fprintf

So how flat is flat for a building board? I have a table that has a very slight bow in it that I cannot seem to remove via shimming. At this point I have tried putting a 36" straightedge along various axis of the table and find one axis where there must still be a dip - it is not large enough to put a peice of paper through, but I can tell it is not quite flat because if I wiggle the ruler (a flexible metal yard stick) there is more resistance at the ends of the ruler than there is in the middle. Also looking at the shadow the edge of the ruler creates it looks (perhaps an optical illusion?) that the shadow is just slightly bigger in the center.

With all the suggestions for building boards I see on here, from glass to hollow core doors to laminated shelving, in order to acccurately build a plane how flat does it have to be? I do understand that the flatter the better, but is there some degree of precision I need to achieve and some point of diminishing returns where getting any flatter doesn't help?

This is my first built up plane and is a 115" wingspan sailplane - I would assume any errors along the way will translate to big errors along 9 1/2 feet of wingspan!

touch and go

My Feedback: (11)

IMHO, if the gap is so small that you cannot slip a piece of paper under it then you have no problem--build away.

MinnFlyer

My Feedback: (4)

Ditto. A piece of paper is only a few thousanths of an inch thick. You won't have any problems with that. I'd be willing to say that anyone who uses a ceiling tile (or the like) has as much or more deviation

Mike James

The answers above are good, and I agree.

But, I'm reminded of a quote by Billy Carter, infamous brother of ex-President Jimmy Carter.
-------------------------------------------
In an interview, it was mentioned that he was a peanut farmer, and the question was...
"In these times of health conscious people, tell me Billy, are peanuts fattening?"

Billy's reply was "Welp... It depends on how many ya eat."
-------------------------------------------
I think each person has to decide what's "good enough" for building. "Perfect" is probably the best answer, but most of us stop somewhat short of that, depending on "things". Personally, I think a flat and level workbench is the best tool we can have, and hope for the day when I can afford the stainless steel, or precision-ground granite, or whatever. In the meantime, it's (for me) a lot of measuring with incidence meters, digital calipers, etc., and striving for the best.

SeditiousCanary

Most paper you get out of a copier is 20 pound and is between 3 and 5 thousandths. This is pretty normal tolerance for machining parts. Plus of minus .005 is something most people wouldn't notice, but if you hold that to the whole plane, it's sure going to fly better for it. I am designing a plan in AutoCAD right now, and I have my prefs set to hold accuracy to ten thousandths. Your plans and tools should always be at least ten times as accurate as you want the final product to be.

kdheath

I saw an interesting building surface. It is a six foot long piece of tombstone granite ground flat and polished. Guaranteed not to warp.......

I'm not convinced that you can build a wooden structure that is truly accurate to better than two decimal places.

SeditiousCanary

I can make a person guaranty that any thing I build is.

We're talking about miles... Right?

gus

Surprised that no-one has mentioned the use of thick glass. Much cheaper than granite, more available, and at least as strong.

I have a 2x4 sheet of tempered 1/2inch glass. Pretty darn flat.

gus

JimTrainor

I have an old door that is close enough.

I've thought that if I really wanted to get a perfectly flat and level surface, I would build a small frame around my old door such that a self leveling floor compound can be poured to a depth of a inch or so. That would create a perfectly flat, level, surface for little cost.

I'm not talking about the stuff that has to be trowled flat. I'm referring to the liquid slurry stuff that finds it own level.

fprintf

A friend recommended I think about that approach - the floor slurry kind. I have no idea where to find it - I would assume a home center. And I hope that it dries perfectly flat - god forbid the cat walks across it late at night. :-)

How heavy would that be? Certainly no heavier than a piece of granite, but otherwise pretty heavy, no?

SST

My Feedback: (2)

Oh STOP!...STOP! I can't stand it! Building a model to tolerances of ten thousandths (BTW, is that .010 or multiples of .0001)? Over what distance? My god that's good! I'm a journeyman toolmaker. I make my living on precision, OK? Mill .100" off one side of a 1" thick, 6" long piece of cold roll STEEL and you will have a measurable ammount of warpage next morning. Your Monokote can cause serious warpage on a hot day. Flat is cool, but be reasonable. Humid days will make your structure move, so don't sweat the table...If it doesn't have sags that allow pencils to roll under your straightedge (or at least almost ), you'll survive. I've seen fuselages that must have been 1" or more out of line (.40 size plane) that still were flyable...Like I said, you want to do the best you can, but you cannot build a perfect structure from the woods we use, and even if you could, it wouldn't stay that way.

Bob101

I use a peice of tool granite. You can order them from various too places like Grizzly etc.. that's been polished down - up to really big sizes but I use a 24" x 36" piece which is big enough for my stuff ifyou build the wings in halves.

Sure it's really straight but I use it for other stuff and it's really heavy (which helps in the - oops I bumped the table area as well).

Why not start as straight as possible so when the monokote does put in a slight warp - it's not warp added to already warped structure?

linclogs

Right on, SST! (Quote: "...you want to do the best you can, but you cannot build a perfect structure from the woods we use, and even if you could, it wouldn't stay that way.")

I remember when I used to build free flight and glider models on my Mom's card table (too many years ago to even try to count) with a piece of fiber bulletin board as the building board. Don't know if it was flat or level 'cause - ohmigod! - I never checked it!!! But you know what? I built 'em anyway and every one of them flew! If there was a warp or bow, I steamed it out. Or adjusted the rudder trim tab to make up for the imperfection.

Gosh - no one told me I wasn't SUPPOSED to be having fun because my building job MAY have been a "few thousandths" off!!!

spokman

My Feedback: (2)

Maybe your straight edge is warped! : )

SeditiousCanary

Sorry guys, living in Tucson, I can't tell you what humidity is. While .001 is unrealistic for building a wood plane, it's not that unrealistic of you are building with foam and glass.

I should also point out that it is unrealistic to expect this sort of accuracy for a hobby that you crash. If you did, you should get a new job at NASA's JPL. Consider that I am talking about accuracy that an inspection room's temperature is controled so you can measure that accuratly. I just draw things that exact so there is no confusion for someone else reading my plans.

Have I mentioned that I need GD&T classes just to finish my AutoCAD101 class final project (which I got an A on) because it was that exact?
Now, where was I?
Ahh, angularity off the...

Jimmbbo

Agree with SST - no need to measure with a micrometer if you're gonna cut with a chain saw...

No visible warps in the building surface works for me - I use 5/8 x 12" veneered particle board shelves with 1/4 inch bulletin board cork contact cemented on the building side... Cheap and effective! They have remained dimensionally stable and flat for over 15 years (stored indoors). Made two 6' long boards for wing and fus construction, one 2 ft long for stabs and one 12" x 12" for vertical fins and rudders... allows me to move the assmbly to elsewhere while working on the next assembly....

HTH

Jim

Patriot

I have a 2'x10' workbench that is pretty flat. Made of a steel angle iron storage shelf, with a kitchen counter top for a surface. It's flat to the eye, with about a 1/16-1/8" dip near the end of the bench, so I don't build there, I use that area for my cutting board.

I was all concerned about the wing coming out warped, but she's turned out just fine, no problems or warps that are visible.

So, BUILD AWAY!!!!!!

Patriot

DMyer

My Feedback: (2)

"I saw an interesting building surface. It is a six foot long piece of tombstone granite ground flat and polished. Guaranteed not to warp....... "

Did he turn it over so the writing was on the bottom?

kdheath

Good question. I didn't see the bottom, but it looked like a piece that never got finished into a headstone. I can't see Windy stealing a tombstone, but he did say he got it for free.......Not a commonly handy option, but shows the lengths people will go to. His stuff is so good, it is positively embarrassing to merely excellent builders.
Kelvin

MikeSell

My Feedback: (2)

In my area you are much more likely to find a 9" or thicker granite inspection surface scrapped out than a tombstone. Many auto factories and machine shops have closed and left these "stones" without a home. I measure on a 7'x4' one daily certified to plus or minus .00005 flat. If you think that granite plate is guaranteed not to warp you are mistaken, they even have to be resurfaced every year for settling and wear.
Use something else. Granite doesn't take pins well. A dollup of glue throws it way out. Balsa dust even takes it out of spec.
Find the flattest most usable surface you can and have fun. Thats what this hobby is all about.

jollyroger

My Feedback: (6)

About hollow core doors: Do they need some reinforcement or are they sagproof? I have one but before I use it can it be made as nearly flat as possible with this type of material. At this time I am using a 25"X49" piece of 1 1/2 thick particle board with a laminate surface I picked up where I work. I just checked and discovered ther is a slight bow enough to easily slide a sheet of copier paper under a straight edge. The gap is close to .5mm. I am going to hardware to locate some angle steel or aluminum to reinforce the bottom of the sheet in hopes of bringing the surface true. jollyroger

Ernie Misner

Good grief you guys! Just build your wings right side up and a tiny bit of dihedral will be built in..... a "good" thing for sailplanes!

Ernie

O2man

My Feedback: (5)

As a practicing fussy person, I am humbled by the level of obsessive behavior reported here. I used to think that if my structure looked strait, then I was OK with it. I would look at it and check it with benchmarks and all that good stuff. Then I would cover it and check it again. Then I would go to fly it and come home and check it again. When I got all through the good news is that I had a straight plane and hadn't used 6 ounces of fuel in a season. BIG savings in costs.

Do the job well and then go out and fly it. Otherwise you can plan to use the plane as a display item and apply to the Guiness Book of World Records for an honorable mention in the World's Fussiest category.

fprintf

Ok, thank you very much. It is just that for those of us who have never built a plane before and hear all these stories of "make sure you have a flat building board or your plane will not fly straight", or "a little attention to detail now will pay off in very little trim needed on that first flight".

This is a $130 kit not just a $44 Gentle Lady, and over 115 inches a small error at the root translates into a whole lot of misaligned parts at the tip end. Just being anal, overly cautious but also a complete newbie on this one.

So by being off a paper width it looks like I am ok. But if I didn't ask I wouldn't know 'cause everyone makes it seem like a really really flat building board is essential.

Thanks again guys! I can't wait for my kit to arrive in the next few days. Off to the fabric store to try and find some dressmaker pins. I bought the $3.50 SIG package of 25 T-pins from the local hobby shop anyway - it keeps him in business.

CafeenMan

Gus - I build on 1/2 glass, but it will bow significantly if it's not supported. I used a long level to make sure it's flat by putting shims between the glass and the frame where needed. It was a pain and tedious and after a month I had to do it again, but it's pretty darn flat now.