Initial design advise wanted

[fifa]

Drive should be defined based of the performance.

Gantry mass play role, however if acceleration is 1200steps/second assuming 200 steps/rev this means 6 rev/s, acceleration of the gantry will be 60mm/s2, assuming gantry's mass is 100 kg - results 6N inertia force ... Correct designed drive can run additional load of human body (100kg) without problem - I am testing drive in this way.

For better understanding: side force of 6mm mill can not exceed 40N, typically it is bellow 25N.


For hobby machines main inertia forces (torques) are defined with spindle diameter. Relation between Spindle Inertia and its diameter is power of 4 (double diameter requires 16 time higher torque), same rule is applying for pulleys and gears. All others relations are linear.

Do not expect stepper motor to run more than 10m/min. It is hobby machine. 10m/min means 1000rpm (assuming 10mm pitch), which is ca 7kHz with half step mode. Not an issue for cheap drivers. For 2 motors you need ca 40kHz cheap Arduino type breakboards can do the job (96kHz).
However they are not good enough if controller need to do also micro-stepping.

Motor performance depends on motor inertia. Therefore motor rotor inertia shall be as big as possible (rotor accumulates an energy). With servo you can afford ratio up to 4 with stepper it shall be always less than one (to avoid risk of loosing steps)


Start designing from "top" , do not go too much in details.

2kW main spindle is quiet (no cooling fan), it is strong enough to continuously cut oak with 10 mm dia 8 mm deep speed aprox 4m/min.

I am assuming you have in mind cheap Chinese spindles. Front bearings are small (15mm dia), consequently bearings are loaded "eccentrically" - not all the balls take a load, some of them are unloaded. If ball is unloaded it start skitting (due to friction between ball and lubricant), consequently bearings worn out relatively quick.

Regarding noise: Whatever you will do, most likely workshop has a window, which will work like horn. The best way is to "absorb" as much noise as possible (for example carpets on the wall - but then walls becomes also dust collectors...). Machine housing (envelope) will do the job.


regards

[driftspin]

Drive should be defined based of the performance.

Gantry mass play role, however if acceleration is 1200steps/second assuming 200 steps/rev this means 6 rev/s, acceleration of the gantry will be 60mm/s2, assuming gantry's mass is 100 kg - results 6N inertia force ... Correct designed drive can run additional load of human body (100kg) without problem - I am testing drive in this way.

For better understanding: side force of 6mm mill can not exceed 40N, typically it is bellow 25N.


For hobby machines main inertia forces (torques) are defined with spindle diameter. Relation between Spindle Inertia and its diameter is power of 4 (double diameter requires 16 time higher torque), same rule is applying for pulleys and gears. All others relations are linear.

Do not expect stepper motor to run more than 10m/min. It is hobby machine. 10m/min means 1000rpm (assuming 10mm pitch), which is ca 7kHz with half step mode. Not an issue for cheap drivers. For 2 motors you need ca 40kHz cheap Arduino type breakboards can do the job (96kHz).
However they are not good enough if controller need to do also micro-stepping.

Motor performance depends on motor inertia. Therefore motor rotor inertia shall be as big as possible (rotor accumulates an energy). With servo you can afford ratio up to 4 with stepper it shall be always less than one (to avoid risk of loosing steps)


Start designing from "top" , do not go too much in details.

2kW main spindle is quiet (no cooling fan), it is strong enough to continuously cut oak with 10 mm dia 8 mm deep speed aprox 4m/min.

I am assuming you have in mind cheap Chinese spindles. Front bearings are small (15mm dia), consequently bearings are loaded "eccentrically" - not all the balls take a load, some of them are unloaded. If ball is unloaded it start skitting (due to friction between ball and lubricant), consequently bearings worn out relatively quick.

Regarding noise: Whatever you will do, most likely workshop has a window, which will work like horn. The best way is to "absorb" as much noise as possible (for example carpets on the wall - but then walls becomes also dust collectors...). Machine housing (envelope) will do the job.


regards

Dear fifa,

I have kinda refined the ballscrew selection.

Next station would be steppers and so on.

looks like its gonna be steppers for economical reasons, or servos if i decide to go on a spending spree

I am eyeballing the uccnc controller.


I hope mechanical stuff is done by end of this year.

ESo electronics in 2018 !

Grtz. Bert.

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[fifa]

No, every ball-nut allow
angular misalignment, and ballnut is not designed to support radial loads.

You shall not drive screw up to critical speed, 70% limit is recommended, this is due straightness tolerances (screw is not ideally straight)...

2010 robust, however, I would say for such type of machine too big. You will not be able to work with high speeds. Frame is too flexible.

I am assuming you are from Netherlands, threrefore German language shall not be an issue:

Check this site:


Search for Minerlaguss...

kr

[driftspin]

No, every ball-nut allow
angular misalignment, and ballnut is not designed to support radial loads.

You shall not drive screw up to critical speed, 70% limit is recommended, this is due straightness tolerances (screw is not ideally straight)...

2010 robust, however, I would say for such type of machine too big. You will not be able to work with high speeds. Frame is too flexible.

I am assuming you are from Netherlands, threrefore German language shall not be an issue:

Check this site:


Search for Minerlaguss...

kr

Dear fifa,

Thanx for taking interest in my project.


"2010 robust, however, I would say for such type of machine too big. You will not be able to work with high speeds. Frame is too flexible."

I have actually selected the 20mm over 1610 ballscrew based on lenght and critical rpm vs speed, based on the zapp automation calculator.
And some other docs i found to verify outcome.

1610 screw looked a bit thin on a 1500mm + span.

Thanx for clearing things up on using the balnutt for a virtual span reduction.


Since i am unsure about max speeds i need... i designed towards >5m/min speeds. Did i mis something here?

Although i might only need that speed for rapids?

2010 will be harder to get moving though..
but is more rigid.
trade offs..

You might be right on maybe not being able to take advantage of 2010 over 1610 on my machine.
Do you feel it is over the top?

Will i lose much speed on small movements?

1610 screw looked a bit thin on a 1500mm + span

Or will i need more powerful motors: step up to servo's.

Now working towards diy 68v psu /80volts digital drivers using 4 x nema 23's 2:1 belt reduction 3.1 / 4nm... probably zapps. not final on this.

Or will better motors be ott compared to "flexible" frame design?
Is 120/80 x80x4mm box section considered flexible? 🤤

About my design.. there will be additional diagonal bracing towards the legs.

I am starting to think i need a 50ton battle tank chassis to get a " rigid enough " base. :-)



Please specify.
Don't be gentle.


The whole idea was to skip a typical mk1 and mk2 build and go straight for a mk3 quality and size machine.



Grtz Bert.








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[fifa]

It is your design, your money.

5m/min with 10mm pitch is 500rpm.

It is better to choose 2020 screw...

I do not understand why you need "rapid" speed. It is hobby machine, and it is CNC. You do not need to be with machine to do the work...

It is relative very easy to "limit" the spindle "wobbling" - 2 nylon bushes do the job (bushes must be splitted axially and loaded with spring to omit radial clearance...

Check the motor torque curve - steppers are not good at high speeds, therefore belt pulley reduction does not work....

For whatever reason link do not works. I am suggesting to do the "bed" form cheap "cast" (epoxy concrete - cheaper than welded structure), gantry must be designed to be torsional stiff, however it is better that laterally it is flexible - to reduce loads caused by rail misalignment...

regards

martin

[driftspin]

It is your design, your money.

5m/min with 10mm pitch is 500rpm.

It is better to choose 2020 screw...

I do not understand why you need "rapid" speed. It is hobby machine, and it is CNC. You do not need to be with machine to do the work...

It is relative very easy to "limit" the spindle "wobbling" - 2 nylon bushes do the job (bushes must be splitted axially and loaded with spring to omit radial clearance...

Check the motor torque curve - steppers are not good at high speeds, therefore belt pulley reduction does not work....

For whatever reason link do not works. I am suggesting to do the "bed" form cheap "cast" (epoxy concrete - cheaper than welded structure), gantry must be designed to be torsional stiff, however it is better that laterally it is flexible - to reduce loads caused by rail misalignment...

regards

martin

Dear martin.

It is as always a trade of isnt it?

stepper : ballscrew pully's 1:2 results in 400 steps per 10 mm right .. so... resolution would be 10/400=0.025 mm per full step right...

Microstepping does not count for repeatability or precision right?

I did actually think 1:2 reduced 1610 and 2010 was generally accepted as the better solution here for alu and hard wood machines diy builds on this forum.
Or am i wrong here?

When one would use 20mm pitch ballscrews... what would you suggest the reduction should be like and keep some resolution?

If i would not need speed i better go for 1605? low rpm and low speed high resolution.


Using 2010 ballscrew on a 1500mm span looked like the best trade of to me. speed vs critical speed vs inertia price.

And yes... it is gonna be expensive

For controls it looks like uccnc 300eth has the affordable price quality i like... not decided on this though...

The most amount of money is not in C7 ballscrews anyway.. it is in linearguides and and bearings ... shipping and tax...


Do you feel i should make a radical move / design change?


Scrap the frame buy an used tank chassis?


And if yes... can you suggest what you would change at this point in the build.



I am open to any suggestions...
The build of the machine is part of the hobby right :-)



Grtz Bert.





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