Monday, February 10, 2014

Project CNC - Start of Build, Materials


An overview of the materials I've used in my build as well as some the rationale I used for my choices.

General Structure
  • 1/2” MDF – 2'x4' piece was about $6.

Motors:
  • Initially I hoped to use ones from old printers and scanners
    • First problem that I ran into was that most of the steppers I've salvaged were unipolar and I needed bipolar motors
    • Next – motors were could not produce enough torque
    • Last – the steppers that I salvaged were all fairly low resolution (about 50 steps per revolution and I wanted more in the range of 200 steps per revolution).
  • Decided on 3 x NEMA 17 (picked up on ebay – they came with vibration dampeners which was a nice extra).
  • Spindle Motor
    • Old cordless Dremel (10-12 years old). When the battery pack died I removed the motor and eventually remounted it in PVC piping along with power cord and a toggle switch to control direction. The shaft on the motor has a rotary bearing and collet holder already attached. This is the motor I used in my initial prototyping.

    • Expanded to using a knockoff brand rotary tool (Toolshop – picked up on sale at Menards for about $15).


Guide rails (rods):
  • From the start I thought of using the steel rods out of printers and scanners
  • I've kept old ones with the idea that I could use them for something someday (they are precision machined).
  • Only issue was that I found that I had 3 different diameters 6, 8, 10mm.
Decided on the 8mm rods since they seemed to be a common size for linear slide bearings when I did online searches.
Initial Y-axis frame with 8mm guide rods

Searching through my salvaged parts I determined that I needed at least 2 more 8mm hardened steel rods. So...I made a trip to Goodwill (with a small section of rod in my pocket to compare) and purchased a a few old scanners that I found ($2-3 dollars each – plus they contain Cold Cathode Fluorescent Tubes, with the boards to power them, additional stepper motors, glass plates, tactile buttons, as well as other goodies).

Currently I am waiting on some additional 8mm rods that I ordered off of ebay (since I decided to use a full size rotary tool for my spindle I wanted to enlarge the size of the work area). When they arrive I should have a work area of about 8” x 13”.

Linear Slide Bearings:
Due to my using 8mm rods for guides I decided on using SC8UU Linear Slide Ball Bearing Block Bearings (this was based on them being the most common slide bearings for purchase – on ebay on other sites). I did do searches for 6 and 10mm slide bearings but they were less common and more expensive. In addition; I wanted “Block Bearings” since I could secure them easily with screws.
Bottom view of Y-axis - square blocks are the linear bearings

Screw Drive vs/Belt Drive:
Initially I intended to use screw drives for the x, y and z axis – later I switched to thinking about using belt drives with gears and belts from old scanners for x and y axis (after reading an article on ebay). After looking at all the different belts I have (and gaining a better understanding of sizes, pitch, spacing, etc. - which came about from my research on what other people have used, why they used it and what is available) I realized that I don't know what belt came with what gear system – I salvaged them from old printers and scanners but did not keep each set together.

Further research lead me to switch back to using a screw drive, since I could use use regular all-thread rod with some additional elements to reduce backlash. In addition I did the math – 24 turns per inch comes out to about 1mm per revolution and with my stepper motors that is 200 steps per revolution or, about, 200 steps per millimeter which should be plenty good resolution. 

I continue this overview in my next post and then get into the nitty gritty of the build process.