Drives are a diamond's best friend

Drives on a diamond-cutting machine automate the process to achieve the largest possible diamond from the rough original.

NPS Drive Automation is using Lenze SMD drives on a diamond-cutting machine to automate the process and achieve the largest possible diamond from the rough original.

An SMD drive is used to rotate a rough diamond slowly while its circumference is being polished.

A second drive on the same machine controls the speed of the polishing wheel.

The process of making a rough diamond round before it is faceted is called bruting or rounding a diamond to form its girdle (the edge of its circumference at its widest point) before the facets are cut and polished.

It is achieved by grinding the diamond using a rotating wheel.

The rough diamond is turned at low speed requiring very high torque and smooth movement, while the grinding wheel also requires high torque operation at higher speed.

The aim of the process is to achieve the maximum diameter from a rough diamond; the machine consists of a double spindle lathe fitted with a camera.

One spindle is used to rotate the job (rough diamond) while the second spindle is used to rotate the polishing wheel.

The polishing wheel is also given a horizontal stroke (along its axis of rotation) achieved by synchronising the drive with stepper motor control.

The maximum diameter is achieved by properly centring the stone.

The exact centring is achieved by the use of a CCD camera connected to a PC and monitor.

The image on the screen is compared with a library of possible diamond polishing pattern templates and software analyses the image (comparing it with the overlay) and then selects the best fit before the grinding process begins.

The speed of both spindles is altered depending upon the size, roughness and hardness of the uncut diamond.

For harder or more irregular diamonds, the speeds need to be reduced and vice versa using the drives.

A smooth change in the speed is required for uniform polishing of the diamond and the direction of rotation of spindle motor had to be reversible.

The Vmin boost function on the 0.75kW drive is used to increase the torque output at the lower speeds, allowing the most compact model to fit into the panel space.

Senior Applications Engineer with NPS, Hitesh Sarvaiya, said: "The torque boost function alone sets the drive apart, effectively preventing other small AC drives from being considered".

"The optional high performance EMC filters however were also essential; we used them to eradicate any distortion on the monitor under high loads, which was one of the main problems with the machine when we took the project on".

"In some instances the distortion of the picture was actually affecting the diamond pattern choice and reducing the size and value of the finished product".

"Using the EMC filters to prevent distortion on the monitor has kept the image sharp and is maximising the effectiveness of the image analysis software".

The SMD range is available in power ratings from 0.25 to 22kW with an input voltage from 200 to 480V in single and three phase versions.

A removable EPM memory chip is standard on all models.

Options include high performance EMC line filters, dynamic braking kit, remote keypad control and through-hole mounting for the heatsink.

Back to top