Planning concrete horizontal mini-mill build, advice?

[mactec54]

Thanks for the info, it seems like various ways of clamping the top surface to the table are pretty simple.


Yes, current plan is to cast an angle out of concrete, face and bore after the machine is running 3 axis, then bond in an aluminum sleeve to hold a harmonic reducer. Currently lurking ebay searching for a good and cheap one, I really want a thru-spindle bore on the reducer so I can tighten an expanding arbor from the rear of the head stock instead of having to go through the part. I have not done any CAD on this yet but it should be pretty simple once I find a suitable reducer. Hopefully a cheap NEMA17 I have laying around will be fine for this at 50:1 reduction.

Also look at Megatorque Servo Motors NSK have some good units, if you find one make sure you get the drive as well as the motor, you can find these new sometimes on Ebay they can be better than a harmonic drive
https://www.nsk.com/products/megatorque/

[j2345]

So you will put the stepper not inline with the reducer but I'm guessing run it with a belt from the side or top? Make sure the reducer is big enough for your application, you wanna pay attention to torsional stiffness. You might need a break, but don't take my word for it.

There are geared reducers where the hollow shaft isn't obscured but they are quite expensive starting at $600.

Yes, belt drive so I can access the hollow shaft, or change reduction as needed. Looking at

https://www.ebay.ca/itm/224431151408...item3441239930
https://www.harmonicdrive.net/_hd/co..._GearUnits.pdf

Seems like backdriving torque is actually lower than I would have thought at 1.6nm, with 80:1 reduction the small NEMA17 with reduction a 0.3nm motor gives 24nm holding torque, while the limit for the gearbox is 28nm for "repeated peak torque". My widest part is probably machining at 80mm radius, does anyone have numbers for typical machining forces? Using a 10mm endmill in 6061. In any case, peak momentary torque is limited to 54nm for the gearbox so I can just throw on another NEMA24 since I was considering ordering a spare anyways, 2.2nm holding torque at 80:1 would surely be more than enough. EDIT whoops I was looking at the wrong datasheet, https://www.harmonicdrive.net/produc...csf-14-80-2uh# is the right one. Figures are slightly lower.


Thanks for the tip on megatorque, unfortunately I searched ebay and it seems none are available with driver for a decent price right now.

[ardenum]

Thanks for the tip on megatorque, unfortunately I searched ebay and it seems none are available with driver for a decent price right now.

Torque motors, or direct drive motors are pretty expensive, eg. cheapest one from CTB is $1500 and another $500 for the drive. You could build your own but even then a drive would cost $200-300 on top of that at least because you usually run them with VOC(vector control) or a newer method DTC(direct torque control). Going that route would be like starting a whole new project though.

[mactec54]

Torque motors, or direct drive motors are pretty expensive, eg. cheapest one from CTB is $1500 and another $500 for the drive. You could build your own but even then a drive would cost $200-300 on top of that at least because you usually run them with VOC(vector control) or a newer method DTC(direct torque control). Going that route would be like starting a whole new project though.

You can find them new motor and drive from $800 to $1000 NSK I have brought them, you have to be looking for them as they go fast

[Buoyen]

I recommend getting at least 3Nm steppers, but try to keep the inductance low. This gives you better rapid speeds than a 2Nm.
I'd also recommend 5mm screws for a metal mill with 200step/rev steppers to get extra resolution.
Drives like the Gecko g214 with full step morphing at high speed and electronic gearing at low speed to increase resolution is probaly of the best out there. I have a couple of them and they are great. But keep in mind the recommended supply voltage rule. The steppers get really hot if you increase the voltage.with these drives compared to some comparable DSP drives.

[j2345]

Uh oh, I've already ordered the 2.2Nm steppers. Hopefully they will be torquey enough to run the 10mm pitch screws. I did cheap out and get 1605 for the horizontal z-axis. The steppers come with DM556 drivers which are said to be decently efficient and supports various microstepping for increased resolution. I plan to run at the 50v top limit of the drivers. If the 10mm pitch is too much for the holding torque of the motors I will integrate a belt reduction to the screws. I don't need super rapid speeds, just good enough feed and accel to do some HEM techniques.

[Buoyen]

I think you will be fine. And with 10mm screws you do get good rapids anyway.

[j2345]

I received the linear actuator stages today, some things I wanted to note:
1. The ballnut mount is slightly higher than the rail cars, so so shims will have to be placed on top of the cars to avoid bending the screw and binding
2. The cars move rather freely on the rails, I worry that means there isn't any preload?
3. Cars one the shorter actuator do not have oil/grease fittings installed. The rails are oily so I assume they were lubricated with oil when in use previously.
4. The existing motor mount seems to be sized for NEMA17, will need an adapter plate or new mount fabricated. No big deal, should be easy.
Otherwise it seems good, the screw moves nicely with low drag and no backlash is perceptible by hand and eye. The rail/car interface seems reasonably stiff.