New Build -- Moving Gantry Milling machine
Hello everyone. Newbie here.
I am starting a build log of a moving gantry milling machine.
I am not completely new to CNC as I have built one in the past. I have learnt so much from the CNC veterans on this site, and I hope to combine my learning here with my mistakes from an earlier build to come up with something better.
Your inputs are highly appreciated.
Here are some stats:
Machine type: I am aiming for a 36"X24" moving gantry mahine built from hollow rectangular steel tubes
Usage: Mostly used for milling aluminum and a few plastics. Occasional steel drilling.
Accuracy: I am choosing repeatability over accuracy. I am happy with a very rigid and repeatable machine at 0.01" accuracy.
Mechanical components:
42" hiwin linear guide rails on the Y axiz
32" THK linear guide rails on the X
some other (unknown) 12.5" linear guide rail for Z
Acme screw or ballscrew for X and Y, ballscrew for Z
2 nema 34 640 oz-in motors for Y, and 1 for X
1 Nema 23 motor for Z
Electronics:
4 microstepping driver (M542H). I believe its chinese.
Power supply:
48V 7.5A for X and Y, 24V 30A for Z and my spindle.
I have been working on a CAD model for a month now. I will post design pictures shortly.
Your critique/inputs are highly appreciated.
Thanks
I suspect that you will be far better off with a ball screw.
Your friction goes up considerably with acme screws and as a result that impact speeds, wear and noise. If positioning accuracy isn't that important to you, you could look at higher lead ball screws.
Frankly I consider 0.010" positioning accuracy to be pretty terrible. Maybe that is all you need for now but I'd strive for better. It appears that you already have parts to make such accuracy possible. I just don't see you having much luck with acme screws and the high speed rapids when it comes to repeatability.
To use ballscrew or not to use ballscrew...
That was the question I pondered on for days.
I had started down the path of acme screws. I bought a set of 3/4 dia 4 start acme screws and plastic nuts from Roton. Felt that the 0.5" lead was a good ballance between speed and resolution for my application.
The point I couldnt get past with acme screw was the issue of backlash and how to deal with it. I wanted a solution outside of compensation in software. most of the solutions I came across required 2 nuts with some kind of spring compression in between. With this setup, I was worried about premature wear on the nut.
The other split nut aproach just doesnt seem that it will hold up well to a lot of axial force while cutting.
Finally, I decided to go for a Chinese anti backlash ballscrew -- 16mm dia with a 10mm lead. I am hoping that when they say anti backlash ballnuts in Chinese, it really means anti backlash ballnuts. The order is probably about a week away (ordered a little over a week ago)
With this change, I had to redo my design and scrap all the alu pieces I had already cut to use with the acme srews.
I will probably throw the 3 acme screws and 6 plastic nuts on ebay whenever I have time.
4 Attachment(s)
Painted and started assembly...
The base frame, gantry, and alu pieces are all painted. There are a few imperfect spots, but the overall paint job is ok.
If I had to do it again, I would probably powdercoat it, especially the smaller aluminum pieces.
Attachment 162696 Attachment 162697 Attachment 162698 Attachment 162699
4 Attachment(s)
Slowly coming together...
The welder must have poured a lot of heat on the metal because the smaller tube (4"X1") is slightly bent right at the weld joint.
I anticipated this since the tube is the thinest of the tubes welded.
I confirmed this while trying to square up the ballscrew end bearing mount with the linear rails. This is not a big deal as that tube is not a critical piece. The tube holds the ballscrew end mount and stepper motor mount.
I was able to square it up with a couple of thin whashers used as shims.
The tubes carrying the linear rails and gantry are much beefier and looks to be unaffected by the weld.
Attachment 162706
One late design change I made (which I was glad I did) was to cut out mount slots (about 0.75" lenght) on the stepper motor mount and ballscrew end mount, rather than just drill holes. This helped me with sliding the mounts horizontally to get a perfect allignment on the ballscrew between the front and rear mounts.
Attachment 162707
So far, the gantry is mounted and sliding smoothly from front to back.
Attachment 162708 Attachment 162709
I do have a few plastic pieces that I will be using to support the wire carrier and to cover some of the frame just to give it a better look. I hope to have those painted and mounted next.
Any recommendation on using gap filler...
I read somewhere here that you can fill the square tubes with some type of gap filler to help with reducing vibration. Any recommendation on any particular type to use?
I saw a couple of gap fillers in the local home depot store. One cures to a hard solid mass: Here
The other cures to a flexible mass: Here
Any recommendation is appreciated. Also let me know if I should be looking at something else other than thse.