One Fein III or Two?

[jeffreymcgrew]

OK, so after working with our Shopbot for a while, and using mechanical hold-downs (clamps & screws) for our jobs, we're thinking of stepping up to a vacuum hold-down system.

Due to power and money constraints, we can't do the Universal blower-based system, we're going to have to go the Shopvac route.

We've got enough money to get two Fein III's. But will two be a waste? I see some posts pro and con, and am just wondering.

What I'd like, ideally, is to be able to hold parts down to about 12" square or slightly smaller, maybe down to 8" x 11". I'd like to be able to cut a deep pass, down to a 'skin' so as to not loose vacuum pressure, and then do a cleanup pass with a downspiral cut that cuts the parts free. I don't have a big shop, so pulling a sheet off the table and cutting the parts with a laminate trimmer or something isn't a workable option for me, I need to be able to cut the part free and clear on the table, with the 'bot.

We'd like to have six, maybe seven zones. Mostly cutting plywood, solid woods, plastic, and some aluminum. Don't mind if we still need to have some level of mechanical hold-downs for some jobs, just trying to keep parts from moving on us and to make it easier to work with the 'bot.

Thanks a ton!

[patricktoomey]

Jeffrey, I use 2 shopvacs split accross 8 zones. I usually run them with one vac per 4 zones. Using an MDF spoilboard I cut parts out of 3/4 plywood sheets in 2 passes leaving a .030 skin, then a final pass takes off the skin. This way the parts come off the table complete and ready to go with no trimming. If my sheets are flat I can get away with one vac but if they are warped at all one vac doesn't pull them flat so I run the second vac just for security. I cut parts down to around 8x11 with no trouble. For bits I generally use a compression cutter so that I get the down force from the majority of the bit but I still get a nice edge on the bottom of the sheet from the short upspiral section at the tip of the bit. I still do use mechanical hold-downs for odd jobs but the vacs do 90% of my holding.

[beacon14]

I use the same strategy as Patrick, except I run both vacs open to all 8 zones. The sawdust that remains in the kerf, even after the 2nd pass, helps restrict the airflow and hold the parts in place.

For the amount invested I feel I got a lot of bang for the buck. One vac would probably work most of the time but I want to be able to work across the shop while the machine cuts without worrying, so I use both.

[richards]

In theory, a single Fein III should generate about 2-1/2 lbs of hold-down per square inch. (My vacuum guage shows 5-inches of mercury, which is about 2-1/2 lbs. per square inch.) An 8X11 piece of material (88 square inches) would be held by over 200 lbs of pressure. Normally that would work; however, just today, I had a piece of 60X60 baltic-birch slip on me. That should have been held by about 9,000 lbs, but it was bowed just enough so that vacuum wasn't maintained, even though I left a 0.040-inch skin on all parts.

I guess what I'm saying is that in an ideal world, either one or two Fein III vacuums would work, but sometimes they don't. I have two Fein III's and usually only use one of them. Today I used both, but warped/bowed sheets need more. I probably should have laid down some gasketing tape (AllStar) on top of the spoil board to assure a better seal, but I didn't. Oh well, the parts are going to be painted, so Bondo will fix the mess and, in time, I'll forget the gouge beneath the paint in my bookcase.