Cutting Processes

It has been stated that a longwall advances by cutting slices off the block. This is relatively simple for hand worked faces and with a coal plough, but with mechanized longwalls using shearers the means of doing this is not as straightforward as it would at first appear because of the complexity of the equipment. To start with, the cutting machine has to cut into the face after each slice is taken to line itself up to cut the next web. It is incapable of cutting at right angles to the face, so has to be eased in at an angle. This is achieved by "snaking" the AFC on which the shearer travels, so that the cutter drum can cut a wedge shaped section of coal until the full depth of the web is attained.
It is possible to cut a full web in one pass and to do this in either direction, a process known as "Bi-directional or Bi-di cutting". An alternative is to cut in one direction only, known as "Uni-directional or Uni-di cutting", often actually cutting only part of the web height in one direction and the remainder in the reverse direction. There are other more complicated processes involving only taking half the web width in one pass and the remainder on the return.
At first glance Bi-di cutting would appear to be the quickest way to advance and frequently this is the case. However there are advantages with the other processes, relating to simplicity of operation, "steering" the face equipment, effect on the coal haulage system, power requirements on the shearer, location of operators in relation to dust sources, etc which can result in better productivity overall. The best system will often be different for different mines, particularly in different seam thicknesses and possibly even with different personnel.
If the full web is cut in one pass, more power for cutting is required and the shearer will move slower than is possible if only part of the seam is cut. This, and the simpler overall process, may allow Uni-di or other processes to attain similar production levels to Bi-di.
If a shearer cuts coal at a given rate, the effect on the haulage system will be different depending on the direction of cut – when cutting towards the tailgate the coal is carried away from the shearer while the shearer is moving away from the cut location; when cutting to the maingate the shearer is travelling in the same direction as the cut coal and so tends to load fresh coal on top of that already on the AFC. The total coal load for each web cut is the same, but there are higher peak loads on part of the AFC when cutting to the maingate. This variation can be evened-out with methods which do not cut the full web in one pass.
There are two primary sources of dust on a longwall face, the cutting machine and during support advance. With Uni-di operations it is possible to keep operators on the intake side of these sources most of the time, particularly with remote or automatic chock operation. With Bi-di cutting this is not possible at all times.
Another aspect of the cutting process which requires strict attention is the straightness of the face and its angle to the gate roads. To begin with, the face equipment is a fixed overall length apart from a small amount of play between items. If the face contains excessive curvature, particularly in the plane of the seam but also to some extent perpendicular to it, it could happen that the face end(s) will be within the longwall block and short of the gate roads. If curvature of the face is too great it is also possible for the pans to become locked and unable to be advanced.
Face alignment is maintained by checks with a string line across the face. If out of alignment, a "straightening cut" is done whereby the AFC pans are only partially advanced by different amounts up to the string line and only a part web is cut across most of the face.
A face will seldom move in exactly the required direction when advanced. If there is a dip across the face, the chocks and pans will tend to move down dip all the time. If the snake is always in the one direction the face will tend to move towards the face end where the snake begins. If chock side shields only touch one adjacent chock, that chock will tend to be pushed away from the one it is touching. If the face line is not perpendicular to the gate roads, the face will tend to move towards the gate which is lagging. All these factors can act together to tend to move the face towards one gate or the other, this movement being known as "face creep". If it is allowed to go too far the maingate equipment can run into one of the ribs and the tailgate end may either run into the rib away from the face or will not reach the edge of the block.
The usual method of "steering" the face is to intentionally cut it at an angle other than a right angle to the gate roads so one end leads the other and the movement so caused counteracts the unintentional movement which is occurring. The required angle is created by cutting a "fly cut", whereby the AFC is set up to a string line set to the required angle. A wedge shaped web is thus cut. It may be necessary to take more than one fly cut to achieve the desired angle.
Note that if too great an angle is present this can have the same effect as curvature on the face where the equipment may not reach the gate roads.
A further aspect of cutting relates to grading. The need for this may arise from:

• Presence of seam discontinuities (faults)
• Gate roads floors cut below normal face cutting height for whatever reason
• Need to cut extra face height (possibly for a number of reasons, but frequently when approaching the end of a block)

Shearer drums have the ability to cut some distance below the bottom of the AFC as well as above the normal roof height, so any profile can be cut within these limits. Whenever grading is required however it is necessary to keep in mind that changes must be made gradually. Longwalls cannot handle sudden changes well in any direction. Grades along the face must be within the limits of vertical movement between pans. In the direction of face advance, large steps are to be avoided (the AFC may jam up against a large step or chock bases may bridge across steps and have poor floor contact or the AFC toe may dig into the floor, especially if soft, and be difficult to level off again).
The position of the drum cutting the roof level can usually be observed visually, especially if there is a good "marker bed" in the strata (which may be the seam roof) or can be judged relative to the chock canopy position. The floor drum may not be so easy as it cannot be readily seen. The use of a measuring stick is a simple but effective means of checking.
Attempts have been made to control cutting height automatically by sensing some strata level (eg seam roof) and maintaining a constant height relative to this, but these attempts have not been entirely successful
Another method of control involves carrying out a manual shear during which a control computer "learns" the cut profile and will then repeat the profile automatically. The "learning" has to be repeated whenever a changed profile is needed for whatever reason.
When cutting with a shearer, an important aspect to be kept in mind at all times is that the angle of the cutting drums is fixed relative to the shearer body and the latter is governed by the alignment of the AFC. If the AFC is tilted towards or away from the face, then so will be the drums and to a position exaggerated by the distance they extend in front of the AFC. If not controlled carefully it can become very difficult to guide a face back to its desired position within a seam, especially if a face begins to dive into a soft floor.