NordicCNC's build log

[jarjar]

It all depends on your requirements.
But if you're after speed, good tolerances and surface finish in aluminium, it is, indeed, huge.

[jarjar]

1. You need to add a safety factor.
2. The root diameter of a 16mm ballscrew is not 16, but 14-15 (outer diameter minus ball grooves).
3. Don't use fixed-fixed unless you know exactly what you're doing. The ballscrew needs to be pre-tensioned properly to account for thermal expansion or you will introduce backlash.

[NordicCnc]

1. You need to add a safety factor.
2. The root diameter of a 16mm ballscrew is not 16, but 14-15 (outer diameter minus ball grooves).
3. Don't use fixed-fixed unless you know exactly what you're doing. The ballscrew needs to be pre-tensioned properly to account for thermal expansion or you will introduce backlash.

Good points. A 2:1 ratio resulting in the ballscrew rotating with 1500rpm, I will have to go for either 2010 or 2510 ballscrews, and decrease the ballscrew length to about 1100-1200mm.. This will give me a safety factor of around 1.2 with fixed-floating bearings.

Seems ridiculous to have to use any larger diameter ballscrews. I have not seen anyone else using that here on the forum. How does everyone else achieve 15-20m/min?

[pippin88]

Add a second spindle bracket up high on the spindle.

Add ribs on the edge of the Z axis plates. This will add a huge amount to stiffness. (Essentially make them into C channels).
Nowhere on the machine should you have a simple flat plate - everything should be box sections of at all possible, otherwise should have stiffening ribs.

[NordicCnc]

Add a second spindle bracket up high on the spindle.

According to the spindle specification, the clamping area is defined here:



I am afraid that clamping higher up (I am at the highest allowed point already) could potentially damage the spindle. Please correct me if I am wrong? The reason why the spindle is clamped like this is for gantry clearance.

Add ribs on the edge of the Z axis plates. This will add a huge amount to stiffness. (Essentially make them into C channels).
Nowhere on the machine should you have a simple flat plate - everything should be box sections of at all possible, otherwise should have stiffening ribs.

Good point! While I am trying to keep the Z-axis weight low, I think adding 15mm aluminium stiffening ribs wouldn't be too harmful (only around 1.2kg added). If I understood you correctly, you means something similar to this:



Additionally I could also move the Z-axis linear rail spacers to the front plate. This would increase the moving weight by another 1kg. I am not sure fi I should keep them on the rear plate or not. I could move them to the front plate and add stiffening ribs to the back of the rear plate. The ribs would be bolted to the rear plate, top bearing plate and motor bracket, effectively forming a rigid box. Is this just adding dead weight?

[AndyUK]

The ribs would be bolted to the rear plate, top bearing plate and motor bracket, effectively forming a rigid box. Is this just adding dead weight?

Machines only as strong as its weakest point. Currently what is keeping the back of your Z axis still? Its those carriages, which are attached via a 90 degree but-joint. So I'd say those two or so screws are the weakest point! That's why I created the side plates on my Z axis to give some support to the joints.

[pippin88]

Regarding spindle, I didn't realise it was an ATC spindle, which are a lot longer. Clamping high up may not be appropriate (though with a decent tolerance clamp I doubt you'd be crushing the spindle). The point is the longer the clamping area (or the further apart the clamping points) the better.

Yes, that's what I meant about the ribs. You don't need 15mm thick plate, even 5mm would be fine. The stiffness comes from the dimension in the direction of the force.

You should also add stiffening ribs to the carriage .

[NordicCnc]

Regarding spindle, I didn't realise it was an ATC spindle, which are a lot longer. Clamping high up may not be appropriate (though with a decent tolerance clamp I doubt you'd be crushing the spindle). The point is the longer the clamping area (or the further apart the clamping points) the better.

Yes, that's what I meant about the ribs. You don't need 15mm thick plate, even 5mm would be fine. The stiffness comes from the dimension in the direction of the force.

You should also add stiffening ribs to the carriage .

Machines only as strong as its weakest point. Currently what is keeping the back of your Z axis still? Its those carriages, which are attached via a 90 degree but-joint. So I'd say those two or so screws are the weakest point! That's why I created the side plates on my Z axis to give some support to the joints.

Thanks both of you pippin and Andy! I took your advice and now I've added 10mm aluminium ribs bolted to the front plate. I've also added 10mm side plates that are bolted to the rear plate, motor bracket and top bearing plate.



New total weights did not change much, while I think I have added quite a bit of stiffness!

Z-axis moving weight: 26.8kg -> 26.4kg
Z-axis total weight: 39.7kg -> 42.4kg



I am now debating, should I do something of the following things:

1. Remove the spacer plates used for the linear rails completely and add machining (on both the rear and front plate)? This would reduce weight (maybe 3-4kg?) and fewer parts needed. More machining though.



2. Move the spacers from the rear plate to the front plate. Linear rails would then go directly on the rear plate. Shorter spacers would be added behind the guide carriages and the front plate. This would increase the Z-axis moving weight by like 1kg, but reduce total weight by 1kg. I would gain some stiffness on the front plate, but lose some on the rear plate. Not sure where the weak point is and where the spacers are needed the most.

[JAZZCNC]

Ok well first, the ribs on the front plate add virtually nothing but weight because the plate is fastened directly to the bearings with no overhang, it's also short and strong so very little vibrations come from that area.

Regards the plates on the sides then these would help, however, I have a better suggestion. If you look at the pic and at the other machines I've built I always put a cover over the Z-axis. This isn't just to protect from debris it's also structural as it does the same job those plates. You don't need a 10mm plate, I use 3mm for the covers and it's more than strong enough.

The spindle mounting area is limited to the lower area (see the pic, it's the silver area, the black is drawbar cylinders.) However, they are steel and would easily handle another clamp.

Regards putting the spacer plates on the front plate to save weight then I wouldn't do that either because of your shifting weight to the wrong area.
To be honest I wouldn't use spacer plates as it lowers precision and makes it complicated.

Regards Motor brake then I can't answer that because I don't know those motors, but servos are not like steppers where they have a shaft sticking out the back, mostly because this is often used for the encoder.

How the motor handles the weight will depend on usage, but if you plan on doing 3D type work with lots of short moves with high acceleration then I think they 180W motors will struggle with the inertia and you'll get some following errors. Because again servos are unlike steppers where if they lose the position you visually see the lost steps, instead the encoders along with the drives close the loop and catch back up so will always return to exact same place when stopped provided they don't go outside of parameters set in the drives.

However, what you get is a following error which if it stays within following error parameters can trick people into thinking they don't have a problem because if they do a type of work which is mostly 2D the Z-axis isn't whizzing up & down so it gets missed.

[NordicCnc]

Ok well first, the ribs on the front plate add virtually nothing but weight because the plate is fastened directly to the bearings with no overhang, it's also short and strong so very little vibrations come from that area.

Regards the plates on the sides then these would help, however, I have a better suggestion. If you look at the pic and at the other machines I've built I always put a cover over the Z-axis. This isn't just to protect from debris it's also structural as it does the same job those plates. You don't need a 10mm plate, I use 3mm for the covers and it's more than strong enough.

The spindle mounting area is limited to the lower area (see the pic, it's the silver area, the black is drawbar cylinders.) However, they are steel and would easily handle another clamp.

Regards putting the spacer plates on the front plate to save weight then I wouldn't do that either because of your shifting weight to the wrong area.
To be honest I wouldn't use spacer plates as it lowers precision and makes it complicated.

Regards Motor brake then I can't answer that because I don't know those motors, but servos are not like steppers where they have a shaft sticking out the back, mostly because this is often used for the encoder.

How the motor handles the weight will depend on usage, but if you plan on doing 3D type work with lots of short moves with high acceleration then I think they 180W motors will struggle with the inertia and you'll get some following errors. Because again servos are unlike steppers where if they lose the position you visually see the lost steps, instead the encoders along with the drives close the loop and catch back up so will always return to exact same place when stopped provided they don't go outside of parameters set in the drives.

However, what you get is a following error which if it stays within following error parameters can trick people into thinking they don't have a problem because if they do a type of work which is mostly 2D the Z-axis isn't whizzing up & down so it gets missed.

Thanks for your advice. I think you forgot to add a picture though?

I will reply back once you have added, as I think things will make more sense to me then.

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