Cutting out a 'Onion Skin' without a vacuum
 

Has anyone had success in leaving an 'onion skin' (.020") in 3/4" MDF or melamine on a table WITHOUT a vacuum and cutting the final .020" in the last pass? Will the part stay put without moving? Does it depend on how large the part is? I am currently using clamps on the outer perimeter of 4'x8' sheets and using tabs to keep the parts in place before cutting through the tabs on the last pass. The software I use forces me to manually insert the tabs, whereas all of the cut paths (G-Code)for the parts are programmically written. If I use a 'onion skin' approach the G-code will be written programmically also.

I have very little success with onion skinning without vacuum hold down.
The problem is the sheet movement, with a moderate sheet, at places where the sheet belly up, sometimes as much as 1~2mm(0.04~0.08"), enough for the bit to cut clearly through & the work piece will displace itself due to the cutting force & ruin...
You can nail or screw down the sheet onto the spoil board but I personally feels it is way too time consuming... imagine the hassle to get rid of 20 nails... every sheet for 50 sheets... Also, one really can't tell where to nail for sure... not forgetting the nail holes you leave behind...
That is why I went through the trouble to build a vacuum hold down... :(

Ken,
Thanks for the reply! I felt that the material to be cut may have a problem lying flat in the center of the board without vacuum. I will see if I can program 'tabs' into my G-code without having to do it manually.

What software are you using? most CAM do have a TAB function.

One additional note, I can consistently leave 0.1mm~0.2mm (0.004~0.008") onion skin with vacuum hold down. :) I don't even have to cut these through with a final pass. a light sand paper treatment is all it requires.

This is how I go about cutting multiple parts on a sheet mdf :
1) in the design i place markings in the waste material, safe distance from cut out vectors and space for the cutter to avoid the markings,
2) clamp the sheet to the table,
3) 3 mm endmill, first cut the markings in step (1) about 1mm into material
4) hammer nails through sheet material into sac-ficial board -
all this to prevent the sheet material from moving on the table because i found the more parts that is cut out from the sheet the more flimsey the sheet becomes and the flimsy ness makes the sheet slide as the cutter changes direction = odd shape parts.

I also dont vacuum the chips from the cut groves until im ready to remove the parts, the chips assist the onion skin to keep the parts in place,

I found that this tool and sand paper works fine to remove the onion skin if tabs are not used http://www.deburringtools.com/deburr...thhandles.html

Chris

Tabs
 

I'm resurrecting this thread to ask opinions of the amount of space needed between parts if I place tabs on the North, South, East and West corners of each part I cut from a 4' x 8' sheet of 3/4" particle board, melamine, MDF and ply and for 1/2" material of the same type as above. I'll have the G-Code written to have the router go back and cut the tabs at the end of the run for a sheet. I have tried placing the parts (rectangles) close together without success. I'm thinking I need to maintain at least a 1/4" of sheet material between each part. Any advice would be appreciated. These are cabinet and drawer parts.

1/4" is my minimum, but it depends greatly on the type & thickness of the said material as well. Something I practice is to connect the waste part of the sheet to the work piece with lots of tabs. As I feel that the 1/4" wide between is too flimsy to hold anything in place seriously.
I also learnt that a tab at the beginning/end of the cut is most beneficial, from my observation, it is that very last bit of cut that moves the work piece. even with vacuum hold down.
Never have any success in Going-back-to-cut-tabs routine. Pen knife & chisel works wonders but they are time consuming.
Did you observed a 0.2mm or 0.0079" slag in cut depth when doing onion skinning, i.e the onion skin is always 0.2mm or 0.0079" thicker then you wanted.

I though my comments will be more precise if I mention the bit , speed, feed & depth I use.
All bits are 2 straight flute carbide tipped router bit, Climb cut.
1/2" or 12mm 13000rpm, 3800~4500mm/min 15000rpm, 12~15mm depth of cut
5/16" or 8mm (my current favourite of the month) 15000rpm, 2500~3800mm/min, 5mm depth of cut
1/4" or 6mm 18000rpm, 1500~2500mm/min (breaks often) 3mm depth of cut
1/8" or 3mm 24000rpm, 1400~1800mm/min )break too often) 1mm depth of cut.

Thanks Ken. I'll do some sample cuts and report back.
Steve

thanks:-)

Using a solid carbide upcut, two flute, sprial 1/4" cutter with a 1/2" shank at 22k rpm at 170 ipm. Cutting 1/2" 2 sided melamine with PB core. Tabs are 1/2" long and 1/4" tall from spoil board (tab length is shortened by 1/8" on each side because the cutter is round, not square, and the radius protrudes into the 1/2" length a 1/8" each side making for a 1/4" connection to the material). Placing the 4 tabs for each rectangle at the N,E,S,W of the part and going back and cutting out the tabs the parts mostly moved on the last tab and gouged the part. Placing 8 tabs per part at each corner of the rectangle as Ken suggested ( I spaced them one tab length away from each corner) and going back and cutting the tabs produced very little part movement and no visible gouging of the parts. 42 parts per sheet. I will try baltic birch ply next.

SWEEEEET!
Are your cutting only straight lines.

Pardon my ignorant, what is PB core?

PB = Particle Board

Ken,
Yes, straight lines, dados and shelf holes are being all cut with this cutter per sheet. No curves for cabinet parts. This process replaces three seperate processes/setups that I was doing manually and using cut sheet print outs to produce parts. Now I just load a full sheet of material and let automation take over. Much better accuracy!
Yes, PB=Particle Board

Great!
Remember to do ramp in, for the sake of router longevity. You may not see the effect immediately, but its what the bearing like.