DIY Desktop CNC

This design was inspired by a YouTube entry that included a CNC design which was about to go into production.
This is the second CNC design that I have produced and both aim to produce a desktop CNC which is easy to build at relatively low cost.
Note - to date this has not been built but I have a high confidence level that it will achieve the stated aims with few problems.

The design uses 50mm (3mm thick) aluminium angle and 625VV 'V' bearings for the X and Y rails. This rail and V bearing arrangement convieniently: -
---gives good separation between the V bearings
---puts the lead screw on the center line of the V bearings
---and keeps the bearing support plates (100mm*4mm aluminium) very close to the lead screw center line
Linear rails are used for the Z axis, once again, to keep the spindle/router center line as close to the X plate and V bearings as possible.
This X,Y and Z arrangement aims to minimise any lever arms which provoke unwanted movements.

NEMA17 stepper motors are used, and in order to best use the limited torque from NEMA17, single start (2mm pitch) lead screws are used.
The Z axis uses a toothed belt reduction ratio of 16:60 to increase the resolution and reduce the torque requirement.
This arrangement will be slow but acceptable for a home desktop CNC like this.
However the steppers could easily be beefed up to NEMA23 (a suitable adaptor is included) and 4 start (8mm pitch) lead screws used instead.

The longest lead screw is slightly less than 500mm and this gives an effective X range of 390mm and Y range of 360mm. Z range is around 75mm. Anti backlash blocks are used throughout and a 0.1mm allowance is made for several layers of paper shim to ensure good alignment.
It would easily be possible to increase the X/Y size. If this is done consideration should be given to strengthening the rails by epoxying (and/or bolting) an aditional 40mm angle inside the 50mm angle.

3D printed parts are used to support the rails and stepper motors etc., and to suitably mount the V bearings. 'Spacer Array' and 'Z Nut Spacer' should be printed at 0.1mm layer height, all other parts can be printed at 0.2 or 0.3mm layer height.
pdf's of the X, Y and Z plates are included. These should be printed at actual size and used as a guide for drilling all the holes.
Before drilling the X and Y plates, assemble 2 625VV bearings either side of the aluminium angle and measure the bearing center distance. This should be 85.71mm or 85.71-5mm to the closest bearing surface. Note that the eccentrics allow +/-0.4mm of adjustment.
If necessary make a minor adjustment to the spacing of the 4 bearing holes (8mm dia) on the X and Y plates.

The design was completed in Onshape and anyone can visualise the design by following this link
https://cad.onshape.com/documents/d5df1b483d27c4f3e0663c31/w/7176886e3c5335a6360f9906/e/ecab29bb5648e917ba3498c4
Use right mouse+Alt to orbit and right mouse+Ctrl to pan.
All possible movements can be simulated in the 'Ass Total' tab by clicking and dragging the router. Or copy the design to your own workspace and modify as necessary.

One of the Onshape tabs shows a potential router spindle design using a brushless RC motor (MT Series 5206 320KV). This is very experimental and it will be necessary to purchase a motor and make adjustments to the 3D printed part before proceeding.

23/5/2019 605 bearings were incorrectly sized. 'Z Stepper Mount' and BOM updated.
14/03/2020 It has come to my notice that 2" angle (1/8" thick) is sometimes easier to obtain. If there is any interest I will produce a modified design to suit.