Alternative to tabs for work holding? Double-sided tap? 0.5mm thick, long tabs?

Hi,

Does anybody have any alternative part-holding suggestions to using tabs please? (cutting a part from a whole sheet that's screwed down). I can't use screws, clamps, nails (the part comes off 'finished') or a vacuum bed (I think the parts are too thin)...

I've been finding that when I come to cut the tabs off with a flush-trim hand-router, where the hand-router has cut, it ends up a different shade when the light bounces off it because it's cut at a different feed/speed to the rest of it which was CNC machined... I'd like to be able to not need to sand and blend in this part and leave the parts finished with a machine-cut surface.

Unfortunately the parts I've been cutting can't be screwed down or have clamps on them.

I've been using 20mm long, 4mm deep 3D tabs with good work holding success and was wondering whether I could maybe use say 100mm+ long, 0.5mm deep non-3D tabs perhaps? Maybe I could then run around with a knife and then the trim router set to a fraction of a mm.

Does anyone have any experience with this?

I was even wondering if anyone might use some kind of double-sided tape?

Thanks in advance, Matthew

I might have found a solution - I can't find any double-sided tape larger than an inch or two in width but a different Google search to the searches I had been trying brought me to this:


http://www.cnczone.com/forums/genera...utting-2d.html


Which I thought might be useful for anyone who comes across this post in the future.


"Onion skinning" (first time I've heard of it!) commented by Ger21 in that post seems like it could be well worth a try... It's just a lot lower (1/2 to a 1/4!) to the 0.5mm 'onion skin' I was contemplating using (at between 0.127 - 0.254mm or 0.005 - 0.01").

Hi Matthew

If its wood parts then I have used double sided tape recently to hold down guitar neck blanks on both sides and rosewood fingerboards. The stuff I have is fabric reinforced and sometimes I have trouble getting the parts back off the bed afterwards so seems to be pretty safe.

I'm not sure I'd trust it when doing aluminium, but might do a test on scrap at some point, just to see.

I use onion skin method all the time on aluminium 0.2mm but you must ensure your bed is perfectly flat because it's very easy to break thru.
I also use lot of double sided tape for smaller sized material where can't clamp or screw and don't have excess waste material. You don't need to cover the whole material area with tape just in several places, depending on the type cutting your doing.

Washout DST works great with aluminium and brass. I trust it enough to have stuck large expensive sheets of brass down for engraving.

Thanks Washout - I might have to have a more in-depth search for sources of wider double sided for a try. Over on cnczone I posted the same question and dogsop suggested using blue tape to the back of the wood-to-be-cut and then gluing that to some hardboard with 3M spray adhesive which seems like a good idea too - it might be easier to remove than double sided too?

Cheers Jazz - It's wood I'm looking at the moment but I did wonder about that problem of breaking through. I've experienced the machine not quite cutting far enough through because of places where the natural flex in the wood has it standing slightly proud of the bed (unbeknownst to me at the time).

Out of curiosity what kind of feeds and speeds are you using to cut aluminium and brass? And do you use single flutes and coolant? I think it would push my machine too far but I wouldn't mind the experience of having a go.

Quote:

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Originally Posted by mattnedgus

Out of curiosity what kind of feeds and speeds are you using to cut aluminium and brass? And do you use single flutes and coolant? I think it would push my machine too far but I wouldn't mind the experience of having a go.

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Oh so many variables that what works for me doesn't mean it will work for you. But I'll give a little idea.
Firstly I use method where I rough using 6 or 8mm 3 flute serated edge carbide roughing cutters so can push deep and hard. DOC will be anywhere between 3-8mm @ 1200-1700mm/min 15,000-20,000rpm.
Then I'll finish with polished 6mm Single flute full Depth of material.

Also I Often use Trochoidal toolpaths which cut mostly with full flute length or material depth and very fast.

In all cases I use Odourless mist or at least blown Air to remove chips and cooling.

Every machine is different so don't go by my or anyone else's settings without at least doing a few tests. Also don't be afraid to cut deep. One of the bigger mistakes in Ali or other materials is cutting too shallow and overheating the cutter. You want nice big chips that take the heat with them. If your making dust your doing it wrong.!!

Quote:

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Originally Posted by JAZZCNC

Also don't be afraid to cut deep. One of the bigger mistakes in Ali or other materials is cutting too shallow and overheating the cutter. You want nice big chips that take the heat with them. If your making dust your doing it wrong.!!

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Thanks Jazz - it's interesting to get an idea of what others use. One of the problem-areas I've had is in my DOC's, feeds and speeds cutting baltic birch - I cut as deep as the cutter is wide (typically 6 and 8mm) with speeds of 13500RPM and feeds of 5000mm/min at the moment. I've been trying to get those chips nice and big while reducing some chatter and some instances where the cutter bites too deep and creates something similar to chatter but slightly deeper and further apart. It's frustrating trying to 'dial in' settings that work reliably to produce parts that need less finishing by hand! I'm considering rebuilding the Z axis to try and help eliminate some of the flex that must be present in the machine.