Small workshop CNC router, steel base, planning on ATC

[spresv]

Hello!

This is not so much a build thread as the machine is mostly complete and operational, but i wanted to share and document any further development. There are so many people over here with much more experience than me, so i'm open to criticism, suggestions, anything.

A bit of backstory, i started with a chinese 3040 router frame, which was mostly horrible. I started upgrading and reinforcing it here and there but soon realized that it was a lost cause, so i started building a machine from scratch. Goal was to have a machine rigid enough to mill aluminium somewhat reliably, but since i'm a student currently without a job, it had to be as budget minded as possible.

First thought was to make the entire frame out of 80x80 aluminium extrusions, but that thought quickly went away due to cost and the fact that i somehow got it in my head that it wouldn't be rigid enough. So i made a frame from 10mm mild steel plates with a 12mm mild steel top, with holes for mounting a vice or an aluminium t-slot table. The base ended up weighing about 50kg alone. Complete machine comes about 75kg.

Then i adapted the gantry i had made for my previous machine. Double ball screws for the x-axis, 16mm ball screws and 16mm supported shafts all round, coupled with 270oz stepper motors. I'd love to have gone with hiwin rails or something like that, but it was more than double the cost and i really couldn't afford it. I really regret it now though. Spindle is a generic 2.2kw chinese spindle. Travel ended up being 550mm x 250mm x 125mm.

Now at that point i intended to get the steel frame stress relieved and the top skimmed but the machine shop wouldn't be available for a few weeks, so i couldn't hold myself from assembling and testing it. First impressions are really satisfactory. There is a bit of ringing from the steel frame, but much less than expected. I'll probably be filling the base with epoxy granite, or something to that effect. I've made a couple of samples (like the carburetor mounts you see in the pics), and i'm quite pleased with the result. These were made with a 2 flute 8mm carbide end mill on ~12k rpms, with a 6.8mm depth of cut and 0.7mm width of cut and 1600mm/min feed rate with no chatter. That's not the limit, it's just the first combination i happened to use randomly, and worked good.

Next step is to disassemble the whole thing, get the base relieved and skimmed, put it back together and square everything up for good. I'll then fabricate some sort of enclosure, because i intend to use both mist cooling and flood cooling. I haven't made up my mind if it'lll just be a plain polycarbonate enclosure, so that the flood cooling won't be making a mess, or if it will be a substantially larger double-walled wooden soundproof cabinet. Also, i played around a bit with the VFD's torque boost function, which does make a difference and will definitely be of use, but the spindle gets really hot really quickly, so a proper water cooling system is in order, instead of just a bucket with a recirculating pump.

I'd love to hear your thoughts! Excuse the crappy quality pics, i'll be posting better ones.
Steve

[JAZZCNC]

Hi Steve,

Nothing much to comment on really other than well done looks nice strong machine. Good start for first build make it earn and you'll soon have enough money for Mk2 which can have few more bells and whistles like profiled rails.

Only comment regarding spindle cooling is that if bucket of water isn't cooling it then probably pushing settings little too hard. I've run my spindle for BIG hours with 20ltr bucket of water cutting aluminium and never gets much above warm.
I've experimented with lots of setups and found that for short cycle times in 2-3hr range then just PC rad and 1ltr of water works fine. On machines I build fit aluminium tank on the back with pump inside which holds aprox 3-4ltr water which is good for 99% of users needs.(Those in UK) If need more cooling then PC Rad & Fan can be fitted on outside but never needed to fit one yet.! (see pic)
With you being in Hot country then maybe worth running it thru small radiator with Fan on. Probably need something little larger than PC rad but small Scooter rad would work.

[spresv]

Hello

Thanks for your comments. If there ever will be a mk2 it will definitely be a vertical mill type machine. Maybe I didn't make myself clear on the cooling. Under normal conditions, it barely even gets warmer than room temp, like you say. It only got much hotter when I decided to test the VFD's torque boost function, which effectively alters the V/F curve. I had it as low as 800rpm , and torque boost seemed to make a difference, but I didn't try cutting anything yet.

And now, onto the ATC portion of the thread. It really bugs me to have to change tools manually, it's time consuming, it's a pain to zero every tool, even with the auto tool zero function, and most of the times doing all that takes up more time than the actual operation itself. I've looked into ATC spindles (since the generic chinese ones can't be retrofitted with any ATC system), but first of all the cost is way high(1.5-3k €), but most importantly there is no real reports from people actually using them, so I'm not going to take that risk.

I then had an idea based on the people that convert the X2 machines to cnc and ATC. The idea is very simple, so simple in fact that there must be something massively obviously wrong with it and I still can't see it. Basically I want to make a new head into which i'll plant an R8 spindle, seated on a couple of angular contact bearings. The R8 spindle will be mated to a two step pulley that will be driven by the 2.2kw chinese spindle. My intention is not to exceed 12k rpm (or thereabouts) on the chinese spindle, the noise is tolerable on that rpm level and it puts out decent power. Pulley ratios will be 1:1.66 on the high gearing and 1:3.75 on the low gearing, so I can get about 8k rpm on the high gearing and 3k rpm on the low gearing. Belt will probably be a poly-v 4 rib belt, since I can easily make the pulleys myself on the mini-lathe and belt speed will be around 20m/s max, which is well within what they can take. Here is a quick mockup of that



One consideration is weight, I've no idea how much weight the gantry could support without deflecting. I've been thinking to make it out of 7075 aluminium (head with spindle will weigh about 5.5kg). People tend not to prefer using aluminium, partly because it will expand a bit more than the bearing in a given temperature rise (I seem to recall reading somewhere it is 1 thou more over 40 degree temp rise). That could very easily be rectified with watercooling for the head, since there is already a system in place for the spindle.

I mentioned deflection earlier, I did a simple measurement. I put a dial gauge upside down on the outer lower face of the spindle body. Then I rested a 5kg weight on top of the stepper motor. Gauge moved down 0.01mm. I then hang the same weight in front of the spindle and gauge moved down 0.07mm. Open ended bearings have inherent weaknesses, and I think most of that slack comes from the Z-axis. I need more travel in the z-axis anyway, so i'll be replacing the rails on the Z with profiled ones. Oh, and the stepper motor (nema23 270oz) didn't even flinch with the extra weight.

About the actual auto tool change, if the head is a success the rest is a piece of cake. R8 spindle can take a Tormach R8 collet, which can take Tormach TTS toolholders (or indeed homemade ones). Couple that with a drawbar preloaded with 3 pairs of belleville washers and a pneumatic actuated lever and you're there. LMS used to offer it as a conversion kit for the X2 mill called the Z-bot, and a lot of people have done a similar conversion like the one below from Hossmachines.



If you can poke holes in my plan please do, it seems to good and simple to be true. The total cost will be around 500€, which is a risk i'm willing to take.

[JAZZCNC]

Nothing wrong with that setup and thought about doing something very similair but would allow 2.2kw to still be used.
Would be side by side twin head with 2.2Kw upside down when low speed cutting. Slide out and turn around and it's back usable has high speed spindle without needing high speed bearings in milling head.

[spresv]

I think i've seen something similar in a drawing somewhere, don't know if it was you or somebody else. I have no real use for high rpms, never do wood or plastics and rarely use small end mills, which have no problem running slower, so i don't need that interchangeability. If i need to do some engraving or a pcb or something like that, i can still setup my old 3040 and spare 1hp 24k rpm spindle and do it there.

I'm glad people with more experience than me don't see much wrong in that, i'll be putting it together as soon as possible.

[Clive S]

Hi Steve and welcome to the forum. I for one will be keeping a close eye on the way you achieve this, and well done with the machine.

[spresv]

Thank you!

12k rpm is asking much, Chris @ LMS says it's good for about 10k rpm. Angular bearings are rated at 12k rpm, if you double up they drop to about 9600rpm and if you decide to fit a radial shaft seal their limit is a surface speed of 18m/s. At 40mm diameter (the lower part of the LMS spindle) that's about 8400rpm.

Still, my main concern is how much and by what means to preload the bearings. I hope I get to the bottom of this.

[spresv]

Ok, I think I concluded to a design I'm comfortable enough with testing. It requires threading the 35mm part of the R8 spindle, so I can use a set of 7207BEGAP angular contact bearings. They are universally matchable, so the inner races are factory ground for a light preload, so fitting a KM7 nut will secure and preload them. Top bearing will be a standard deep groove 6206. The lower gap of the pair will have a flange that will secure the outer races and house a 40x62x8 radial shaft seal, series CRW1 or CRW1A.



Also, (assuming the rest of the design works) the more I look at it, the more I don't like the design of the lever operated drawbar. The benefit is that you can use a small and cheap pneumatic cylinder, but the end result will look a bit bulky and amateurish to me, although fully functional. If I can spare the expense I might change the design to a 100mm bore 10mm stroke air cylinder. I think I'll need about 6mm of travel for 3 pairs of washers to release the collet, but I'll test as soon as i get them.

[Davek0974]

Try and get those lower bearings as close to the end of the spindle nose as possible, it will reduce deflection.

I think either operating method is good, both have merits, the lever and small cylinder will possibly be lighter.

[spresv]

Yes, i've read that recommendation elsewhere, and it really makes sense to me, but it poses 2 problems. By far the bigger one is that I have no machinery to turn it down and grind it. The second problem is a design flaw that I couldn't predict. The end of the spindle head will be flush with the bottom of the Z-plate which is about 135mm from the table. That means that at the end of the z-travel, assuming a toolholder with an end mill with 20mm stickout, it will still need another 40mm or so to reach the table. If I get the bearings 20mm closer to the spindle nose, that becomes 60mm. I could maybe build longer toolholders, or use ones with er collets, or mount the head below flush from the z-plate, but wouldn't that be a problem?