As we described in the previous article, There are many pros to designing and building a custom hobby CNC machine.
When it comes to designing and building any machine you will come across many different problems so easy to fix others not so.
When designing a CNC machine you will realise how simple they are, the concept is pretty straight forward, but you can quickly run into issues in the process, but don’t let that deter you.
That’s is why I have created this Guide, to keep you away from the possible errors you will make along the way this will help buy keeping your costs and spent time.
This guide will mainly concentrate on the hobby CNC router machine, because these machines are relatively simple the guide could easily be adapted to most linear motion systems, for example you could remove the Z-axis and router and add just a laser to create a CNC laser cutter.
By reading this page you are probably considering building your own CNC router machine. You probably even have a design in mind, this is to be expected, at the end of the day its your machine build it as you wish.
This guide is not in place to change your design or tell you, you must build it this way for it to work, but rather supplement your design with useful and valuable information on your CNC router machine.
I have wrote this guide, in the belief that you have some general knowledge of how a CNC works, if not you may want to take a look at CNC Routers the Basics.
So Lets Get Started
1) Choosing the right designing
There are two main designs you can choose from for the hobby CNC, this may seem to contradict the fact that every machine build is different, but when you look deeper you realise there are only two you will ever encounter, they are.
1) Fixed Gantry, Moveable Cutting Bed.
2) Mobile Gantry, Fixed Cutting Bed.
You can see the difference in the two designs below.
Now will explain in more detail the difference in design.
The Mobile Bed
You don’t see this all to often in the hobby CNC world, but there are still reasons for picking this type of CNC machine because they can be built strong and sturdy they are ideal for PCB or engraving machines, for these size machines they work well.
As mentioned above this type is very sturdy, because the gantry is fixed there is no flex In the gantry under load, this is because there are no components in place to allow flex ie linear rails and bearing sliders.
When using a mobile gantry you must take into consideration the mass of the gantry and use the approprate linear bearings on the X-axis. With a moble bed design the gantry its self does not move this mean you can build the ganrty as heavy as you need to reduce vibration to a minimum.
With ever pro there is a con and the con of this CNC design style. And one of the main ones is that when you increase the length of the X-axis the mobile bed becomes less efficient.
If your unfamiliar with the X,Y and Z Axis notation I have added a picture to reference better, as described in the basics of CNC router machines the X,Y and Z are always in this configuration.
On the mobile bed design when you increase the length of the x Axis you’re design will become less efficient this is due to the size of the bed, on average you will find the general size of these machines to be around 100 to 300 mm, I have seen larger but the design of them becomes more and more complicated.
With the Y and Z-axis you don’t really seen many changes in them.
In conclusion if you are after a small machine for engraving or for PCB work these are ideal, personally I have never built one a I don’t like the design.
The Mobile Gantry
These probably make up around 95% of hobby CNC machine designs and this is the design style I would choose over the other. In this style the bed is fixed and has no moving parts the only thing that moves is the gantry is is beneficial because the whole cutting bed becomes useable unlike the mobile bed version.
The problem you will find is building a gantry that is light weight but sturdy, it will also need to be designed to fit some for of linear bearing assembly.
If you are not considering building a small machine then this would be the ideal design to go for. We will go into more detail about this later.
But for now you need to consider which size best to suit your needs.
2) The Cutting Area
The cutting area is the overall distance in which you can cut over, so if on your X-axis you have a 700mm linear bear you deduct the length of the two combine linear bearings so if X = 700mm – 200mm of the linear bearings you have a tool tip that can move 500mm on the X-axis.
So when it comes to your design you want to build it the right size these are things to consider which we will cover more on a later topic structural considerations.
But first off we will want to workout what we wish to cut its size maybe just A4 size or maybe a full sheet of plywood 2440mm x 1220mm we could build a machine this large but it will become expensive and your find many issues along the way so I suggest keeping your design small to start learn from your mistakes and then go from there.
Common issues with large machines can be that the mobile gantry that you would have to use could sag and have a lot of flex in it.
The cutting area is also known as the machines travel and this is the movement along the axis, this will be smaller than your overall machine size which will explain more in stage 3.
3) The Machine Size
Everyone one I know that has built a CNC machine has tried to build it as big as they can get even I did when I first started designing my first project, but there’s a few issues to cover.
Do you have the space for such a large machine whether you built a shed solely for it or its going in to your garage.
Allow room so you can easily move around your machine when it come to repairs or modds do you want to have an easy job doing them.
Money, the root of all evil and the same with you massive design, problem is ACME lead screws and linear rails don’t come cheap and more weight bigger steppers.
So with my current machine I have a space 700mm x 700mm so my machine would be this size to fit its also an easy size to manage and repair.
My Footprint is 700mm x 700mm so how big is my cutting area I cover this in more detail here, but briefly if your gantry is 400mm you would want your bearings to be around half that distance so 200mm this is taken away from the overall linear rail size. 700-200=500mm cutting travel.
Aim your design at having as much travel distance as you can possible achieve.
4) The Machine Tolerance
The ideal of having a CNC router machine is to complete your jobs accurately more accurately than if you was doing it by hand.
When designing your machine you must consider accuracy and precision and these 2 things should not be confused as the same thing.
The image below shows.
Accuracy is the machines ability to machine with tight tolerances.
Precision is the machines ability to perform the above repeatable.
Your machine should be both accurate and precise and hold and maintain this. For example you maybe able to cut a piece is within 0.01mm but repeatability might be 0.10mm.
Most people design there machines and live with how they perform overs try bettering the accuracy and precision.
There are ways to build your machine so that it is both accurate and precise if you are happy with a 0.10 mm machine then and that’s all your ever need so be it.
But if you want a machine that is 0.01mm repeatable then there are some design requirements you must include into your design in order for it to perform so well.
Typical hobby CNC routers can hold tolerances of between 0.01mm to 0.10mm however, this is your decision on how tight you want your tolerances, but make note the larger the machine becomes the more costly it becomes to reduce them tolerances.
5) Materials and Tools
Materials first so what are you going to use for your design Aluminium, MDF, Plywood Plastics Steel.
The material of which you choose should fit your budget building a machine fully of aluminium can become very expensive quickly.
Does it matter is MDF worse than aluminium “no I have seen MDF machines hold tighter tolerances than one of aluminium.
It is crucial to pick your materials before designing your machine reason being is the right material for a wood machine is not always the right material for an aluminium machine.
Tools steel is not to hard to work with but you will need a steel cutter and posible a welder.
Aluminium can be tricky requiring a special cutting disk and drill bits and router bits can become clog to.
Plywood MDF and Plastics are the easiest to work with easy to cut and drill common toolling can be used.
Most people build a UGLY machine first normally made of wood they then use the UGLY machine to build a nice fancy aluminium machine.
If you do not have the right tools to build your CNC this will show in your machines tolerances.
We will look at the properties of materials later, this will allow us to calculate deflection within SolidWorks find weak spots and amend them before we start are build.
It will also help you in designing your machine to the load ratings you specify.
Last one on the list, for most of us me included this is the biggest restriction in your build for instance I would love a machine that had a ATC spindle but my budget suggested otherwise.
You can easily make an expense list in spreadsheet covering all costs.
After all what’s the point in spending loads on a hobby machine when it might all accumulate to a cost you could of got a low range machine prebuilt.
Example of a Budget Spread Sheet.
Summary of what we covered in part 1
1) Choosing the right designed
2) The Cutting Area
3) Machine Size
5) Material and Tools