Vacuum table questions

[jshoes51]

After using my Bot for a couple years, I finally need to add a vacuum hold down system to accomplish a certain job. The job involves cutting planks for small rowing craft from full sheets of mostly 1/4" plywood.

The plank cutting is nothing new, as I would usually drill hold-down holes in the fall-off and screw the sheet down and even add tabs if needed. The problem comes with routing the 3-step, blind, dog-bone scarfs on the very ends of the planks. Some of the planks use the whole length of the plywood, so there is no room for hold down screws just beyond the end of the scarf.

I have a 5'x10' table with a single 3/4" particleboard spoil board which has only been surfaced one time. I'm pretty sure I can route the plenum into this layer. Is the only layer on top of the plenum the "bleeder", and is this also the new spoilboard? I can get trupan locally and hear it's good for this.

It sounds like after doing a search that I'll need at least two shop vacs to hold down a 4x8 sheet of plywood. If I route two quadrants into the plenum, is there a ready-made file for this tool path or do I just create my own?

Thanks,

Jim

[ken_rychlik]

For the most part a hold down consist of 3 layers.

The bottom layer is your stable support board.

The next layer has the plenum grid cut into it.

The final top layer is the spoil board.

When you cut your plenum, you will remove to much of the stable strength of your current base sheet.

If you feel the base is ok after a fly cut, then that would be your first sheet. Next attach a board for the plenum. Most use 3/4 mdf, but some use hdu or other plastic so they don't have to seal it.

Then your trupan goes on top of the plenum.

Seal,, Seal,, SEAL EVERYTHING GOOD

There are several ready to cut plenum files in your sb3 software or you can make your own if you don't care for any of them.

Kenneth

[harryball]

I'm now sure what you are cutting when you say plank but it sounds like a vac mask might be useful to you. You can still use your vac table with spoil board as described but place a mask over it to limit the suction to your plank, it will hold VERY firm.

I use this method to cut bat house sides out of 2x8x4' cedar or cypress. the hold down is power is impressive.

/RB

[dana_swift]

Jim, look around the forum, there are lots of posts on vacuum "how to". There are also many helpful hints.

My vacuum system works with both the trupan general hold down, and vacuum mask for odd shaped stock. My comment on Ken's note about "seal seal seal" is 3M masking tape is all I needed to get the edges of my trupan to seal. I pull 24" Hg regularly on wide area sheets.

When I cut through the vacuum level drops quickly, so when I am doing vacuum hold down, I break up my part files, all non-cut-through cutting happens in the first file and ends with the smallest holes cut through. Then I end the cut file, put masking tape over the holes so the vacuum level is restored. Then run as many part files as needed to finish the piece. Each time the cut through gets up to about 4sq in, I break the cut file into another section. I want the router off, and not moving when I am reaching in to put tape on the holes.

Resist the idea of taping leaks while the vacuum is running. Its a great way to lose body parts. Yes I have done it. A few close calls made me come up with the method of making many cut files so the operation can be done safely.

"I have done it".. refers to trying to keep my fingers away from the router, not losing parts. That is a club whose membership I wish to avoid.

Good luck-

D

[jshoes51]

I should clarify about the planks. These are long narrow boat parts, for example, 8" wide by 96" long, usually with curvature to both edges. As many of these as will fit will be cut from a 4x8 sheet of ply.

I can now see why I should just re-surface my existing deck and cut a plenum pattern into a new sheet of mdf. I bought my Bot used, however, and am not sure where to look in my documentation for the plenum files, so I'm thinking I'll write my own program. How deep should the air channels be?

When people say to "seal" the plenum and trupan. do you just mean to seal the edges with paint, shellac etc? I have several feet of iron-on edgebanding. Can I use that?

Thanks

Jim

[curtiss]

.....When people say to "seal" the plenum and trupan. do you just mean to seal the edges with paint, shellac etc?

....From what I read, and did... seal the entire plenum with 3+ coats of shellac.

Seal "just the edges" of the top trupan with 3+ coats of shellac as you want air/ vaccuum to pass through the trupan.

Is there a science to the grid pattern/ groove depth in the plenum ?
1 inch squares seem to be the most common.

I understand that it is best to glue / not screw the mdf plenum to the base plywood below.

[matney_models]

This is what I made

[dana_swift]

Here is a shot of the system I came up with:

http://www.talkshopbot.com/forum/showpost.php?p=22179&postcount=16

As I wrote earlier sealing the edges of the trupan is now done with masking tape. No drying time. Works great, I get 24 to 25 inches of mercury regularly. When I "only" get 15"Hg it feels like something has gone wrong, most vacuum systems wont get up that far.

You can see my support pillars are only 1/8" above the valleys that distribute the flow to the trupan.

Holding the trupan in place with dowels has also proven to be a good idea. They don't resist the router, and make replacement trivial.

About the only thing I would change is make access holes in the trupan so I can remove the vacuum layer from the ALS without lifting the trupan to get access to the bolts. Because I use masking tape to seal the edges, it hardly matters tho.

D

[jshoes51]

Thank you to everyone for all the help. I read somewhere (Brady Watson?) that the plenum grid can be routed into the back side of the trupan thus saving a layer of sheet goods on the table. I think I'll try this unless this is outdated info. I found it in a search from a few years ago.

Thanks

Jim

[dlcw]

I seal my Trupan edges with melamine pre-glued, heat activated edge banding. Seals REAL well and is simple to do.

[jerry_stanek]

Here are a few premade vacuum cuts.

[jshoes51]

Jerry,

thanks for those files. I think the 5x10 4 zone would be a great help, but I can't open them in partworks or Aspire to see them. I have to place the vac holes at the right locations to miss the joists holding up my table. Is it possible for me to change those file types to a .dxf on my end?

Jim

[bleeth]

Those are shop bot files. After saving them you can use the file converter under file tab in your control software.

[gundog]

It is pretty simple to make your own file. I made mine and it does not take long. Use the Copy feature in partworks. I defined my zone size and then you make either circles or squares copy them with the tool that allows you to choose how many parts X & Y. Make extra colums and rows if you want then highlight the ones outside the zone and delete them. Next click on the zone and write down the coordinates of the center of that zone now deselect the zone but highlight the shapes you made for that zone use the move feature in center mode and enter the coordinates you wrote earlier and walla you have that zone almost done. Find where you want your vacuum ports add them delete the shapes that overlay the ports and repeat for the other zones 20 minutes work tops. Why use what someone else made to fit their needs make it fit yours.

Mike

[jerry_stanek]

You can preview them in your sb3 software.