CARP Box Buyers Guide
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- 1 Full Kit
- 2 Building Our Own Setup
- 3 Recommendation
CARP Box is not commercially available at the moment. It should not be, as I do not have the production capacity and it is published under CC-BY-NC-SA license. It was meant to be an universal basic set-up which you can customize and build on your own, so you can achieve the best price and features that suites your needs. The total price of required material for dual head version is way under 350 EUR (400 USD) when you use parts imported from China with free shipping.
Building Our Own Setup
As with other DIY 3D printers, construction requires some basic skills like drilling, fastening, soldering, hooking up electronics, measuring with calliper and multimeter, editing and uploading firmware.
To build the a complete printer from scratch requires approximately 8 to 12 hours so it can be done during a weekend.
Most parts required for the construction are made of laser cut aluminium sheets. Other materials, like steel or plastic may be used too, but have its disadvantages for which I would rather recommend to avoid them.
My parts were ordered on internet from company that have 3 and 4 mm thick aluminium sheets available on stock and ships all over my country. I think that with a bit of googling you may find similar workshop in your neighbourhood.
The easiest and cheapest way to get hands on electronics like, the controller board, drivers, thermistor, endstops, heatbed, display and even fully assembled hotend and LED power supplies is to order it from aliexpress or similar e-shop. Just check what duty and VAT will be added to the total sum to avoid unpleasant surprises. Eventually you may divide the order to smaller parts that will fit under certain limit.
Wires, Sockets, Plugs, Switches
Well equipped local store should be the easiest way to go. If not, I am sure there are plenty of internet shops with required stuff all over the world.
Fasteners, Rods, Bearings
Most of the screws nuts, smooth and threaded rods are common stuff in local hardware store. If you will find difficult to obtain for eg. bearings, they may still be found on internet.
Belts, Pulleys, Z Joints
Again another set of parts that are the best to be ordered from China.
For stepper motors you should rather check if local shop near you is not cheaper than aliexpress as they are usually not shipped with free shipping.
From the most part you can get the gears in the same workshop that would make the aluminium frame. With one exception. Gears are best to be made of stainless steel 1mm thick. Worm gear is machined from a common M12x25 screw, though it may be difficult to find a company that would be eager to do such small job. Due to high precision required I do not recommend to drill it at home, unless you have a turning lathe.
Before you will purchase any parts, there are few thing to consider. First of all, if you plan to print quickly of precisely. Unfortunately you cannot have both. At least not with common or discount parts.
The major problem is in widely used electronics based on Atmel chips. Even the quickest ones cannot do more than 40000 microsteps per second in each axis. If you use 1.8° stepper motors (200 steps per turn) and drivers with Alegro A4988 chips, that can do 1/16 microstepping, than you can achieve 12.5 turns per second on each stepper motor. If you use for eg. GT2 pulleys with 20 teeth, than each turn of motor represents 40mm distance, so the maximal speed you can get is 12.5*40=500mm/s. That is impressive, but as you have to use acceleration/deceleration and print area of CARP Box is only 200x200mm, speeds above 250mm/s are hardly reachable. Still that is very nice for travelling speeds, when not printing. With M5 threaded rods in Z Axis if the top speed 12.5*0.8=10mm/s. This is not so good and thanks to the fact two stepper motors are connected to one driver, real speed will be lower. Contrary, if you use 0.9° stepper motors (400 steps per turn) and Texas Instruments DRV8825 driver with 1/32 microstepping, With the same 40000 microsteps per second limit you can reach only 3.125 turns per second. Again with GT2 pulley with 20 teeth 3.125*40=125mm/s. You will get 4 times better precision but also 4 times lower speeds. 125mm/s won't hurt you much, when you are printing, but for travelling speeds it is a bit low. M3 threaded rod in Z axis will reach 3.125*0.8=2.5mm/s at best. That is still usable when printing, but homing and travelling is truly annoying.
Voltage and Current
The typical set up is with 12V PSU, heatbed, ceramics cartridge in extruder and fans used in computers. You may use power supply from computer or made for LED lights. Second option is better as it is smaller and you don't have to deal with 12V branches. They will also allow you to use slightly higher voltage and current that what they are designed for. Typically you need 12V/20A = 240W for printer with MK2b heatbed and one or two extruders with ceramics cartridges. But I would rather recommend 24V/15V = 360W PSU with voltage reduced to 18-20V. Motors will run smoother and hotend/heatbed will be ready much quicker. It will make you some troubles too as you will find bellow.
Motor wise the higher voltage, the higher speed you can achieve. With higher voltage, you can even use lover current, so that can save your drivers from overheating. We measured with oscilloscope that to overcome the electric induction of the coils in motors, at least 15V is required for low resistance motors (rated for 2.5A) and even more than 32V for motors with higher resistance (1.0A and lover). You can still use 12V power supply but forget about reaching speed limits written above. On the other hand when going slow you don't have to take care about voltage so much but current have to be set precisely so the motor will not skip steps.
With sufficient PSU controlling the current is mostly matter of used drivers. There are two widespread drivers on the market commonly used in 3D printers. Alegro A4988 and Texas Instrument DRV8825. Both of them were originally designed and sold by Pololu Robotics and Electronics, but there are numerous clones out there too and they are hard to distinguish from the original. Especially with the DRV8825 you may run into a big trouble with current control.
A4988 is very small and hot chip that gets overheated very easily. Passive cooler have to be used and fan mounted above it is highly recommended. Still you can burn them very easily. Even when they don't burn, their overheating may ruin your printed model. Their characteristic is up to 35V, 2.0A max and 1/16 microstepping. Their biggest advantage is, that the trimmer is controlling the current through the whole length, it have feed stops on both ends and you add current going clockwise. Whenever you bought them from Pololu or China, the functionality was quite the same. May seem ridiculous, but you will understand my point soon.
DRV8825 is superior in every aspect. It is not overheating so much, still passive cooler is required. Without active cooling you will not burn it though. It may control motors with characteristic 8-45V, up to 2.5A and have more precise 1/32 microstepping ... on paper. Now even original Pololu is made so, it would be possible to set the current up 3.3A. But the driver actually works only till 2.5A. So you can use only 75% of the scale. Because the current in the driver is set by voltage changed by the trimmer and under 1V it is not working linear bottom 30% are useless too. So instead of 270° scale you can use 120° at best. The Chines clones have this operating area even narrower. And as a cherry on top, trimmer is without end stops and to rise current it have to go counter clockwise. Trying to set these precisely is a nightmare. You will most likely end up measuring reference voltage with multimeter. Even the fact it do not require 5V for internal logics is somehow spoiled as on the A4988 5V pin it have Fault pin instead. If you try to remove it, the driver will not work and with this pin installed, when the driver will reach fault state, it will pull this pin but instead of sending signal it will get hit by 5V. Does not cause much damage so far, but making unpleasant noises when motor suddenly stops.
With higher voltage you can reach desired temperatures quicker but have to be careful not to overdo things too much as heat bed or cartridges in extruder can burn quite easily too. This was especially true when MK1 headbed was used. With this older version was impossible to print ABS with 12V power supply and with 24V you needed sizable 350-400W PSU. Try to avoid this first iteration of heatbed at any cost. MK2 is designed so 110-115°C can be managed even with 12V PSU, but it takes ages. If you have LED PSU with voltage control, try to make it running 13-14V instead of 12V. Caution !!! Make sure that the rest of the components will sustain this voltage too !!! They are selling 24W ceramics cartridges for hot ends too, but you have to buy them extra. You can even use 12V cartridge with hither voltage, but you MUST set the PWM control and PID of hotend in firmware !!! Don't try to do this unless you know what you are doing.
You will need fans, believe me. For electronics, for extruder motors and hot end, eventually to cool down X and Y axis motor and extruded material. But cooling with computer fans have one big flaw. You cannot go above 12V. Either you have to use fans designed for higher voltages or make some voltage control. Using resistor is also an option, but trying to pull down from 24V to 12V will make them really hot even when you will use the right size, so you have to cool them. Rather use PWM voltage control or additional 12V PSU just for fans.
For eg. Sanguinololu can be used with voltage up to 35V. It will also allow you to burn your computer's USB (or even motherboard} much easier as this electronics have almost no protection of any kind installed. It cost me two notebooks already. Default RAMPS with Arduino can be used with PSUs up to 12V. With a bit of tweaking you can run Arduino from your computers USB or own PSU and RAMPS may then work with higher voltages.