EMuT: 3d Printed Electronic MUltiTool (Power Bank - Mini Drill - Welder)

Today we have a lot of electronics objects in our home and sometimes they break and we replace them with new and powerful models. But in the complexity of these objects there are a lot of componentes that can be reused to create new objects more close to our needs.

In this instructables I show how to r euse a small dc motor, recovered by a laser printer, and a solder stylus to create a new small (pocket size) tool useful for small repairs when we are outdoor.

In particular this to ol (3D printed) is a battery bank, a drill and a welder with two containers for the drill bits and the solder tin in 95x70x35 mm size.

Let's enjoy its fabrication.

First I got myself a small and low cost battery bank, it is important find a good trade-off between the size of the battery and its capacitance. In this case I used a small cylinder shape battery (65x18mm) of 8800 mAh.

I unmounted the battery bank and recovered the battery and the charger circuit which will be mounted in the final tool.

As said I recovered a small dc motor from an older laser printer, from this I removed the encoder leaving only the motor. This motor is small size and small power, however when used as core of a drill and with small drill bits, it returns good perfomance on materials as wood or plastic with a low consuption of power.

The motor is controlled by a low cost, commercial driver (max output current 800mA), this choice allows, in future developments, to change the direction of the rotation (currently it is not expected) and mostly to limitate the current consuption of the motor. In fact in some case, e.g. when we are trying to drill some hard materials, the power consuption of the motor may grow quickly, damaging the battery, in this case, instead, the maximum consuption of current is limited by the motor driver and in case of failures the motor driver works as a fuse, preserving the battery.

The voltage value which commands the motor is given by a voltage partitor circuit (shown at the step 4) composed by a fixed value resistor and a variable one. In particular as variable resistor was chosen a pressure sensor FSR400. In this sensor the value the resistor is inversely proportional to the pressure in its area. Trivially when the force exerted by user on the sensor grows the voltage on the motor grows and this one rotates faster.

Summirizing, (follow the pictures to better understand):

1. Mount the FSR sensor on the "chuck_support" piece (I used a double-sided adhesive tape) and a silicone cap to uniform distribute the pressure on the surface on the sensor;
2. Put the bearing of the chuck inside the "chuck_support" piece;
3. Fix the chuck inside the "chuck_support" piece;
4. Mount the two magnets to the "motor_support" piece (I used the super glue);
5. Mount the motor inside the "motor_suppor" piece;
6. Connect the "motor_suppor" part with the "chuck_support" part, using four KA30x25 screws.
7. Join the shaft on the motor with the chuck via a grub screw;
8. Mount the two external brearings on the "motor_support" piece.

For the welder I reused a solder stylus, of a micro solder station, which has small size (3x55mm) and works with 12 Volts.

The stylus is inserted in a own support which can slide along the chassis of the tool, the right voltage for the welder is obtained by a dc-dc step-up converter, which takes the voltage from the battery and gives the desired voltage to the stylus.

The "solder_support" piece has inside the tool two positions: rest and work.
When the "solder_support" is in the work position there is a voltage on the resistor of the welder and it starts to heat, in the rest position instead the resistor is not connected to the circuit. In this manner the welder works only when it is in the working position.

Summirizing, (follow the pictures to better understand):

1. Unmount the stylus from its original container;

2. Place the NTC resistor close to the tip of the welder and fix the two piece with a shrinkable guaine or with a kapton tape;

3. Insert the welder in the "solder_support" piece;

4. Connect the two wires of the resistor of the welder with their supports.

The electronics of the tool is very simple, it is composed by a few of small analog components subdivided it three macro areas.

The first area is composed by one led, thres resistors, a diode (1N4001) and a transistor (BC547) and gives information about the charge of the battery. The working principle is simple, when the voltage of the battery goes under a threshold (about 3.7V) the voltage on the resistor R5 goes under the value needed to turn on the transistor and this last one turns off and so the led. In this manner the user always can check the charge of battery and in case turn off the device without damaging this last one.

In the second area, which is composed by five resistors, a led, an operational amplifiers (LM358), a transistor (BC547) and the ntc resistor, the led turns on when the welder reach a desired temperature.

I recovered the ntc resistor by an older extruder of a 3D printer, I do not know its name but I measured a value of 100 KOhm a 25°C and a value of about 400 Ohm at 300°C. So when the temperature grows on the ntc resistor, the value of the resistence in ohm of this one drops.

Two voltage partitors are connected to an operational amplifiers used as a comparator. When the value of the ntc resistor drops under the value of resistor R9, there is an higher voltage value on the positive input of the amplifier and its output goes high, turning on the transistor and so the led. In this manner the user can set (modifying the value of R9) and check if and when the welder reach the desired temperature.

In the last area the motor of the drill is managed, as described in the step 2, trivially a pressure sensor (FSR 400) is inserted in a voltage partitor, when receives a pressure the resistance of the sensor drops, in the same manner its voltage while the voltage on the resistor R1 grows as the input voltage of the motor driver. Summarizing an higher pressure on the sensor, gives an higher voltage value on the motor.

1 - The size of the holes:

Many times two or more parts are assembled by using screws. Depends by the needs can be used auto-tapping screws for plastic (KA) or metric (M) with their nuts.
The right sizing of the holes when you design they, sure, can improve the final results and reduce, any, problems during the assembly process. Choosed the kind of screw and its nominal size it is important sizing the hole in the design. The self-tapping screws are made to create a thread in the material ,e.g. plastic, when you screw them, in these cases normally the size of the holes should be a bit smaller with respect the nominal size of the screws. There is not a perfect rule about the size of the hole, which depends by the material, but a good rule is to multiply the nominal size of the screw for a coefficient of 0.85. So for example if you choose a screw KA25 which has a nominal size of 2.5mm a right hole should be 2.5*0.85=2.125mm, in this manner the size of hole should guarentees a good grip during the screw process.

A metric screw, instead,is used with its nut and a pass through hole. In this case during the assembly process the screw has to pass through the hole and this last one has not to resist to the screw. In this case a good rule should be to give at the hole the same size of the screw. But the 3D printers normally works with FDM (Fused Deposition Modeling) process which brings a little to crush the layer one to the other, so an hole of 3mm in the design becomes a little bit smaller, e.g. 2.8mm, in the final piece. A good rules is to design the hole a little bit bigger, there is not a perfect rule (depends by the material and the quality of the printer), but a good rules is to multiply the size of the hole for a coefficient which depends by the size of the hole. Or rather, for a M3 screw which has nominal size of 3mm a good hole could be 3.6mm with a coefficient of 1.2 while for a M6 screw which has nominal size of 6mm a good holes could be 6.5mm with a coefficient of 1.08.

While the size of the screw grows the coefficient decreases according to the resolution of the printer. Summarizing you has to design the size of the hole considering if it has to be passer-by or not and the resolution of your machine.

2 - How to print a circle:

Printing in 3D a perfect circle is never simple and sometimes it is impossible but respecting these two simple rules, of sure your circles become better.

• The plan where is the circle: if it is the same of the printing plan it will be possible avoid the effects due to the gravity. In fact, normally the 3D printers deposit the layers one on the other along their z axis, so, when are still hot, the layers are affected by the gravity. In the case of the circles which lie of the x-z or y-z plans the gravity can stretchs or crush them. While in the case of circles on the x-y plan the gravity has no effect of them. In the figures are shown two examples of 3d printed circles where gravity crushed the roundness of these ones.
• The exact roundness of the circle: this factor could seem not important for our applications but in many cases is fundamental. For example if you design a part which has to be inserted a bearing or has to contain a bearing its perfect roundness simplifies the coupling between the pieces and avoid the need to refine the pieces after the printing. The slicing process generates the full closed path which the extruder has to follow to print the piece layer after layer. In the case of a circle the path starts from a point and the then it moves in circular to the initial point, when the final point touches the initial one the extruder moves inside or outside this last circle and starts to print another one. The continuous overlapping of the final and initial points creates a small swell which preclude the exact roundness of the circles. The right mode to avoid this effect and to obtain a perfect roundness is to draw an imperfect circle. In fact moving the junction between the initial and final points (as shown in the figure) it is possible to move its effect inside the circle obtaining a perfect external roundness (e.g the piece goes inside a bearing), or outside the circle obtaining a perfect internal roundness (e.g the piece contains a bearing).

3 - Rounding the edges:

As said a 3D printing object is composed by different layers one on the others, and are attached using the adhesive ability of the printing material; when the total surface of the next layer is smaller with respect the previous one the adherence between the two layers decreases. For example, when you print in 3D a box, which has to contain something, there will be the last layer of the bottom of the box and the first layer of the lateral walls of the box, in this case the surface helpful to join the bottom of the box with its lateral walls is the base surface of the walls. If the walls are narrow this surface will be small and so the adherence between the layers which can detach. A good solution to avoid this problem is to reduce gradually the surface of contact between the layers (from the bottom to the walls) rounding the edges (see the figure), in this manner the reduction of the surface of contact between the previous layer and the next is small and the edge is more compact.

After you 3D printed and acquired all the pieces (at the bottom of the page, it is possible to download the stl files), we can start to mount the tool.

Summirizing, (follow the pictures to better understand):

1. Place in the "bottom_box" piece the magnets of the motor block, the magnets of the welder block (two of these are then connected to the circuit) and the four inserts used to close the tool;
2. Fix in the "bottom_box" piece the contacts of the battery;
3. Place the battery in the box;

4. Connect the contacts of the battery to the battery recharger circuit and insert this last one inside a non-conducting shrink;

5. Insert the switch inside the box and fix it (I used the hot glue);

6. Insert the "sprocket_piece" in the box using a 2x26 mm pin;

7. Insert the motor block;

8. Insert the welder block;

9. Connect the electronics boards: motor driver, step-up and a custom board which contains all the other components.

10. Clean the "Top_Box" piece and mount this on the "Bottom_Box" piece using four M3x16 screws;

11. Paste the letters of the text on the "Top_Box" piece (optional).