SHAPEOKO 2: ELECTRONICS AND EXTRAS

Keeping cost low was important as i am a poor.. poor student. It went quite a bit over budget. At this stage i had not thought of ordering from aliexpress or cheap Chinese sites. Hence some of the components being a bit overpriced and probably same quality as the stuff from China is anyway. Ill explain what i had to do with each component under each heading.

Here are the electronics and extras:

  1. Protoneer GRBL Arduino Shield (22.70 euro)
  2. 4 NEMA 17 42STH47-1684AC, 43.1N.cm (4.40Kg.cm) Holding torque,1.8° Stepper Motors (17.58 euro/motor)
  3. 4 DRV8825 stepper motor driver carriers (9.98 euro/driver)
  4. 3 GT2 Pulley (20 Teeth) and 5m GT2 Belt (25.25 euro/set)
  5. Arduino UNO (already had one)
  6. 2 Electronic Cases
  7. Miscellaneous items such as wiring, connectors etc
  8. 19v 3.43A Laptop Power Supply

Protoneer GRBL Arduino Shield

I done lots of research on what would be a suitable board to control the shapeoko. There were a ton of options for 3d printers which are very similar. But only a few open source cnc mill boards out there. I already had a arduino uno laying about so i opted for a protoneer GRBL board. I saw a few people on forums using this board with good success. Its low price was good. It had an array of decent options such as connecting abort, hold, resume buttons. Of course 6 endstop connections are on the board. Board supports a voltage range from 12-36v which allowed me to use a 19v 3.43A laptop power supply laying about. I ordered the board from protoneers ebay site from New Zealand. Took about 2 weeks to arrive to Ireland. It came un-soldered. But was easy to solder all the components to the circuit board. Check out protoneers blog for more details on this awesome little board. http://blog.protoneer.co.nz/arduino-cnc-shield/

NEMA 17 Stepper Motors

The shapeoko 2 can handle up to nema 23 size stepper motors, but nema 23 are more expensive and  would draw to much power to be usable with the DRV8825 stepper carriers. So i opted for generic nema 17 motors with enough holding torque. Minimum of 40N.m. Knowing the degree per step was important when it came to inputing the steps/mm in the GRBL code in the arduino uno. The set of motors i purchased are 1.8 degree per step. Which means 200 steps per revolution. When i first got the motors i shorted the wires to find out which ones where connected to the same coil. When you short the wires of the same coil and try and twist the shaft, you can feel a notable resistance in comparison to the wires not being shorted. I noted the colour of the wires in relation to this. The diameter of the shaft of these stepper motors is 5mm so when ordering the gt2 pulleys i insured the bore diameter matched this. There is a connector for the wires, this was added bonus when setting up the wiring to the motors. I ordered the motors from http://www.ooznest.co.uk. Items arrived quickly and where well packaged.

Some specs on the motors

Frame size: 42 x 42mm – NEMA 17
Step angle: 1.8° ±5%
Holding torque: 43.1 N.cm (4.4kg.cm, 62oz.in)
Inertia: 68g.cm
Voltage: 2.8V
Phase Number: 2
Current/Phase: 1.68A
Resistance: 1.65Ω/Phase ±10%
Inductance: 2.8mH/Phase ±20%
Number of Wires: 4
Wire Length: 1m
Insulation Resistance: 100MΩMin. 500VDC
Weight: 0.35kg
DRV8825 stepper motor driver carriers
There were only two common stepper carrier to choose from the a4988 or the drv8825. The drv8825 could supply more current and could step to 1/32 compared to 1/16 of the a4988. Both where at a similar price range so i opted for the drv8825.
It got packaged with the heatsink separately. I had some akasa thermal double sided tape laying around so i scrapped of the low quality OEM sticky thermal stuff from the bottom of the heatsink and applied the akasa tape to it and then stuck the heatsink to the drv8825 chip. I assume the akasa tape has better thermal properties.  Now! Finding the orientation of which way to plug the drv8825 to the protoneer board caused some headaches. Placing it in the wrong direction could fry everything so i googled for ages. Found plenty info on a4988 as this is one of the most common drivers out there. With the a4988 and the protoneer board the potentiometer is closer to the power socket. For the drv8825 the potentiometer is on the opposite side of the board in relation to the pin layout. So basically when inserting the drv8825 , the pot is directed away from power input.
Limiting the current to the stepper motor is a vital step. Maximum current for the stepper motors i am using is 1.4 amps.
I adjusted the pot in this project by trail and error. Basically dropped the current to its lowest and then incrementally raised it and see how well the steppers functioned. Later on i used a reference voltage got from the pot to the ground.
Using Vref = current/2 / I adjusted the driver to the correct current. More info on the drv8825 visit: https://www.pololu.com/product/2133
GT2 Pulley and GT2 Belt Set

Bought a set of 3 GT2 pulleys and 5m of GT2 belt for about 25 euro. The GT2 pulleys have 20 teeth and a bore of 5mm. Secured to the shaft of the stepper motor by 2 grubscrews. 5 meters of GT2 belts was plenty. Only needed just over 1.5 meters of belt for the shapeoko 2. But i wanted to use the left over belt for other projects. I found that the Shapeoko 2 method of attaching the belt to the machine wasnt very good. Belts kept slipping. I came up with improved version. ill post this later.

Arduino UNO

Already had one around. One i have is a genuine Ardiuno, not a clone. Don’t really think that makes a difference anyways.

Electronic Cases

Had some laying around, one big enough for the arduino and protoneer board another for organizing the cables connection to the moving gantry.

Miscellaneous items such as wiring, connectors etc

Had a bunch wire from previous projects, connectors taken from scrap electronics. Finding connectors that would fit the stepper pin on the protoneer board proved to be difficult, but i found that the 4 pin wire floppy disk player power connector worked perfectly. Had a few old power supplies laying about with this connector.

19v 3.43A Laptop Power Supply

I must have 3-4 of these around the house. Worked just fine. Stepper rarely ever reach full current rating especially all at the same time. So 3.43A was enough.

Feature picture shows some of the wiring which i will cover in the next post!!

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