Back-Cuts, Hinges & Control (Article 6)

 

This is the last in a series where I have been discussing a sequenced planning method to tree felling.  When read in sequence, the articles combine to describe an effective tree felling planning process. It has been my intention to encourage readers to think about many aspects when felling trees – with the focus being on tree faller safety.

It is important to remember that some tree felling techniques developed from years of trial and error and in field practice and in all cases deserve our respect and acknowledgement. It is difficult to produce conclusive scientific data that verifies certain aspects of tree felling techniques, due to the many variables that are at work when dealing with living organisms such as trees.

Remember that new tools cannot get into a toolbox when the lid is closed.  An open mind is like an open toolbox and new tools when tried and proven are a wise addition.

In most cases, the back-cut is the final step when felling a tree.  The back-cut removes the wood left preventing the notch from closing, it also forms the hinge wood. The hinge wood is the wood left uncut behind the notch and is what provides control and guidance to a felled tree. It is important to understand that the hinge can only offer control as long as it can work; when the hinge fails, control is lost.

Cutting & Forming the Hinge
 

Traditional Back Cuts

A crosscut saw was the tool of choice and for generations back-cuts were cut by hand with a crosscut saw, cutting from the back of the tree towards the notch.  The modern chainsaw and tooth design offers us alternatives to this traditional method. The notch selected should be considered when deciding where to place the back-cut. Traditionally it has been recommended to raise back-cuts above the apex of the notch.  This technique (called the stepped back-cut) forms a step or back stop and minimizes a tree from sliding backwards off the stump causing great risk to the faller. It is important to raise back-cuts whenever felling a tree with a small notch opening such as 45 degrees or less. In these cases, the hinge will have to break when the face notch closes. After all a notch with a 45 degree or less notch aperture will have to break the hinge and as a result will lose control when the tree in most cases is only half way to the ground. A limitation of a small notch aperture is that it forces hinge failure before the tree reaches the ground, thus causing loss of control and gravity takes over before the tree is on the ground. If limb tying or felling obstacles are present that could snag the falling tree a stepped back-cut is also advisable regardless of notch opening.

Open Face Notch

Forming the hinge and choosing your back cut

If the notch opening is 70 degrees or greater (called an Open Face Notch), the likelihood of hinge failure is greatly diminished and the risk of the tree sliding off the stump along with it. A simple advantage of an open face notch is that it gives the hinge the opportunity to work through the entire arc of the fall thereby giving a greater degree of control of the falling tree and improving faller safety. In some standards, this advantage also allows for back cuts made in conjunction with open face notches to be placed lower and in some cases in line with the notch apex. The reasons behind this relate directly to the inevitability of hinge failure and the many risks associated with this loss of control associated with notches with openings less than 45 degrees.

 One safety advantage of a back cut that is more in line with the notch apex is that the operator’s chainsaw is less likely to be caught and pulled along with a falling tree. A raised back-cut, stepped up from the notch aperture can create a kerf in the section of the trunk that is falling. This is especially dangerous when felling or topping large treetops. Another advantage of using a back cut that is in line with the notch apex is that it is easier to avoid by-pass when forming the hinge, as your back cut line is in line with the notch apex.  A raised back cut takes focus away from the notch apex. Finally it is suggested by some that an even hinge performs more predictably than one formed in a staggered fashion. I have found both back cut methods useful, but I must mention that certain standards state and require a raised back-cut when felling trees. In all situations, you must comply with your local regulations and standards. Changes and improvements to regulations require the involvement of industry professionals such as you.  I encourage everyone to stay active on industry committees and review boards as this is how you can effect changes and modifications made to regulations.  Often the review process can be lengthy, and it takes time to introduce alternative valid methods and techniques into regulations.

Back cut placement in relation to the notch aperture is of significance. The other important consideration is the method used to make the back cut. The most traditional and common back cutting method is cutting from the back of the tree towards the notch.  In some cases, sight lines scribed into the bark can help ensure proper height and avoid hinge by pass. This traditional back cut method requires the saw operator to cut until the tree begins to fall, at which point simultaneously the notch begins to close and the hinge begins bending and providing control.  All of this happens when the hinge wood is cut thin enough to begin bending or hinging and applies to forward leaning trees.  When felling back leaning trees or trees that are otherwise loaded against the notch, it may be necessary to use wedges, jacks or pull lines to initiate the fall. 

Always remember – when the tree begins to fall is the most dangerous time for any faller. On average, it takes 15 seconds from when the tree starts to fall until it settles down on the ground and studies have shown that 90% of all faller fatalities and injuries occur in these 15 seconds and within 5 feet of the base of the falling tree. I explain this as the 5-15-90 rule in my article regarding escape routes.  Always get down your escape route as quickly as you can and get distance between you and the falling tree – the more distance, the better.

Bore Cut Back Cut

Another back cut method that can offer a faller a quicker option for heading down the escape route is the bore cut back cut method. This method is limited to larger trees and is generally not suitable for trees less than 6- 10 inches in diameter. With the bore-cut back cut technique, the cut begins behind the notch with the bar of the saw being bored through the center of the remaining holding wood. A sound understanding of chainsaw reaction forces is necessary whenever bore cutting and the bore must be started with the lower quadrant of the bar tip and with correctly sharpened cutters and filed depth gauges.

Bore Cut Back Cut Technique

 

Generally, the bore cut passes through the entire tree and then the cut is made forward towards the notch forming to create a desired hinge wood thickness.  A common rule of thumb for hinge wood thickness is 5- 10% of the tree’s diameter. This leaves a section of uncut wood left behind the initial bore cut.  The cut can continue backwards from the bore cut towards the back of the tree until a small section of holding wood remains. At this point, the faller can take time to ensure the falling zone is clear and free of incoming wanderers and to assess and prepare his escape. With one simple and final cut of the saw through this holding wood the tree will be released and begin to fall. This description again applies mostly to forward leaning trees.  In the event the tree is back leaning or loaded too much to initiate the felling process, it is important to have wedges, jacks or pull lines ready and available.

The bore cut is a cutting method that allows saw operators various advantages when falling large diameter trees where the saw bar is not long enough to cut through the entire trunk and used when falling very large diameter trees. When bucking logs that have a diameter that exceed bar length, the bore cut affords some advantages also.

This concludes this series and I hope you have enjoyed reading and have expanded your mental toolbox.  Education is a journey – not a destination.

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