NordicCNC's build log

[AndyUK]

Regards the weight then I think you must have got something wrong because a machine this size will easily weigh more than 200kg, even 300Kg by time it's all finished.

Whats the maximum weight you'd expect though? I think a 1,500 Kg minimum as suggested in post #2 is a little overkill.

[JAZZCNC]

Whats the maximum weight you'd expect though? I think a 1,500 Kg minimum as suggested in post #2 is a little overkill.

Think JarJar is talking about a different strength machine, esp when he's comparing to a Datron which is a big lump of epoxy Granite. Thou that's like comparing apples with pears when a Datron cost's like £100K.

I'd expect a machine this size to be around 400Kg when finished using the materials and components suggested. It's amazing just how much weight fasteners etc add.

[NordicCnc]

Thought you said wanted a good surface finish.!!. . . . Any decently built machine will give you that finish.

Hehe, seems like we have different standards! I think that finish is quite good and I would be happy with that. Of course, I will try to achieve the best finish I can with my DIY build. I will not settle with decent, if I can get good.

Also:
If you go with 30mm rails and 25 or 32mm ball-screws then you can forget 180w or 400w servos. The extra inertia will freak a 400w servo motor when trying to stop 25mm screws from 3000rpm that are attached to a reasonably heavy Gantry which is sat on heavy bearings with a heavy ATC spindle hanging off it.

The best machines are those that have the right balance of power and weight which means getting the design and components matched. The choices you are looking at now will give you a very unbalanced machine.!

Yes, I got confused with the advice from jarjar. It seemed quite contradicting to the general advice I've seen on the forums. He mentioned that a 400W would probably be the bare minimum. Anyway I don't plan to rotate any ballscrews with 3000rpm. I will use at least a 2:1 ratio, resulting in max. 1500rpm.

20mm screws and rails are more than enough for a machine this size. Regards the screw size and whip etc then forget whip calculators because they can't and don't account for the whole machine.

As I planned from then beginning, to use 20mm rails. Thanks for confirming!

If you want proof if 20mm won't whip at 15mtr/min then just ask anyone who's using them on similar sized machine.! . . . .

I've seen those builds, which is why I was skeptical and questioned jarjar's advice about the huge ballscrews.

I've built dozens and trust me they don't if properly aligned and with correct end bearings. However, I've told you how to eliminate any chance of whip and still get the speed you require.

Correct. About preloading the ballscrew with tension, using double fixed bearings. Is there any guide how to do this? I guess the applied force by tightening the fixed bearing nuts will be critical?

Regards the weight then I think you must have got something wrong because a machine this size will easily weigh more than 200kg, even 300Kg by time it's all finished.

That is exactly what I said, that I expect it to be around the 300kg mark once finished. Probably above! :)

Also on that note, you have to factor in the Servos and the extra resolution and power they offer over steppers. This means you can use higher pitch screws and still end up with good resolution compared to steppers. Use this along with applying a ratio and the screw speed can be lowered greatly.

My plan is now the following:

- I will build the machine with motor brackets than can be replaced. This will allow me to at least first test the 180W JMC servo motors. If they are not powerful enough even experimenting with ratios, I will have to buy new motors - probably some 400W servo motors or get stepper motors. This will be an expensive learning lesson, but that is life! I will most likely never buy any parts before design is ready again, lol.
- 20mm HIWIN linear rails on all axis's.
- 2010 ball screws on the Y-axis (base frame).
- 1610 or 2010 ball screw on the X-axis (gantry).
- 1605 ball screw on the Z-axis.

[NordicCnc]

Alright first actual design post with some pictures.

Z-axis design

- 3kW ATC spindle & Ø100mm spindle bracket. The spindle alone weights about 19kg according to the supplier (RattmMotors).



- 20x200x500mm aluminium rear plate, onto which spacers, BK12/BF12 bearings and motor bracket are mounted.
- 15x50x500mm aluminium spacers to make room for BK12/BF12 bearings and 1605 ballscrew. Linear rails are mounted to the spacers.
- 20x200x171mm aluminium top plate for HIWIN narrow guide carriages that goes on top of the gantry. Gantry ballscrew nut bracket will be mounted to the same plate.
- 20x200x240mm aluminium front plate, onto which the HIWIN narrow guide carriages and ballscrew nut bracket DSG16H are mounted.


- About 200mm gantry clearance
- 146mm overhang
- Spindle can be raised about 60mm above gantry
- 1605 ballscrew, 400m long with 25mm F-length end machining for BK12 fixed bearing to allow for pulley. Floating bearing in the other end.
- 20T HTD 5M pulley on the servomotor and 40T HTD 5M pulley on the ballscrew.
- 180W JMC servo motor.
- Motor bracket with oval holes to allow belt tensioning.



- Moving weight in Z-axis direction, about 26kg - still missing cables, water pipes, proximity switches etc
- Moving weight in X-axis (gantry) direction: 40kg - still missing cables, water pipes, proximity switches etc.



I am pretty happy with the total weight of the Z-axis. I don't think it'll go over 45-50kg with everything added.

I could still trim down the X-axis direction moving weight, by removing the linear rail spacers and adding machining to the front and rear plates. But I am thinking that I will need that stiffness which the 20mm plates gives me. Would it weaken the structure a lot by adding machining of 7x60mm along the middle of the plates, to allow room for the BK/BF bearings and ballscrew nut bracket?