Saturday, July 31, 2010

Friday, July 30, 2010

Beta Table - IT WORKS!

I finally got the cnc up and running! To test it out and practice using Cambam and Mach3 I cut some trial projects in insulation foam.

1. Accuracy Test

To determine the accuracy of the machine a 2" square and circle were cut. The results are shown in the picture below.

Beta Table - Trolley

Beta Table - Gantry

Beta Table - Base

The base for this machine is pretty much as easy as it gets. Everything on the machine is made from 1/2" MDF. The base is a single sheet with groves cut for 3/4" aluminum tube as the linear rails. In the corners there are mounting block to secure the tube as well as braces glued to the sides to resist the tension of the timing belt. To finish it all off there are clamping blocks on top of the mounting blocks to hold the timing belt.

Routing the groves for the linear rails. 1/4" wide, 1/8" deep.

CNC - Beta Table

With my first attempt at building a CNC machine not successful and beyond repair I returned to the drawing board to design a new one from scratch. This time I took note and carefully studied the design intent of published designs that have been built and proven. Returning to my previous research gave me two paths:

CNC plan on posted by oomlout:
This is simply the best set of plans I've seen for a DIY CNC machine. The instructions are clear, include LEGO style assembly drawings and include a BOM and pricing (from American suppliers). I learnt a heck of a lot from walking through the plans step by step. If anyone wants to build a CNC that has the best chance for success I wouldn't hesitate to recommend this design.

Joe's CNC:
This appears to be a very popular design and has notable features such as a torsion box base and duel guide rods each side. The down side for we was the use of leadscrews/threaded rod for the drive mechanism. From my previous attempt I found that 1/4 threaded rod developed too much whip if unsupported over 1 1/2' or more and I wasn't willing to invest the money into proper leadscrews or 1/2" threaded rod for another test.

With these two sources in mind I went about designing a table that combined my favorite aspects of the two. It would have a torsion box base to resist warping and remain flat and accurate but would also utilize a timing belt drive system. I completed the design and sat back and reviewed my work. The design would have been stiff but included many duplicate pieces that would have been tedious to cut and nearly impossible to duplicate accurately. The heavily over constrained design made me return to the drawing board with a new mentality to keep the design as simple as possible.

This time I used the design posted on as my starting point and only made changes that would simplify the building process. The design had a fairly complicated gantry with lots of small tabs holding everything together. While the author claimed it could be made with a bandsaw, drill press and hand tools I was sceptically at how accurate it would turn out. I chose to keep maintain the same base design but to use my design for the gantry and trolley. Having learnt my lessons with MDF the gantry design is centered around an 'L' section beam which would be the simplest part to make and would have more than adequate stiffness in the principle directions.

I was finally happy with the design which give me confidence in its build-ability, precision/accuracy and aesthetics (I know this one shouldn't count but I'm not going to invest my time into an ugly duckling). Time to start building!

Building logs can be found following the links below:

First Attempt - Lessons Learnt

I am a believer that setbacks are not failures as long as you learn from your mistakes. By reviewing the mistakes I made and endeavoring to learn from them I hope to put my first attempt at a CNC machine under the "learning from experience" column.

There are four major points that lead to the failure of this machine. I will address them individually and propose simple solutions for their remediation.