new build opinions required

another question.

Bolting a frame together, prior to weilding or bolting instead of wielding

Am thinking that i could probably make a JIG for the bolt holes, and am confident that i could make the machine straight and true with bolts. and then just tac it were required, once nearly assembled.

I have been mainly thinking this because my X will have an overhang, were the rails will be placed. and am thinking that, at least that bit, will be easyer to get exactly level, by bolting and shimming first.

Again, nobody seem to do this, and it seams a no brainer as an easy way to ensure a straight and level machine, but, its probably me not engaging my brain, am open to any opinions on this,

People do use that method, such as my build here;
http://www.mycncuk.com/forums/gantry...html#post50664

which actually turned out like this;
http://www.mycncuk.com/forums/gantry...html#post52425

Jazzcnc also uploaded some photos once showing a part bolted, part welded frame. I would think that if it's a large and heavy machine it's a good idea to make sides and ends then bolt these sub frames together.
Regarding shimming, there are two lines of thought. The first as you say, and the method I'll be using, is to bolt the top beam that supports the rails, this way it can be shimmed level. Also within this method are two ideas that I've seen mentioned, one is to use standard metal shims, the other Jazzcnc method is to use epoxy putty where the rails are bolted down until level thus squeezing the putty until it sets. A piece of cling film or similar is stretched over one component to prevent the putty sticking it all together.
The other way is to fully weld the frame and use the 'epoxy method' to level the top prior to fixing the rails, you can search the forum for that.
My own take on it is this; if accuracy is your main concern and as such you will likely be cutting metal to high tolerance, then the epoxy method is best. If you are cutting mainly wood and your tolerances are not so high then the shimming method would most likely work, after all the wood will shrink and expand probably beyond a fine tolerance.

Eddy is quite correct and most of the larger machines I've built have been welded and bolted. It just makes good sense when you think about.!!
Also I've said this many many times.!! . . The more adjustabilty you build into a DIY machine the easier it will be to set it up. Bolting makes for another area where you can tweak a machine into alignment etc.

I also mostly Agree with Eddy's take on it except that full epoxy is better for high tolerances than Shimming because it's not really, it's just easier(Quicker). I'd actually go has far to say it's potentially less accurate because your soley relying on the epoxy with no room for error or improvement where has with shimming you can tweak out any error it just takes a lot of time. Personaly I'd do a mixture of both on a large or long machine that requires higher accuracy.!!

I believe there must be available movement to Get the Highest precision as possible. especially on the rails. so have a preference to Shims.

Shim steel is more expensive that I thought, do people just cut up food/drink cans or is there somewhere to buy an assortment cheap ?

both have advantages and disadvantages.

Epoxy makes sence if your floor is level and machine is square=ish, to take out imperfections,

Shims give you a very high level of presision, but take time to get right

but , then i start thinking back to the days i had a go-cart circuit, and the jigs we built for straightening the frames, we could easly achive very high levels of precision, at complex angles, much more than you would need for rails.

So am thinking why not build a simple jig, into the top of the frame, both sides down the X ( 80mm or 100mm box ). a simple bolt pattern jig, Fix a bit of say 80-160-4, on that just leaving enough overhang to fit rails to, use the jig to get the 80-160 perfect, with a laser level and a few mirriors, you could get a very long X, very accurate, and make sure the opposite x was perfectly alligned.

I do get that this would be a waste for any machine with less then 1600mm X, but, you could be close to unlimited on X length andd get very accurate, with just a very basic tool kit.

My proposed build plan goes like this.

wield

Build 2 ends in Jig ( Y ) 1400mm
Build 2 sides in jig, ( X ) 2500mm , with Jig/bolt patterns

Assmeble square, by bolting together

Fit the 80-160 to the X, by adjusting down on the bolt/jig, to get it perfect as possible, using simple laser.

Then, bolt a bed, on, using you X to ensure your fitting the bed as close to perfectrly level with the x.

further reasons for the above build plan.

1. can be manufactured and assembled at differnt places
2. Easier to get into workshop/transport
3. Final truing is simple, a few spanners, a few cups of tea, and a bit of time.
4. it can be moved, if required, and is simple to true up again
5. It can be built very accurately with simple tools ( pillar drill, cutters and grinders and weilder ),
6. it will be a lot easyer to get a large but solid bed level to the x/y, possibly even have a replacable bed for different configurations,

Cons

1, it uses a lot more steel, but if you want you bed to hold a lot of weight, its not a loss
2. It takes time to make sure all your cut pieces are exactly teh right size, and to get your bolt patters accurate, but, accurate pattens could be printed from a standerd desktop printer, and overlaid,

i can not think of many more cons,

you will notice that i have not mentioned the gantry yet, its because am working on a very differnt type of design, but i need the frame built, to make the final tweeks, my Y is going to be more like a cartridge than a carriage. giving me much better force resitance in all dirrections. Again, for standard XYZ, this may seem overkill, but, if i want to build a turret on one of the Y sides, or strap a B/C head to the Z, then it makes sense, .

any opinions welcome

Quote:

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Originally Posted by JAZZCNC

I'd actually go has far to say it's potentially less accurate because your soley relying on the epoxy with no room for error or improvement where has with shimming you can tweak out any error it just takes a lot of time.

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The problem with shimming is you can't compensate for rail twist with it. I discussed it at length in the recent build log. I didn't include the readings from the shimmed rail in that post, however if you want I can dig out the graphs obtained when the rail was shimmed and you'll see the error was still far worse than the final result obtained using self-levelling epoxy. I'll quote myself, from the build log:

Quote:

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Originally Posted by Jonathan

At this stage it is interesting to note that the bearings slide smoothly on the rails, so although compensating for the height error with shims or a machined profile is sufficient for the bearings to run smoothly, this does not by any means assure accuracy since neither method compensates for the angular error.

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It all depends on how accurate you want the machine to be and its intended use. The error at the tool introduced due to rail twist will be exceptionally small, so you'd need a very strong machine to notice it. The additional wear on the bearings caused by the rail twist will depend on the stiffness and accuracy of the structure they're mounted to, so it's hard to judge, but in a DIY CNC situation is unlikely to be too significant. I'd still choose epoxy though as it's so much quicker and easier than messing about with shims and enables one to get very good accuracy with less measuring equipment.

hi

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I'd still choose epoxy though as it's so much quicker and easier than messing about with shims

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yes i am doing, i think you did post the readings from your shims then your epoxy, iether that or it was the bare metal and the epoxy.

I was thinking, epoxy on top to take out surface imperfections, ( although a real lot of that would be taken out by the bolt jig ). Am also haveing quite a wide ( down the X ) gantry ( sacrificing cutting area ), so alot of minor imperfections will taken out becasue of the space between the Y barings to the X

Quote:

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Originally Posted by EddyCurrent

Shim steel is more expensive that I thought, do people just cut up food/drink cans or is there somewhere to buy an assortment cheap ?

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Often I use tin foil has it's thin cheap and won't rust.!! If I need more than 2 layers I'm upset.!!

Quote:

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Originally Posted by Jonathan

The problem with shimming is you can't compensate for rail twist with it. I discussed it at length in the recent build log. I didn't include the readings from the shimmed rail in that post, however if you want I can dig out the graphs obtained when the rail was shimmed and you'll see the error was still far worse than the final result obtained using self-levelling epoxy. I'll quote myself, from the build log:

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Why not.? Shimming is all about where you place the shims.! I agree it's not easy in comparison to epoxy but it can be done, I've done it many times.

Your experiences with epoxy are on a short machine but on a longer machine then things are slightly more exagerated and just relying on one method that is fixed or not very flexible is not so clever IMO. That's why I say for a Long machine(requiring higher precision), longer than 4ft I use both methods.!

Why make life hard for the sake of some plates and bolts, which if Epoxy hits the mark first time you don't need to touch, if it doesn't then fine it can still be tweaked if needed. Adjustabilty is Key and the more of it you have without compromising the machines strength the better at DIY level.!!