Tabbing Versus Onion-Skinning for Small Parts

      CNC owners discuss various strategies for maintaining vacuum while you cut out small parts. October 26, 2013

When cutting small parts from a nested job on a CNC, is tabbing or full sheet onion skinning a better method for keeping small parts from moving on the table? The software we use gives us both options and we have been using the full sheet skin but still get a few parts that move on us and mess up the sheet. I know that the pump we are using is undersized for the size of the table we have, but replacing it isn't an option right now.

Forum Responses
(CNC Forum)
From contributor J

I now have purchased a larger vac, but before I was almost there, I would run a sheet (all parts) to say .70 (in a .75 sheet), then run a full path at the balance. Always cut small parts first (most vac available) then the larger ones. Tabs work as well. Make them thin so you can cut them with a knife. Mozaik has the by the part or full sheet onion skin method.

From contributor B

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Depending on how thin you cut your onion skin, you could find that tabs are much stronger. Contributor J's suggestion is a great way to increase the strength of the sheet for the first cut around when using onion skins.

Sometimes we'll make tabs 1/4" thick, which totally locks everything in place. We only make them that thick, though, when it will be quick and easy to sand them away on the edge sander after cutting the parts free.

From the original questioner:
I first need to get the materials and the cutters dialed in. I spoke with Vortex yesterday about the tooling I was using. They had originally sent me a 3/8 downshear bit that I was using to cut the parts out, but that didn't last too long and the cut quality was not good at the bottom of the sheet, so I called and they said I should be using a compression bit instead, which should be here today.

At first I didn't really have any issues with parts moving, but the last few sheets were having lots of parts move. That may have been because the cutter was getting dull and taking more force to get through the parts. Maybe the new cutter will help things out and we are going to take the spoilboard off and look at closing off a bunch of the table, which is now 72x144, and limiting all of the vacuum to the 4x8 area that we cut on. We have also been using 5x5 BB for drawers, but I figure we can cut them in half and nest them on 2.5x5 sheets instead and focus the vacuum on that area to increase hold down too. At some point I certainly will change this machine out for a 4x8 or 5x8 to reduce the floor space it eats up.

From contributor B

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Just cover the unused area of the table with scrap plywood or even cardboard. We usually use cardboard and it works great.

From contributor O:
Use a compression for the larger parts that you know won't move and a really sharp Vortex Xp or equivalent 1/4" diameter down-shear for the small parts.

If your software allows it, space the really small parts out so there is about .5-.75" thick wood frame left between cut profiles. This will increase waste but will help keep them from moving without tabs and will prevent a chain reaction if a part does move.

Also cut in two passes. Cut the majority with the first pass, packing in the chips, then remove the onion skin left while in the same location. This will save time versus having to run the sheet and then having to go back around an re-cut the paths. Turn off the dust collector and use an even smaller diameter bit and more passes if needed.

I used to tab every small part until I started using the method above; tabs work but are time consuming to have to rout off if you're doing a lot of parts that way.

Other than that, check and make sure your vacuum system is working properly, check for leaks, etc. Get some golf grip tape; always comes in handy. Maybe try a dedicated board if you have a lot of the same sized parts to make.

Make sure to store material so it remains flat and will vacuum down flat. If the sheet won't vacuum completely, flat parts are likely to fly. You may have to carefully screw down unruly plywood.

From contributor J

I am going to ask Mozaik to give us a click on/click off item list for toe kick and small stretchers, etc. We run these on a table saw edgeband the full length then cut to fit on a chopsaw. When vac is at a premium this has been our preferred method and we just cut larger parts on the CNC.

From contributor Y:
What I did was change the material for stretchers and nailers to a different material name so that they nest together away from the regular parts so I can cut them out once together and watch them or do them manually with scrap.

From contributor J

I am still learning this method - I just love magic buttons though!

From contributor N:
I have tried just covering the unused part of the table and while it is quick, it is not as effective as gridding off the unused part. We run a lot of small parts. I just finished a test run of parts that were 4.75x16.25 nested 44 on a sheet. Could have gotten a couple more, but I was in a hurry. It was nested as a two step - cut the first at .70 and the second at .75. The first tool was a .5 vortex comp spiral chip breaker and the second was a 3/8 chip breaker. The material was 3/4 shop ply.

1700 ipm both passes and not one part moved. The table is 5x12 but was gridded to 4x8 with a 40hp Travini vacuum pump. This was nested by using Mircovelum Toolbox 7 and using a separate toolfile set for the increased feed rates. I'm happy with this and the mill shop super is happy. Small parts can be a task at times.

From contributor K:
Is the 1700 IPM a typo? I don't think a machine could even get up to that speed cutting small parts.

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