anyone use a vac bag for stair stringers
mathewson
| Posted in Construction Techniques on
I’m thinking of using a vacuum veneering bag for gluing up curved stair stringers. Has anyone used these? Does it work better than the dozens of clamps?
Since the bag has to be clamped to the form along with the bag I’m wondering how long the bag will last.It looks like to bag has to be clamped in only a few locations instead of at every riser.
Replies
Seek out Armin.
A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
you can buy heavy duty plastic tube on a roll they use it around here to flood bean/rice fields... look on the net people use a 100 different ways to seal the ends and pull a vac...
what i have played with is old political signs the hollow core white plactic.... you need something like that to allow the air to be drawn out evenly... even if it's only strips of it the full lenght of whatever it is you are glue'n...
i've played with pull'n a vac on my cast concrete... just on small stuff... sure gets all the air bubbles out...
p
Thanks PonytlI'd be happy to find something cheaper than what is offered from the online outlets. Where is "around here", I don't know the first thing about rice or bean fields, so I wouldn't know what to ask for.I'm pretty well setup for flat bag work, but would hate to drop $500 on a stair-stringer bag if they don't last very long.
Pretty easy to make your own bag with 20 mil vinyl from joann fabrics. Pretty cheap to buy and they often have 50% off coupons available
Tom,That sounds like a solution that fits the budget. Even if it fails after a couple of uses the cost still makes sense.
The vac bag works well however to achieve optimum results you still need the clamps. Even with the bag I use spring clamps along the edges as it seems to produce a tighter finished seam. I still build the form the old way, one vertical stud for each riser. Where it becomes difficult is when the veneer lumber is not long enough to make a full length laminate. Maintaining a tight splice while stretching around a curve takes a bit of thought. One thing you might try is after applying the glue wrap the finished stack of laminates tightly with stretch wrap. This keeps everything together while you push the stringer into the bag. I use a plastic screen between the stringer and the bag to prevent a trapped air pocket in the bag. Try a dry run before you apply the glue as you may be surprised as to heavy and unyielding the assembly could be. Have plenty of help on hand and don't underestimate the potential spring back during bending. One of my earlier attempts resulted in launching me off the ladder and darn near slapping me silly. Would have made a great U-tube clip had someone caught it on tape. I now tie stuff off with a safty strap to keep things from getting out of hand. Get the 30 mil poly bag. A bit more money but mine has been in use about 15 years now and shows no sign of quitting. Remember don't over tighten the clamps, just snug enough to hold the assembly in place. Again try a dry run as it comes with a bit of a learning curve.
Hope this helps, anything else I can do for you ask away.
Thanks for the advice, looks like a bag will last the remainder of my career. At the moment there is a flyer being built in the shop with veneer on on both inside and outside. It turned out to be a 3 stage glue-up process. 1 day for the core, 1 day for one side of the veneer, then remove the form and glue the veneer on the side where the form was. Made me think that a bag would have saved 2 days.After my first glue-up I use the stretch wrap to transport the wet plies. I once heard it described as trying wrestle a python covered in gorilla snot.Thanks for the warning on the springback, I'll bet that came as quite a suprise.I'm sure I'll have more questions before starting the next one.
Armin is Riverman, you've got a good source there for information. Best of luck.A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
You might want to pick up a copy of Darryl Kiels (Vacupress) DVD "Working in a Vacuum." He does a step by step demo of using a vac for a curved stringer and shows a comparison of the clamp vs. vac formwork. About one tenth the clamps and the formwork looked like something you could toss up far quicker. Looked like a great solution to me, but I only use my vac bag for flat work.
Brad
Sorry for the delayed response. I have one of his dvds and had to go back to see if I missed that part, turns out I don't have that dvd. Going to order it on line tonight.
It is an interesting DVD, but it is not greatly detailed when it comes to fabricating the stringer. Honestly, Darryl makes it look fairly straight forward, but I know that is far from the truth.
After about 600sqft of vac bag flatwork with crotches, burls and common veneers I am finally trying a fairly simple curved project. I am gaining a huge amount of respect for any woodworker such as Riverman that does this work regularily. Lots of little hickups in curved work. I agree with Rivermans idea of starting with a project where it doesnt matter if it takes too long or parts wind up in the burn pile.
Brad
When reading one of Stan Foster's photo descriptions of his curved work (he's another local at Breaktime) I asked if he ever used a vac bag. As I recall, his response was that the bag can't keep the stack of veneer from cupping. He even showed a photo of this effect from a prior project. Since then, I helped a pal do a curved stair stringer with my bag. Sure enough, there is cupping after the stringer is glued, set, and removed from the bag. Its not an enormous amount, but enough to become a factor in some applications. We were able to work with it but we had to be clever. I'm sure the homeowner, or anyone else, ever noticed. The jillions of clamps will prevent the cupping.
I wonder how others with a lot of experience deal with this or is it factor enough to be of concern?
The cupping problem, at least in flat work, most often occurs when both sides of the substrate are not veneered. I believe this is to balance the rate of evaporation, wood cups toward the direction of greatest moisture loss. When you experienced the cupping did you glue everything up in one step? If so I don't know what would explain it, but would like to hear from someone who knows.
I doubt we have any rookies in this discussion that would try to veneer one side only:)
After Sapwood pointed out the cupping that made a lot of sense. The cupping is a reality of curving a section. On any curve you have a long side and a short side. This would be even worse on a curved stringer as it is a two dimensional curve. The stock needs to stretch and compress in different areas. The radii will control how much. In the DVD I mentioned, the author shows how to veneer mouldings, rolled shapes and even a circular table top. He uses a very small amount of moisture to relax the wood on the surface (not so much to ruin the glue bond) and applies the adhesive direct to the veneer to promote some wood relaxation. I cant see that trick working in the case of a curved stringer, but I thought it might interest some. Some veneers will be better than others for curved work, but I sure as heck do not know which ones.
Brad
Edited 3/10/2009 6:40 pm ET by brad805
Some of the cupping is from the inconsistent moisture evacuation, through many variables. One is the flow of "air" from the bag. As moisture is put into a vacuum, it turns to gas. When the evacuation is not even, certain areas will dry out at different rates, causing cupping.
The same thing happens when there is uneven temperatures of the material. This is because liquids turn to gas at lower temperatures in a vacuum atmosphere. A warmer side of material will have less moisture. Basically, the boiling point temperature is reduced as the vacuum increases. I learned this one the hard way.
Funny thing is may dad new the reason from the Korean War. Seems that the military tried to dry the old silk parachutes with low heat and vacuum to prevent weakening/burning. Didn't work too well for the military.
I hate it when my dad is right.
I still remember the parachute hangers when I was a young kid in the late 60's,
As for veneers on only one side. Veneer/lumber only expands/contracts in the width not the length. Always band in odd layers. Keeps both sides under equal tension
Part of the problem with cupping comes from the type of glue used. As I recall Stan, along with most tradesmen are big fans of yellow glue. Yellow glue has a lot of great applications but large scale laminations is not one of them since the water based glue injects a lot of moisture into the core and can cause a whole host of long term problems after it dries. Try soaking your stringer with a gallon of water and see what happens after it dries. That said I never use yellow glue for lamination's of any kind. Unibond 800 or some other two part canalized resin glue will minimize the problem. Also the number and type of plies has a lot to do with cupping. Luan is a no no, much too porous, absorbs to much moisture and has no strength worth mentioning. For the core I use 1/8 inch thick Baltic birch and the face veneer is solid 1/8 thick veneer same thickness same species both sides of the stringer. I have built over 70 stairs and once I got away from using water based glue the cupping has not been an issue.
Edited 3/10/2009 8:01 am ET by riverman
First off, my apologies to you, Stan Foster, everyone else here, and my dog..... I remembered the prior discussion here incorrectly. Twas you, Riverman, who answered the question and it wasn't me who asked... but I'd been thinking it.....The topic I'm referencing is: 70866.40Here is the photo I remember:
That is a photo of one of my handrails and the curve you see is not cupping but rather the shape the rail takes on from being wrapped around the pipe used as a form. Keep in mind the the laminations are bent around the form in a helix, if you were to check a plumb line in the installed rail it would show as a straight line not a cup. Check out the link to my website for the completed stair. The main stringer is the same way, cut out a section lay it flat and it has the illusion of being cupped however in its intended position the plumb line is straight. A pretty funky explanation I know but its early morning so what do you expect.http://www.northernsunwoodworks.com/custgal2a.htm
I've done a lot of vacuum pressing, but the one thing that I never have done is a stair stringer. I have to agree that there is a great potential for cupping, specifically when you use glues that cure by absorbtion, if I can call it that. Titebond or other PVA glues are some of them.
The best glues I have found were the urea resin(as in plastic resin glue) that uses a catalyst for curing/hardening. One such catalyst is formaldhyde based. BEven better is epoxy, but too expensive.
Cure times are still in the 8hr/one day range. You can cheat on your times, but I have had more spring back. The titebond also has a more elastic/flex bond. I have heard the term is called creaping.
The other thing that I have found is that the glue laminations are not as tight as pressure clamping. Vacuum pumps get down some where in the 12-14psi over large areas and regular clamps can get way over the 100-200psi over smaller areas, but still are greater in the end.
As for the bags that are used for bagging stringers, they have small veining grooves into interior surface area to help promote air evacuation. I have done sort of the same thing with using a piece of fiberglass window screen.
You can make your own bags with 15mil or 30 mil patio vinyl. The stuff is reusable and will last along time. Glue seams with vinyl adhesive. Ends can easily be clamped with 2) 3/4"x1" sticks a 2 spring clamps at each end. Once the vacumm has started, the ends pretty much seal themselves.
A rotary pump, such as one from Gast(Graingers sells them)is what I prefer. Venturi pupms are recommended by others, I just don't like having to run them off the compressor. I really, really, really like the controllers that Quality Vak sells. Plain old 1/2"ID nylon reinforced tubbing works. If you want, a check valve and an empty compressor tank will help keep the pump from cycling on/off.
Any more questions, I can fill you in some more.
Heck, if you were close to Sacramento, Ca, I have a bag you could have to play around with. I even have a large pump available for $100.
Thanks for the response.
After looking at Armins stair job from last year I've gotten a little carried away with vacuum pumps, I now have 4. The largest a 5 hp 70 cfm, a 25 cfm a 15 cfm and a 5 cfm. I'm not sure what compelled me to buy them all, but it seemed like a good idea at the time...I am currently using plastic resign glues but a friend of mine swears by the West One system because it does not introduce any water. Neither of us have used a bag for stringers before, so I'm grateful for all input.
I agree on the apoxies, but the cost. The pot life doesn't help either.
This is the place that I buy from. They will send you samples to try. And they ship. UPS only allows something in the 4 1/2 gal for liquids.
National Casein of California
3435 W MacArthur Blvd, Santa Ana, CA 92704
(714) 979-8400
I find other than the cost the big negative to epoxy is the fat glue joint and the MESS. Maybe it's me but I have not been able to get an invisible joint using epoxy. Also the way a vac bag system works is it applies a lot of pressure across the entire surface of the work piece and not so much near the edges where you need it most. That is why I still use clamps along the edges on tight bends. To extend the life of the bag soften the work edges to minimize the chance of a corner cutting into the bag and use a screen to help evacuate the air. Everything has its limitations and learning curve, what works for one person may not work for someone else.
My advise would be don't try something new if you don't have the time to do it over and above all don't get to discouraged if it fails the first time around. Every winter I heat my shop with plenty of great ideas that never panned out.
My earlier reply to you should have been addressed to Migraine sorry.
The only benefit I have found with the epoxy is its gapping ability, otherwise I sure prefer the Unibond 800. I did a few straight Cherry stringers recently with West, and the squeeze out was a bit of a mess to clean up. Very hard on planer cutters too.
Good luck.
My experience with vacuum laminations was in cold molding for boat construction. There the "bag" is simply a sheet of 6 mil visqueen sealed at the perimeter to a solid wood frame which formed the male part of the mold. The sealant was simple window caulk, that you rolled into a rope to make the perimeter seal. An old milking pump supplied to vacuum necessary.
But the key point is the quality of "clamping" as compared to anything you do with mechanical clamps is far, far superior. Atmospheric pressure is about 15# per square inch or over 2,000# per square foot. It is evenly applied to every surface.
We were molding four layers of 1/8" veneers, with epoxy between each. The result was like a very high quality plywood, extremely strong and incredibly dense. Though fir veneers, it behaved like hardwood after curing. Once the vacuum was created, the laminations continued to be forced down, and you could see the epoxy being squeezed out of every joint and edge. It is remarkable to see first hand; unlike anything you have ever experienced when using conventional clamps