Im looking at building a CNC machine that will primarily be used for milling wood and maybe some aluminium. I have been doing some reading of books watching you tube videos and the such.
The question I have is in relation to the x axis and using rack and pinion with the linear tracks. I understand that the linear track has to be on both sides of the frame and needs to be 100% level or as close as can be.
The table I am looking at building will be 4-5 foot long and probably 3-4foot wide. Should there be rack and pinion tracks and gears on both sides of the frame as well or is this driven from just one side.
Last edited by ikhan42; 05-18-2011 at 08:39 PM.
Thanks for the info I am thinking about using linear rails on bother sides of the x axis. I am a at a loss as to the material to use I was thinking about structural steel that is welded together the steel is like 14kg per meter so it will be heavy but from what I can tell 8020 aluminium is also available I found 40-8016 t slotted profile which is big and solid and seems like it would be easier to work with then steel as I dont need a welderer but from what I have read I am better with a welded gantry as this is a stiffer gantry and will allow for a more accurate machine. I also found this rail system that works with the 8020 aluminium but dont know if this would be better then using linear rails. The link for the rail system is here The 80/20 Training Center - T-Slot Accessory Introduction
Any advise would be appreciated.
A stiffer machine is not inherently more accurate. It just allows you to take deeper and or faster cuts.but from what I have read I am better with a welded gantry as this is a stiffer gantry and will allow for a more accurate machine.
Accuracy is mostly dictated by how precise you can build the machine, and the components you use.
Mach3 2010 Screenset
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
While the SR30 rails and the 4590H are pleasant "finds" from eBay, they also work really, really well together (on paper). The SR30 rail takes an M8 socket head screw, and Bosch makes a 1m long "super nut" that comes pre-tapped for M8 screws in the exact spacing that is required by the THK SR30 rail. This should be a really strong connection. Since I lack many of the machine tool toys that others here have, I'm pretty excited by how this will come together (note: I'm waiting on the critical nut bracket before getting serious on assembly -- and documentation).
Other people use things like cold-rolled steel (CRS) and aluminum carriages with skate bearings for linear motion, while yet others do things with steel pipe. There are a bunch of options, though the linear guides are pretty much at the top of the heap for quality -- and expense.
There is an old adage in racing: Speed costs. How fast do you want to pay? For a homebrew CNC, the same is basically true about the various component options. If you want higher accuracy? $$. If you want easier assembly, $$. The question is really what your goals are, what your capabilities are and what your time is worth.
And a whole lot of reading.
You are correct about the quality for the rails otherwise its a waste of money doing the project and that's why I am researching rails to see what is the best option as I want a machine as accurate as possible, I am seriously considering the redi-rail linear guides by Pacific Bearing Co it looks like a cool little setup am trying to work out costs so I can budget for them and also if it will do what I want. I have seen some of the thk stuff aswell.
Will do some plans tonight, have moved away from the idea of using the 80/20 40-8016 t slotted profile extruded aluminium framework as they are charging $1100 per 4 meters. I am going to work out exactly how much I need and then look at what the other sizes are for the table. May do a aluminium gantry and bolt it to a steel table for strength.
I am currently reading the from this site to try and work out torque, motor size, speed etc. This document isnt making sence to me when they talk about weight are they referring to the weight of the gantry with tooling device mounted or are they referring to the weight of the gantry as well as the weight of the material being milled.
Last edited by ikhan42; 05-25-2011 at 03:33 AM.
It's is really all of the above. Your X-axis motor will need to push the fully-loaded gantry plus your cutting force. If you have a 50kg gantry when it is fully-loaded (gantry, spindle, milling bit, vacuum hose, etc.) and you want to accelerate that gantry at 0.5g, then you have 50*9.8*0.5 = 240 Newtons of force that have to be generated by the motor -- and that's before you add cutting force. The upside is that if you end up with a heavy gantry, your cutting force basically becomes an almost negligible part of the equation once you add in a safety factor. There are various software tools (e.g., Kollmorgen) and articles available out on the web to help with motor sizing. With stepper motors, you can "go big" with worry about negative consequences -- just Go Big enough. With servos, life is more complicated.I am currently reading the from this site to try and work out torque, motor size, speed etc. This document isnt making sence to me when they talk about weight are they referring to the weight of the gantry with tooling device mounted or are they referring to the weight of the gantry as well as the weight of the material being milled.
Thought I would give an update I think I have chosen the steel for the base I am tossing up between the following steel posts I believe they use these for fence posts.
89x89x2400 I like this one cause its very heavy so will give stability to the machine and at $94.66 per 2.4 metres it seems like a very good option at a great price allowing me to spent more on the linear slides, bearings, motors etc.
There is also a smaller one that is considerably lighter the dimensions are 65x64x1800 I am thinking of using this for the bracing in the center of the table and also maybe for the gantry. Though I am thinking the gantry should be the 8020 aluminium to reduce the weight on the bearings and sliders.
Oh yeah and I have found a welder who is a friend of my dads who is willing to come to my house and weld it all together for me, he doesnt want any $$$$ but I am going to give him some money for doing the welding as his time is money and I dont believe in not paying people for their precious time.
Any comments on the posts would be greatly appreciated..
I don't know how much time you've spent planning/designing/thinking about your machine, but I think that some of your questions might benefit from a read through this excellent FAQ:
Homemade CNC Router The Builder's Guide (FREE!)
Some key factors covered in that FAQ are the various drive mechanisms and the trade-offs associated with supported and unsupported axes. A few thoughts to consider:
1) Straight, true and plumb - accuracy in your machine will first and foremost be determined by how level and square the structural members of the machine are and how straight they are overall. Having X-axis stringers that are magically square at the corners don't do you a lot of good for accuracy if they twist or bow in the middle. If you are trying to hold a millimeter tolerance, then most good quality stock will do. If you are trying to hold sub-millimeter accuracy and precision, then you will want to do a lot of research. Having the ability to surface mill the stock also helps.
2) Deflection - Any time you have a horizontal member that is unsupported, you have deflection. For short spans, this may be a micron or three. However, over 1.5 - 2m, you may be into a tenth or three of a millimeter. This can be made worse once you add in the weight of the gantry. Again, probably not a big deal, though annoying, if you are trying to peg a millimeter tolerance. It is something to account for in the design if you are trying to hold a tighter tolerance. The upside is that a couple of well-placed supports will virtually eliminate deflection on the X-axis.
3) Y-axis/Gantry - Extra supports work great for the X-axis, but aren't really an option for the Gantry, whether it is a fixed or moving one. The gantry beam is generally your biggest, most structural beam/member for the obvious reason that it can only be supported on two ends. Go as big as you practically can here.
4) Drive/Motion - The Y and the Z axes are generally easier than the X-axis. For the X-axis, you need to find a way to have the right amount of support happen in a way that doesn't compromise your space envelope or your work envelope. In other words, a ball screw will have a hard time pushing a drive bar through a support column if you haven't taken into account the need to have an unobstructed sweep under the table/bed surface of the machine -- when using a single drive axis. One way around this is to use dual drive mechanisms (which also helps prevent "racking"), but this is only one way of many. The link I posted, above, will show various ways of driving the X-axis. For R&P, dual drive seems to be the popular choice and gives a very compact footprint. You trade racking issues for synchronization issues, but such issues are a relatively straightforward issue to solve in software.
Note: unless you get lucky at an auction, a screw-driven x-axis gets expensive in a hurry with longer spans. This goes even with Chinese ball screws (a 1.5m 25-10 ball screw is still going to be a few hundred dollars, and that's before getting the nut bracket housing built!). For this reason, a lot of people are going with rack and pinion drive systems for longer X-axes. You add-in some backlash, but the drive rail, an extra carriage and an extra motor are usually cheaper than a screw, BK and BF supports, plus the nut housing.
Ultimately, a lot of this comes down to your budget and tolerance/capability to manufacture high-quality parts. Given that you are looking at R&P for both X and Y axes, you can probably go with a bit bigger of a footprint than, say, 4x3, but only if you have the space. Take a look at some of the builds using aluminum extrusion over at CNCRouterParts:
These should give you some ideas of what basic designs are popular and should be relatively bug-free.
Hopefully this helps somewhat.
Last edited by Bear5k; 06-08-2011 at 12:55 AM.
Bear, thanks for the recommendation! Ikhan, we'd be happy to help you out with rack and pinion drives for your machine. We even offer module 1.5 pinion gears on both our Nema 23 and Nema 34 systems for European and Australian customers, who often have easier access to metric gear rack than the 20 pitch rack more commonly available here in the states.