help me with the practice problems — Sharp AI

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help me with the practice problems

Which of the three questions are you working on first, and what do you have down so far?

If you are stuck on the first one, take a look at the section on the one rule behind all of it—it explains exactly how much room a screw needs on each side so it doesn't bind up and act like a pin.

For the second one, there is a handy table of standard sizes right in the middle of this lesson; check the M3 row for the close clearance hole size.

For the third one, look at the section about what to do when 0.3 mm is not enough, which talks about how to handle patterns with three or more holes so you don't end up with an unusable part.

Let me know what you think, and we can check your answers together!

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1

Overview

Telemark·Team owned

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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2

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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3

The standard sizes

Telemark·Team owned

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

When 0.3 mm is not enough

Telemark·Team owned

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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5

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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6

Fill-in-the-Blank Practice

Telemark·Team owned

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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7

Exercise

Telemark·Team owned

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.

8

Fill-in-the-Blank Practice

Telemark·Team owned

During the generating block of a brainstorm, evaluation of ideas should be __________.
Describing the problem as a function rather than a mechanism keeps the team from committing to one __________ too early.
Concepts that violate a hard constraint should be removed during the __________ step, before any weighted comparison.

Show answers

deferred (postponed to the judging block)
solution (mechanism or family of mechanisms)
screening (the constraint screen)

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9

Fill-in-the-Blank Practice

Telemark·Team owned

The four parts of a standard entry are Problem, What We Tried, Result, and __________.
Recording that a mechanism "worked better" is insufficient because it is an opinion rather than __________.
Determining whether a mechanism is reliable enough to compete with requires at least __________ trials.

Show answers

Next (the decision or next step)
data (a number with a trial count)
25 (or more)

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10

Fill-in-the-Blank Practice

Telemark·Team owned

Design reviews are most valuable at transitions where the cost of change __________.
Naming one person as the explicit __________ makes finding problems a duty rather than a social risk.
The output of a review is a written list where every item has a description, an __________, and a date.

Show answers

jumps (increases)
skeptic (critic)
owner

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11

Fill-in-the-Blank Practice

Telemark·Team owned

Drawing the mechanism at both its retracted and extended positions catches problems that are invisible in the __________ of the travel.
Hatching or a ground symbol on a part tells the reader that the part is __________.
Including the game element in the sketch at roughly correct scale exposes __________ problems that a mechanism-only drawing hides.

Show answers

middle
fixed (anchored, does not move)
geometry (interaction, clearance, or reach problems)

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12

Fill-in-the-Blank Practice

Telemark·Team owned

Hardware lookup strings in the get() method are __________-sensitive, meaning "Motor" and "motor" are different configuration names.
If a motor variable is not successfully mapped, it retains a value of __________.
To avoid mismatch errors across multiple files, programmers store configuration names as static final __________ variables in a shared class.

Show answers

case
null
String

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13

Fill-in-the-Blank Practice

Telemark·Team owned

Every rotating shaft is supported in __________ places.
Wrap on the small pulley should be at least __________ degrees.
A cantilevered load should be no longer than the __________ between the bearings.

Show answers

two
120
spacing

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14

Fill-in-the-Blank Practice

Telemark·Team owned

A named value defined once and reused throughout a model is a __________.
Entering #plateThickness - 1 mm in a depth field rather than a typed number records the __________ between the two dimensions.
In Onshape a length variable must carry its __________, so 15 mm rather than 15.
Making a pattern's instance count follow the length of a rail requires an __________ rather than a typed count.

Show answers

variable
relationship
units
equation

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Graded exercises with hard numbers and self-checks are in [CAD Practice Exercises](/mechanical/cad-practice).

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15

Fill-in-the-Blank Practice

Telemark·Team owned

Aim for a static safety factor of at least __________.
Sustained motor load should stay under __________ percent of stall torque.
Reserve about __________ percent of the weight budget for what nobody predicted.

Show answers

2 (two times)
60
10

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16

Fill-in-the-Blank Practice

Telemark·Team owned

A notebook entry describing a test should contain numbers rather than __________.
Judges consider a notebook that records only successes to be less credible because it lacks documented __________.
Every photograph in the notebook needs a __________ explaining what it shows and why it is there.

Show answers

adjectives (subjective descriptions such as "fast" or "reliable")
failures (or iteration, problems encountered)
caption

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17

Fill-in-the-Blank Practice

Telemark·Team owned

The robot must be measured in its __________ configuration, with every mechanism stowed as it will be at match start.
The full inspection checklist should be run twice: once in the shop and once in the __________ before queueing.
Running a hand carefully over every exposed surface finds __________ edges and snag hazards.

Show answers

starting
pit
sharp

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18

Fill-in-the-Blank Practice

Telemark·Team owned

Before building a prototype, the team should write down the single __________ it is meant to answer.
A prototype should be built from the cheapest materials that answer the question, and should be __________ so values can be swept in one session.
Prototypes must be tested with the real game element because weight, texture, and how it __________ all affect the result.

Show answers

question
adjustable
tumbles (behaves, moves)

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19

Fill-in-the-Blank Practice

Telemark·Team owned

A pivot joint should be modeled with a __________ mate so the mechanism can be driven through its full travel.
Interference must be checked throughout the range of motion, not only at the __________.
The center of gravity should be evaluated in the worst case position, which for an arm robot means fully __________ and loaded.

Show answers

revolute
extremes (endpoints)
extended

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Graded exercises with hard numbers and self-checks are in [CAD Practice Exercises](/mechanical/cad-practice).

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20

Fill-in-the-Blank Practice

Telemark·Team owned

A break at a sharp internal corner with no deformation suggests a stress __________.
The why chain should continue until the answer is a design decision the team can __________.
A failure caused by a part that has a finite life, such as surgical tubing, is a __________ failure and calls for a replacement schedule rather than a redesign.

Show answers

concentration
change (control)
wear

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21

Fill-in-the-Blank Practice

Telemark·Team owned

An extrude end condition of __________ __________ follows the geometry it stops at when that geometry moves.
A revolve sketch must lie entirely on one side of the __________.
A __________ requires both a profile sketch and a separate path sketch.
Blending between two different cross-sections is done with a __________.

Show answers

up to (face)
axis
sweep
loft

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Graded exercises with hard numbers and self-checks are in [CAD Practice Exercises](/mechanical/cad-practice).

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22

Fill-in-the-Blank Practice

Telemark·Team owned

A __________ on an internal corner reduces the stress concentration that causes brackets to crack.
Hollowing a solid to a set wall thickness while removing chosen faces is done with __________.
Angling a face so a part releases from a mould, or so an overhang prints without support, is called __________.
A thin web that stiffens a part along the line where it bends is a __________.
The clearance hole for an M3 screw is about __________ mm, not 3.0 mm.

Show answers

fillet
shell
draft
rib
3.2 (3.2 to 3.4 is fine)

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Graded exercises with hard numbers and self-checks are in [CAD Practice Exercises](/mechanical/cad-practice).

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23

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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