Holding parts without »bridges«?

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Holding parts without »bridges«?

Post by typomaniac » Tue Mar 26, 2013 11:55 am

i am not sure if bridges is the correct term for what i mean: I am talking about the little connections between a cutout part and the rest of the material.
How do you make sure that the cutout parts do not get caught by the spindle if you do not use those bridges? I have seen videos (e.g. http://www.youtube.com/watch?v=yUaGZh4WK0Y) where the parts were being picked up manually after being cut but most time this did not work for me because the parts were »sucked«
into the end mill or at least damaged by it.

thanks a lot,

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Re: Holding parts without »bridges«?

Post by cvoinescu » Tue Mar 26, 2013 1:28 pm

The bridges are called "holding tabs". You can sometimes use double-sided tape to hold down the stock; if the part is relatively large, it'll remain stuck down once you've cut it. Or, if your part has holes in it, you can mill the holes first, then screw the part to the spoilboard through the holes, and then mill the perimeter (profile). The trick, as with manual tool changes, is to keep the motors engaged (and use the software controls to jog the machine away), so that you don't lose the position. Professionals often use a vacuum table, sometimes sucking right through the MDF spoilboard (if milled on both faces to expose the porous interior).
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Re: Holding parts without »bridges«?

Post by madmanmoe » Fri Mar 29, 2013 11:42 am

Double sided tape definitely holds the piece down, but it gums up your end mill and can be tricky to get off your part. I find it far easier to sand down a couple of tabs, than to have to pick away at tape, and clean the part / mill with alcohol.

But if you really hate tabs then sometimes you don't need them if your final pass is very thin. e.g. 0.2mm . However you can't always guarantee that'll work.

If it's just for the odd job, or you neglected to figure something else out before starting then there's always the old 'hold the piece down with a long screwdriver on the final pass' technique, crude but effective.*

*I can't in any good conscience recommend this, although I've done it before, it still doesn't seem like a good idea.
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Re: Holding parts without »bridges«?

Post by edwardrford » Sat Mar 30, 2013 9:51 pm

Using a long screwdriver is a pretty common technique. Everyone does it but everyone says you shouldn't do it. The term in "the biz" is called using a chicken stick....

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Re: Holding parts without »bridges«?

Post by cvoinescu » Sat Mar 30, 2013 11:22 pm

People have done worse things with screwdrivers. They also had a cuter name for it.
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Re: Holding parts without »bridges«?

Post by cchristianson » Sun Apr 14, 2013 2:04 am

This is pretty basic, but if you have a regular old fashion router table you can throw a flush trim bit on it and set the bearing just above your normal tab thickness to trim them off perfectly every time. I just keep that bit in my router table all the time unless I need a different bit for something, but then switch back to the flush trim right away for the next time I need it.

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Post by mjohn » Mon Apr 22, 2013 6:55 pm

I've tried everything except a vacuum table and I keep coming back to holding tabs.

Since I err on the side of not cutting into my waste board, even though I do use card stock between the waste board and my material I always have my holding tabs and some flashing here and there (profile cuts that didn't make it all the way through). The flashing I cut with an Exacto blade, the tabs are cut away from my part with a thin keyhole saw.

For wood, depending on the size of my piece and tabs I'll either rasp and file or just file. Literally takes a few seconds to get the tab flush. The rest is finished when I sand the edges. For Acrylic, I put a piece of 100 grit sandpaper on the table and run the tab against that. You can do this with wood as well. Hard to over-do it because once the tab is gone the feel and sound of the material on the sandpaper (and the file too) changes.

So, tabs aren't so bad and I think they're still the best thing out there at least for my work.
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