Vacuum loss while machining is a real problem. Vacuum drops with each pierce and cut. Since the coefficient of friction is based on the area of the part, smaller parts are the most affected. Unfortunately, the lateral force applied only varies by material and feed so what was ok for large parts may not work at all for small parts.
Step one is to ensure that there are no machine vacuum leaks and than the spoilboard edges are seals etc. Here are several strategies that will help with the movement problem. Two of these strategies try to conserver vacuum for when it needed most and the third reduced the force feed.
Option 1: Conserve vacuum by not cutting through the material.
Leave an ‘onion skin’ to reduce the amount of air passing through the spoilboard. Parts hold more firmly. Two passed are required to fully cut the parts. The first does do most of the cutting where the lateral forces are the greatest. The second only has to remove the ‘onion skin’ which applies only minimal lateral force.
When programming the parts in ONGAA CAM the programmer orders the processes appropriately, roughing and the finishing etc. In the case of cutting the skin, generally last, the programmer will also set the ‘Finalizing Process’ flag. This ensures that all other machining processes are done (on all parts) before cutting the skin. This will happen even if the same tool is used. (Note.. During nesting, processes from one part program are interleaved with others but the order of the steps are never rearranged)
Option2: Cut the smallest parts before too much vacuum is lost.
By cutting the smallest parts first it ensures that the most vacuum available is holding the part while it is being cut.
While nesting, selecting ‘Small Parts First’ will order the part cutting based on area and rectangular circumference. Although this option can be used with tool change optimization, using both options together will minimize the effectiveness. In general, if small parts are a problem, use only the part ordering option.
Option 3: Slow down while cutting small parts.
Since ONGAA CAM supports variables as do many machines like Biesse, Homag and Holzher, it is possible to use the part area as a FEED factor ‘per part’.
ONGAA CAM supports this by using variable indexing while they are added to the nested program. FEEDFAC for example in the part program would become FEEDFAC1 the first time it is encountered. Each parts L,W, would also become L1 W1 etc.
Making FEEDFAC = IF (( L * W ) / 92000) > 1 THEN 1 ELSE ((L*W)/92000) would provide a slowdown factor for parts smaller than 92,000 sq mm (about 1 sq ft).
Options 3 can be combined with option 1 or 2.
