CNC Machines

Oliver Steeples and Stuart Winstanley

Caveat:

This article mentions the Ooznest Workbee quite a bit.  I had planned to get another CNC for comparison, however I wasn’t able too.  This is not an advert for Ooznest and please do research before buying a CNC to verify what is right for you.

In the past few years 3D printing has taken off exponentially and now they are common place in every hobbyists workshop.  CNC machines have been around for a lot longer, but are less widely used.

This is not a comprehensive guide, but more of a basics of CNC, what people have learnt and differences between machines.

Basically a CNC is a computer controlled router than can repeatable cut the same shape over and over, something that’s great if you want to cut droid parts.  Typically four stepper motors are used, 1 for Z, 1 for x and 2 for Y (although cheaper machines have a single Y stepper motor).

The stepper motors are connected to either belts/pulleys or leadscrews that allow the cutter to move in all three dimensions with the help of rails; typically aluminium extrusion.  The CNC size can vary and you are generally looking at an A4 to 1.5m square machine for hobbyist use.  As a note the cutting area is different from the CNC size so this is important to factor in. 

Oliver Steeples

About 6 years ago I started with an A4 sized eShapeoko that I should call Trigger; due to multiple upgrades and modifications since it doesn’t resemble the original machine at all. 

It was very similar to the picture below and used a Dremel for the cutter, belts for the X/Y axis and threaded rod for the Z axis, a cheap TB6550 parallel port CNC controller was used for the electronics.

It didn’t stay that way for long was quickly upgraded to a 1000*750  machine, with the overheads of the rails and Z axis gantry that holds the router this allowed a usable size of 530*770mm which is enough for R2 skins.

The Z axis M8 screw thread was upgraded to an acme leadscrew to reduce on backlash/play.  The Dremel was also replaced very quickly with a 400W spindle which had a lot less wobble at the cutting bit (technically an end mill).  A Dremel is not a good option for a CNC and best avoided.  Play and wobble is your worst enemy on a CNC machine as you want the cuts to be repeatable on every layer as you cut down into the material.

I found out after using it for a while that the TB6050s aren’t very good CNC controllers and with parallel port speeds I was never able to cut anything very fast and it was really slow on circles (for a CNC a circle is lots of straight lines so it needs to work this out and move the router in both X and Y axis as it cuts).  At the time the hobbyists controller of choice was the Gecko G540 and was expensive at the time, however 5 years on it’s still going strong and hasn’t missed a beat, but more about this later.

With all the above I learnt a lot, including about different cutting bits/end mills.  MDF and wood are relatively easy to cut and hard to mess up, they are really forgiving (apart from lots of dust with MDF).  When moving to styrene the feed rates (how fast the router moves when cutting) and the cutting bit size needed lots of adjustment until I was happy.  As a warning when you start up you will break lots of cutting bits.  A standard wood cutting bit will melt styrene and this caused me a few breakages of bits as the styrene clogged into a molten ball, even reducing the feed rate failed to fix this.  As styrene has a low melting point a carbide single flute upcut bit is needed; the carbide doesn’t heat up as much as HSS (High Speed Steel) and the upcut with a single cutting blade allows the chips to be ejected away from the work so they don’t get in the way.  Even recently I got caught out cutting acrylic using the same profile I use with styrene and the plastic melting, I was quick enough to stop the machine before the bit broke.  

There are charts online with the best feed rates and spindle speeds which I used as a start and then partial trial and error to find the best settings for my machine.

I learnt that large diameter cutters aren’t always best.  A cutting bit can typically cut to a depth of ½ it’s diameter so a 6mm diameter bit can cut 3mm at a go, great as numerous R2 parts are made from 3mm styrene.  However this is great on paper, but I found that using a 1.5mm cutter with cut depths of 0.75mm and using multiple passes (3mm/0.75 = 4) produces a better finish on the edges.

The major downside of the eShapeoko using belts was the X axis wobbled back and forth on the Y-Axis mounts, and needed squaring up every time it was used, it was never stiff enough to be square all the time and wanted to work on the skew.  Also with time the belts became slack and this introduced play on both the X and y axis.

Securing material to the bed was also a massive leaving curve, wood and thick material are easy to clamp down using spare bits of wood, however thin styrene and upcut bits want to pull the material away from the bed.  Double sided tape gums up the bits so not very good, recently I’ve learnt that low tack masking take on the bed and workpiece with a dab of superglue in the middle works wonders, but am yet to try this.

As a DIY solution I made a vacuum bed using an old vacuum cleaner.  Using holes and channels the vacuum helps hold down the material and is essential for cutting 1mm styrene for skins.

Due to the issues I had with the belts and stiffness of the eShapeoko I recently upgraded to a 1000*750mm Workbee from Ooznest.  As it’s the same size I was able to use the existing vacuum table, electronics, stepper motors and Kress spindle.

If you buy a hobby CNC now your budget choice of router is typically a Dewalt or Makita that can be mounted on the Z axis.  Water cooled spindles are the next step up, but this quickly gets expensive.  Earlier on in my CNC journey I found the Chinese air cooled 400W spindle I was using was underpowered, especially when the feed rates were increased to cut material quicker.  The Kress 1050 at the time was the hobbyists choice (along with the Gecko G540) so I use one of these on my setup, like the G540 the Kress never misses a beat.

Software

With CNC work there are typically 3 different pieces of software involved, these are:

CAD to produce the dxf/dwg (potentially pdf)

CAM to change the vector line work of the dxf for example to gcode.  This takes into account feed rates, cutter diameters and maximising how much you can cut from a single piece of material.

CNC software that takes the gcode and translates this to movement of the router.

There is enough out there about CAD software so skipping over this, but if you are a Fusion 360 user this has the CAM software built in so you can export gcode to save you money on the CAM software.

For the CAM software I use a really old version of Vectric Cut 2D, which is very antiquated but still works and is relatively easy to use and runs on a low spec PC.  The newer versions of Cut 2D have a 24” limitation so not enough to cut skins, and the PRO version is a lot more expensive.  They also do other software that allows 3D profile cuts, but this is more expensive. 

Mach3 is a ‘popular’ choice for converting the gcode into stepper motor steps, however this needs to be purchased.  There are free solutions available such as LinuxCNC for anyone with a spare Linux desktop and UGS (Universal Gcode Sender) if you want to send commands via USB to the controller.  Some controllers such as the Duet 2 have the CAM software and controller as a single software/hardware solution.

Controllers

The Duet 2 is a modern controller that can work independently of a PC and work from Ethernet/wireless, the gcode is uploaded and then the on-board hardware also controls the steppers.

A lower cost solution is RAMPs that combines an Arduino and stepper motor controllers, the software sends gcode to the Arduinno via serial which is then processed and sent to the stepper motors.

Duet 2

Here is a picture of my cobbled together controller (during partial rebuild of the limit switches), it takes inbound USB from UGS (Universal Gcode Sender) and then sends this to the Gecko G540 via the parallel port.  It’s not an elegant solution, but works given the older G540 hardware.

And here is the current machine I have, the electronics are mid rebuild due to changing the limit switches from passive on/off to active ones with noise filtering.  For some reason the upgraded machine has a lot more noise than the older one, despite the electronics being the same.

The base of the CNC is a DIY torsion table to minimise flex, and works incredibly well and also acts as storage.

There are two main changes to the machine from stock, the first being the taller Y-gantry plates and the second being the slight change to one end.  Both were needed to retrofit the vacuum table in and maintain the z working height.

Stuart Winstanley

I have been around CNCs since my early adult life. Back in the day I used to have me own sign company, and although I didn’t have a CNC myself, I often used external CNC companies when simply manually cutting around a vinyl template wasn’t good enough. So I was already familiar in how to setup artwork ready to cut. After I closed the company in the early 2000’s and moved on to what I do now, my brother who worked for me at the time, continued on in the sign business and now is the manager of a local sign company in Leigh. They have a large format industrial CNC, which without access to, I certainly would have never built my R2. Almost all of my R2 (with the expection of the dome and greeblies) has been cut on that machine out of mainly 19mm Foamex and 2mm Aluminium.

So as I have ventured further into the droid building rabbit hole, and having transformed my garage into a fully-fledged workshop along the way, I decided I “needed” my own CNC.

After days of research I finally opted for a Workbee 1500mm x 1500mm machine with a Duet 2 Controller. The Duet 2 controller comes in 2 versions, one with an Ethernet port and one with WIFI.

I opted for the Ethernet version. The Duet 2 is a great controller. It has very nice web interface which you can access by any PC on the same network. The web interface is mobile responsive, which is great. Most of the time I operate it from my mobile phone when stood in front of the machine.

Gcode files are uploaded direct to the Duet 2 and run from there. The controller also has a power loss resume feature which is really handy. You can run the “controlled power off” macro manually to put the machine in a safe resume state before powering off the machine. This is particularly useful if you’re running a long job and need to pause it for a later.

For the router itself I opted for was the Dewalt D26200. There was no real reason behind my choice, other that I like Dewalt tools. Nearly a year in and it’s been great so far.

When I bought the Workbee 1500mm x 1500mm, at the time and at that size, the only option was a belt drive the X and Y axis. The maximum screw length was 1000mm due to the whip in the screw at lengths over this.

Although I got reasonable results on large cuts there was too much play and backlash in the belt to get accurate results in smaller details. I tried having the tension in the belt so tight it snapped the belt. I tried the double belt modification, where you glue a second belt to the V-slot and it acts like a rack and pinion. This give the best results, but there were still issues when the drive changed directions.

Luckily for me at the same time, someone came up with a simple solution to the reduce the whip in 1500mm screws. The clamps where fitted to the outside of the Y gantry plates, inside of on the inside, enabling enough tension to be added to the screw to reduce the whip to a workable level. Oozenest (the company I bought the Workbee from, and can’t rate highly enough) were quick to adopt this solution and upgraded me free of charge). Since then the machine has been unbelievably brilliant, delivering highly accurate, consistent results.

My Workbee is fitted on top of 2 1500 x 750mm standard workbench. I have a dust shoe fitted which is essential in my option. I bought the one from Oozenest, which is really nice. It is in 2 parts and held together will magnets for quick release. It is quite expensive but there are cheaper options available. You can even make your own. I’ve seen CNC’d ones and 3D printer ones.

I created a spoiler board with screw fitting for clamps, which are great for thick material, but doesn’t work so well for thin stuff. For the thin stuff I made a vacuum table run off a Titan workshop hover to clamp the material down.

My first vacuum table was created in MDF. The bottom grid section was cut and sealed and a 6mm porous sheet was glued to the top. I removed 0.2mm from both sides of the sheet before attaching to remove the treated top surface and allow the air to be sucked through better.

This vacuum table has worked really well for me but I’ve recently decided to update it. The first one took up a lot of unnecessary Z height and the vacuum area was small.

I created a new and improved version out of 6mm Styrene and so far it works really well. I was able to fit the vacuum hose to the centre between the workbenches.

Once the material is on and the edges sealed with tape the hold it really strong. If you break the seal though the hold is reduced drastically. For this reason, if I need to cut through the material I always glue it to a 1mm spoiler piece enabling me to cut through the material an extra 0.5mm, giving a clean edge and keeping the seal intact.

For a spoiler piece I generally use 1mm styrene and fix using this repositionable spray adhesive:

It offers a great hold but is easily removed. If I’m cutting styrene, it normally comes with 1 gloss side with a protective film. Unless I need the gloss side to be upward facing, I glue both protective film sides together, so when cut, I can simply peel off the film and there are no traces of the glue at all.

One of the biggest issue that I’ve had to deal with, and I didn’t realise beforehand, is the actual operating noise. When the router, vacuum and extraction unit are on the noise level reaches 90 DBs, so ear protection is essential.

Because of this I’ve build a room around the machine in sound-block plasterboard with acoustic foam inside. This has worked quite well. Now, just stood outside the room, with the door open, the noise level is down to 70 DBs, and with the door, shut it’s reduced even further to 60 DBs. Outside the garage is now around 54 DBS, so shouldn’t get any complaints from the neighbours.