Robot Type 1 45 cm - belt gear - direct drive - R0
#1 of 15 Overview
The Type 1 robot printer has the following properties:
- easy to be built in mind: with a few tools like boring machine, hack or band saw, some measuring tools it is possible to build the robot 3D printer
- print area about 45 cm in every direction
- belt gear based
- direct drive extruder
- R0 mode, i. e. Axis 4 has an actuator, needing 5+1 stepper drivers in total
- no rail
- precision is expected to be middle
Duet 3 has enough drivers (6), Duet 2 needs one additional driver with Duex or as external driver, Duet 3 Mini 5+ needs one additional driver with a 3HC, 1LC, 1XD or similar board. I have no Maestro, so I could not test whether performance is sufficient, and I have no knowledge about which driver extensions are needed.
The robot project is accompanied by the following links and documents:
- main discussion thread https://forum.duet3d.com/topic/17421/robotic-kinematics
- configuration documentation: https://duet3d.dozuki.com/Wiki/Configuring_RepRapFirmware_for_a_FiveAxisRobot?revisionid=HEAD
- homing documentation: https://duet3d.dozuki.com/Wiki/Homing_for_a_FiveAxisRobot
#2 to #15 target specific topics (work in process).
#2 Weights, Torques, Stepper dimensioning
Belt based gears have no brakes like harmonic, planetary or cycloidal drives. The arms would fall down when power off. The goal is to balance the weights of the arms and axes so that the arms are in balance even when powered off. The steppers have detent torque, which is holding torque when they are powered off. The inbalance must be smaller than the detent torque. Detent Torque is stepper spedific, but a typical value is 2 Nm for a Nema 17 and 12 Nm for a Nema 23. The weight is best planned from end to begin, beginning with axis 5's weight.
The torques are calculated by adding up all elements of a specific axis with their distances from the axis. A N (newton) is about 100 g, the distance is in m. E. g. 500 g Nema 17 stepper 20 cm away from the axis is 5 N * 0.2 m = 1 Nm = 100 Ncm. Summing up left and right results in the total torque. Weights of or vertical above or below the axis do not count for torque calculation (0 distance). A Nema 17 stepper with detent torque of 2 Nm and a gear with ratio 1:24 can take an inbalance of 48 Nm up- or downward. The torque calculations is for horizontal elements. Rotating dimishes the torques (inclined plane), but the same factor on both sides.
Arm 4 is vertical, so there is no static torque if Axis 5 is balanced.
A higher weight has a negative effect on acceleration and speed, even if the torque is balanced.
Current stepper dimensioning is
- Extruder: Nema 17 short with 1:3 gear (1 stage)
- Axis 5: Nema 11 with 1:9 belt gear (2 stage)
- Axis 4: Nema 17 with 1:24 belt gear (2 stage)
- Axis 3: Nema 17 with 1:24 belt gear (2 stage)
- Axis 2: Nema 23 with 1:40 belt gear (2 stage)
- Axis 1: Nema 17 with 1:40 belt gear (2 stage)
For a detailed weight and torque calculation see in step #14. But roughly Axis 5 is balanced: 550 g, from axis 4 total weight (including Axis 5): 790 g, from Axis 3: 2.3 kg, from Axis 2: 5 kg and the total robot weight without foundation and print bed about 6 kg.
#3 Axis 5, Extruder, Hotend
4/11 axes and arms 2, 3, 4
Axis 2 to 4 are assembled by
- the arms are connected by shafts and F688 ball bearings in 16 mm holes
- to give additional sideways stability and function as gear, the arms are connected by rotation plates
- the arms are constructed from aluminium 20x10x2 hollow square tubes
- the holes inside the arms have raster 1 cm distance each, an exception are the holes for the rotation plates. Holes of 4.5 are for M4 screws, holes of 16 mm are holding the F688 ball bearings
- arm 2 length is 27 cm, arm 3 is 23 cm, arm 4 is currently 15 cm. The aluminium square tubes must be longer to take the steppers on the other sides of the axes: tube 2 has 60 cm length, tube 3 has 60 cm, tube 4 has 20 cm, arm 5 is only virtual. When using Nema 17 at axis 2 is used, a longer tube 2 may be useful for for counterweight torque.
#5 Axis 1, plate calibration, fundament, print bed
#6 Tools, Hole Processing
This is an overview and explanation. See BOM list below for details what is needed and different topics how to build gearboxes etc. in detail.
Use maximum security, e.g.
- eye, ear, breath protection
- boring without gloves, no juwelry, no long hair, no dangerous clothes. Better a wound on skin than a rolled up arm due to high torque of boring machine!
- secure workpiece by two clamps (to hinder rotation)
- when something goes wrong: where will it fly? => don't stand there
- boring and sawing heat the parts up, even with a hacksaw! Grasp the parts elsewhere (or use tool french or similar) and let them cool down
- first aid nearby
The robot is built by
- arms: aluminium square tube hollow 20x10x2, with 4.5, 5 and 16 mm holes
- gearboxes: aluminium 40x40x40x2x50 length U profile (Axis 2: 40x60x40x2, Axis 1 special) with 4.5, 5 (shafts) and 16 mm (for F688) holes
- shafts: steel or aluminium 5 mm and 8 mm full material shafts which fit into the 5H7 holes and F688 ball bearings
- flanged ball bearings F688 8x16xx
- steppers and common black steel based stepper holders
- connection stepper holder to arms: aluminium L profile 3 to 5 mm (2 mm is too weak) with 4.5 holes
- belts: 6 mm glassfiber based partly closed, partly open
- belt tensioner: aluminium 3...5 (2 is too weak) with 4.5 mm holes
- pulleys with teeth: 20/40/60, boring 8 or for stepper shaft
- pulleys without teeth with ball bearing: boring 5
- open belt fix at rotation plates: aluminium L profile 5 mm, 4 mm threads.
- screws: M3, M4, M5, M8, normal forms, a few countersunk. I prefer DIN 912 for M3 and M8, DIN 933 for M4, countersunk for M5. All fully threaded.
- washers: DIN 6798 form A (DIN 127 A, B not recommended)
- DIN 988 8x12 0.5 mm thick and 5x10 0.5 mm thick washers to let ball bearings or pulleys rotate better
- adjusting rings: 5 and 8 mm, springs. Partly alternative safety rings. For safety rings a specific tool is necessary.
- rotation plates ball bearings: aluminium with steel balls, 250 (or 225) mm and 120 mm diameter, aluminium with steel balls
Holes are bored with 1 cm raster (distance from hole center to hole center) (exception: stepper holder, rotation plates).
- marking with centre punch or calipher
- it's easiest to bore a 3 mm hole first. The bigger holes need less force then, and the precision to center the hole is better. With a long drill, it's possible to bore both holes front and back side of aluminium 20x10x2 and the 40 width gearboxes with one boring. The holes are at the same position then.
- 5 and 16 mm holes for holding shaft and F688 ball bearing are finished with reamers
- countersinks are created best with countersinks, but drill with higher diameter is also possible (beware not to drill too deep).
- always deburr bored or sawed parts. For safety, but for precision also.
Preparation is done by mainly boring and sawing:
- Aluminium boring is best with HSS-G Type W, but normal metal drill will also do. Cooling is not necessary (but has advantages). Reamer should be fixed size 5H7 and 16H7, not an adjustable reamer (they make conical holes). Boring normal speed, reamer with low speed (low rotation speed and low sink speed). Be careful if big drill or reamer gets stuck, protect hands! Boring low vertical speed with low force is good: less heat, less danger to get stuck. For deeper holes, redraw while boring often to break the shavings.
- boring M4 thread with a tap, alternatively M3.
- sawing will be easiest with a band saw, but with table saw or hacksay sawing is also possible. Table saw 2 mm is no problem, but I would saw with hacksaw thicker aluminium if a band saw is not available
Boring 16 mm hole:
First mark the point, then 3 mm, then 6 mm which is the diameter of the next tool, then 15.2 mm hole (better would be nearer to 16), then reamer. Last deburr and test fit. The reamer is too big to fit into my boring machine. I fixed it on the table and rotate the aluminium part through it clockwise.
7/11 Rotation Plates
Rotation plates are big ball bearings from aluminium material with steel balls.
Plate for Axis 1 has 250 mm diameter, Axis 2 250 mm also, Axis 3 and 4 120 mm each. Weight of 250 mm is 450 g, of 120 mm is 180 g each.
As example plate for Axis 4:
- connecting two arms each by 8 mm shaft, connecting to arm by 16 mm hole and F688 ball bearing
- pressing inner and outer ring of plate by spring in middle on shaft
- arms connected to plate at two holes in plate each, countersink screws
- Axis 4 special connection of arm 4 (arm 4 is a 8 mm shaft)
Arm-plate and Arm-arm connection precision (hole positions) is important for the rotation to be smooth.
In the image, one arm has only a 8 mm hole for the shaft. This will be changed to 16 mm with F688 in next iteration.
Guidelines for construction
- gear ratio 1/2 (20/40 teeth) or 1/3 (20/60 teeth) * ratio 20 to big wheel
- gearbox from aluminium U profile 40x40x40x2 with length 50 mm (gear 5 longer)
- teethless pulleys are on 5 mm aluminium shaft
- teethes geared pulleys are on 8 mm aluminium shaft, at aluminium case with F688 ball bearings
- screws are M4 all
Gear 1 will be reworked.
Gear 1 is the most complex one, because the 20/60 is belt is routed under the rotation plate to the side under the plate.
The box has size 40x40x40x2 with length about 5 cm. The gear is still 1:3, but will change in next iteration to 1:2 to give a total of 1:40.
Teethless pulleys near the rotation plate:
The gear is connected to Nema 23. For Nema 23 to be placed in the middle of the arm for better balance, the gearbox has size 40x60x40x2 with length 5 cm.
This version is how I like it:
- the two teethless pulleay near rotation plate have minimum distance to it
- minimum 19 mm distance of teethless pulleys
- minimum distance between teethless 5 mm pulley shaft to 8 mm shaft is 25 mm.
- the 20 and 40 pulleys are on a 8 mm shaft, connected to the gearbox with a F688 and in future with 8x12x0.5 washer between
Gears 3, 4:
- based on 40x40x2 U profile with 50 mm length
- teethless pulleys are on 5 mm shaft
- 20/60 pulley gear is on 8 mm shaft
5 mm screws will be replaced by shaft in next iteration.
The gearbox is from 40x40x40x2 with length 13 cm. Not finished yet, right will be connected the Extruder. An additional tensioner for the lower 20/60 gear is needed. Size of right Arm 4 axis to left is 10 cm, right will be about 5 cm for Extruder. I let enough room on top to install Nema 17 if necessary. Length from top to bottom is maximum 10 cm, right Extruder + heatbreak + hotend will be about 12 cm. The assembly allows free rotation of axis 5 under arm 3 (free is relative, because wiring and filament will hinder it). Placing the steppers as near to arm 4 as possible for low torque. I'll try to balance arm 4 when finished building.
Belt and distance measuring
To find closed belts which fit well and then measure the distances of the pulleys, I used a little "tool":
9/11 Belts, Tensioning
The belts on the rotation plate to the gearbox are open belts, fixed on the rotation plate:
Aluminium L profile 4 mm or more. M4 to fix it at the plate. Two M4 threads with M4 screws on top. GT2 short belt (here steel belt) to make sure belts do not slip. I have no good belt tensioning at the moment.
The belts between gearbox and stepper are closed belts. The belt tension is handled by:
First use screw 1 to place the stepper in a distance so belt has a bit tension. Then screw 2 by rotating the stepper so that belt has good tension. The screw 2 has a slot as counterpart instead of a hole:
Second the already good tensioned belt with higher tension:
The tensioner is at the middle between gearbox and stepper. Aluminium 2 mm is too weak, 3 to 5 mm needed. The connector between arm and stepper holder should be 3 to 5 mm also, instead of 2 mm, which bends.
Closed Belt lengths:
- Gearbox 1: 20/40, distance xx mm, belt 350 mm (175 teeth).
- Gearbox 2: 20/40, distance 90 mm, belt 240 mm (120 teeth).
- Gearbox 3: 20/60, distance 180 mm, belt 450 mm (225 teeth).
- Gearbox 4: 20/60, distance about 340 mm, belt 760 mm (380 teeth)
- Gearbox 5 first at stepper: 20/60, distance 47.8 mm, belt 180 (90 teeth).
- Gearbox 5 second stage: 20/60. distance 78.5 mm, belt 240 (120 teeth).
10/11 Endstops, Calibration
11/11 Electronics, Duet Configuration
#12 precision analysis
#14 BOM lists, weight calculation
#15 images of printed things