Build Table Question: How flat is flat enough?

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Autodidact

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I thought of another idea to get a level smooth table.

First build a table in the conventional manner, and make it as level as possible to +/-1/4 inch or better.

Put a 1/2 inch tall lip around it.

Mix up a bunch of low viscocity acrylic casting resin and poor it on the table to a depth just below the 1/2 inch lip.

Let it sit undisturbed for a couple of days. Remove the lip.

And there you have a perfectly level and smooth table!:gig:
Excellent, but be sure to time the curing of the resin with the phases of the moon - wouldn't want it to harden at high tide!
 

Jay Kempf

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The most accurate way I can think of is to flood the surface with coherent light from a laser, and then use a Michelson Interferometer set up to look at the interference pattern, using the same laser as the reference---+/-several hundred nanometers. But that's getting a bit ridiculus. The next time it rains you would have to do it over again!



Or a piece of string.

I'm starting to think that +/-1/32 inch (1/16th inch peak to valley) when you are building a wood aircraft in Ohio (high humidity in the summer, dry in the winter) is about all you need.
I think the best way you are going to assess flatness is to get a very straight bar and prop it above the table and roll a truck down it with a dial indicator to find out your total flatness deviation.

But we are back to what tolerances do you need to hold. Let's say you need to measure something sitting on the table. If you have a reference plane, any reference plane you can measure relative points on the object. So the table at that point doesn't matter. Your laser could be used as the reference plane. If you wish to certify the table to be used as a datum then you need to know how accurate it is. Once you know that then you can only measure to that level of accuracy. So if your table is say .015 then you will only be able to spot points on whatever you are building to .015 or more.
 

Hot Wings

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"Or a piece of string."

This, and a plumb bob, has for decades proven to be perfectly adequate. Wood aircraft move with changes in humidity and aluminum aircraft move around with changes in temperature after built.

The table is only a gross reference. A 1/4 (6mm) inch over 16 or 20 feet (6m) is most likely going to be good enough. Reduce that tolerance proportionally - 1/8" per 8 to 10 feet - 1/16 per 4'. The normal procedure it to make critical alignments with reference to the plane as it gets built, not the table. Unless you are making multiple copies of something the table is only a convenient area to work. If it has enough strength and stability to to keep each sub-assembly fixed during construction that is all that is really needed, no matter how twisted or out of level it may have been to start.

When you build in the washout in a wing you use the root wing rib as the datum. The fuselage sides are jigged up using the plum bob or a simple square, relative to each other. When the empennage is getting attached to make it square it's common to use the upper longerons with a straight edge laid on them as the reference.

All this applies to a composite planes as well. Ever seen the divots left in a wood table after the glued on jig boards are removed? We just fill them in, sand over the area, glue on the next jig, and keep building.

Don't sweat the small stuff!! Go build an airplane, not a table ;)
 
Last edited:

deskpilot

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I thought of another idea to get a level smooth table.

First build a table in the conventional manner, and make it as level as possible to +/-1/4 inch or better.

Put a 1/2 inch tall lip around it.

Mix up a bunch of low viscocity acrylic casting resin and poor it on the table to a depth just below the 1/2 inch lip.

Let it sit undisturbed for a couple of days. Remove the lip.

And there you have a perfectly level and smooth table!:gig:
The floor of my metrology lab was done this way with the exception that the powers that be at the time thought it would be too slippery so the stippled the surface as it cured. End result was a floor that was a ***** to clean.
 
E

ekimneirbo

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Lots of interesting discussion, but what about my original question?

I don't think 'level' is as important as variations or ripple in the surface

For someone building a wood or aluminum plane, using the table surface as a reference, what is an acceptable variation? (and plans don't always give tolerances) 1/16"? 1/32"? Working with wood and its variablility, I doubt you could get any construction tolerances tighter than 1/32" anyway, and maybe even 1/16" would be difficult.
OK, here is a direct answer to your question. No matter what you use that is common to a homebuilders jig table, it will move some with changes in humidity and temperature. Plywood is going to be a certain specified thickness but may be wavy and not in one single plane. Many many many airplanes have been built using this material and quite a few of them flew acceptably. Those that had inherent flaws in their dimensions and never flew or never flew well.........we don't know about them........so most everyone figures that plywood is the acceptable choice. That's up to you to decide, but whatever you choose, figure that the flatter it is when you buy it, and the less that it is affected by temperature and humidity.......the better chance you have to get and keep a reasonably accurate work surface. For those reasons I like the MDF (multidensity fiberboard) that's available. Its very very flat. If you span a four foot wide table you will need several cross supports under the work surface and at least a few well placed ones under a narrower table. This will allow some shimming if needed, but if you are careful with your table build, it should need very little if any shims. NOW, once you have the table assembled, you can place a lazer level in one corner of the table and by moving it to shine in different directions you can have a straight line of light projected slightly above and parallel to the surface. Take a framing square and put it on the table and mark where the light hits it. Move the square to different points on the table and see if the light hits the same spot everywhere. Then try getting a long straight piece of metal. Six feet or more is best and if you can find one of the longer rulers you can be sure its pretty straight. I'm not really sure what they are used for, but I came across a 6' long ruler somewhere in my life, and its hanging in my shop. Anyway, turn the lights off in your shop and shine a small flashlight at the point where the straightedge touches the table. Yes you will see some light passing under it but you are just looking for the worst places. If you have no really bad places, then you are good to go. Myself, I would shoot for less than .030 (slide a feeler gage under the straight edge at the worst points). That's a subjective matter of opinion recommendation, but I realize we are dealing with less than professional equipment. If you can get something better than that....great.....but remember that the material you select will have a large effect on how difficult it will be to get a flat surface and it may change with climate change....but so will your projects parts.Personally I think 1/16 (.0625) is too much for some of the dimensions you will be trying to hold but will not matter on others. Its best to shoot for the .030 than to be too sloppy on needed dimensions.
 
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ekimneirbo

Guest
I'm not saying that slope is unimportant, just pointing out the differences and what to do if you HAVE to put up with it. Going way back to the days of hand measurement, our 10 x 5 foot cast iron table had 6 adjusting jacks supporting it. It was approximately 9 inches thick and we hand scrapped it to keep it flat.

When CMM's (Co-ordinate Measuring Machines) came on line, cast iron was replaced by Granite and datums were where ever we wanted to put them.
View attachment 24707 Due to the number of variant's and size of our product, we opted for a hand operated machine.

View attachment 24708 As the years went by, auto companies wanted fewer parts so they gradually got bigger than the table. This was a temporary method to fit the exhaust system (Holden Commodore/Pontiac GTO) into the measurement envelope. We soon had a frame on which the complete system, from headers to tail pipe could be mounted flat and by including datum points roughly in the middle, the whole thing could measured in two halves simply by remeasuring the datums after the frame had been moved. Accuracy? 0.00000mm. We only worked to 0.000mm. You'd be surprised at the amount of geometric dimensioning and tolerancing that is involved in what people generally never see under their car.
I remember using some of the CMMs when I worked for the Navy. They were great machines for their time. Pick up three points on a radius and it could calculate the size of the arc and where its center existed out in space. Touch a couple of straight line points and it established a datum, drop a probe in a hole and it told you locations, angularity..........really great stuff, and that was 30 years ago. To people that haven't used this equipment, it sounds difficult but it was actually very simple to use. One of the local companies I dealt with just before retiring had a couple of small ones that sat on a table. They were about the size of an old Cadillac gage (18" ?) with several ball socket type joints like a robot arm would have.. They were truly amazing and fast..........did I mention expensive. The fun thing about geometric dimensioning and tolerancing was that it was a technological way to use redneck engineering and make it sound highly technical. The basic premise as I remember it was that if a bolt wouldn't go into a hole you could make the hole the maximum size within the tolerance range to see if it would then fit. If undersize fasteners were available you could try that too. Farmers had been doing that for a long time, they just didn't have blueprints to deal with. Actually there was a lot more to it on other types of dimensions, but we bored a lot of holes out to maximum size. You also have to remember that many engineers overengineered parts either because they didn't understand the purpose of the part, or the wanted to CYA (cover your A**) on everything. I remember one job where they designed a part with 8 locating pins located by a .001 true position requirement on every one of them......which boils down to a few tenths of a thousandth to where they could be located. He tied them to another dimension that had a .010 location tolerance which made it impossible to manufacture the part. No matter where the locating pins were placed, they would be wrong unless the original datum was perfect.......and they never were. After some consultation we were able to convince him that 8 locating pins were no more accurate than 2. These holes were on the inside of a segment of material that looked like you had cut a 10" long section out of a 3" pipe and were just using a 90 degree arc from it. I went back to the assembly building to see what these extremely tight tolerances were used for. Your gonna love this........ The section of pipe was placed on a round shaft and the welded in place so they could attach the end of a large hydraulic cylinder to it. Picture the cylinder used to raise the bucket on a high lift bulldozer. That thing could have been off a quarter of an inch and it wouldn't have mattered.
 

Jay Kempf

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I remember using some of the CMMs when I worked for the Navy. They were great machines for their time. Pick up three points on a radius and it could calculate the size of the arc and where its center existed out in space. Touch a couple of straight line points and it established a datum, drop a probe in a hole and it told you locations, angularity..........really great stuff, and that was 30 years ago. To people that haven't used this equipment, it sounds difficult but it was actually very simple to use. One of the local companies I dealt with just before retiring had a couple of small ones that sat on a table. They were about the size of an old Cadillac gage (18" ?) with several ball socket type joints like a robot arm would have.. They were truly amazing and fast..........did I mention expensive. The fun thing about geometric dimensioning and tolerancing was that it was a technological way to use redneck engineering and make it sound highly technical. The basic premise as I remember it was that if a bolt wouldn't go into a hole you could make the hole the maximum size within the tolerance range to see if it would then fit. If undersize fasteners were available you could try that too. Farmers had been doing that for a long time, they just didn't have blueprints to deal with. Actually there was a lot more to it on other types of dimensions, but we bored a lot of holes out to maximum size. You also have to remember that many engineers overengineered parts either because they didn't understand the purpose of the part, or the wanted to CYA (cover your A**) on everything. I remember one job where they designed a part with 8 locating pins located by a .001 true position requirement on every one of them......which boils down to a few tenths of a thousandth to where they could be located. He tied them to another dimension that had a .010 location tolerance which made it impossible to manufacture the part. No matter where the locating pins were placed, they would be wrong unless the original datum was perfect.......and they never were. After some consultation we were able to convince him that 8 locating pins were no more accurate than 2. These holes were on the inside of a segment of material that looked like you had cut a 10" long section out of a 3" pipe and were just using a 90 degree arc from it. I went back to the assembly building to see what these extremely tight tolerances were used for. Your gonna love this........ The section of pipe was placed on a round shaft and the welded in place so they could attach the end of a large hydraulic cylinder to it. Picture the cylinder used to raise the bucket on a high lift bulldozer. That thing could have been off a quarter of an inch and it wouldn't have mattered.
Geometric Tolerancing was crucial to being able to source parts of an assembly from multiple sources that weren't allowed to know about each other. It's just a math convention like any other math convention with it's own notation and interpretation. I have been around it for so long that it I forget how abstract it is to most people who don't understand it's purpose. In general it is a system that allows designers, fabricators/machinists, and QA/AC people to all look at the same set of symbols and numbers and get what they need to get out of it. And it has been misconstrued and misapplied as much as anything that any standards bureau has ever put to paper. In general it is still the best and most comprehensive system ever devised for manufacturers of goods to use to guarantee that designs turn into methods and turn into finished parts that fit as planned. Without it you have to write paragraphs of notes with what sounds like legalese to describe relationships between bit of geometry. I just signed up to a job that is all about getting this stuff right on a project so it is right in front of me and I am refreshing myself on the rulebook again. Sheesh.
 
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ekimneirbo

Guest
Geometric Tolerancing was crucial to being able to source parts of an assembly from multiple sources that weren't allowed to know about each other. It's just a math convention like any other math convention with it's own notation and interpretation. I have been around it for so long that it I forget how abstract it is to most people who don't understand it's purpose. In general it is a system that allows designers, fabricators/machinists, and QA/AC people to all look at the same set of symbols and numbers and get what they need to get out of it. And it has been misconstrued and misapplied as much as anything that any standards bureau has ever put to paper. In general it is still the best and most comprehensive system ever devised for manufacturers of goods to use to guarantee that designs turn into methods and turn into finished parts that fit as planned. Without it you have to write paragraphs of notes with what sounds like legalese to describe relationships between bit of geometry. I just signed up to a job that is all about getting this stuff right on a project so it is right in front of me and I am refreshing myself on the rulebook again. Sheesh.
You are right Jay, it was a good system. The point I meant to make was that many expensive parts would have been scrapped because .......lets say someone had drilled a 1/2 inch hole and there was a tolerance on the size of the hole of +.005/-.002. If the hole were drilled perfectly on location and exactly 1/2" (.500) in diameter you would have a good usuable part. If the mating part was manufactured somewhere else and the hole was exactly 1/2" (.500) but was only .001 from being perfectly on location, there would be a mismatch and the .500 diameter pin would not go through the two holes. Since hole locations always have some tolerance for location both parts would be good to the drawing specification but would not work during the assembly buildup. By boring one or both of the holes to its maximum size, the pin would now fit and both parts would be useable. The problem often came from the fact that holes were generally off location in two directions (X and Y) Geometric dimensioning allowed for both of these directions by trigging the true position of the hole and seeing if that dimension would yield a usuable component. I know this is an over simplification because there is a whole lot more to applying it to other types of dimensions. Its been 30 years since I used to do this stuff and the memory banks are fading fast. Geometric Dimensioning was a big improvement over standard drawing tolerances. Good Luck using it, I'm kinda glad I don't have to fool with it any longer.
 

wsimpso1

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Holy cow, that is some serious thread drift...

I built a 4'x10' torsion box for building my wings and control surfaces. Half inch MDF, 4" thick, built it to the plans on a woodworker's website, cut all of the internal spacers on one setup, assembled with carpenter's glue and 15 gauge airnailer. With tight string we can not measure the out of flat, even after three years of it sitting there. I would definitely recommend this approach if you want a flat jig table to build wings that are mirror images of each other.

Billski
 

entropy76

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Mr. Billski, interesting thread and post. Could you describe the torsion box method for me in laymans terms? What are the benefits of a torsion box structure? Thanks.
 

Old Koreelah

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For all it's disadvantages, you can't beat the flatness of a bloody big sheet of glass. Talk to the blokes who replace shop windows.
 

Aerowerx

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For all it's disadvantages, you can't beat the flatness of a bloody big sheet of glass. Talk to the blokes who replace shop windows.
But unless it is really thick it will sag an unacceptable amount from its own weight. Since you then have to build a support table to keep it from sagging, you might as well save some money and use plywood.
 

Jay Kempf

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For all it's disadvantages, you can't beat the flatness of a bloody big sheet of glass. Talk to the blokes who replace shop windows.
But you can't clamp of screw anything to it. Good for layups though. Can't beat the surface finish of a large sheet of glass. MD40 with Mylar almost as good.
 

DangerZone

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OK, here is a direct answer to your question. No matter what you use that is common to a homebuilders jig table, it will move some with changes in humidity and temperature. Plywood is going to be a certain specified thickness but may be wavy and not in one single plane. Many many many airplanes have been built using this material and quite a few of them flew acceptably. Those that had inherent flaws in their dimensions and never flew or never flew well.........we don't know about them........so most everyone figures that plywood is the acceptable choice. That's up to you to decide, but whatever you choose, figure that the flatter it is when you buy it, and the less that it is affected by temperature and humidity.......the better chance you have to get and keep a reasonably accurate work surface. For those reasons I like the MDF (multidensity fiberboard) that's available. Its very very flat. If you span a four foot wide table you will need several cross supports under the work surface and at least a few well placed ones under a narrower table. This will allow some shimming if needed, but if you are careful with your table build, it should need very little if any shims. NOW, once you have the table assembled, you can place a lazer level in one corner of the table and by moving it to shine in different directions you can have a straight line of light projected slightly above and parallel to the surface. Take a framing square and put it on the table and mark where the light hits it. Move the square to different points on the table and see if the light hits the same spot everywhere. Then try getting a long straight piece of metal. Six feet or more is best and if you can find one of the longer rulers you can be sure its pretty straight. I'm not really sure what they are used for, but I came across a 6' long ruler somewhere in my life, and its hanging in my shop. Anyway, turn the lights off in your shop and shine a small flashlight at the point where the straightedge touches the table. Yes you will see some light passing under it but you are just looking for the worst places. If you have no really bad places, then you are good to go. Myself, I would shoot for less than .030 (slide a feeler gage under the straight edge at the worst points). That's a subjective matter of opinion recommendation, but I realize we are dealing with less than professional equipment. If you can get something better than that....great.....but remember that the material you select will have a large effect on how difficult it will be to get a flat surface and it may change with climate change....but so will your projects parts.Personally I think 1/16 (.0625) is too much for some of the dimensions you will be trying to hold but will not matter on others. Its best to shoot for the .030 than to be too sloppy on needed dimensions.
Good points and advice.

If a flat and level table is needed then the MDF boards are the cheapest and most accurate solution because holes can be drilled into the table with an electric screwdriver without sticking out leaving a flat surface for your jigs and materials. I used MDF boards of 28mm thickness which were heavy enough to be moved around by one to two persons and allow good drilling through to attach to the wooden supports. I inserted paper to the places that were not entirely level so this allows to get the levelness up to the best tolerance. The thickness of paper determines how much a certain place of your table would be elevated or lowered and by simple inserting of the thin A4 paper a true level table can be made. It would vary with moisture and temperature but since the paper has similar characteristics as the MDF board and massive wooden table then the levelness is kept almost constant up to around 0.05mm to 0.1mm precision.

Glass can be put on top of such a table and held with simple small wooden blocks from the side that are screwed into the table. The mass of the MDF boards and glass add to a very solid platform for doing something. For example, fine grinding paper can be duct taped to the glass in perfect alignment when needing very flat surface as when fine grinding the top of a crankcase or a cylinder head. It is amazing how precise such a finish can be without needing to visit an expensive machine shop. When not needing the glass surface anymore, simply unscrew the wooden blocks which allows the glass to be removed, it's like Lego we played with as kids.

If better flatness or levelness would be needed it might be useful to contact someone who has a milling router cause they know how to obtain perfect flatness, but I wonder if more precision than that would be needed for a homebuilt.
 
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