Wednesday, 12 September 2012

CNC: Building a DIY CNC Z-Axis without a CNC

Building a DIY CNC Z-Axis without a CNC



Arguable one of the hardest parts of any CNC router build, the Z-Axis!

Basic Tools You Will Need

M6 Tap (long)
Home Made Tap Guide (black rectangle with a hole in the middle).This will aid in starting the thread square to the work piece)

Light Machine Oil or Cutting Oil,
Toothbrush (For cleaning the Tap threads)

6mm/5mm Allen/Hex Keys
29 mm Hole saw (I chose a Starrett and a A1 Arbour)
Assorted Metal drills (4.8mm or 5mm for the M6 Tap Hole)
Hammer, Centre Punch (I used an optical centre punch)
Engineers Reference Square (Important,Or something that is known to be perfectly square)
Engineers Metal Ruler
Digital Callipers (Optional)
Craft Knife (For precise marking)

Power Tools

Table Saw (Non Ferrous Blade)
Pillar Drill

Parts

A Chunk of 19 or 20mm 6020 Aluminium Plate 
M6 Bolts and Washers
M5 Bolts and Washers
2x SBR16 Supported Linear Rail 
8x SBR16UU Linear Bearing Pillow Blocks
1x 350mm RM1605 Anti Backlash Ballscrew
1x BK, 1x BF  Ballscrew Mounts
1x RM1605 Ball Nut

Z-Axis Back Plate Bolts (for Pillow Blocks/Ballscrew Mounts)

16x 30mm M5
6x  40mm M6
6x  45mm M6

Z-Axis Front Plate Bolts (for Supported Rails)

20x M5 20mm

Z Axis Ball Nut Housing Bolts

2x M6 50mm
6x M5 20mm

Z Axis to Y Axis Bearing Plate Connectors  Bolts (For Pillow Blocks)

16x M5 30mm


Z Axis Universal Stepper/Pulley Plate Bolts (INC Stepper Motor)

3x M6 40mm
4x M5 15mm




The Design


Z Axis Back Plate


Z Axis Front Plate


Z Axis Ball Nut Housing


Z Axis to Y Axis Bearing Plate Connectors (x2)


Z Axis Universal Stepper/Pulley Plate


SBR 16mm Supported Linear Rail Drill Hole Spacing

I'm quite a big fan of printing templates to scale, using double sided tape to stick the design to the material , meticulously cutting the sides of the template perfectly square with a very sharp craft knife.
I then form a corner using some waste material and align one of the square sides of the template to the form.
This method for me at least saves a lot of time marking , I doubt that I could mark any more accurately than a modern printer can, Before using a center punch!

A word of warning here! after setting your printer to "Print to Scale" in the printer preferences, do a test print and double check with a ruler or digital callipers that a few of the lines are of the correct measurement, I usually check from one drill centre hole to another.





Cutting the aluminium setting up the saw





In the following pictures, the different between a Non Ferrous Blade (Top) and a normal wood saw blade (Bottom) can be seen by looking at the difference in tooth profiles.

Non Ferrous blade 
(notice the chamfered teeth ,also the angle on the normal teeth is reduced)




Normal Wood Blade



Drilling


Notes on setting up the pillar drill for square

Before we can start using the pillar drill, we need to check the table to ensure it is square to the incoming tool.Using a reference square and a long shaft fixed in the chuck check for everything is parallel on at least 2 sides of the table.






If any adjustment of the table is needed it can be performed underneath.Using a combination of slackening  and tightening the large centre bolt and grub screw everything can be brought in to line so that the reference square is perfectly square with the shaft.
Every so often I like to run the drill and let it come to a halt in-between checking to allow for any backlash of the pillar drill.





Dummy Mock up Nema 23 Stepper Motor 3Nm (425 Oz-in)

As the size of stepper motor is still undecided I thought it useful to make a wooden scale mock up of the largest stepper motor I would use.This way I can physically see and potential clearance issues as I hone the design.

I quite often will make certain parts from MDF to check the design before wasting expensive materials.
I would rather waste a glued blocked of cheap MDF , than a chunk of aluminium.






Z Axis Universal Stepper/Pulley Plate

By using pulleys and a belt, the speed,torque and resolution can be adjusted, this also helps reduce resonance and vibration when cutting.Another advantage of using pulleys is that the motor is mounted behind the Z Axis .This has 2 advantages, one of them being to reduce the overhanging weight of the Z Axis, the second being to reduce the overall height of the Z Axis.At the rear of the mount the holes are slotted to allow for adjustment of the belt tension.

The reason the plate is universal?, in the design I allowed for the stepper motor to be mounted to the rear using pulleys or overhead in an upright position over the end of the ball screw using an appropriate riser and shaft coupler.






The pulley design principal can be seen here, mocked up with wood scale 30 teeth pulleys with a ratio of 1:1.One of the pulleys could be changed  for a 60 or 15 tooth pulley increasing the resolution of the ballscrew from a pitch of 5 to 2.5, a ratio 1:2. I read that Going below 15T pulleys is not a good idea in a reasonably sized CNC machine as the belt curve is rather acute and can lead to stress. 




Here you can see the connection to the Z Front Plate and the Ball Nut housing.









You can see here in the design the Supported rails/Ball nut housing are actually connected to the front plate rather than the back plate, this serves to give greater rigidity to the the Z-Axis when extending towards the material to be cut .





To be continued.....