Some thoughts on the Mechanician

As written the DragonQuest rules provide little incentive for an adventurer to take up the Mechanician skill.  The brief description of the skill focuses largely on the making of traps, locks and safes.  Not things you might make outside of the Mechanician's workshop.  There are no abilities ascribed to the skill that might be regularly employed by the adventuring Mechanician.  Those devices described in the rules all require tools, materials and, in most cases, a workshop. The Mechanician's ability to create locks, traps and other mechanical devices should give them some insight into disabling or disarming the traditional pit and arrow traps found in dungeons but nothing is said of this in the rules.  The skill can definitely use some love in the form of more detailed abilities and some adventuring-targeted abilities.
The introduction for Mechanician has this to say:

A mechanician's most complex products will involve pulley-, hydraulic-, or spring-based motor systems, which cause the operation of well-greased moving parts. The mechanician is most often called on to devise locks and traps to foil the best efforts of thieves. His second most popular line includes mechanisms for domestic use (e.g., windmills).

I've never really thought of a windmill as being a domestic item but I guess the writers were thinking of machines used in support of the work-a-day activities of the general populace.  Under [56.5], domestic devices are mentioned again but the only examples are children's toys, mechanical scythes and pulley lifts.  Just from what few examples are provided in the rules it is apparent that Mechanicians will need knowledge of, if not be skilled in, a variety of disciplines including carpentry, blacksmithing, metallurgy, forging and casting. They will also need some understanding of basic physics, geometry and trigonometry.  In the construction of large projects, like a windmill, the Mechanician would hire other skilled craftsmen along with general laborers to do the actual work while the Mechanician would provide the design and supervision as mentioned in [56.5].
The rules allow for Mechanicians to remove - and presumably disarm - traps of their own construction.  The text doesn't say but given that the rules allow for the removal and storage of traps I have to assume that these traps are of a size such that may be concealed next to a lock within a door or chest.  The rules don't address the creation of larger, more involved traps such as a wall launching a barrage of darts or giant rolling balls of doom but I think such constructs would also fall within their skill set.  The cost and time required to produce these larger traps would be proportionately greater.

Adventuring Mechanicians

What would be the best way to give the Mechanician some adventuring chops without taking away from other skills?  Here are some of my thoughts on it.
Per [56.7] Mechanicians may open their own locks and safes but not those of others.  It stretches credibility a bit to think that a skilled locksmith wouldn't be able to open the locks and safes of others.  Perhaps they would be slower at it or have a lower chance of success than that of a thief.  Treating them as a Spy of the same Rank for purposes of opening locks and safes wouldn't be out of line in my opinion.
The same argument can be made for Detecting/Removing Traps and for Detecting Secret/Hidden Apertures.  I would limit these abilities to those Mechanicians that have taken the associated specialties (see below).
Much like the bonuses granted by Assassin, Spy & Thief for Stealth I would grant a bonus of 2% per Rank with Mechanician when using the Spy/Thief abilities under [61.2 - 4] assuming the corresponding Mechanician specialties were taken.
Near the end of the Mechanician section is the following:

[56.6] The GM may, at his discretion, allow a mechanician character to construct devices of use on adventures. Under no circumstances may a mechanician build post-Renaissance weaponry.

Though the wording seems to have been chosen to discourage GMs and players, limiting the constructed devices to that of the Renaissance and earlier actually allows for a very extensive collection of sophisticated machines.  Below I've listed some of the devices that had been invented in our own world by the end of the Renaissance and in many cases much earlier.  I split the devices into various categories and included a couple of ideas for Adventuring Devices.
A number of the DragonQuest skills allow for the selection of additional abilities (Courtesan, Troubadour, Alchemist) or specialties (Beast Master, Merchant, Ranger) as the character advances in Rank.  The Mechanician could be made more interesting with a similar setup.  At Rank 0 the Mechanician would be familiar with the six fundamental Simple Machines (see below) and other essential knowledge appropriate to the skill.  Lock Mechanisms might also be granted at Rank 0 and then other specialties could be chosen at some interval such as every even Rank or multiple of 3 Ranks.  Additional specialties after Rank 10 could be gained by spending 2500 experience points per specialty.

Inventing

While there are a number of machines and devices that would already exist and have been produced for centuries in a campaign with a Middle-Ages or Renaissance technological level, some, such as one-of-a-kind Automatons or cutting-edge (for the period) technology would need to be invented by the Mechanician.  The GM would be the arbiter of when a device would require the Mechanician to invent a new technology.
A system and rules could be devised, much like Spell Research from Arcane Wisdom, whereby Mechanicians would have to experiment and research those devices with time and cost being determined by complexity of the proposed device along with perhaps technological level.  I think that for the typical DragonQuest campaign that would probably be too much but perhaps for a steampunk DragonQuest campaign that level of complexity would make sense.
As a more general rule I think something along the lines of a GM chosen Difficulty Factor (0.5 - 5) applied to a formula like [Success % = (DF x Rank) + PC] would be sufficient. One D100 roll would be made per week. Success means that the device was created.  A roll greater than the success % but less than or equal to (success % + PC) means progress was made and some new insight was gained. Add the Mechanicians Rank to the success chance for subsequent rolls.  A roll greater than 90+Rank means the design is flawed and the Mechanician will have to start over.  Any other roll would mean no real progress was made but the Mechanician can continue for another week.
Optionally critical successes could be allowed for rolls less than 5% or 15% of the success chance.  If the dice roll is 5% or less of the success chance, one attribute of the device could be doubled (or halved) such as weight, speed, efficiency, or cost.  Or two attributes could improved by 50% or decreased by 25%.  For rolls above 5% of the success chance but less than or equal to 15% of the success chance, one attribute could improved by 50% or decreased by 25%.

Starting Specialties

As mentioned above, the Mechanician would start at Rank 0 with the knowledge of Simple Machines and their applications along with  Lock Mechanisms.

• Simple Machines - Lever, Wheel & Axle, Pulley, Inclined Plane, Wedge and Screw.
• Lock Mechanisms - padlocks (keyed, rotary dial, multiple cylinders), deadbolts, built-in locks for chests or other containers.

Other Specialties

It is up to the GM to determine what types of devices are common enough in his world such that the Mechanician can produce them without having to research or invent them first.

1. Traps (small) - locks, chests, doorknobs.
2. Traps (large) - trap door, arrow, blade/scythe, gas, etc.
3. Clockwork Mechanisms - operated by springs, water, sand, pendulums. Includes other timekeeping devices like sundials, candle clocks and the horologium nocturnum.
4. Automatons* - mechanical toys, musical instruments, animated figures, self-propelled wagons and carts.
5. Siege Engines - catapult, ballista, onagers, trebuchet, battering ram, siege tower.
6. Rotating/Lifting Devices - capstan, winch, crane, tread-wheel, treadmill, windmill.
7. Hydraulics/Pneumatics - including steam operated devices, pumps, siphons.
8. Navigational Instruments - sextant, quadrant, compass, cross-staff, traverse board.
9. Astrological Instruments - sextant, orrery, astrarium, astrolabe, armillary sphere.
10. Lens - telescopes, eyeglasses, magnifying glasses, projectors.
11. Printing devices - single plate (wood or ceramic), movable type (wood, ceramic, metal).
12. Scales - balance, spring.
13. Weather Devices - thermometer, barometer, anemometer, weathercock.
14. Surveying Instruments - groma, dioptra, theodolite, surveyor's square, pantometer.
15. Hidden Apertures - secret doors, false bottoms, hidden compartments, rotating bookcases.
16. Adventuring Devices - collapsible or folding grapple, lock picks, screw jacks for bending/spreading bars, auto-cocking/loading crossbows,  take-down bows & crossbows, pneumatic blowguns.
17. Enchanted Machines - machines/devices that operate by magic, devices to detect magic, devices that employ magic (a lantern with a light spell that is turned on/off by a switch), devices that store magic.

The GM will have to determine the costs of materials and whether or not any additional specialists (such as a Navigator, Astrologer or Magic Adept) would be needed to produce the devices listed above.
*Automatons will almost certainly require one or more of the other specialties such as a Clockwork Mechanisms, Hydraulics/Pneumatics or Enchanted Machines in order to be produced.