I would pour the concrete on my garage floor. Then build a form around the my hardened slab and fill that with a cast polymer. Once hardened and form removed, I would knock off the meniscus on the cast polymer. There are some urethane sand epoxies that are pretty thin if warmed before pouring.

 

Why does some bushman need a "perfectly" flat slab of concrete?

Have a look at this thread. I'm trying to design a CNC milling machine that can be built from scratch in the absence of a machine shop with which to build it. Also with the most inexpensive materials possible. I've been investigating making my own CNC machine for a long time and the trend I've noticed is that the most expensive parts are the tight-tolerance linear parts like flat machined ways, linear rails, linear bearings, ballscrews, etc. I'm trying eliminate as many of these linear components as possible, but I just can't seem to get away from the need for "perfect" flatness/straightness in one way or another. So I'm trying to find the easiest way to do it that doesn't involve a milling machine.

How flat is truly needed for the intended purpose?

I would like the machine to hold a working tolerance of at least .001" so in order for that, the total of all deviations in all my axes needs to be <.001". I'm thinking I should shoot for .0001".

Is your bushman going to have the capability to lap two large slabs of concrete together?

I don't see why not. At least not yet I don't; I haven't looked into it much yet aside from some videos on youtube. Looks pretty simple to me; check out the video in post 12. Seems easily scalable/adaptable to me.

What about the aggregate in the concrete? Won't some of that tend to abrade the surface cement in the other slab? Seems like lapping is only going to work at anything approaching the scale you are talking about when both slabs are homogeneous down to the scale you are talking about.

I'm not sure. one side of my head says that the aggregate in the slabs isn't what's doing the abrading; it's the harder, finer, lapping grit between the slabs that's doing the abrading, and since that grit is harder than both the aggregate and the cement, it should cut both equally without prejudice. The other side of my head says that's stupid, and of course the cement will be worn away faster, leaving chunks of aggregate sticking out and digging into the cement of the other piece. I can't find any examples of concrete lapping concrete, so it might just have to be something I learn empirically. If it does prove to be a problem, I could just lay a layer of cement on top with no aggregate.

What about mechanical deformations when these slabs are moved around? Might not that dwarf the scale of the flatness you are trying to achieve?

I'm not sure what to expect, but my gut says it won't be a problem, especially if internally reinforced with steel wire.

 

Why would cement bound particle board?

Did you mean why NOT cement bound particle board? In any case, I'm not familiar with it. When I google, all the links are European, so that leads me to believe it's something we call by a different name. Hardie Plank/Board maybe? If so, I went to the big orange store and looked at it. It seemed to have a lot of flex. I want something as rigid as possible.

Surveyors use the following formula for the divergence of a circular curve from a straight. (offset distance of the curve from the tangent)

R = radius, L = distance along tangent, D = divergence, all measured in the same units.


\(D = {\frac{(L)}{{2R}}^2}\)

The mean radius of the Earth is 3959 miles = 20903520 feet.


So the deviation between the laser straight line and a level line is 12*(1000)^2/(2*20903520) = 0.287 inches.

Thanks for the confirmation.

 

If this is for your machine I would separate the precision parts from those that provide rigidity. Look at a typical machine tool, it has massive cast parts that provide rigidity and only small areas of these are ground to provide accurate surfaces. So you could embed a steel track into a concrete bed and then just grind and lap the track.

This rotary method (see video in post 12) seems to me to be the only method that will give true flatness. Regarding lapping in a straight line; what I've read so far on the all-knowing infallible interwebs, indicates that it yields a "perfect" "flatness" between the two surfaces being lapped, but not a "straightness" (actual flatness in two planes).

I guess I should disambiguate terms and confess that I haven't made any distinction between "flatness", "squareness," and "straightness" when they really are different things and I've been discussing them as if the were the same. Here's a pic to illustrate:

That's over-simplified, but good enough for me.

If I could get a perfectly flat and straight (flat in both 2 dimensional planes) piece it could be used for my purposes. However, if it were lapped only in one plane, it could be higher at one end than the other, and still be "flat"

That said I wonder if standard concrete will fall apart under the vibrations?

I don't see it falling apart in my mind's eye; it will be full of reinforcing wire and rod.

 

Please note you need three workpieces to lap together to form a 'perfect' straight edge or surface, not two.

There was a time when toolmaker apprentices would regularly prepare straight /flat tools in this way by hand.

Since we now know the application I'm sure you will get lots more useful help, but I like sirch2's idea of setting the guide rails in something massive but less accurate.
Again, however best practice was to use 'jib strips' to finally align a machine tool straight edges for 'truth'.
The other advantage of guide rails like this is they can be made relatively temperature insensitive.

Truth for a lathe is tested by turning something and then reversing the workpiece and comparing against a DTI or other measuring device whilst slowly rotating the workpiece.

A resin machine base is only for those in a hurry and with lots of money.

All you have to do is use an appropriate fine aggregate concrete and be patient. Wait the full 30 day curing time.
Also fibre reinforcement would be more appropriate than rods or mesh.

 

Clay. self hardening, not kiln fired. roll out with a rolling pin with s smooth mdf backing. refine the surface by pressing out small wrinkles using s piece of plate glass. (coated with silicone grease to prevent sticking. when hardened cast the concrete.

 

Quote strantor, "About 5'x5' ", this is one of your first problems, with the way your attacking your machine design. As a retired machinist/die maker I made many many things over the years. The reason for no one making a machine like your proposing is the size. Most of the things people need to make don't take a table that big, at least not with conventional machines. Using a circle to make a straight line/cut is not economical. For the most part a piece 12"-18" is considered big, unless your making another machine. Do you think your "bushman" is going to have the computing power to even run a machine to do this? Are you prepared to write software needed to support it?

To get back to your flat surface casting, why not use the age old standby of plate glass for the flat 'mold surface' ?

 

http://gingerybookstore.com/MetalWorkingShopFromScrap.html

 

Please note you need three workpieces to lap together to form a 'perfect' straight edge or surface, not two.

There was a time when toolmaker apprentices would regularly prepare straight /flat tools in this way by hand.

[Note] I do not say the following with any authority. It is conjecture:

I understand why 3 surfaces are needed for lapping, and I believe that the rotary lapping method show in the video in post 12 precludes the need for a 3rd surface. By that method, the two surfaces will not be passing back and forth over each other in the same spot as is the case with traditional lapping. The reason is because with every rotation, the face of Piece#1 will be rubbing the face of Piece #2 in a different place than it rubbed in the previous rotation. Therefore I see no opportunity for either piece to rub the other into a concave/convex.

[Note] Yes, I realize that in the video, they are using the method to make a convex shape. But the method of lapping they are using involves only one surface being lapped, and the other surface is convex already. I think that switching that method up to lap both plates would result in a flat surface. But I am open minded to the contrary.

Since we now know the application I'm sure you will get lots more useful help, but I like sirch2's idea of setting the guide rails in something massive but less accurate.
Again, however best practice was to use 'jib strips' to finally align a machine tool straight edges for 'truth'.
The other advantage of guide rails like this is they can be made relatively temperature insensitive.

Truth for a lathe is tested by turning something and then reversing the workpiece and comparing against a DTI or other measuring device whilst slowly rotating the workpiece.

A resin machine base is only for those in a hurry and with lots of money.

All you have to do is use an appropriate fine aggregate concrete and be patient. Wait the full 30 day curing time.
Also fibre reinforcement would be more appropriate than rods or mesh.

Thanks for the tips. Gibs would be useful, but I would need gib adjustment in mutliple axes which complicates things. But, it's complicated already and I think maybe the application confounds any/all attempts to simplify it.

 

Quote strantor, "About 5'x5' ", this is one of your first problems, with the way your attacking your machine design. As a retired machinist/die maker I made many many things over the years. The reason for no one making a machine like your proposing is the size. Most of the things people need to make don't take a table that big, at least not with conventional machines. Using a circle to make a straight line/cut is not economical. For the most part a piece 12"-18" is considered big, unless your making another machine.

Yes, the size is an obstacle. It is an unfortunate byproduct of the circular design; the X-axis must always be the same size as the Y-axis, and the structure of the machine must be a few inches wider still. The planned dimensions are not set in stone (or concrete ); I basically just pulled them out of my rear. I would like it to have working dimensions similar to a Bridgeport. So taking this mill for example, with a table size of 9"X42" and a travel of 30" in one axis and 13" in the other, I would need 15" radius from epicenter to spindle center. The spindle is inside a ring inside another ring, and if I keep the "thin spots" to 1" thick then I have to add 2" to the radius plus gaps in between, I'll call it a 18" radius to be safe. 36" diameter disk, call it 40" wide machine. So not quite as large as 5'X5', but still quite large heavy concrete blocks being handled.

It will surely be scaled down for the first model.

Do you think your "bushman" is going to have the computing power to even run a machine to do this?

Well, he's going to have to. I can't think of a way to cast a computer out of concrete. It will be the one thing that he has to give up a few goats to acquire.

Are you prepared to write software needed to support it?

No, but neither am I prepared to do any of the things that I do, until after I've done them.

To get back to your flat surface casting, why not use the age old standby of plate glass for the flat 'mold surface' ?

What I read about plate glass is that anymore all it is is float glass, and what I read about float glass is that it's flat, but not flat enough. But I haven't laid a single instrument against any surface yet; MDF, glass, granite, nada. So if I ever get out of theory mode and into DOING mode, I might just find that it is acceptable.

 

http://gingerybookstore.com/MetalWorkingShopFromScrap.html

I've been eyeing that set for a few years now. I used to see it in the Lindsay Publications catalog every time, and every time I almost pulled the trigger. I wasn't aware it came in a consolidated hardback. Your post prodded me to finally buy it. Found it on Amazon Prime for $50, done. Thank you for pushing me to it.

 

I meant to add one more line to my last post.

I said that the old established workshop technique to create true flat surfaces and straight lines requires 3 workpieces was because you cannot distinguish whether you are increasing or decreasing the curvature with only two, but with three you can guard against creating matching curvater between two workpieces since either will only match the third if they are both not curved.

But what I meant to add was that these days you don't need to do this all you need is to purchase standard section and have a machine shop run them down to any desired degree of accuracy. That need not cost an arm and a leg.

Jib or Gib strips also allow periodic readjustment as any machine will go out of line.

 

www.use-enco.com

a 24 x 36 inch granite surface plate is $201.00
they also have sales codes like free shipping or 20% off an order that happen every month. i buy lathe and mill tooling from them often. no concrete, no molds and less than half of a thousandth inch overall tolerance.

 

Blank (unetched) tombstones are flat also. Not in a surface plate tolerance but close enough that you can't tell without an interferometer.

 

What I read about plate glass is that anymore all it is is float glass, and what I read about float glass is that it's flat, but not flat enough. But I haven't laid a single instrument against any surface yet; MDF, glass, granite, nada. So if I ever get out of theory mode and into DOING mode, I might just find that it is acceptable.

New from the factory glass is also flatter than old used glass. Due to the glass being a 'liquid', and gravity causing it to flow over time. It actually gets thicker at the lowest point.

 

Due to the glass being a 'liquid', and gravity causing it to flow over time. It actually gets thicker at the lowest point.

I always thought it was the whisky caused that, not gravity.

 

New from the factory glass is also flatter than old used glass. Due to the glass being a 'liquid', and gravity causing it to flow over time. It actually gets thicker at the lowest point.

I believed that and went looking for a nice example, but instead I found this.

I get stupider every day.

 

Wow, another myth busted. I've heard and read the glass flow thing since way before the internet, back when I was a boy. The 'flow' wasn't said to be much, but in the thousandths of an inch range, and this was for modern plate type float glass, not the older crown glass.

 

Hello,

The surface of the water will not be flat near the edge of the tub.
Depending on the material of the tub, the surface will bend up or down:
http://superhydrophobiccoating.com/hydrophilic-hydrophobic/

Bertus

 

I believed that and went looking for a nice example, but instead I found this.

Excellent. I only thought the old glass distortion looked pretty. Never really thought about how it was made. I learn something almost every day, here!