Hi everyone.
I have more than 2 years XQ6232A milling machine, bought from china fob price was ~4600$ without options, i paid around ~6000$ with options like automatic oil lubrication, digital readouts, and more stuff.. which last year I converted to cnc, with ac servos, and X,Y ball screw, and now I need more spindle speed, currently I have max 1500 RPM 2.2kw ac motor and that is too slow, + that gearbox is very very noisy, and all machine vibrates at max RPM, that leaves poor surface finish.
So I started to think about new spindle, I want 4000-6000 rpm, belt drive, I made research and new cartridge spindle BT40 cost ~1200$ from chine + shipping 250-300$ that's 1500$, + taxes, papers,and if adding pneumatic draw bar that all will go close to 2000 euros, that's a lot of money for such machine. on low cost side there was cheep spindle about 350$, but shipping is the same and it has 3000 rpm max, and at the end will cost more than 600-700euros. + very long shipping, close to two month. at the end I decided to try and make spindle myself.
Already made spindle housing for first spindle shaft design, it is in the picture below it took 6 days, drawing, milling, turning, and supply materials.
Problem is that few days ago I started to rethink my spindle axis design, and learned to make axis inertia calculations, and did some research of other 40 tapper cartridge spindles and they all have bigger inertia values, for example, belt drive cartridge spindle bt40 lowest power range has 8000 kg/mm2 inertia, higher Kw spindles with lower Rpm have >10 000 kg/mm2, but very interesting was 22kw high speed spindle with integrated motor has 20000-25000 kg/mm2 inertia, So I guess spindles with integrated motor have much higher shaft inertia, and I think this is good thing.
My first designed axis with inner bearing spacer has inertia ~3000 kg/mm2 if I add 32 tooth HDT M8 30mm wide pulley I get ~4250 kg/mm2 and that is very low number, (2.2kw 1500 rpm 60hz motor has 5400 kg/mm2 rotor inertia) currently my milling head with all thous gears have very huge inertia , and currently I can cut with indexable endmills like 16mm diameter 2 insert and even 50mm 4 insert cutters steel very well with only 2.2kw motor, i have doubts that if I make that first low inertia shaft that i won't be able to cut with indexable end mills steel, to explain my concerns I included print screen picture of ISCAR power calculator where you can see that 16mm indexable endmill can cut at 4000 rpm steel 2mm wide 10mm deep, and feed 0.15mm/tooth at 0.9kw of power, but at power time variation plot you can see that max power is 7.5 kw and the same is with torque, average is 2.2nm but max is 17.9 nm so how can under powered spindle 2.2kw take such a cut, and answer is inertia, inertia stores kinetic energy and smooth things out, and I made some calculations for 2.2kw inertia number (0.0054kg/m2), and at 1500 rpm it has kinetic energy 66 joules(or Nm) at 3000rpm 266J, at 6000rpm 1058J, Formula is Er=1/2* Inertia*W2 (angular velocity 2square), so faster you rotate more energy you store and I guess it also smooth torque/ power ripples and vibrations when cutting with indexable inserts, Now my milling machine gearbox does that kinetic energy storage function, but will new spindle do that ?
Solid carbide endmills don't have that torque ripple if cutting with full flute depth, so it has very low vibrations, this is my video https://www.youtube.com/watch?v=dI8iaeI95xk but I want my new spindle to be able to cut as my old gearboxed spindle, so my conclusion is to make spindle shaft as heavy as I can to increase shaft inertia, that will also damp vibrations, and I think it will also extend bearing life significantly so i redesigned my shaft and got 12145 kg/mm2 inertia that is 4x more than first design, with 32t pulley I got 13345kg/mm2 , and if I add 2.2kw motor shaft inertia, and other 44t pulley inertia I get 23400 kg/mm2, that is something I start to like. it could be close to some 5.5-10 kw lowest end CNC machining center with bt40 spindle inertia numbers (I think so), or direct drive internal motor high power spindles, thous all cut steel very good with indexable insert cutters.
I got so low inertia number in first design because spindle shaft length is just 304mm, compared to cartridge spindle 450-460mm overall length, I can't drill hole so deep , and my mill capacity is already on the edge, I have 17.5mm spade drill with coolant holes, it is 180mm long, I got it at discount Sales long time ago, I will try to make that spindle shaft, and inner tapper I will turn at the end on milling machine, by milling machine with lathe tool vertically, I think I read about that in this forum some spindle building topic long time ago, so I will try that too.
what do you think about my thoughts on this spindle shaft, inertia, other things ?
any comments suggestions are welcome.
I know that everyone in industry want spindle with low inertia, but i think this is not my case, spindle starting and stopping time doesn't concern me. I need speed, long tool life, low vibration, low noise level.
i will update my progress next week when I will start to make shaft.