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 stepped joint details.|
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.
See our range of designs at http://dixdesign.com/ .