Introduction: CoreXY CNC Plotter
This instructable describes an A2 plotter made from low cost aluminium extrusion, a piece of particle board, two NEMA17 stepper motors, and a few belts and pulleys.
The plotter is:
- high resolution (80 steps/mm),
- and scaleable
The plotter has an on-board interpreter that recognizes the g-code output from "Inkscape" .
Metal work is simple ... all you need is a hacksaw, three drills, a rat-tail file, and a screwdriver.
12 October 2017:
An improved pen-lift is described in instructable https://www.instructables.com/id/CNC-Pen-Lift/
Step 1: Circuit
The wiring diagram for this plotter is shown in photo 1.
Adjusting the motor current(s)
Set your CPS-3205 power supply to 12 volts.
Attach a 12 volt 30 ohm NEMA17 stepping motor to a Big EasyDriver module and apply power to the motor assemby.
Now adjust the small potentiometer on the Big EasyDriver module for a current reading of 0.4 amps (400mA) on the CPS-3205 power supply.
Repeat this process for the remaining motor and Big EasyDriver module.
Disconnect the power.
The plotter also works with 6 volt 8 ohm stepping motors in which case the CPS-3205 power supply should be set to 6 volts and the motor currents set to 0.6 amps (600mA).
Step 2: Software
The source code for this plotter is attached.
- copy the contents of coreXY_plotter.ino into an arduino sketch.
- save the file as "coreXY_plotter" (without the quotes).
- compile and upload the sketch to your arduino
Step 3: Parts List
The parts list for either an A4 or A2 coreXY plotter is attached ...
Wood saws are not required if you ask your timber merchant to cut the particle board and and pine supports to length.
Step 4: Theory
The timing belts are arranged in a "coreXY" configuration.
An excellent explanation of the "coreXY" principle may be found at http://corexy.com/theory.html
The pen moves:
- horizontally when the motors rotate in the same direction.
- vertically when the motors rotate in opposite directions.
- diagonally if only one motor rotates.
Step 5: The Mathematics
This step derives the pen motion equations and may be ignored ...
If we rotate motor A counter-clockwise by an amount ∆A then the pen carriage will move horizontally to the right by an amount ∆X and vertically upwards by an amount ∆Y. The equation for this is:
∆A = ∆X + ∆Y ...................................................... (1)
If we rotate motor B counter-clockwise by an amount ∆B then the pen carriage will move horizontally to the right by an amount ∆X and vertically down by an amount ∆Y. The equation for this is:
∆B = ∆X - ∆Y ....................................................... (2)
The reason for the ∆Y sign/direction change is that the two timing-belts are moving in opposite directions... the timing-belt for motor A is being fed towards the pen which lets the pen to move upwards, whereas the timing-belt for motor B drags the pen downwards.
Rearranging equations (1) and (2) we get:
∆Y = ∆A - ∆X ....................................................... (3)
∆Y = -∆B + ∆X ..................................................... (4)
Equating equations (3) and (4) we get:
∆A - ∆X = -∆B + ∆X ............................................. (5)
∆X = (∆A + ∆B)/2 ................................................. (6)
Translating: the pen moves horizontally when both motors rotate in the same direction
Rearranging equations (1) and (2) we get:
∆X = ∆A + ∆Y ....................................................... (7)
∆X = ∆B - ∆Y ...................................................... (8)
Equating equations (7) and (8) we get:
∆A + ∆Y = ∆B - ∆Y .............................................. (9)
∆Y = (∆A - ∆B)/2 ..................................................(10)
Translating: the pen moves vertically when the motors to rotate in opposite directions.
Collectively equations (6) and (10) infer diagonal movement if only one motor rotates.
Step 6: Evolution ... a Story of Trial and Error
My first build comprised a single timing belt arranged in the shape of a letter 'H', or "H-Bot" configuration, as shown in photo1.
I quickly abandoned this design for three reasons:
- If both motors rotate in the same direction the tension at opposite ends of the gantry caused unacceptable "wracking" .
- pen wobble was excessive
- the timing-belt tension spring was not a good idea ... the belt tension needs to be firm.
A few modfications eliminated all of the above problems.
- The timing belt was cut in half and arranged in a "coreXY" configuration . This arrangement has the advantage that all unbalanced timing-belt tension is in the direction of pen movement as shown by the arrows in photo 2 and photo 3.
- Pen wobble was eliminated by using an extra guide rail for the pen assembly.
- Variations in belt tension were eliminated by replacing the spring with cable-ties.
The design is scalable
Photo 4 compares the original A4 plotter with the base of a larger A2 plotter described in step 7.
The gantry and pen carriage dimensions are common to all plotters. The only additional parts required for the larger plotter shown in photo 5 are:
- a larger base board.
- longer side rails.
- longer timing belts
An H-Bot timing belt pulls the gantry ends in opposite directions whenever the pen moves horizontally (see arrow directions in photo 1). This causes a twisting motion known as "wracking".
CoreXY requires two full-length timing-belts. Halving the original H-bot timing belt allowed the coreXY concept to be tested before longer timing belts were ordered. It also accounts for the reduced plot area shown in photo 2.
Step 7: Construction ... the Base
Decide on your plot area:
- An A4 plotter requires a 6mm x 600mm x 400mm piece of particle board.
- An A3 plotter requires a 6mm x 800mm x 600mm piece of particle board.
- or larger ... the design is scalable
Bolt a length of 1.5mm x 20mm x 20mm aluminium "Tee Section" extrusion along opposite sides of the base. Ensure that both rails are parallel.
The extrusions should be flush with the long edges of the base and positioned under the base such that the edges just protrude (see photo). These extrusions act as railway lines for the overhead gantry.
Screw 65mm x 18mm timber end supports to the base. The timber supports will need a rebate for the aluminium extrusions ... make a shallow cut then knock a sliver of timber out using a chisel positioned against the end grain.
Step 8: Adding the Fixed Pulleys and Motors
The two timing-belts are stacked vertically above each other.
This is achieved as follows:
- One motor pulley is inverted as shown in photos 1, 2, & 3.
- The toothed idler pulleys are stacked in sets of two as shown in photos 4 & 5.
Mounting the motors
Drill four x 3mm mounting holes, and one spindle clearance hole, for each NEMA17 stepping motor as shown in photos 2 & 3.
The outside 3mm holes are 25mm from each edge of the base. This distance ensures the the motors avoid the wooden support and the aluminium side rails.
Mounting the fixed pulleys
Drill two 4mm holes for the fixed pulleys.
These holes are located 25mm from the opposite end to the motors and 40.5mm from the side rails. This distance ensures that the pulleys avoid the wooden support. It also ensures that the timing-belts are parallel with the side rails.
- Timing belts stretched between each motor and the associated fixed pulleys MUST be parallel to the side rails.
- Separate the pulleys with a 4mm washer. The washer prevents the pulley edges from rubbing.
Step 9: The Gantry
Vertical (Y-axis) motion is provided by a movable gantry that runs along the fixed side-rails attached to the base.
Horizontal (X-axis) motion is obtained by running a pen carriage-assembly across two rails attached to the gantry end brackets.
The gantry wheels are fashioned from VZ624ZZ V-groove sewing machine pulleys (photo 3). The V-groove prevents the gantry moving sideways.
Cut two 60mm x 130mm brackets from a sheet of 18 gauge aluminium using the method described in instructable https://www.instructables.com/id/How-to-Cut-Fold-S...
Drill two 3mm holes and four 4mm holes at the locations shown in photo 1.
Fold a 50mm top for each bracket.
Mounting the wheels
Sandwich four "V-groove" pulleys between a 4mm nut and bolt. The nut prevents the sides of the pulleys touching the aluminium.
Now bolt these wheels through the 4mm diameter holes as shown in photo 2.
Attaching the end brackets to the base
Attach a gantry end plate to each side of the plotter.
To do this:
- the grooved wheels must straddle the side-rails.
- elongate the bottom holes as required to eliminate any vertical play in the gantry.
When correctly adjusted the gantry end-brackets should roll freely when the base is tilted slightly.
The over-head gantry rails are attached when we attach the pen carriage-assembly ...
Step 10: The Gantry Pulley Brackets
The gantry pulley brackets
The gantry pulley brackets are cut from 18 gauge aluminium sheet. An approximate  drilling template is shown in photo 1.
Photo 2 shows the pulleys attached to the motor 1 (left-hand) bracket.
Photo 3 shows the pulleys attached to the motor 2 (right-hand) bracket.
- The smooth side of the timing belts always sees a smooth pulley.
- The ribbed side of the timing belts always sees a toothed pulley.
- One of the double pulleys on each bracket is NOT used ... it simply acts as a spacer.
Attaching the pulley brackets to the gantry
Press each pulley bracket against the inside of the gantry bracket such that the bracket is centered and the heads of the pulley bolts are clear of the base.
Mark these positions by means of a pencil through the two existing holes in each of the gantry brackets.
Now drill 3mm mounting holes and bolt the bracket pairs together.
The 4mm hole positions for the idler wheels depend on the sharpness of the upturned folds.
Fold each pulley bracket then custom position the holes such that:
- both timing-belts over-lap when viewed from above.
- the idler wheels used as spacers don't touch any belts.
Step 11: Pen Support
The pen support is made from 18 gauge aluminium sheet. The dimensions are shown in photo 1
Custom size the large holes such that your pen slides freely but without any sideways wobble.
Attach a pen-lift collar to your pen. Mine was made from the brass fitting found inside a radio knob and a circular piece of plastic but anything will do ... so long as the position can be adjusted.
Step 12: Pen Carriage Assembly
The pen carriage is made from 18 gauge aluminium sheet. A drilling template (not to scale) is shown in photo 1.
Assemble as follows:
- drill and fold the metal
- attach 8 x v-groove pulleys as shown in photo 2. Elongate the lower holes such that there is no vertical play when the aluminium "tee section" rails are fitted.
- use a cable-tie to attach the SG90 servo as shown in photo 2. Drill the cable-tie holes to suit.
- use the 3mm holes in the pen-support as a drilling template when attaching the pen.
Fitting the top gantry rails
Attaching the rails:
- Pass two 600mm lengths of aluminium "tee section" between the "V-groove" pulleys as shown in photo 2.
- Attach each gantry bracket to one of the rails by means of two 3mm nuts and bolts.
- Position the second rail such that the pen carriage-assembly moves freely. Now drill and bolt the rail to the gantry brackets.
Step 13: Attaching the Timing Belts
- the plotter has TWO timing belts.
- each timing belt forms a continuous loop (see photo 2)
- the timing belts are mounted at different heights ... the pulleys share a common shaft.
- the timing belts are tensioned using cable-ties.
- both timing belts are attached to a common 3mm bolt (see photo 3)
- push the pen carriage-assembly to one side.
- tape both gantry ends to prevent movement (photo 1).
- thread the bottom timing belts and fix the ends (photo 2).
- thread the top timing belt and fix the ends (photo 3).
- the timing belts should look like this (photo 4)
- remove the tape.
- tension the belts such that the carriage-rails are the parallel to the base end.
Step 14: Attaching the Pen Carriage Assembly
Two 25mm long threaded spacers are joined together (by means of a 3mm head-less bolt) to form a solid rod.
The belts are attached to this rod as shown in photo 1.
Attach the top of the rod to the center of the pen carriage-assembly using a 3mm bolt.
The pen should:
- move to the left when BOTH motors are turned clock-wise.
- move to the right when BOTH motors are turned counter-clockwise.
- move diagonally if only one motor is rotated.
Step 15: The Menu
Upload coreXY_plotter.ino to your arduino if you haven't already done so. Instructions for this are given in step 2.
Connect a USB cable to your arduino and left-click "Tools|Serial Monitor" ... a menu similar to photo 1 should appear.
Apply 12 volts power to your motors and your plotter is ready to use.
The menu is not case sensitive. Typing:
- MENU brings up the menu
- G00 allows you to send the pen to a specific XY co-ordinate with the pen raised.
- G01 allows you to send the pen to a specific XY co-ordinate with the pen lowered.
- T1 allows you to position your pen over your 0,0 co-ordinate. Type 'E' to exit.
- T2 allows you to scale your drawiing. For example "T2 S2.5" will scale your drawing 250%. The default scale is 100%.All pen moves use the drawing scale last set using this menu option
- T3 and T4 allow you to raise or lower the pen.
- T5 draws an "ABC" test pattern.
- T6 draws a "target".
- T7 draws a set of radial lines
The internal wiring of some motor brands are reversed. If your motor directions are reversed then use the alternate code in the step_motors() function.
Step 16: Creating and Sending Gcode Files to Your Plotter
This plotter assumes that co-ordinate (0,0) is at the lower-left corner of your paper. As such it is 100% Inkscape compatible.
Instructions for creating gcode files are given in the following instructables:
- https://www.instructables.com/id/CNC-Robot-Plotter... (step 9)
Instructions for sending your gcode file to this plotter are given in instructable: