Hi just a quick hello as although i have been lurking doing research on here for a while this is my first post. I am in the process of designing a large 6m x 1m x 400 cnc. the main use will be for cutting foam but with the occasional job in wood and looking for around .5 - 1mm accuracy.
i have designed the frame which is to be a single sided cantilever design which is something i have seen used a number of times before in this application and due to the large block of foam will make life a lot easier getting them on to the machine
I have spoken to a number of companies recently and have been given a very mix opinion on what type of drive to use for the 16' long x axis. i will be using a aluminium profile for the frame and hywin rails.
Some of the people i have spoken to have said use rack and pinion as although i will have to join 3 x 2m pieces together and possibly harder to mount it will be a lot more accurate and the all so said that no mater how good a belt drive is over that distance accuracy is going to vary
The other people i have spoken to have said belt drive will be fine as cutting forces are relatively low and it will be a lot cheaper and simpler to set up. They did say some accuracy may need to be accounted for in setting up but the repeatability would still be accurate.
These people were also saying if i was to use rack and pinion that i would need some sort of way to reduce back lash. I have looked a a number of machines and from what i can see some do have mechanisms to reduce back lash and others don't.
Is there anyone on here how has any experience with belt drive over a large distance?
Would there be much saving in time and money in the long run using belt drive?
Do all rack and pinion on horizontal axis require anti back lash devices?
If anyone can offer any advice that would be much appreciated
many thanks charlie
My experience, with belt drive on that size you will suffer belt stretch over time resulting in continuous adjustment.
R&P how it's done is you have the pinion in a aping loaded mechanism. From what I've read the correct tension on R&P is that you put a piece of paper between so it only fold and not tears.
At this length and your requirements I would go with belt drive if just cutting foam but done in a slightly different way to what your probably thinking.? Check these out. . .Bell-Everman Dual Carriage Servo Belt.wmv - YouTube
ServoBelt: amazing belt drive - YouTube
It's one belt on top of another with rollers and pinion, it's easy enough to duplicate. I've played with it a little before for long machine but found R&P was better suited to router with a heavy gantry but for light weight gantry cutting foam then it would work fine I'm sure.
If you do a search for Mike Everman beltdrive you'll find more about it.
That does look very interesting however my fear would be it would be prone to getting clogged up with foam dust under the belt. I would also like it to be substantial enough to use for cutting wood which i assume the belt should be all be it you would have to cut slower.
ignoring cost and setting up what are the disadvantages of rack and pinion on something this long?
many thanks charlie
One of things you need to be aware of with R&P is that it's less efficient with much more friction so will need more powerful motors/drives unless gantry is light weight.
Regards the gantry you say your having a cantilever design and would like possibility to cut wood.? Well unless you substantially build the gantry then you will struggle to cut wood has it will deflect at the outer ends. This will definitely take you into Nema 34 size motors which require larger drives with high voltage so bigger PSU to get the best from.
This leads to the next issue with R&P (And Belt drive.!!).? . . Resolution, Torque and Direct drive. The pinion size determines Resolution and even the smallest pinion size still gives a resolution that is too large to be useful. IE: If you use common Mod1 rack the smallest pinion is 12Teeth and 12mm DP so the affective pitch would be 12xPi= 37.7mm so your pitch would be 37mm which is quite large but nearly usable.!! . . .But it gets worse.??
12T pinion dimensions are too small and won't fit on the motor shafts so in reality you'll end up around 24-25T but now the DP is also 25 so 25xPi=78.5mm affective pitch.!! . . . . Massive and useless for anything other than linear speed.
Also on top of this you have friction to over come and R&P is quite inefficient at around 50-60%, compared to 80-90% for belt and 90-95% for ballscrews so this is why larger motors are required but with direct drive they would need to be larger still.
So to help reduce the motor size/load along with increasing resolution then you need to have a ratio applied, what ratio will depend on several factors.? Resolution and feeds required, Weight to move, motors used, sticktion to overcome.!! etc
Often 3:1 ratio is used which at 25DP is still around 25mm pitch which will give high feed rates around 20Mtr/min and given an average micro stepping of 800Ms give theoretical resolution of 0.03mm, say theoretical because you won't actually get that but even if it was double still ok.
You mostly need the ratio to give the torque and get the Resolution down to a level that the motors work best with.? One of the problems with Low resolution is the speed, the motors struggle and are very unhappy when low feed rates are required has they are hardly spinning so can give very rough action. The ratio needs to be a balance of providing the best torque and resolution while still keeping the motors spinning in speed range they like at lower feed rates.
So has you see it's not just simple has R&P or Belt drive, which by the way you still need a ratio for belt drive has most of the above applies with the exception it's more efficient so less Sticktion to overcome and can often use smaller Nema23 motors.
Hope this helps rather than blows your mind. . .Lol
Last edited by JAZZCNC; 07-11-2013 at 04:03 PM.
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