CNC frame design considerations

The real challenge in building a CNC is to choose right type of CNC for the desired task. Remember something. The CNC will (a homemade low cost at least) not respond well to every situation. It will be good for the main type of work it was created for and be poor for almost everything else.

The lower priorities might change from builder to builder but the main goals are (in this order):

1-Rigidity

2-Light Weight

3-Vibration tolerance

4-Size efficiency

…

After building my CNC I ended up getting a CNC with:

1-Light Weight

2-Size efficiency

…

9-Vibration tolerance

10-Rigidity

In other words my CNC sucks big time lol. It will allow me to do very, very light milling or plastic printing and that’s it. My previous MDF CNC was more stiff than this one although it had other kinds of defects, like to many places where some degree of detuning was induced by vibration.

Despite all defects my CNC is very light weight. The frame itself is about 1/3 of the total weight (the rest is from the lead screw system and motors). Here is a 3D sketch of my machine.

I’ve learned this the hard way: IF YOU DON’T PLAN AHEAD EVERYTHING YOU WILL SPEND ALOT OF MONEY FIXING AND REBUILD THE CNC.

So what are my advices to build a CNC:

1º - What is the purpose of the CNC and what kind of materials are you going to work?

-Think this through it will influence the whole design. What’s the maximum size of the machine, the material it’s made of, etc…The harder and heavier the material you are planning to work the more important rigidity becomes. The bigger the volume of the biggest piece of material you intend to work more efficient the area of work the machine has to be and of course the harder the rigidity is to achieve.

-There are different typologies of CNC. The movable/fixed part (gantry, bed), type axis of support (fully, partial). If you’re not trying to reinvent the wheel check existing CNC plans (JGRO, and others) for references. My constant decision changing during the execution did not helped the final result. There are advantages/tradeoffs in every single option. Google-it, read learn.

Resuming the more popular solutions:

• Small milling jobs, and/or lightweight materials, with very high precision can be done in a movable bed CNC (X axis only). It’s quite easy to build to. Nice rigidity.
• Larger parts, and/or heavy materials call for a movable gantry with partial/fully supported approach. Rigidity harder to achieve.
• For very lightweight materials and very small jobs and small machine size consider a X and Y movable bed design. This design is more sensible to alignment issues.

From my research and trial/fail experience the Tweaky CNC seems to be a good starting point. The way the frame was designed seems to assure a good compromise between all factors and most situations above.

2º What are the available raw materials available to you?

For me this was a major setback. I don’t have access to extruded aluminum or steel profiles (like the T-slot). So I had to make everything from very basic hardware, that allied to my weak experience made a poor machine.

3º Budget?

Don’t fool yourself. It will be expensive no matter what. But it can be more or less expensive after all the design considerations taken.

4º Approach?

Draw, measure, draw again measure, check for flaws, measure and draw some more. PLAN PLAN PLAN. To keep the costs down plan the whole machine before building. This will reduce mistakes and unforeseen mistakes (that will surely happen in custom made designs).

This is a learning adventure. You will become better. Don’t be afraid to fail (and you will fail), but be smart (smarter then I was lolol). Research a lot before committing. It’s very hard to undo a poor decision, once the ball is rolling.

Here are some usefull references to start with:
CNC Basics
Linear Motion Ideas
CNC Forums
Free CNC Control Software
Self Replicating CNC Project
DIY CNC (one of the many exemples in the WWW)

Hope this helps.