Building my CNC Router

Drive.

Backlash. Backlash is the sideways movement in the nut along the thread, the "slop", and its a problem. You want to remove as much backlash as possible, without increasing friction too much. There are a few ways to do this, one way is to cut the nut half way through, and then squash it down onto the thread to take up the backlash, another way I used is two nuts on the same thread, one nut adjusted slightly away from the other untill the backlash is reduced without creating too much friction.

I later changed the all-thread for Acme thread. Acme thread has a much higher pitch, 3.5mm in my case, so this means the axis will travel 3.5mm per revolution. When I was running the 1.5mm pitch all-thread, my stepper motors were running at their maximum speed, so by going to the 3.5mm pitch Acme thread I could expect to at least double the axis speed. I bought my acme thread from United Fasteners in 6 foot lengths for about $50 per length, cheap enough. The problem was the nuts that came with the thread were rubbish! About 1mm of backlash and poor finish on the thread, so I had to make my own nuts.

I has some scrap HDPE ( you can use those plastic kitchen cutting boards, same sort of stuff. ), 12mm thick. When heated HDPE turns into a clear slimey liquid, can be shaped, and then cooled to retain the shape. I cut some blocks as shown, the center hole is slightly smaller than the Acme threads inside diameter. This block was then wrapped around a scrap length of the Acme thread. Using a gas blowtorch, I heated the Acme thread about 2 inches away from my HDPE block. As the thread heated up, the HDPE started melting into the thread. Using a big pair of multigrips, I squashed the block down untill I felt the HDPE had fully melted into the thread, then been carefull to hold everything nice and square, I dunked the whole thing into a bucket of cold water until the Acme thread had cooled. Its then just a case of un-winding the newly formed nut, seperating the two halves and cleaning up the edges with a file and sharp knife. The first few didn't work too well, it takes a bit of practice to get it right. The result is a HDPE nut with no backlash ( the top self tapping screws adjust the backlash ).

I use chain and sprockets for the connection between stepper motor and drive thread, but I could have also used toothed belts or direct drive, so why chain?

Chain has a few advantages for the hobby CNC machine. First up, its cheap, the chain I use has a 1/4 inch pitch, and comes in length of 10 feet for about $30. The sprockets to match are about $5 for the smallest 11 tooth, to $15 for a 22 tooth from memory.

2nd advantage is its very easy to change drive ratio. I can swap sprockets to play around with the axis speed and torque, and its easy to shorten or lengthen the chain with connecting links.

The last big advantage for me is chain has a tollerance of incorrectly lined up sprockets. My stepper motor may not be in perfect alignment with the drive thread, but the chain drive will still work OK.

Dissadvantages? Chains need a drop of oil occasionally, they need to be kept clean, and they can be noisy. Idealy, now that I have set up the machine with the optimum drive ratio's, I should replace the chain and sprockets with a toothed beld setup.

Direct drive coupling is best, if you can run a 1:1 drive ratio. The coupling needs to allow for missalignment between the stepper motor and drive shaft, and this can get a little tricky, but once set up a direct drive will be reliable and quiet.

To add in the near future...

• CAD drawings of the lead screw end layout, bearings and pre-load adustment.