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Article 3

ARTICLE 3

Cutting Optimiser - gimmick or necessity?

Echidna has developed an accessory called a Cutting Optimiser, which can be attached to an excavator to improve the efficiency of cutting with rock saws, both diamond tipped and tungsten-carbide. For more information about the Cutting Optimiser Accessory, Click here. To find out how it works, read on.....

In rock cutting, as with any other cutting or grinding operation, efficiency is largely given by the feed rate of the tool as it runs over the workpiece. In other words each tooth should take consistently the same bite as it passes through the material. This is extremely hard to achieve with an excavator, where the machine is tremendeously strong and the operator has no feel, no direct feedback.

Operator controlled cutting with excavator driven rock saw

Graph 1

Optimised cutting with excavator driven rock saw

Graph 2

Consequently, the process of cutting with an excavator hydraulic rock saw is very often characterised by a never ending series of stopping and restarting as the blade jams and stalls as too much pressure has been applied and then has to be retracted back a little, let the blade start spinning and start again.

The only feedback an operator has is what he can hear, which often is not much value as his workplace is often a very noisy place to start with, and what he see the blade doing. More often than not this audio-visual information comes too slowly for the operator to be able to react in time and correct in the right amount in order to keep the cutting performance at its optimum. Besides all else this places an enormous load on the operator's concentration and the seemingly boring rock cutting job quickly becomes very tiring.

All our hydraulic diamond rock saws and carbide rock saws come equipped with hydraulic test points where flow and pressure can be measured during the cutting operation. The result of an example of such measurement is shown Graph 1 (below).

Operator Driven Rock Cutting

The red coloured trace shows the pressure applied on the blade. It is seen how, initially, pressure increases until it reaches a peak val

The aqua coloured line shows the rate of material removal, i.e. the cutting performance. One can see the most efficient cutting happens when about 15 MPa is applied to the blade. At that point there is enough pressure for the tips to take a good bite, and not too much for the excavator pumps to deliver enough flow to keep the blade moving at good speed.

As the pressure goes up, the teeth are trying to get even a bigger bite, but the excavator pumps fail to deliver enough oil and the blade rapidly slows down and stops, i.e. the cutting rate drops to zero. Below the magic value of 15MPa, the blade spins happily, but because there is no pressure on the teeth to take any decent purchase, the cutting rate also drops to zero very rapidly.

As the graph clearly shows, the cutting performance. i.e. the height of the aqua coloured line, is very sensitive to the blade pressure as soon as it deviates a little bit from the optimum 15MPa the cutting rate slows down very fast. The red line shows the average cutting rate achieved. In this real example the average cutting rate value on the graph is about 9.3 which for a 500mm deep cut with a 1200mm diamond tipped blade corresponds to a cutting speed of about 20m/hr - which is a fairly average speed achieved for a 5t excavator with a good rock saw working in hard sandstone.

Please note the 15MPa is not some magic number universaly valid for all kinds of rocks saws, cutting blades and materials! This optimum varies from machine to machine and from one rock type to another, but importantly, there always is such one value for each combination of equipment and rock .

Optimised Rock Cutting

The next illustration, Graph 2 shows the same kind of data taken on an excavator equipped with our Cutting Optimiser or auto driver as it is sometimes also called. This device continuously reads the pressure applied to the blade at very fast time intervals and instantly applies changes to the movement of the excavator arm(s) it is connected to. The way it works is that as soon as the saw starts spinning a clever circuitry detects it and starts controlling the movement of the excavator arm. The operator keeps the appropriate joy stick at "full on" position and the Optimiser adjusts the flow to the arm so that it advances at a rate which produces the right amount of blade pressure. If the material get softer it goes faster to keep up, if it gets harder it automatically slows down. These adjustments are so fast they are invisible to the human eye. An observer simply sees a blade that spins at a constant speed.