Perhaps slightly off-topic but I was wondering if half the problem with unsupported rails comes not from the rail itself, but from the commonly used clamping method to hold it?
I can imagine those rather immaterial aluminium screw clamps allow a fair bit of movement and flex under use. On the other hand, if the rails precisely slot into a much more solid piece of metal and are directly threaded and screwed up tight, I bet the movement is reduced a lot. This is how the CNC3040 and 6040 do it.
I'm guessing most who have used unsupported rail in DIY machines have gone the clamping route, because the cost of end-machining the rail to have a thread and perfect flat end makes it easier to buy supported rails.
Supported rail will be better, of course, but I'm just wondering where the main area of weakness might be.
Think about a springy length of piano wire sitting on a support at each end:
Imagine pushing down with your finger in the middle of the wire.
Now clamp each end of the wire firmly to each end support and assume the supports cannot move. Imagine once again pushing down with your finger in the middle of the wire.
Finally put a third support in middle of the wire and imagine pushing down directly over the middle support...
You may like http://www.clag.org.uk/beam.html , see 'Centre load on beam with two simple supports' and 'Centre load on beam with 2 fixed supports'
I'm not convinced that is the main weakness. Hardened solid steel is very stiff! I remember reading a post in which a chap calculated the deflection under load and it was tiny, nothing like the amount of cutting inaccuracy others have reported in practice. Hence I wonder if the rails modulus is in fact the primary cause of problems.
From the equations you can see that the difference in deflection between a beam simply resting on end supports and one rigidly supported at both ends is about a factor of 4 (192 / 48 taking simple cases). So the difference in deflection between fully rigid and 'kind of rigid but with poor bolts or wobbly walls" must be less than that factor of 4.
δ = FL3/192EI vs δ = FL3/48EI for those who don't like links.
Anyway, I dug this great resource out of my bookmarks, http://web.mit.edu/2.75/resources/FUNdaMENTALS.html . It is heavy on the maths but has lots of pictures so well worth a quick once over. The videos don't appear to work any more which is a shame, or maybe I accessed them by other means. If I remember correctly, one of the guys main tenets is "stop thinking of everything as rigid and firm, starting thinking elastic and floppy".
Last edited by ecat; 30-03-2012 at 02:15 PM.
Just to jump in and have my twopenith worth, Building has to be the way to go. I started out with an £800 project and have to now find £1800 for a solid 1200 x 600 cnc. But when it is finished it will be able to cut ally and all the wood I want. I also know how to expand it, repair it, and why it works. I stood on the gantry and pushed along. Yes it even took my weight. This machine will last and as it would be nice to cover some of the cost, make me money.
As I am self employed my build time is scarce and so the build may take longer; but if i had the time this could be built in a month especially with the help you will find here. I use a cheap drill press, an under the bench router table and hand tools. probably a lot of shims but it can be done.
I know that time is money but is the time + £1200 spent on producing a machine worth say £3 - £4000 better value than spending £500 - £1200 on something that is guaranteed to break down. I can only suggest that you listen to the wise words of those that know.
Which ever direction you decide to take I really hope that you have success.
BruceThe more I know, I know, I know the less. (John Owen)
Where is all this talk about breaking down coming from? I don't think anyone here actually owns a CNC6040. What parts are going to break down? The motors or drivers maybe, but then you can replace with better.
All the information needed to build a perfectly good CNC machine is here on this forum and can be glean'd within a few nights searching. Also there's enough folks like me or Jonathan etc who could point someone in a rush to the right way to do it and where to buy the components and what's needed.
Me personally I'm more than prepared to help someone either via email, PM or my preferred ancient method "The telephone" with how to go about it, I often do this with people.
Obviously if they are in a rush then they will have to accept my interpritation's and ideas to what makes a good machine, but equaly I'm more than happy to discuss/advise on there interpritation and help put together a plan of attack.!! I could tell them all that's needed to build and where to get it and the pits falls to watch out for while building all within a, phone call.!! (All be it longone or maybe 2 short.!!)
So basicly some one new and in a rush with the time to build could be armed with the right info and pointing in the right direction within a day or two and within a just over week have all the components in hand ready to go and if equiped with the correct tools and skills using off the shelf materials have a within the month a working decent quality machine.!
To me the argument about prebuilt and cutting quickly so therefore earning quicker would hold up well with any decent quality machine but these are far from it and they wouldn't IMO stand business type production runs where they need to be running constant thru the day to earn there keep because they would die very quickly.
In this enviroment the down time alone would cost more more than the extra to buy a decent machine or justify any extra time it takes to build your own. Not to mention the damage it would do to the business Rep letting customers down.?? . . . . Imo buying one of these 6040 machines to run a business from would be commercial suicide and domed to failure before getting off the ground. .
Also it's not all about parts used, strutual integrity comes into it and while it's not exactly flimsy it's far from good and will show it's true weakness's in short order when it comes down to the nitty gritty of getting good quality work from it in a timely fashion. . . . . .In a 8hrs a day 6 days a week enviroment I'd give it 2 weeks MAX before it was sloppier than an Pro's flu.!!:naughty: . . . . By the end of the month it would be the most expensive door stop you had ever bought. .:rofl:
A lot of the DIY machines on here are built with Chinese ballscews, ballnut and linear bearings. Do you have evidence the ones in the 6040 are inferior? The rest of the structure is metal and screws, probably machined on very good industrial kit.
I'm not saying the 6040 is a perfect machine but I think you are going way over the top with your comments in the context of a small run machine for wood parts or light aluminium. Yes DIY will give better results, but only if you are willing and able.
Anyway enough of Hi-jacking threads.!! . . . . Me personally I'm saying no more on it and lets concentrate on Mr Millers ponderings.????
Andy. . . . Here's an option for you.?. . . If you have the time to wait then get intouch and I will be more than happy to help you build a machine that will knock one of these 6040 machines into the middle of next week for around the same money.
By Richard in forum Machine DiscussionReplies: 2Last Post: 18-02-2014, 03:18 PM