Rigidity about cnc machine
Hello, hope you can help me Im starting to build my own cnc machine for aluminum and steel, trying to maximize precision, belt drive its out of the equation which leaves us to ball screw and lead screw. I know some ups and downs about each other but what concerns me is the price versus the actual work. Based on that, i know that for cutting steel the machine must not have play at all, so, can the body of the machine be made of v slots? Or can it but reinforced with steel sheets? Would you recommend lead screws over ball screws?
Would you recommend this? Im trying to build a 3' x 4' CNC router.
Sorry for my bad english, not my native language.
Re: Rigidity about cnc machine
Firstly I know little to nothing regarding routers but what I do know is that you will struggle to cut steel with any precision
on a 4 x 3' table. Rigid routers have small movements and are massive to gain rigidity. You may be ok with aluminium but
subject to the precision / tolerances required may be up against it!. Ballscrews are more accurate generally and don't tend
to wear 'loose' as quickly as lead screws. Nothing wrong with a toothed belt drive if tensioned correctly every movement will
be transmitted - any joints in the driveline is where the slop and backlash creep in, messing everything up.
Re: Rigidity about cnc machine
But loosing tension is faster than backlash right? I mean i would have to adjust the tension often than adjusting backlash right?
Re: Rigidity about cnc machine
If you want to work on metals like steel, a router is not the tool for the job. Even the most massive factory-built routers won't do a good job on steel. Find an old knee mill or VMC and retrofit it with new controls; that's much easier than building a whole machine, and will likely work much better.
Re: Rigidity about cnc machine
The question here is that im trying to build one, i thought that build the gantry was better because of the weight of the plate, but i didnt know it took away rigidity of the machine.
Re: Rigidity about cnc machine
Moving table designs are generally more rigid than equivalent moving gantry designs. They take up more space, though. Neither one will do a good job with steel, if you need precision. But if you do a good job with your build, you have a chance of being able to machine aluminum if your tolerances aren't too high.
When you've completed your router, please write back with some pictures of your results.
Re: Rigidity about cnc machine
What would you desing to have a good job with steel.
Right know im buying ballscrews in alibaba, im having real trouble importing stuff to Mexico (Where i live). So im looking to buy products and get them send to USA and then go get them.
Right know a router is concerning for the rigidity but a knee machine will take a lot more space but its better than the router.
Re: Rigidity about cnc machine
Machines designed to cut steel are usually made from cast iron. If you have an iron foundry in your shop, perhaps you'll be able to build a machine that does a good job on it. They are based on a big casting (or several castings bolted together) in the shape of the letter "C". A 3'x4' working area is rather small for a router, but it's large for a mill, especially on the Y axis. A knee mill with a 3' X axis is normal, but it won't usually have nearly a 4' travel in Y; half that would be large.
If you look for an older mill with a broken or obsolete control, you can often find them for about their value as scrap metal. Machine-shop owners usually want them hauled away to make room for new equipment. I don't know how true that is in Mexico, but if you're willing to wait for a good deal and travel north, then you can find them in the USA. Ball screws are generally included, often the motors and spindles will work; all you have to do is a "brain transplant". That's a lot easier than building a mill from scratch.
Re: Rigidity about cnc machine
Aloha!
Old thread - month ago you posted question - yet let me drop a few coins.
It's possible to buid a reasonable gantry that works on steel. The whole thing is about how much time you want it to cut such sheet.
The trick is to use a very lightweight spindel motor.
For example my spindel motor is very heavy. About 9 kilo. That means if i put it in a steel box, that box is soon 40 kilo if not more. If that box then hangs onto a long tube on a gantry, that means i need some very strong stiff tube.
Now next is to limit the Y-travel.
Deflection of your gantry bridge that moves over the machine is Newtons force times length of the Y axis to the power 3.
For example simplistic point deflection:
Say we have my 9 kilo spindel that in total has a weight of 60 kilo with motor and box and so on and then we add 50 kilo of Z-axis force it is using to hammer onto the workpiece.
Far over 1000 newton. Let's use 2000 newton. Now let's use a cheap tube as our moving gantries bridge that rolls over X axis. Let's use a tube of 100 x 100 millimeter thickness 5 millimeter.
First we calculate how strong that tube is: (100 ^ 4 -90 ^ 4) / 12 = 2,865,833
We then take length of our spanning of the gantry bridge say 3.5 feet => lets take 1000 mm.
I would then want to calculate with deflection (millimeters) = (2000 newton * 1000 ^ 3) / (48 * 210000 (steel) * 2865833) = 0.069 mm deflection
And we didn't even add the weight of the tube itself yet, though we did calculate as if all force is concentrated in the middle of the tube whereas it probably
is not entirely in the middle located. So in reality probably we measure 0.05 mm deflection.
That is a big problem in short if we get nearby 0.05mm as human eye will see that easily.
Now we do the same calculation for a cheapskate spindel of 1.5 kilo that we hang at a light structure of 4 kilo and because we mill with a smaller mill we also generate less force onto workpiece. Say maximum 10 kilo.
That's a weight of something over 15 kilo. Let's take double that in force. 300 newton.
So now if we do exactly same calculation with 300 newton, we suddenly are having a deflection that is nearly 7x less, or in short less than 0.01 mm.
Easy as a pie.
Yet of course milling time suddenly is very long as you can take away very little steel from the steel plate each second.
Re: Rigidity about cnc machine
Quote:
Originally Posted by
awerby
Machines designed to cut steel are usually made from cast iron. If you have an iron foundry in your shop, perhaps you'll be able to build a machine that does a good job on it. They are based on a big casting (or several castings bolted together) in the shape of the letter "C". A 3'x4' working area is rather small for a router, but it's large for a mill, especially on the Y axis. A knee mill with a 3' X axis is normal, but it won't usually have nearly a 4' travel in Y; half that would be large.
If you look for an older mill with a broken or obsolete control, you can often find them for about their value as scrap metal. Machine-shop owners usually want them hauled away to make room for new equipment. I don't know how true that is in Mexico, but if you're willing to wait for a good deal and travel north, then you can find them in the USA. Ball screws are generally included, often the motors and spindles will work; all you have to do is a "brain transplant". That's a lot easier than building a mill from scratch.
Andrew - we fully agree - the problem when you want to cut sheets is of course that if i take aluminum here in Netherlands. If i buy in small quantity of aluminium plates 1x1 meter the price is 4 euro a kilo. If i order part of it say 30 cm x 30cm, then i pay 10 euro a kilo. The cheaper price starts from 1x1 meter and longer (say 1 x 2 meters etc).
With aluminum so very expensive of course there is no option yet to order 1x1 meter. Cutting that by hand into smaller pieces is a lot of work of course. Easier is to have gantry that cuts out the shapes. Then you clamp the cut out pieces into a more accurate metal milling CNC machine that is doing the more accurate milling :)
Re: Rigidity about cnc machine
Diepchess,
If your talking the el cheapo spindles on Ebay, good luck with that!
At 1.5kg weight for the spindle however, I'm not sure what you think you'd find as even a weak little 800w spindle weighs around 2-3kg. Unless you mean a die grinder?
In short, machining steel requires brute force, and a LOT of mass and rigidity. Without the horsepower to cut, and the mass and rigidity to dampen the vibrations and keep the cutter on target, you'll find the cutter will be shrieking like a screaming banshee, and will be skating all over the material. You'll also destroy cutters very, very fast this way.
For practical hobbyist purposes in most user designs, rigidity, and mass to a degree, is generally inversely proportional to cutting area. The bigger you make the work area, trade-offs are usually made to lighten the gantry and X axis in order to still be able to move it adequately.
A bigger machine could be made to cut aluminium to varying degrees of success depending on mass, rigidity, size, and quality of machine components, but to machine steel is a very different ball game.
cheers, Ian
Re: Rigidity about cnc machine
Yeah we fully agree upon the ideal situation, which is why i got something that's 380 volt and a bunch of kilowatts.
Yet he could buy a kress spindle which has very little runout and weighs 1.4 - 1.5 kilo and 1050 watt. Costs over here (Netherlands) a 150-160 euro. Of course milling some mild steel with that takes forever and lots of cooleant, yet milling with very small diameter carbide tools is also possible. Of course that thing is bloody noisy and milling at such high RPM requires some good calculations on how to mill :)
The ebay stuff in general has a very bad runout - that's asking for problems on tool wear isn't it?
No one said If you want to mill cheapskate that you do not need to be a very good machinist to do it :)
Furthermore if he finds even a couple of hundreds of dollar too much for shipment of something great it's obvious that has to rethink to pay for 500+ kilo of steel and a lot more in concrete which he should to build such gantry :)
Re: Rigidity about cnc machine
Hi Yodda - follow the Maximus thread it details the design of a router but could be a mill, same process. In it5 there are references to info about machine building and calculations. Cheers Peter
Re: Rigidity about cnc machine
Yoddha - the huge difference is whether you want to build 1 machine for yourself - or whether you want to build a series of machines.
We can talk endlessly about the advantages of cheaply pouring pot metal - yet how useful is it if you do not have a professional factory that's doing a series production?
It needs casting, pouring the metal - then sit and waiting 3+ months until the pot metals internal strengths have settled - then have some giant machine to make it very flat for the linear rails.
If you just want to build 1 machine - go the epoxy concrete method. That's easy to do on your own. Yet pretty expensive for a heavy 10k+ kilo machine.
Doesn't matter what sort of machine you build - if you just build 1 - then epoxy concrete is going to win it in rigidity of everything and it's easy to pour couple of hundreds of kilo's.
Add a few percent of epoxy (not 10% - like 5% is enough already). Yet that epoxy is expensive.
You want to overdo rigidity. Obviously with any form of CONCRETE every wall needs to be very thick. Not 1 centimeter like steel - you want 10 centimeter or more at least.
For a gantry with say 600mm of X range and say 400mm of Y range it's easy. Just pour a block of 160mm thick and 1000x600mm or so. Then 4 rubber/alu legs at around 22% of the weight at both ends.
The moving gantry on top of it there is more choice.
Easist thought is an U shape structure. So 160mm in the middle and like 260mm height left and right. I give rough guesses here. Have an example worked out in CAD here if you're interested.
It's all about : what can you produce yourself, because someone else isn't gonna do it for you.
If you got 40k euro on other hand - you could buy one ready to go from epoxy concrete. In Zhermany some company sells a line there.
It is a lot of work though to build something yourself.
It's not only about rigidity it's also about how much sound do you want it to make?
You end up at water cooled milling motors then - but those are heavier.
Just do not touch aluminium extrusions - that's having a different expansion coefficient than the steel linears for example.
No math will work for you if you have different sorts of metal combined with each other.
Epoxy concrete has the same linear expansion coefficient like steel (it is very similar).
Notmal concrete is not possible by the way - it keeps working for years to come.
It's not only about rigidity and calculating that - that's the easy part - some sort of crabble notion there is enough with respect to milling.
You get onto a different league though if you want to design a machining center. That's a different league from building one (or a couple) at home in your own workshop.
Those machining centers in itself do not need to be 15000 kilo from a rigidity viewpoint seen - yet they are - and with good reasons.
