Small workshop CNC router, steel base, planning on ATC

[Davek0974]

Nice ideas, but i have serious doubts about using the 2.2kw spindle as a power source for this type of setup. The bearings are poor and won't take the constant side-load of a belt, torque at lower speeds is terrible as well. I looked into ATC stuff recently on my Bridgeport conversion and gave up as it was beyond difficult - I need an ATC that can handle 24k + but at those rpm's everything needs proper balancing which is beyond the home shop.

I would go for a BT30 spindle, definitely not R8 in any case.

[spresv]

What would be a bigger load, the load of driving the belt or the radial load of a 7mm doc/1mm woc cut? The torque at 12k rpm is enough for the cut itself, wouldn't it be enough when underdriven via a set of pulleys?

It is quickly becoming apparent that the most demanding aspect of the design is the part I overlooked the most, the bearing arrangement. I spent hours and hours reading up on it and it's still kind of a mystery to me. I've narrowed it down to two possible designs.

The one is a couple of back-to-back universally matchable angular contact bearings at the bottom and a common deep grove up top. The inner races of the bottom set will be held together by a KM nut (top portion of the 35mm dia part of the R8 shaft will need to be threaded), and the outer races by a flange on the bottom of the housing, which will incorporate a radial oil seal as well. How much will the bearings will be preloaded and how the preload can be measured is still unknown to me.

The other design is sort of a clone of the Tormach spinle, ie a couple of angular contact bearings at each end of the shaft. The tormach design incorporates preload washers at each end for the bearings, but weather they are used to have the best possible contact on the inner races or as a seal (or indeed anything else) is unclear to me. Again, how much the bearings need to be preloaded is a bit of a mystery to me.

If anyone has any ideas, please share. I'll try to post a drawing of each design soon.

[Davek0974]

The difference to me is that the cutter load is random in position and loading as well as intermittent, the belt is constant in position and load, it may well work out but don't forget these spindles are seen as disposable - they are pretty much dirt cheap and a set of decent quality matched bearings will be around 2 x the cost of the spindle whole - there is no free lunch.

Preload is a subject in itself, it should be documented somewhere on the bearing manuf. sites.

[spresv]

You're right in that, I guess i'll have to try.

SKF has published a huge pdf with info about bearings, setups etc, I spent a lot of time going through it. Some things are still a bit unclear to me though, and quite outside the spectrum of a hobby/home machinist in my opinion. I'll do my best to figure it out.

[routercnc]

Hi spresv

Nice first machine, well done. I've sketched, deleted, pondered and wondered about doing an ATC spindle driven by my chinese spindle, along the lines you describe. Things I thought about:
Running the spindle upside down (Like Deans says) - but didn't think about turning it the other way for direct machining!

Limit to 12,000rpm to make the cassette bearing spec manageable (plus limit centrifugal forces in the the ATC mechanism)

Tormach TTS system based on R8 spindle (but hadn't seen the LMS one, was thinking of making my own before you pointed that one out, thanks)
In case you missed it they added the drawings - and they are metric:


Gear down with at least 1 pulley set, maybe 2, to get the speed way down whilst multiplying the torque (hadn't decided on lower speed but maybe to 2000rpm ?)

I also thought about the constant side loading but I was prepared to give it a try. Could rotate the spindle body around from time to time I suppose to load a different part of the outer race?

The main thing stopping me doing the detailed work was that I should really finish my current machine (which is direct drive) - . . . although designing it in now before making the Y and Z parts would be better than a retro fit . . . . hum . . .