NordicCNC's build log

[NordicCnc]

Hi,

Hi and thanks for your comments, very much appreciated! I will add my replies below.

Some comments in no particular order, assuming you want to build a decent machine and not a toy:

- 1400 x 1000 x 200 mm is a _huge_ working area. I would expect a machine that size to weighs at least 1500 kg.

Yes it is a huge working area! About the weight, I've quickly added all components to my build with assigned material weights etc. and it sums up to about 200kg right now. I excpect it to be around the 300kg mark once everything is mounted (with aluminium profiles as base frame). Not much is missing so I am not sure if you thought that the whole machine would be built out of steel? Then the 1500kg would make sense.

About servos:
- those 180W JMC servos are toys and seriously underpowered if you want decent speeds. If you want to go servos, use some proper ones. Delta servos are cheap and good. You need to size the motors depending on the max feed/rapid rates and accelerations you seek. 400W is probably the bare minimum.
- Personally I expect a router this size to be fast, I would target 30m/min (1000 ipm) and maybe 0.5g accel.

Yep, I realized this later (NEVER BUY PARTS BEFORE DESIGN IS READY) that I would've been better of getting either 400W JMC servo motors or 400W delta servo motors (seems to be pretty much the same price and specs!) or even stepper motors.

Targeted speed is 15m/min and about 0.25-0.5G acceleration. This I plan to achieve with a 2:1 ratio with 3000rpm and 10mm pitched ballscrew. I will also achieve some increased torque with the 2:1 ratio, which will be helpful for these small beasts of servos.

Here are some references where some people are testing these specific servo motors. According to description section he is running at 12m/min at 0.25G at a 2:1 ratio with a 10mm pitch.
https://www.youtube.com/watch?v=xyVr35pgx7s
https://www.youtube.com/watch?v=nd19IBllD2I&t=32s

Here is another one with some amusing testing. Same video in both links but at different time stamps.
https://youtu.be/ZMxzRn9GrKY?t=330
https://youtu.be/ZMxzRn9GrKY?t=420

About ballscrews:
- 10mm pitch ballscrew would be the minimum I'd use, without any reduction. With servos direct-drive is the best and the cheapest. Good pulleys are expensive, and add unwanted inertia to the system. Servos are usually rated at 3000rpm, with 10mm pitch this gives you a max speed of 30m/min.
- 5mm pitch ballscrew is acceptable for the Z axis. Again without reduction. If you really want pulleys for space issues, use 1:1.
- Ballscrew diameter: you need to calculate it depending on the desired max speed. A ballscrew 1400mm long that don't whip at 3000rpm with fixed-floating ends is around 60mm diameter... If this sounds too big you can go with rotating ballnut (complex) or larger pitch. 20mm pitch would be ideal and you would only need 32mm ballscrews.

I wanted to use belt drive to be able to experiment with ratios, mounting simplicity and to hide the motors so that they are not protruding. Using direct drive with 3000rpm and 10mm would be too much as you said for 16mm ball screws. This is another reason why I intend to use a 2:1 ratio, which would result in 1500rpm.

What calculator are you using to calculate those diameters (link or fomula)? I have not seen anyone in here that size of ball screws! I keep seeing the recommended sizes being 1610 and 2010. I will be happy with 15m/min anyway.

About rails:
- don't bother with anything less than 25mm. For the simple reason that 20 rails use M5 bolts and require tighter drilling positioning tolerances.

If I go for the aluminium profiles on the Y-axis (base frame), then I will only have to drill and tap the M5 bolts onto the rear Z-axis plate. This plate size fits well in my shitty CNC router that I built 10 years ago. It can handle the requires tolerances needed for those holes, I am pretty sure.

Anyway, do you think 25mm would be to prefer over 20mm rails for the sake of stiffness?