Automated nesting programs such as the Microvellum Nesting Optimizer are very efficient at maximizing material usage by placing and rotating a large number of parts as necessary to create a nested pattern that minimizes waste. Along with this efficiency, because of the possibility of many small parts being included in the nest, comes a potential for inaccurate machining for some of those small parts. Or a dangerous situation may arise if some of these small parts are cut free and propelled across the shop.

The reason for this is that the vacuum system of a nesting CNC may diminish with age, or it may simply be inadequate from the beginning. This is not necessarily a problem for large or medium-sized parts. With small parts, it’s a different story.

Small parts do not contain enough surface area to react with the vacuum adequately, and the result is a situation where the parts may move during machining. The movement may be minor, resulting in machining that ‘wanders’ or it may be significant, resulting in the part breaking free prematurely. If that happens, the tool continues along its predetermined path and may catch the part and propel it up off the spoil board and across the shop. To prevent this, many Users utilize the nest tabbing option or the onion skin option of Microvellum. The tabbing option is discussed in this article.

A tab is a small bridge of material that is left remaining during the machining operation. It creates a one-piece framework that immobilizes all parts until the tabs are removed by a secondary machining operation, or the parts are broken free manually. If you can visualize, the routing tool travels along in a level path in the Z-axis until it encounters the predefined tab location in the G-Code. At that point, the tool raises in the Z-axis an amount equal to the Tab Height setting. It continues along with the path, remaining at that height for a distance equal to the Tab Length setting. It then drops back down to the original Z-axis location and continues.

You have the option of manual tab placement in both the spreadsheet type toolfile and the IPP type (Integrated Post Processer). You have the option of auto tab placement only in the IPP type toolfile.

Once the tabs have been placed, Microvellum gives you two choices for removal of the parts from the completed nest. The functionality you have available to you depends on whether you’re using a spreadsheet toolfile or an IPP toolfile.

The first option is available with both the IPP and spreadsheet types of toolfiles. It gives you the ability to remove each tab by dropping the tool a second time only at the tab locations and machining them out. When that is complete, the part is free of any other part or nest material and may easily be removed from the nest.

The second option is available only with IPP type toolfiles and gives the option to leave the tabs in place. This second option requires that the machine operator knock the part free from the material sheet and then do finish work to smooth any remaining tab material.

Small Part Handling Options

Automatic Tab Placement

To set up automatic tab placement, open a Processing Station, and click on the Nest tab. In the Small Part Handling section, click the Tabs option button to enable tabbing. Then you can change the settings for tab height and length 1 and 2. Length 1 is the length of the bottom of the tab. Length 2 is the length of the top of the tab.

If using a spreadsheet type toolfile, there are some of the settings that not available from the Nest tab interface. This includes the Border Methods of “Standard” and “Stay Down.” Also, the option for “Leave for Manual Removal” which prevents the tool from returning to cut out the tabs, and “Qty Auto-Placed Per Part” setting. These latter two settings are set up inside the spreadsheet toolfile. The "Clean Out Tool" feature is also available in this interface. Common Tool number of the router in the associated tool file. This will be used to clean out the tabs after all parts have been cut out.

Manual Tab Placement

To set up a couple of key properties for manual tab placement, open a Processing Station, and click on the Nest tab. In the Small Part Handling section, click the Tabs option button to enable tabbing. Then you can change the settings for tab height and length 1 and 2. Length 1 is the length of the bottom of the tab. Length 2 is the length of the top of the tab. Placing manual tabs is possible with any of the 3 border method settings: Standard, Center/Common Line, and Stay Down. 

After those properties have been verified, open a nest drawing, and click the “2D Machining Tools” button found on the side of the Toolbox palette, under the gear icon.

From the 2D Machining Tools palette, click:  Nest Tools > Add Tab to Nest.

Click on the part borders for the points in model space to place the tabs.

Click “Update Existing Gcode Files from Drawing” to add the tabs into the G-Code files.