How much vacuum do I need

[rww]

I have been reading and reading about spoil boards and vacuum and I still don't have an idea how much I need. I would like to be able to hold down a slightly warped or twisted 1/2" 5'X 5' Baltic birch and cut drawer parts down to 4"X 10". How much vacuum would I need????? Thanks for your time and knowledge.

[Gary Campbell]

Jason...
Without knowing all your plenum and spoilboard specifics, its hard to make a recommendation. I can tell you that we use 4 of the Ametek 117123 motors. They pull near 10 inHg. Close to 100 cfm each. We are at sea level and we still occasionally have a sheet that is bowed enough that we cant hold it down. That being said, we often cut sheets (4X8) with many small parts, but try to leave the chips in the bit kerf.

There are many that use masking and other "tricks" to make up for less vacuum. I recently helped put together a system in the Caribbean that used 2 Fein vacs and it had good peformance also. I am not sure that is would do that well in your scenario.

Your task of cutting many small parts from "not so flat" plywood may not be an easy one to accomplish, especially on the first go around. As the small parts are cut, and if they lift, there will be a substantial vacuum loss. Lots of CFM will help.

Short answer is, there is no short answer. Methodology, material selection, spoilboard makeup, plenum design and your bit characteristics and its toolpath strategy are all important pieces of the puzzle.
Gary

[dlcw]

Jason,

I run almost the same configuration as Gary. A dedicated Ametek 117123 220vac motor connected directly to each of four zones in my plenum board. My plenum board consists of two layers of 3/4" baltic birch plywood sealed with about 4 coats of shellac top and bottom and around the edges. I have no plumbing at all and I get about 7.5 inHg. The big difference is that I'm at 2500' altitude instead of sea level. If I have a board that is slightly bowed or warped I can pull it flat. Any more then slight and I have problems. I tend to put the crown up and I can pull a sheet flatter then if I have the crown down. With the crown down and edges up, my lower vacuum pull just won't get the board flat for me. Of course you get the occasional sheet(s) from the supplier that, no matter how much vacuum you have, you can't pull it flat. If it's this tough to pull flat, I would suggest returning it to the supplier and getting a new sheet. Just finished a job where I had to return 4 1/2" sheets of hickory ply because they looked like an ocean swell. :-(

I am looking into adding a second motor to each of the zones in my plenum in hopes that I can pull boards flat.


8176


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8178

For the spoil board I use 1/2" Trupan LDF and I skin it both sides for maximum airflow possible with my setup. I also edge band it with melamine edge banding to stop air flow through the edges.

Gary brings up some good points about cutting strategy. How you cut your parts will have a big effect on whether they go shooting around the table or hold still while the bit munches away on them. If you cut most of the way through on a first pass, then cut the remaining 1/64" on the second pass the load on the part won't be nearly as great and smaller parts will hold still while being munched. Also, as Gary indicated, if you have lots of small parts then your vacuum will drop rapidly. There's no getting around this other then throw more vacuum at it.

You mention cutting drawers parts from 1/2" baltic birch. The job I just completed used 5/8" prefinished baltic birch (10 sheets) with drawers sizes from about 4" to 10" deep and I only had one part move on me. I use prefinished 1/2" maple plywood for the drawer bottoms. Yes, I'm lazy and hate finishing.....

I know this probably raises more questions then it answers but as the old saying goes "It all depends"

Don
www.diamondlakewoodworks.com (http://www.diamondlakewoodworks.com)

[gene]

Jason,
when you layout the sheets you can designate spots that you can use a screw or a few to help hold down the sheets. I use a 7.5 hp roots blower and lately i have found that i can draw screw locations and not even use the vac at all. If i remember you are a small cab shop? There are also a lot of things that you can cut on the table saw faster. Please tell me more about your applications for the bot and what size you have. also if you use a down spiral cutting bit it helps to hold pieces also. Gene

[curtiss]

The vac motors above look the same as the three stage rebuilt "car wash vacuum" I bought at a local vac repair shop for $90.

Mounted them basically the same way and they run off a switched outlet I mounted to the bot.

I get about 6 in hg with one motor on a 48 48. Probably not for bad plywood but they work well for the flatter stuff. I usually add a hold down clamp or two if I can.

[dana_swift]

My take- you can never have enough vacuum. It is a design flaw that the earth only allows 14.7 psi maximum.

Sooner or later you will cut something that will "lift up" and shift on you. I have a system that gets a reliable 20"Hg on generic flat material, nothing is guaranteed. When I cut through, by the time I cut out the perimeter of a part, the system starts leaking vacuum, which quickly drops down to the 10"Hg range. "Usually" it still holds.

I have thought about the vacuum limitation and have mused about putting the bot in a pressure chamber so I can get 30psi or so of hold-down using compressed air on one side, and atmosphere on the other.

Someday maybe.. then If I find a big pressure chamber in a garage sale.. who knows?

D

[curtiss]

If you lack vacuum, you might try a 20 inch long 1/4 in stick...

I have found that with 30 pounds of pressure on .25 square inches I get 120 psi on small parts on the final pass.

No nicks on the stick as of yet...

[beacon14]

Curtis - the size of the stick doesn't matter - it's still 30 pounds regardless of the contact point size. But I agree sometimes a little help goes a long way.

[gene]

What do you do with the stick? Beat the wood straight? Tell me more.

[ken_rychlik]

Jason,
If you use prefinished 1/2 inch ply for your drawer parts, they seal better and don't move as much. I use mostly prefinished 3/4 ply for my box construction as well and one fein holds them in place very well.

When I run unfinished ply in baltic 1/2 inch with one fein, the small parts move a little. As for using fein's two should work on unfinished baltic most of the time.

Kenneth

[cip]

Gary:
How much noise do those vac motors make?
I currently have a Roots 15HP and the noise is crazy loud.

Mike

[Gary Campbell]

Mike...
Not much more than the flight deck of a carrier! Actually they are fairly loud, but not as loud as some other tools in the shop. Ear protection is a must in any shop.
Gary

[dlcw]

If you attach the available muffler to the motor it cuts the noise level by at least half. When the ShopBot is cutting wood it is a lot louder then the motors. You can't hear the motors above the cutting noise.

Don
www.diamondlakewoodworks.com (http://www.diamondlakewoodworks.com)

[cip]

Thanks for the info.

Mike

[curtiss]

One thing I forgot to mention with the use of a 20 inch stick.... if you find the stick getting shorter and shorter for some reason.... and don't know why...

you may want to abandon the stick and buy more vacuum.

[wberminio]

Mike

I use the Republic 16.9 hp regenerative blower
8 zones.I get 5-7 in/Hg.There is so much air movement,I can hold down nearly anything.
I have also started making drawer parts using 1/2"
maple ply.
Small parts can still be problematic.
I close down the other zones when cutting the smaller parts. I have also notice that since I got my 3hp Cyclone DC,it is removing too much dust.At times I will shut the DC down so to use the chips to keep the parts from shifting.

Erminio

[wberminio]

BTW The noise level is not bad,it is not a high pitch noise, A shop vac is much louder.

But as Gary said-I still use ear protection!

[rww]

Thanks for all the ideas. I was thinking of doing exactly what Don did and have around 10 vacs on a 5'x10' table. Then have a switch to each one instead of PVC with zones.