Plywood substrate...grade?

[myxpykalix]

I'm assembling my table and i am going to have a vacumn table setup with 2 feins, my question is what grade plywood should i use, and thickness? I'd rather not buy $50.00/sheet cabinet grade plywood if i don't need to.

[bleeth]

Jack: You can easily use 3/4" luan/virola/sandeply for your base. Use 3/4" MDF (the heavy stuff) for your vacuum grid, seal it with shellac (2-3 coats) and MDF (ultralight) for your spoilboard. There will be more opinions out there, but the above has been the most common answer to this often asked question.

Dave

[myxpykalix]

ok im a bit confused (not unusual) but I thought I take my 3/4 ply and cut my plenum design into it. On top of that I lay the 3/4 MDF spoilboard. If i understand you correctly I have my 3/4 ply, then on top of that i have a 3/4 MDF that i cut the plenum design into, then on top of that i put the spoilboard? Seems to me since the mdf is so pourous you would want the plenum cut into the plywood so that all the vacumn is forced upwards and not leaking thru the mdf that you cut the plenum in?

[dhunt]

- that's why Dave R. suggested you seal the vac.plenum you cut outa 3/4 MDF with 2-3 coats of shellac.

Your (first)(Base)board is always decent grade 3/4 ply..for strength and durability.
on top of that lies your(second) vac.plenum board,varnished..

and on top of THAT combo
comes yer (third)sacrificial spoilboard,
which you'll replace every so often.

When you're done, you have a sandwich of THREE boards.

If you're real fussy, you'll seal the first/baseboard too,
since that's usually emplaced once and never again:
you don't want that warping.

[beacon14]

Don't skimp on the plywood - you only have to buy it once. If you are spending $700 on two Fein vacs then why try to save $20 on the plywood? You don't need "A" faces (you can use D-3 or D-4 faces), but in my opinion you should at least use a decent cabinet grade of plywood.

[gerald_d]

A hassle we have with plywood in the table is that the sheets aren't available in large enough sizes. That's why we use MDF as the base table as well. (biggest ply here is 8'x4' - MDF is 9'x6')

[richards]

I used two layers of Baltic-Birch plywood for the support boards (staggered so that no seams line up. The bottom board is bolted to the steel table with carriage bolts. The top board is glued to the bottom board with contact cement. The top board is surfaced and then coated with three coats of polyurathane to keep it stable. Holes for my multi-zone vacuum are cut into the baltic-birch.

Then I cut the combined plenum/spoilboard into a sheet of MDF (I can't easily get trupan in Salt Lake City), using Brady Watson's design. After surfacing the top side of the plenum/spoilboard, I'm ready to cut.

[myxpykalix]

ok just so i understand...do I use shellac to coat all sides of plywood and mdf? (top, bottom, sides?) coat the mdf then cut the plenum or coat afterwards? or does it make a difference? Do I glue the top mdf (spoilboard) to the 2nd mdf (plenum)? with what? Also not having the mdf laid on table yet how can i tell (without cutting) where the honeycomb pattern in relation to the holes will be? Also can i only cut the pattern and not the holes till i know i am not located at a cross beam. If I am how can i alter pattern to accomodate that? These might be elementary questions but i'd rather ask and get it right then realize later i screwed up. THANKS!!

[bleeth]

Jack: We learn from our mistakes and we never remember totally if all the answers in the back of the book are looked up before we even try to figure out the problem ourselves. Think about what your goal is and why and the answers will come to you. Once you actually are getting into design and machining you will have so many challenges that you will either answer most of the questions yourself or will be so busy making other people responsible for your challenges that you will be unable to make progress in a timely and profitable manner. JUST TRUST YOURSELF AND GO FOR IT. All of us are here for you when you will really need it, just as many more experienced people were and are here for me when I got/get stuck, but the best advice we can give is had when we are not worn out from repetition. Use the search tools and peruse the forum by scrolling down topics and previous postings and you will usually see that there is no one definitive answer and oftentimes many different good ones which you will then choose either what works for you or, many times, a whole new answer you come up with yourself.

Dave