Securing a Narrow Part to the CNC Table

      A CNC operator learns how to hold down the blank for a narrow curved part. June 5, 2006

Question
I have only about 5 months experience on a Holzher 7120 processing mostly sheet goods (matrix table with MDF spoilboard) and "played" a little with the flat table conversion with pods. I am using TwinCam32 to manually draw parts when needed.

I need to cut a solid wood part measuring 3/4" thick X 1-1/2" tall that has a radius of about 72". The straight line distance from end to end is about 55". This part is simply a solid wood drop edge for a curved plywood shelf.

I plan to start with a solid wood glue-up large enough to cut the entire part. I am not sure how to hold the part during cutting. I am concerned that because the part is tall and narrow, it may flex, tilt or move in some way while the last cut is being made (inner or outer radius). Does anyone have any suggestions on holding this part? Also, any suggestions on the type of cutter to use? I am thinking of a 1/2" downshear, 3 flute, maybe with chippers.

Forum Responses
(CNC Forum)
From contributor B:
Start with wood that is 1 3/4" thick and then set your cut depth to 1 5/8". Use a blank that is significantly larger then you final edge band so you will have plenty of material for holding. You can use whatever hold down method you normally use, but you might have to add some additional holding points and methods. This will be material wasteful but project efficient. Once you cut the part run it through a planer or wide belt sander to remove the waste and bring it to final thickness. If you feel it's more efficient you can band saw away the waste first. An obvious alternative is to use your CNC to cut a form and then do a strip lamination to make up your edge band. We use both methods in the shop on a regular basis.



From contributor B:
We would use a 2-flute upcut spiral 1/2" HSS bit for this. 18,000 rpm at about 200 ipm. We use HSS for this instead of carbide because it holds up better to the extra heat generated by the high rpm and low feed rate.


From contributor J:
Figure out where the path of your cutters is going and then screw the wood to the MDF spoilboard so that the screws are outside the tool path in the "waste" part of your wood. Like BH says, don't cut all the way through because if you do your part will go flying. Fill the screw holes in the spoilboard with Bondo when you're done.


From the original questioner:
I was considering the option of leaving the bottom skin and I feel a little better about it now given that someone else mentioned it. I was also thinking of cutting the outer radius first (completely through), pausing the CNC and attach some hold down fixtures to the curved face of the cut and then resume to cut the inner radius. The screw holes would be hidden when the part is attached to the shelf.


From contributor S:
I don't have a vacuum table so all of our parts are secured with aluminum nails. We use a nailgun which as you know costs very little. After that we just snap off the nail, leaving part of it in the product. It leaves a very small hole to deal with. If you should happen to stray with your router, it cuts right through the nail. Although I have chipped the odd bit, it happens very seldom. Every two months we change the spoilboard and it costs very little. Make sure the nails go in far enough to hold well, as the vibration can loosen them if not.


From the original questioner:
It worked. I started with 1-3/4" stock and screwed it to the spoilboard. Cut both arcs 1-5/8" deep using 4 passes each. Ran the blank thought the widebelt sander to remove the 1/8" skin and bring the part down to the desired 1-1/2" thickness. I used a 2 flute upcut spiral, 18,000 RPM at about 200 IPM.



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