I have decided to completely redo my Gantry.
Initially I was thinking of trying to improve on my stiffening mod. Possibly replacing the steel bar with a steel T. Or filling the void inside the rails with resin to increase stiffness.
But I have noticed a sudden drop in the quality of my cuts. I am getting a lot more flex than I was before. I think something is beginning to bend or wear out as everything seems to be tight and tuned.
Also the eccentric nuts on the X axis just will not stay tight, I am having to readjust them every couple of cuts.
Fixing all that will require new wheels and switching over to eccentric spacers. At that point I might as well replace the rail with something stronger…
So I decided to redo the whole gantry, replacing it with a c-beam and solid wheel setup. This will require me to make new gantry and Y axis plates. I am going to be using the R7 plates for the gantry and a custom hybrid of the XC and R7 plates for the Y axis.
The new gantry c-beam is effectively 40x80 and I will be dramatically increasing the number of wheels supporting it. Hopefuly this will be a more rigid setup. I will also be using a ACME screw with anti-backlash nuts on both the Z and X axis. This will reduce the X axis top speed but I think will be worth it for the greater stiffness and strength. Which will make my Y axis the weakest point. So I will be upgrading that with a G3 Belt and pulleys.
This will be a major undertaking as I will be replacing 2/3rds of my XC. Hopefuly it will be worth it. (Only one way to know for sure I guess. )
I was able to purchase the X Gantry plates already made, but I will have to carve the Y plates myself. I am figuring it will take me a couple of iterations before I get the design and quality right. I plan on making a few test plates in MDF?/wood?/Acrylic? to test layout and fit before taking on aluminum.
I don’t have any hard data. Just the recommendation on the web page that G3 is “better” when using NEMA 23’s as well as the anicdotal coments of others that it is “better”.
The web page does list the specs on the belts.
From what it looks like the G3 is thicker with longer teeth. So I am assuming a bit less “stretch” and more contact area between belt “tooth” and the gear?
Not that I have had much issue with the belt slipping. With my current setup the motor is more likely to slip before the belt. But changing the belt is cheapest and easiest part of this upgrade.
If the eccentric nuts are coming loose, nothing will help, until you get them to stay tight. There are a few mods here. I just got longer bolts, and put a 5mm locknut behind them. No problems so far, and I’ve made close to 600 pcs with the X carve so far.
Belts are another story… Lol
Yeah. I was all set to do a full eccentric spacer replacement and 40x40 mod, then there was the cryptic “coming soon” update. So I held off, giving me more time to do research…
I spent the last few days going over possible mods and alternate designs. But anything that looked like it would significantly stiffen the gantry involved making new Y plates, if not new gantry plates as well. Something I had been trying to avoid.
But I decided to bite the bullet, make new plates and switch to the C-Beam. I would be getting new bolts and spacers anyway, might as well jump to the upgraded solid wheels as well.
It is a bit of a gamble as I don’t know how well the c-beam gantry will hold up against torsion. But I figure it has got to be as good as a 40x40?
Are you getting the C-Beams that glitter in the dark?
I have the same plan lined up, but I am not using the acme shaft, I will keep it belt driven. I’ll also be using the 1500mm c-beam.
With that said, I am still looking real hard at using regular 40-4080 80/20 extrusion because I believe it would be stronger. I just haven’t figured out how I am going to do the wheels… Either add the v-rail or add linear rails and bearings.
I plan on sticking with a belt drive for the Y axis. Either a 1500mm v-slot / c beam (solid wheel) or a 1800mm maker rail / open rail (V wheel) setup.
I am taking a bit of a gamble with the c-beam, Though I think the upgraded solid wheels will be an improvement no mater what. My thought was if I still have torsion issues with the c beam I may be able to attach a second 20x80 rail to the back of the c-beam and widen the gap between the gantry plates. I would have to remake the y plates, but I am already making custom ones anyway, so that is not longer as much of an issue.
A big reason why I went with the acme screw for the X axis is the plates have already been designed and tested. So I didn’t have to design it myself, working off a existing part list and instruction set. I will have to see how it will affect my speed and cut depth settings though.
This is going to take me a while to complete. I will get most of the parts this weekend, but making the Y plates may take me a while.
Some one pointed out the V-Con rails on 40-4080 and by sourcing the 1"x.5" stock locally it is an extremely strong upgrade that will not break the bank.
I have not added it all together, but I figure to upgrade to a 1000mm x 1500mm would cost less than $250
Nice, steel rail and steel wheels, you can’t get stronger than that.
I think I will take a look at that for my y rail upgrade, especially because the v-con rail can be coupled together for longer runs.
That is rich. …good stuff.
I kept my dimensions to the 500mm stock because I primarily want to work with Aluminum. My problem is the Z axis. The torque of the Dewalt 611 is near the hp of mills that have 10 times the mass. I saw somewhere in this forum that someone was considering rods and linear bearings. I used these in my first 3d printer that I have subsequently rebuilt. In a 3d printer low mass and quick response seems to translate into accuracy and I did not think that the linear bearings were appropriate. However, in my CNC mill it seems like mass enhances stability. I am considering using 4 rods and linear bearings surrounding the same ACME threaded rod powering Z. Any comments?
From what I have been able to glean more mass seems to be better for CNC. There is a lot of vibration going on and you can get some weird resonance issues. And more mass seems to help combat the vibrations. The “pro” machines seem to be using larger 40mm based rails which are not just bigger but thicker walled as well. So everything is heaver.
REALLY stiff setups (all steel) can be a problem because if a steeper hits a resonate frequency the whole thing can shudder like mad. But our aluminum based builds seem to be “loose” enough to not experience this as much.
For the smaller format builds in the 500mm range a X, Y table design is often preferred. (as opposed to the gantry design) The work piece is move around and the Z axis just goes up and down. This eliminates a lot of the flex and vibration issues that come form moving the spindle back and forth.
But this increases the foot print of the machine so it is impractical for a larger build in the 1000mm or more range.
I think this is why gantry style CNC are optimized for 2.5D carving. Made for carving thinner stock and wood. Because the longer you make the Z axis the worse the flex issues get.
And X Y table style CNC’s are optimized for metal carving and have much large Z axis ranges.
Openbuilds now as a C-Beam XY Table kit. It is a CNC optimized to carve the metal plates needed to make the larger format Gantry CNC.
I find it amusing and sort of meta that there is a kit to make a machine, to carve the parts you need, to complete the kit, to make the machine you are actually trying to build…
I don’t know how rods and linear bearings compare to other setups. But I have noticed that all the higher end machines like to use rods and linear bearings driven by a screw for the Z axis. And those who have upgraded they XC to a rod based Z axis have had very good results. (See It’s Alive for more info)
It seems to me that is a more solid setup but only workable for shorter distances? Most of the rods I have seem have been around 300mm. I am not sure if this is because it is a common size used for 3d printers or if it is some kind limit of how long they can be before having problems?
I know that the acme screws, while available in longer lengths, are not used past 1000mm because they start to have problems. Something called “screw whip”.
On a positive note I was able to locate a local source that will sell aluminum plate cut to size.
So the aluminum will wind up costing about half of what I would pay to get it on-line with shipping.
Test bracket carved, good thing too, it turns out my holes were a bit small.
I am working out the countersink thickness too. It turns out the remaining thickness of the plate is critical to the beam wheels as they have a very tight clearance on the inside channel.
The part arrived and I have started assembly.
So many parts!
24 wheels in the gantry assembly, 16 more on the Z axis.
And each wheel assembly has 2 more spacers.
The C-Beam seems really strong. Thought I wont know how well it will stand up to torsion until I get it assembled.
I forgot a spacer and cracked some wheels tightening them down.
My mistake as the video instructions were very clear about it.
Replacements are on order
I am looking forward to the holiday weekend to really work in this upgrade.
I tried cutting the brackets in aluminum this weekend with Mixed results.
I broke the job up into lots of smaller operations so I could easily adjust if needed.
Most of the holes and countersink operations went well. Even the larger stepper motor mount hole.
I ran into problems when doing the final outline profile cut.
It cut fine until just as it was cutting through the bottom. It seemed to snag, then jam and broke the bit. My brand new 1 flute coated bit.
I looked it over, it looked line the bit may have snagged on a bit of bur as it broke through?
So I switched to my back up bit a 2 flute coated bit. And it was doing well, until it did the same thing again in a new spot.
This time it really knocked it out of alignment. I should be able to recover alignment enough by rehoming. Fortunately the final outline is mostly cosmetic so if it is slightly misaligned it will still work.
But I am unsure of what when wrong.
*It could be snagging in burs as it breaks through?
I can set up the cut file to run the last few passes at a much lower speed.
*It could be a build up of shavings in the cut?
I did vacuumed it out by hand several times during its cut but I can do more.
*I had tabs on only 2 sides, maybe it is shifting just a bit as it breaks free and binding the bit?
I will add a few more tables.
If there are any aluminum shavings in the cut, they will end up welding themselves to the side or to the bit and causing problems. You’ve got to have constant vacuumage and/or blowage and keep the cut path spotless.
I may be wrong, but I don’t think snagging on burrs would cause problems unless you’re cutting too deep.
I re-did my cut file so the last mm is a separate file going slower so I can easily keep the chips vacuumed clear.
Just ran across this article in the CNC Cookbook that has some good recommendations for aluminum - http://blog.cnccookbook.com/2016/01/12/7-biggest-feeds-speeds-mistakes-cncers-make/.
One thing I was wondering about, is your profile cut a single groove the width of the bit? I was watching a video of someone on the forum who makes paintball triggers and it looked like he was always milling a channel larger than the bit.
Thanks for the link.
I reviewing my setting the second bit break was totally my fault. In adjusting the feed rate when switching from a single flute to a dual flute I screwed up and increased my feed rate instead of cutting it in half.
It was. I have seen other videos of people milling aluminum with a wider channel. I had assumed it was a milling stratagem in Mach 3 or something.
I will look more into this.