Small workshop CNC router, steel base, planning on ATC

[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.