Hi all,

After 2 years of designing and building I’ve finally got around to creating a build thread for my new CNC. I’ve been meaning to write a post up on here for a while, and if I decided to start one at the time of the design stage I certainly would have saved a lot of time and effort in avoidable errors or design flaws.
My CNC is a fixed gantry type, it’s an iteration of my previous cnc that has served me well for 5 years which can be found here: http://www.cnczone.com/forums/uncate...29856-cnc.html

The goal of this machine was to ideally mill ferrous material whilst also being able to do the occasional pieces of wood and plastics (with a spindle swap). I understood from the onset that building a CNC capable of doing this well is difficult and I would be better off buying a C mill and converting it, but I didn’t have the room for a mill with the sort of table size and travel I wanted. I also liked the idea of having a machine that was capable of switching between materials such as wood and metals even though I knew this would mean compromises in performance. + it’s fun making one.

I’ve have since forgotten most of the fine detail with regards to design and some build aspects but I shall do my best to recall them. I have written a brief description about the design followed loads of pics of the build

Design Stage

My travel on the CNC are as follows:
X Axis: 1010mm
Y Axis: 450mm
Z Axis: 115mm
Attachment 384338

X Stage
Attachment 384340
X Stage - The Frame
Attachment 384342

The bulk of the frame is made from 6mm thick 100mm x 60mm cold rolled rectangular box sections. The original design (as shown in CAD) had 3mm wall; rectangle box sections running as cross supports but I have since got rid of them as I don’t think I can weld them in well enough to provide any meaningful benefits (this is mainly down to my lack of welding skill and lack of tools to cut the box sections accurately). This design has the spindle in line with the linear bearings that get bolted in the middle of the frame. This reduces some of the moments acting on the bearings as a portion of the milling force is in line with the bearings. It also means that most forces are transmitted through the middle cross member rather than any cross braces that would be present. This frame without the braces weighs 42.7kg.

The areas that mate with the gantry sides and linear bearings are all machined.

X Stage - Linear rails
Attachment 384344
The linear rails are supported TBR20, there are 8 of these bearings in total. The design is far from ideal and the open bearings when the table slides seems like it’s asking for trouble, but I have used this design on my previous CNC and its worked well so far. On my previous setup (which has 2 bearings in the middle of the frame) I see a 50-80 micron deflection when I push hard against the table. I’m hoping the use of an additional 2 (4 in total in the middle on the new design) will see this deflection drop. I am however planning on adding ‘proper’ linear rails (thk, hiwin etc) in the future to stiffen this further, I’m also toying with the idea of replacing half the ball bearings in the TBR20s with slightly oversized balls but I’m not sure if this is possible to do at home?

X Stage - The Table and Servo motor
The table in the CAD is from my previous CNC. I plan to change this as this table is currently welded box sections and aluminum flat bar. I'm considering casting a table in cast iron and machining the T slots, this may be cast in 3 or 4 parts. From initial designs it seems like it will weigh around 120kg. If I do go down this route I am considering using a 1KW Servo motor to drive the X axis coupled with a seriously stiff coupler. I’ve done some torque calcs and it seems like I can go with a lower power drive option however I still need to do some maths to work out inertia ratios between the servo and the table. Some work will be needed here.

The Y Stage
Attachment 384346
Y Stage -The Gantry Sides
Attachment 384348
The gantry sides are cast iron, the biggest crucible that fits my furnace is an A35 so I’m limited to the weight I can cast these. They are shaped so any forces transmitted in the z direction do not impose any moments about the mating points about the frame. The sides have been through FEA and various iterations to optimize the design for stiffness and the weight of iron I can cast. In hindsight I should have performed some FEA to see if I could have got away with just using box sections instead of casting these. These sides weigh 28kg each
I shall at one point write a detailed post on alloy avenue about these as they were a bit of a challenge to make.

Y Stage -The Gantry Beam
The gantry beam is 8mm 100x150mm cold rolled box section, Weighs in around 18Kgs. The areas that mate with the linear rails are machined flat and so are the areas that mate with the gantry sides. There are 16 M5 bolts that screw this beam to the gantry sides, the beam and the sides have transition fits between their mating surfaces.

Y Stage - Linear Rails
These linear rails are THK SSR25. I’m using these as I got them at a bargain price on ebay however I believe they are not preloaded, which I think is a big part of some of the deflection I see in the Y Axis Assembly. I may consider changing these to preloaded bearings but will need to justify the cost and perform some calcs/tests to understand if this will help.

X and Y Axis Ballscrews
The ballscrews are 25mm in diameter with a 5mm pitch (2505), they are C5 rated. I managed to get them for a good price from Zapp Automation. The diameter is justified through torsional calcs vs inertial forces and the C5 rating is mainly due to error over the large X axis distance. At the time I did not consider that as these ballscrew are larger in diameter than say the popular 16mm ones it relates to a greater packaging area and therfore greater moments on some of the linear rails due to milling forces.

Y Stage - Stepper motor
I’m considering getting a leadshine 12nm closed stepper to drive the Y axis but this is not set in stone and I will probably need some advice to what motor variants may be best here.

The Z Stage
Attachment 384352
Attachment 384350
The main mounting plate is cast iron, it’s currently 30mm thick however in hind sight I should have increased this to maybe 50-60mm. To overcome any deflection ill cast the spindle mount to stiffen the assembly.

The bearings are 25mm linear rails I picked up from Zapp Automation. They are mounted so the rails move up and down rather than the bearings. Although this imposes different moments on the bearings depending on how retracted the z axis is, it does mean the assembly can be stiffer than what’s possible with the bearings and rails mounted the other way. The ballscrew is mounted on the Z plate so it is not exposed to any swarf flying about during machining.

I shall be using one of my current 3N.m Steppers to drive the z stage, I believe the holding torque should be adequate to hold the weight of the stage plus an amble 30kg + of spindle (should it weigh this much). I don’t expect the stage to accelerate fast.

Design Calcs:
I’m not going to dwell on design calculations here too much, it’s something I have written up elsewhere and hope to post on my website one day. In short I have performed some calcs, such as moment analysis for the bearings to work out optimal positions. I have done ballscrew calcs to size up the ballscrews to maximize torsional rigidity vs inertial forces to accelerate them, I have also performed FEA to optimize the castings.

The design stage was mainly supplemented through calcs and understanding what else has been done, I have many useful report/docs/papers that I used lurking in my CNC folder and I can post these if anyone is interested.

Build Stage:
I have misplaced many photos I took during the initial stages of the build, but I have a few from later stages. I could probably talk forever about the problems, lesson learnt and how I went about manufacturing everything but I think I shall let the photos do the describing. In short I have not subcontacted any of the work, and all the casting, machining and welding/fabrication was done by me