Spoilboard Query

[knedert]

Folks -

At Bill Palumbo's suggestion, we've taken the MDF spoilboard off of the top of the 3/4 cabinet plywood base of our table.

The plywood base was out of true/flat by a bit over 3/16" - I was VERY surprised that it was that much... Anyway, I am surfacing the plywood to have a flat base for the sacrificial board.

So - I have some sheet PVC that is flat, but is is also north of $150 a sheet. Tru-Pan, which has been recommended would be a good choice, but time/logistics are not with me on that... What do y'all think about using a sheet or two of Masonite, aka tempered hardboard? I was thinking of using it by itself, or on top of the PVC sheet.

I really want something that is going to be stable in service - I've been having a devil of a time getting accurate cutting depths due to the variations in the spoilboard.

Thanks in advance!

John Moorhead

[jhicks]

I too have been considering PVC, Komacell, Azec, or Celtec for some time for a variety of reasons but mostly stability. The price has been the barrier but my belief is that the options are all about the type of work you plan to accomplish.
Trupan is great for a bunch of things but probably no more or perhaps less stable than MDF. I suppose the more duranble materials like MEDEX, and Extera might be more stable, and of course the "seal coat" on any porous ply, or composite.
I think your concept of a thin stable skin on top of a stable PVC base might prove to be a very effective option assuming its held down to the PVC consistently. Let me know how it works if you go that way as it certainly helps prevent rapid destruction of the PVC and it is quite consistent and stable.

[gerald_d]

All you guys chasing "stability" of the table top, are you sure that the steel underneath and the gantry over the top are "stable"? There are a couple of things in that system that could be moving around.

[wcsg]

My cross bars bow a little right in the middle, so no matter what buy once bolted to the crossbars (angle iron) it's bowed. I always have to surface every couple of weeks

[gamisano]

PVC sheet is great for many things, but I would consider a couple of things before I concluded that is was the optimum spoil board:

First, it is flat and remarkably stable, but as the above posts mention, when everything beneath it is less than perfectly flat, then you will still need to resurface the PVC. If you have ever cut PVC then you know that it ommits fumes, and has a tendency to melt which leads me to my
second point.....

Lets say you got it surfaced ok, survived the fumes, got the spindle speed and feedrate correct such that it does not melt....now your ready to cut parts. What happens when you make your final passes on these parts at differing speeds and feeds? You will eventually encounter the right combination that will melt the PVC which will do at least one, and maybe two of the following things....it will leave a raised edge on the surface of the table where the cut path was, requiring you to resurface yet again.....and it could gum up the bit actually breaking it because some of the melted pvc welds itself to the cutting edge of the bit.

Simply put, you can't set speed and feed rates based on how your spoilboard will react, as the obvious conclusion may be detrimental to your bit and/or parts materials. An expensive proposition in all aspects: PVC sheet ($$!!); bit replacement; and potentially damaging parts materials. Hope this helps.
Regards,
Guido

[Brady Watson]

I setup a Komatex PVC spoilboard for another shopbotter not too long ago. You will not melt the PVC if you throw off big chips when you cut it. Outgassing was not noticable. The one problem that I found is that when you flatten it, the top surface is no longer shiny/smooth like a factory fresh piece is...which doesn't work too well if you are using DS tape to hold parts. The residue sticks to it and it makes a mess.

I always recommend gluing ANY spoilboard down and clamping well/weighting the SBoard while the glue is drying. Screws pretty much suck and cause variations and warpage in the spoilboard. Every botter's shop that I have visited that used screws to hold the spoilboard complains about warpage and inconsistencies in the Z. (including me until I glued it down). By gluing you don't have to worry about hitting screws when you surface the table. You can surface it all the way down to the base board that you glued it to.

I have a 5' wide table and it *appears* that my steel supports are showing a little sag. I think that the table like everything else will 'settle' and it will no longer sag an appreciable amount. So despite sagging supports, my table is dead flat...and consistently flat since I glued it down.

PVC is excellent as a vacuum grid material...but I personally would not use it as a spoilboard. I think that material is falsely blamed for Z inconsistencies, when in my experience, the problem has been the fact that the screws 'pock' the material where the 2 sheets meet or people literally have 12 screws holding down the entire 4X8 spoilboard...that doesn't cut it.

Just my .02,
-B

[don]

Brady,

How do you fasten your base board down to the steel? And do you surface the top of the base board prior to gluing the spoil board to the base board?

Don

[Brady Watson]

With carriage bolts as SB specifies. No, I don't surface the base board as it is not necessary. Only the spoilboard itself needs to be machined square to your Z-axis.

PS- It typically takes about 1/3 to 1/2 gallon of wood glue to glue the sheet down to the base.

-B

[bill.young]

Make sure your tabletop material is rigid as well as stable. If your tabletop stays the same thickness...is stable... but sags between the cross supports then you're still out of luck. The little bit of 3/4" PVC I've cut for house trim is awfully limber.

If sealed well..especially along the edges...a couple of layers of good quality plywood glued together has a lot of things going for it as a table surface...easy to work with, easy to glue, and quite rigid.

Bill

[terryd]

I used 3 layers of 3/8" okoume BS 1088 marine grade ply with 7 plys per. Bolted the first layer to the angle iron as per shopbot specs but using Red Loctite then epoxied the next two layers on. Drilled many through holes and using fender washers and bolts (every bolt in the shop)clamped it all together to cure. Removed the bolts, filled the holes with body filler and surfaced.I then sealed with thinned epoxy until it stopped sucking it up. $150 more than the usual top but living next to an inland sea I did not swelling and distortion to be an issue in the long term. Our climate goes from -40C to + 40 C and humidity in the shop varies from 0 to 95%. @ $20 a sheet for 1/2" MDF I consider it a throw away and at times will surface the table every day as need be. Like Brady and Bill I glue and screw the stuff on and when dry (overnight) I remove the screw and surface away. BTW do not use drywall screws because those little roaches break about 1/8" under the surface just waiting to meet your new router bit. I use 1/2" because a fixed rate of distortion per inch of material is a gimme therefore having less of the material to start with means less distortion in the end. The name is spoilboard for a reason.

[knedert]

Folks -

Okay, here's my plan... let me know what you think of this.

The plywood is surfaced - I picked up a sheet of 1/4 hardboard and I am going to just carpet tape that to the plywood, and save the $$$ PVC sheet for some real work. The ply is held to the table by 1/4" dia nylon flathead screws, so no danger of hitting metal there.

This will give me a spoilboard that will be appx 3/4 thick. I have an extra sheet of hardboard, but don't think that there would be any benefit to putting two sheets on.

Before I put the hardboard over the plywood, should I seal it? The underside of the panel is finished, and I was thinking that a coat or two of shellac would help reduce any moisture/movement issues in the plywood.

John Moorhead

[Brady Watson]

John,
In my opinion...it's a bad idea. You would be 100X better off by gluing a sheet of regular old MDF to the bolted plywood. Taping hardboard to the ply will yield worse results than if you just screwed MDF to the baseboard.

You want 2 sheets. 1 sheet that is bolted to the angle iron supports and another that is glued on top of that. High quality 3/4" plywood will work as your spoilboard as well, but it will not give a hemogenous surface like MDF, Medex or Extira will when machined flat several times.

-B

[gerald_d]

Similar thread (http://www.talkshopbot.com/forum/messages/7/4186.html)

On Spoilboards:
Start out by thinking what you want your spoilboard to do for you.....

If you want it to grip a part that you want to hold down, (sometimes double-sided tape, or vacuum puck, or screws), then you want to be very sure that it in itself is held down securely over the whole area. You don't want it to lift between screws. Glue is the answer for holding something down over a full area. Plastics don't like to be glued. The answer is wood.

If you screw down a thin sheet of anything, it has little resistance to lifting between the screws. You can't use screws to hold things down because the screw needs to go deep and it builds up some crud under the "spoilboard" where it penetrates.

Thin, floating, plastic spoilboards are fine for full vacuum tables with plenty of suckdown, but is that your application?

On support boards:
Our support board is MDF 30mm (1.25") thick, and the heads of the carriage bolts are recessed into it so that the top of the head is about 3mm (0.125") lower than the top surface. The recesses are poured with thin epoxy before finally tightening - bolts are less inclined to work loose with time.

The decision on whether to surface the support board or not will depend on how flat it is - general advice can't be given for that. By dropping the carriage bolt heads a bit further down, you leave yourself the chance to do a surface cut without hitting metal. Drop them too far down, and they might tear out the bottom of the support board. I can't really see a practical support board being much under 30mm (1.25") thick - this also gives the rigidity between the cross-members.

Plywood or MDF? Our preferance is for MDF for 2 reasons:
- Our MDF is much more consistent in its inner structure than the plywoods which sometimes have substandard inner layers. You don't want to recess for a carriage bolt and find a knot or an unglued bubble.

- Our plywood is not big enough to cover the whole table area

[jhicks]

LOTS of stuff here so forgive me if I haven't read it all on my quick morning review. I thought about this PVC base and spoil board subject some more and agree that the AZEC or PVC sounds like a winner. On the spoil board, why not just use a thin sheet of SINTRA or something like it as the Spoil board rather than the Masonite sheet or MDF etc?
Cheap, thin, and stable. HUMMM???
Costly, flat, stable, secure base in 3/4" pvc, topped with 1/4" cheap stable replaceable SINTRA glued to the PVC. Machine away and replace as needed. Odd idea but maybe even Coreplast spoil board? Gotta Go.

[gerald_d]

Why this belief that plastic is stable? I am forever having problems with machinery where plastic components change shape/thickness with temperature or water. Nylon is notorious for swelling with moisture absorption - over 6% by volume. Maybe I trust MDF more than plastic because I have less experience with it.

[gamisano]

Coroplast!!??? Now your kidding...right? The first time you machine the surface for flatness, you'll break through to the corrogated ridges, even on the thicker coroplast sheets. But, then again, at least you were kidding.

Guido

[jhicks]

Yeah Guido! I guess I had a temporary brain freeze with that Coreplast thought. Not every thought that pops into my brain makes sense but thats why all you guys are out there. You need someone to throw out insane ideas to see if you're paying attention.
On the subject of Base boards, and thicker PVC, I was thinking of a base in something like 1" or 3/4" ply maybe even MEDEX or Extera with a top spoil board of PVC. I do believe its more stable than MDF but the technocrats will be able to prove or disprove. My experience with it on vacuum pucks has been excellent so when I get ready to replace with a higher cost material, I keep telling myself its worth the experiment.
Strange someone hasn't tried it yet, don't you think?

[matt_p]

here's what I have done and it seems to work well. I started by bolting 7-ply baltic birch to the 8020 cross members (i went with the baltic because of the table size, 60x120). I surfaced this repeatedly to compensate for any sag and such. I then glued 13mm sintra to the ply. Surfaced again. I then cut my vac grid into the pvc with a bull nose cutter. I then glued "de-skinned" 1/2" mdf to the pvc. the whole set-up is fairly stable, moderate in cost and it will be easy to mill off the mdf when it gets too thin. My vac system is powered by a fein turbo 3 and it works really well for what it cost. It will easily hold a 12x12" square of acrylic in place under heavy cutting force.

[mike_ross]

I have been thinking along the same lines. Plywood to the table frame, then 3/4 Sintra for a vacuum plenum, then a MDF spoilboard. How did you attach the PVC to the Plywood and MDF? Thanks in advance.

[Brady Watson]

Scuff the back of the PVC with 60 grit sandpaper and wipe it down. Then you can use epoxy or gorilla glue (I like epoxy better since it doesn't expand) OR contact adhesive. CA doesn't give you any room for error...once it is down it's staying there!

-B

[matt_p]

I used CA. I'd probably do it differently next time tho. maybe a latex flooring adhesive. sintra usually has a cup to it, so make sure you get it on the downside.

[btk]

I am not sure how well it sticks to plywood, however for PVC-to-PVC adhesion I have been using a new product called Bond-N-Fill http://www.advancedtrimworks.com/bondfill.htm
and it is really great. It dries white, has a longer working time than PVC cement and is sandable once dry. Only drawback is it is pretty expensive.

-Brian

[billp]

Another viable option would be "Roo Glue". It's a water borne adhesive which has numerous formulas for different materials. They recommend their "clear" formula for PVC; http://www.rooglue.com/which.html
It has the added benefit of drying clear (hence the name) with a water cleanup. They claim that it works very well with the following materials;

[jay_p]

I have used Roo Glue clear many times for gluing to the non-porous surface of melamine, and it works very well.

Jay

[denver]

I just ordered my machine (60"x96"), and now I'm trying to figure out the rest of the items I'll need by the time it arrives. I've read many of the support board accounts, and particularily liked Terry Doherty's: http://www.talkshopbot.com/forum/show.cgi?tpc=29&post=35683#POST35683.
Another account that impressed me was Mike Richard's: http://www.talkshopbot.com/forum/show.cgi?tpc=312&post=36639#POST36639

My environment will have large temperature and humidity swings like Terry's - I live in Minnesota. Terry, any new thoughts about your support board design after this additional time? How is it holding up, and did you seal the bottom of the lower layer before bolting it to the machine?

I'm also trying to figure out how to deal with joints in case the material I decide to use isn't available in 5' widths. Are seams better going along the Y axis or X axis? In either case, I would have to have multiple layers, with the seams offset. Perhaps one X and one Y?

Thanks,

Denver