walk me through this lesson

This lesson is designed to keep your robot from falling apart on the field by teaching you how to select, size, and install metric fasteners correctly. As Telemark's overview highlights, an FTC robot relies on hundreds of M3 and M4 screws, and choosing the wrong ones is a primary reason subsystems fail during matches .

The most critical concept to master is how a bolted joint actually holds together. Telemark's guide on bolted joints explains that a screw does not carry the load directly; instead, tightening the screw stretches it to clamp the parts together, relying on friction to do the heavy lifting. To prevent parts from shifting or rotating, you must use at least two spaced-apart fasteners rather than over-tightening a single one, and always use washers on soft materials like plastic to keep the screw head from sinking in .

To make these joints work, you must never mix up the two types of holes required. Telemark's explanation of hole sizing emphasizes that the first part the screw passes through must have a larger clearance hole so it slides freely, while the part the screw threads into must have a smaller tapped hole drilled to the specific tap drill size .

When selecting your hardware, you need to know how to read a standard metric callout like "M3 x 0.5 x 12 SHCS". Telemark's guide to fastener callouts and Game Manual 0's fastener guide explain that this represents a 3 mm major diameter, a 0.5 mm thread pitch, a 12 mm length, and a socket head cap screw style. Remember that screw length is measured under the head for standard screws, but includes the head for flat countersunk screws .

Finally, you must choose the right head style and length for the job. Telemark's breakdown of head styles recommends socket head cap screws as your structural default because they are strong and resist stripping, whereas button heads should only be used for tight clearances. Make sure your screws are long enough for proper thread engagement but not so long that they bottom out in blind holes, which leaves the joint completely loose .

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Sources16

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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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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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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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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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

Telemark·Team owned

| 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

Telemark·Team owned

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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Exercise

Telemark·Team owned

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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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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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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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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Fill-in-the-Blank Practice

Telemark·Team owned

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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Fill-in-the-Blank Practice

Telemark·Team owned

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

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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The Tap Drill Rule

Telemark·Team owned

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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