Hi Justin,
having a look thru the Hiwin catalogue, I don't think the MG series is going to be up to it. The MG series has just two recirculating
ball tracks, one on each side. Dynamic load rating about 5kN. The EG series has four ball tracks and is much stiffer, the same 15mm
rail dynamic load rating about 10kN and the HG series about 12kN.
The guideways and cars I selected for my mill are equivalent to the 15mm EG series and they're fine but I'm glad I didn't choose anything
lighter. 3D printers have vey little load; a lathe has cutting forces and might as well come from a different planet! Don't be duped, rigidity
is paramount, even if you get everything else right if the ways/guides are not rigid you've wasted your money. I can understand the
height limitation. The WE 27mm rails/cars have a height of 27mm and a dynamic load of 12kN. Given their width you may get away without
having a lip to clamp it to. Additionally the extra width will help if the rails are closer together than ideal. It may also allow you to have
the ballscrew along the front of the machine without it skewing.
The 15mm EG rails/cars have a height of 24mm, but should probably have a lip to which to clamp and be as widely spaced as possible.
Would almost require that the ballscrew be centrally located.
but then...there is never any load on them except for directly at the spindle
That's wrong, once you start cutting there's PLENTY of load.
As for a spindle motor, certainly servos are good and just about mandatory if you want to do any serious threading but they're not
cheap. Given that you aren't intending to cut hard materials (yeah right, you will try..I sure have!) modest power (1-1.5kW) should
be enuf. If you want to save a few bucks then a standard 3 phase motor and VFD are pretty good. Don't buy a cheap Chinese or
Indian made one tho...hook them up to a VFD and they go bang. You could probably get a 2-3 hp American/British/German/Japanese
one second hand and mostly they tolerate being driven by a VFD. I have a little 1/2 hp 3 phase 4 pole motor, with 50Hz input rated
1475 rpm for grinding tools. I regularly push it up to 3000 rpm. The rotor is as far as I know the same as is fitted to the 2950 rpm
motor, in fact probably indentical to the 3600 rpm rotor in countries with 60 Hz. Haven't blown it up yet!
Stepper motors run out of torque real quick at speed. Most two phase steppers are likely only have 5% of rated torque at 1000rpm.
Best to consider max speed of a stepper of about 500 rpm and still have enuf torque to accelerate the cross slide. With a 5mm pitch
gives you a rapid of 2500 mm/min. Clearly you could go faster with 10 or even 20mm pitch but then run out of thrust at cutting speeds.
My steppers have 10:1 planetary gear reductions and rapids are fairly slow, 1200mm/min but the thrust at cutting speeds is huge. I probably
went a bit overboard but I sure do like it when I start hooking into a bit of 316 SS.
Post a few more pics and some measurements, I'm wondering if one or both rails could be mounted on the sides of the bed and therefor reduce
the height, I wouldn't consider dropping below 10kN dynamic, I'd have to find another way.
Craig