Robotic CNC Table

I purchased a large CNC table on ebay for $500.

The table was probably made in the 90s. It has a total of 18 inch movement in X and Y directions. The lead screws have a .25inch pitch. The motors that came with the table were too old to be used, and some new servos were retrofitted.

The servos also came from on ebay.

Their specs are the following:

    37.51 Oz-In/A    
    25.0-30.5 V/KRpm
    Motor Terminal Resistance .75-1.02 Ohms @ 4A.
    Maximum Terminal Voltage 60V
    Maximum Continuous Speed 2100 RPM (No Load)
    Maximum Peak Torque 1500 Oz-In.
    Maximum Peak Current 44.5A
    Maximum Continuous Stall Torque At Max Motor Temp 300 Oz-In.
    Maximum Continuous Current 8.9A
	

There are piles of posts of this on yahoo: CAD_CAM_EDM_DRO. Links 1, 2, and 3.

US digital motor encoders were used to detect the position of the motor relative to turns of the lead screw.

The table that supports parts to be cut by the laser was constructed with 12 power hack saw blades. The dimensions of the blades are 24 inches long, two inches wide and 1/16th of an inch thick. They have 4 teeth per inch. The table was constructed by running two foot long, 1/4 inch threaded rod through the holes at the end of the blades. One and a half inch long spacers were placed between each blade.

The only other construction item was the brackets that were placed in the assembly to bolt the blade-unit to my cnc table. The brackets were cut by my table and folded into the a C-shape. Its nice to have a system that can help build itself.

This is a picture of the blades holding up a piece of stainless steel. The t-shaped brackets laying flat on the table have bolts that project down and wing nuts bring up a part below the table to hold down the steel.

If you just used a flat plate to support the steel, the laser would come in and king of splash as it tried to cut. What works better is to have a small number of points of contact for the part. Turns out large two inch wide hacksaw blades work very well in this application. The tips of the points on the blades are machined well enough they form a nice flat surface. Once assembled together with the threaded rod they are very ridged and dont flex in left to right or up and down directions. The blades came from ebay and cost about $100 total.

Motor mounts

The belt tensioning system for the old table was not really optimal, so I redesigned the plates and sent off the CAD file to AyersTech to be cut with a waterjet system. The resulting parts showed up in less than 2 weeks and cost a total of $175. They’re 3/8ths thick aluminum. Neoprene belts were ordered from SDP/SI.

Visit this page to see view the hardware and software that runs the table.