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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Reading Metric and Imperial Without Mixing Them

Telemark·Team owned

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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Exercise

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Walk your robot and count the single-fastener brackets. Every one of them is a future alignment problem. Fix the two that carry the most load this week.

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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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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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