Joining Plywood Stepped Scarph Joints

A stepped scarph joint is the mechanically produced equivalent to a hand-planed sloping scarph joint. It is normally cut on a CNC router that has only three axes of movement, namely forward/backward, side-to-side and up-and-down. The sloping face of the hand-planed scarph is replaced by a series of small steps, each comprising a small vertical face and a broad horizontal face that coincide with mating faces on the other piece to which it will be bonded.

The main problem with stepped scarphs is similar to that with sloping scarphs. That is attaining proper alignment when gluing the joint and keeping it aligned until the glue has set. A sloping scarph can and will slide in almost any direction as soon as the slippery glued surfaces are brought together, so it has to be very carefully clamped. A stepped scarph cannot slide forward because the steps prevent it but it can slide back or sideways. Again, it needs careful alignment and clamping.

Various methods have been developed by different companies to overcome this problem. These methods are not patented, so can be used by others.

Mechtronics in Cape Town, South Africa, use a dowel peg system to lock the joints. The holes for the pegs are drilled by the CNC machine and hold the joint so securely that the panel can be moved right after the joint is made, without having to wait for the glue to set. After the glue has set the joint is sanded smooth with a belt sander, which also trims the dowels flush.

Mechtronics panel as it comes off the CNC machine

Mechtronics stepped scarph joint completed.

Ertug in Istanbul, Turkey, achieve a similar locking effect by programming the CNC machine to form islands in the one half of the stepped scarph that lock into holes that are cut into the other half. This is also a very neat solution.

Ertug stepped joint details.

Stepped scarph joints are good for any size of boat but care needs to be taken in high load situations or where there may be flexing of the panel. The weakness of this joint is in potential cracking along the surface joint lines, either from stress in the panel or from bending. Joints should, where possible, be kept away from edges of openings that might align loads along the joint to promote cracking. Edges of openings must be reinforced with wood perimeter frames or with glass or carbon tapes to spread the loads away from the corners and into the panel surfaces. The weak areas of the joint can also be reinforced with glass tape laid across the surface joints. Joinery shelves or locker tops that span across the joint also help by stiffening the panel, preventing flexing that might initiate surface cracking.

The CNC operator must take extreme care when cutting scarph joints to ensure that the plywood is hard against the sacrificial backing board during cutting, or the accuracy of the stepped surfaces deteriorates. There must be a vacuum on the table to suck the sheet against the table, as a basic requirement. If this is insufficient then plastic nails should be used to mechanically fasten the sheet to the table. All waste material must be efficiently removed as it comes off the router bit so that there is no chance of it getting between the sheet and table. This is best done with vacuum right at the cutting tool. Finally, the cutting paths need to be programmed so that any puncturing of the full depth of the sheet happens as the last stage of the cutting process. If it happens earlier it will increase the chances of waste material getting under the sheet and it will weaken the vacuum that is sucking the sheet against the table.

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