[avarakin]

Hi All,

I am new here and started a CNC build about a week ago.
It will be used mostly for wood and some aluminum.
I bought a rail/screw 1050/650/300mm set from eBay and spent few days agonizing about the material to use for the build - AL vs MDF.
Looks like pretty much everyone is using 8020 nowadays but for few reasons I did not like this approach.
Finally went to local Metal supermarkets and got some good pricing on AL so decided to go with regular AL profiles.

Overall design is attached, it is pretty standard moving gantry with 1 X motor and sliders on the side.
I think I have a good idea about base - it will be built out of 2.5x2.5x0.25 angles. I already cut the pieces and getting ready to bolt them together.
I plan to use 0.375" for gantry side and bottom.

Where I am stuck is gantry beam - trying to build it light and solid without spending too much.
I found this video on youtube which indicates that very common t-slot profiles are not very strong:
https://www.youtube.com/watch?v=2QTZ...ature=youtu.be
So I have few options I am thinkging about:
1. 0.25x6x34" AL plate and attach rails and screws on face of it, add 2.5x2.5x0.25 angle to the back of it if needed
2. Laminated MDF or plywood 1.5x8x34"
3. Start with option 2 and if not strong enought, wrap it with CF

Any suggestions would be welcome.

Alex

[wmgeorge]

Instead of asking a lot of very basic questions, take some time and do some research and reading on your own. Or purchase a set of plans and go off that.

Or better yet, go to CNCRP or Avid and look at the kits they sell. Far better off than something you design on your own,.... unless your an experienced builder.

[fretman_2]

Light isn't necessarily good. I was told when designing my machine that having some weight on the gantry was a very good thing. So I didn't worry about weight and instead built for rigidity. My motors can more than move the gantry I designed and that'll likely be your case. I built a torsion box gantry beam out of MDF and it's super strong. Certainly not the weak point of my machine. I've seen some people that use the big extrusions fill them with sand or lead shot to give them some weight. If you over build your gantry beam, the weak point is likely to be flexibility your Z unit bearings.

[routalot]

I suspect the biggest potential problem with the proposed design is the single lead screw in the centre of the gantry.There have been several instances on this forum of such gantries skewing under cutting load.It isn't too hard to stiffen a gantry if you just imagine a roof truss laid on it's side with a horizontal equivalent of a king post.For a single lead screw it might make more sense to go with a moving table machine.

[peteeng]

Hi Avara - there are many threads here describing successful MDF and plywood machines. Small machines like yours can be built very stiff with these materials. Adding CF to plywood will not significantly change it unless you add quite a bit of thickness. Aluminium is the stiffest lightest material to play with and its easy to machine. Angles are not a good idea as they are not stiff within themselves. Under load they warp as they are not symmetric. Big plywood boxes are much stiffer then the small AL angles you have. Hand laying CF will get you less then half the stiffness of aluminium so you may as well go with AL or steel. Steel is 3x stiffer then Al for the same thickness (but its also 3x denser) so build your first machine to get through the learning curve and get onto your second as soon as possible. Peter

[avarakin]

Thank you all for your feedback.
I saw quite a few designs with the central screw and my understanding is that as long as the distance between the cariers is not too short (at least 1/3 of distance between screw and carier), it should work.

One more question - where would you suggest to put the long rails - on side or on the top of the base? I see people building machines both ways. I understand that having the rails on the side makes alignment of the rails more complex. Any other considerations?

[peteeng]

Hi Avara -The attraction with having rails on the side is that the columns can be plate and then they can bolt straight on. The issue is getting the rails parallel and horizontal when they are on the side. If they are on the top then the top can be made horizontal as best you can and then the rails are relatively easy to get parallel. But then you have to either have an extra plate to get around the "corner" so to speak as the car mount is horizontal. All depends on what you want to do, your materials and skills and machinery resources. Being your first one I'd go on top...Peter

[avarakin]

Thank you Peter!

[Nightlight]

Hello avarakin,

I think your gantry could be improved in some points, to start with your concern, its aspekt ratio of the AL gantry beam. It is rather high compared to its depth which makes it les stiff torsion wise, a square beam might hold up better and then can be filled with epoxy granite if that should not be enough. Also, your gantrys sideplates are just that, plates, which makes them weak for sideway forces as plates tend to bend in that direktion rather easy. An I-Beam or something similar would be better here. So if you want stay with the AL profiles you can but use other form factors which are more apropriate for their loadcases. The basic principle of stiffnes through geometry still holds up regardless of material...

Also, weight should not be an issue here unless you plan on doing some ridiculus high speeds (a few more informations would be helpfull here, you said what you want to work on, but how fast? Which Motors are you planning to use? ect.) an it is more of an vibration topic anyways. One can fill it with just sand and therefore dampen it, but there wont be anny stiffnes gained.

As for the mounting rails on top or sideways, it is also an economical question and depends on the rails. An Ideal machine would only have a footprint as large as its working area as to use it to its maximum efficiency. Sadly, thats physically not possible with a gantry design (or anny other, really..). So to avoid "dead machine footprint" one can mount the rails to the side and saves the volume of the machine bed that would normally sit under there and just exist without a use exept for holding the rails (in your construct not that much the case as the rails seem to be rather large compared to the bed according to the cad), for a higher gantry is cheaper then a wider bed.

Also, it depends on the form of your rails. Yours seem to be some SBR12 derivates if I am correct? Thes are preferable for loads on top of them as their stiffnes in sideway load situations is not optimal (again with the,hererather thic, plate and forces perpedicular to their plane, the groove does not help either...) so mounting them sideways normally puts them in a bad place. But again, in a small machie like this one can get away with it.

I hope that helps you a bit and together with the other tipps you should get a fine machine.

Greetings from germany, Nightlight

PS: On Grabcad there are a lot of models of the elements you used. If you want to step up your CAD game and give us a better feel of what goes where you might want to take a look over there or whereever you like. There is a lot of CAD material out there =)

[avarakin]

I made some progress on the build:
1. Assembled the base. I used 10mm brackets to hold parts together (see attached picture). I think it is very solid. If I ever find that X rail has too much of texture, I will add steel angles or rectangular tubes as support.
2. Cut gantry plate out of MDF and glued 2 pieces together. Not sure it will be the final, may build it out of AL later on.
3. Put more details into CAD drawing. I used FreeCAD for this design and I am totally new with CAD in general, so it was a challenge
4. For now, I have the sliders on the side. My frame does allow some adjustments so I think it will work.


Nightlight,

Many good comments and especially thanks for pointing me to Grabcad!
My rails are SBR20.
I am not planning on very high speed - it will be used for hobby. I picked these 270oz/in motors:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Alex

[Nightlight]

Now THAT is a nice CAD drawing =) Glad I could be helpful.

About 1:
I would go for angles right away or even better, combine them! There is no such thing as a to stiff connection^^

About the motors, 1.9Nm should be plenty of torque for this and even allow for decent speed. I may be overlooking it but what ballscrew diameter and pitch are you using?

Greetings from germany, Nightlight

[avarakin]

Nightlight,

My ballscrews are SFU / RM 1605.
Here is the kit I am using for my build, it has information on all components:

https://www.ebay.com/itm/391930155935


Alex

[Nightlight]

Jup, plenty of torque for that build, first thought that to be an SFU2505 which would be a little overkill.

Annyway, keep us up to date! I am curius where this is going =)

Greetings, Nightlight

[avarakin]

Long time, no posting.
I was working on my build for few months and finally completed most of the work and had a first run today.
At some point I changed the design - instead of 1 X screw, I implemented 2.
I also changed the gantry design.

I tried using simplistic controller, built out of CNC Shield and DRV8825s but it was very weak.
So I went ahead and built a controller out of TB6600s, connected to Arduino Uno.
I am running GRBL on Uno and use Universal Gcode Sender for controlling the board.
The whole controller fits into the old PC case.
The motors run pretty well once I reduced the feed rate to 500mm/min.

I also have Raspberry Pi inside of the controller, which is not hooked up yet. Idea is to run Universal Gcode Sender on the Pi and control it remotely using VNC.

Next steps are to add MDF bed and implement some kind of dust management system. Surprisingly, the vacuume cleaner works pretty well so far, you can see it on picture.

Alex