First Build 5 x 12 Steel Frame CNC router

[Boyan Silyavski]

Boyan Your forgetting the Encoders are Quadrature so that's 2500 x 4 =10,000ppr.!

I am always confused about that info. Why are they never more clear. When i bought my Samsung servos, it stated 2500ppr, then i thought like you say- that they are quadrature, but it went they really meant 2500 pulses per revolution. So i actually for my machine have them programmed in Mach3 at 2500/10=250x(30t:20t)=375steps

Thats why i said he must check in servo manual and every time i think about servos ppr i check in manual.



Scott,
they are talking about controlling the resonance frequency, so if you are near it will be ok.

The bst motor inertia could not be different than typical servo motor inertia. Just make sure you know the correct length of motor and shaft diameter and you could use data from comparable motor

[Scott Damman]

The fact you even considered 5mm pitch screws and Steppers on such large machine shows you need to do much more research.

Sometimes I forget about the speed of the screw also being an important factor. I put together a little table for resolution based on ballscrew, based on my calculations I need to go with a higher pitch or up the gear ratio to keep the speed ~80% max. Upping the ratio of course hurts resolution so Im looking for a 20 pitch screw. Higher pitch also helps keep the ballscrew speed down, but of course hurts resolution per mm.



Thoughts on this?

[Scott Damman]

From my table above I am looking at a 2525 screw for the long axis. If I do a 2:3 ratio I get 37.5 mm travel per one motor revolution. 37.5/10,000 pulses = 0.00375 mm per pulse. This would be the best resolution. Or 0.0001476 inch resolution. This seems plenty good to me but thought I would ask

This reduces the speed of the screw considerably to reach the same machine speed and actually gives me some headroom on the motor max speed as well.

At the moment I am planning on rotating the screw and putting in multiple ballscrew supports. Something like...


If I choose to do rotating ballnut I will need to recalculate the inertia and just make sure everything still matches up. I am leaning the other way as my parts are coming back around $300 usd per side....

Does this sound like a reasonable solution?

[Scott Damman]

From my table above I am looking at a 2525 screw for the long axis. If I do a 2:3 ratio I get 37.5 mm travel per one motor revolution. 37.5/10,000 pulses = 0.00375 mm per pulse. This would be the best resolution. Or 0.0001476 inch resolution. This seems plenty good to me but thought I would ask

This reduces the speed of the screw considerably to reach the same machine speed and actually gives me some headroom on the motor max speed as well.

At the moment I am planning on rotating the screw and putting in multiple ballscrew supports. Something like...


If I choose to do rotating ballnut I will need to recalculate the inertia and just make sure everything still matches up. I am leaning the other way as my parts are coming back around $300 usd per side....

Does this sound like a reasonable solution?

Got my gear ratio backward. I need to redo my calculations...

[Scott Damman]

Alright new table now that I got the gearing correct the ballscrew and motor speeds were correct but I took the ballnut travel per motor rev the wrong way.



So based on these numbers still looking at 25mm lead screw. With a ratio of 2:3

Ballnut travel per motor rev 25*0.66 = 16.5mm
Precision mm = 16.5/10,000 = 0.00165 (is this real? i.e. with a quadrature resolver will I truly get 10,000 counts per motor rev?)
1600 rpm ballscrew speed to get to 40,000mm/min (~1500ipm)
Motor speed of 2424rpm (67% of max motor speed)


So am I looking good here? The precision seems great and the speeds would be very acceptable with some headroom on the motor speed.

Thanks

[AVF]

BST will provide cables upto 7mtr long. This is Video I made showing there 1.8Kw set.

Hi Jazz,
I saw your video which made me curious: would it be possible to use this 1.8kW servo and controller for driving a spindle or could I hit trouble doing so?
BTW: Nice thread - and sorry for my little hijack!

[JAZZCNC]

Hi Jazz,
I saw your video which made me curious: would it be possible to use this 1.8kW servo and controller for driving a spindle or could I hit trouble doing so?
BTW: Nice thread - and sorry for my little hijack!

Don't want to let this run on so please start another thread if you want to know more.!!. . . But yes large Servo's are often used for Lower RPM spindles. Better controllers like the Cslabs IP-A let you control the spindle using Servo. This means you can use the Encoder for spindle orientation for things like ATC or Ridged tapping etc and do things like this.!

[Scott Damman]

Got a little more done on the frame, havent had as much time as I would like in the last few days. My bed is 16 inch on center and I decided to brace in between each of the bed cross members only the first ones in so far.
.


Ballscrew
I need to finalize my plans for the X axis ballscrews so I can make mounts for them. Then motors mounts.
So a couple questions -
Do the numbers for the 2525 screw look good and make sense? On this long screw should I go with fixed:fixed mounts, I have read that this can stiffen things up a bit for a long screw.
Like some of the big guys do, can I get away with intermediate supports along the ballscrew and drive the screw? Or go the rotating ballnut route?
Do I even need the belt ratio on the X axis ballscrew? Would the motor speed be to slow without the gearing (~1600rpm at 1500ipm)? If I spin the screw, I could go direct drive with a 0 backlash coupling. Not that belting is all that difficult to implement it just takes some complexity out if I go direct coupled.

Motors
Ive run my inertia numbers multiple times with a few different numbers to try to account for some unknowns.
What I ended up with is, 2 of the 1.8kw motors for the x, the inertia ratios in my calculations worked out between 1.75 ---> 3.4 depending on the numbers. And the torque worked out between 4.25 -->5 Nm per motor, these motors are 6Nm so I feel like this is a pretty good choice.

Then for the Y I went with the 0.75kW motor 2.4Nm motor, inertia ratio calculations worked out between 2.25 --> 3.5 and the required torque was around 1.5Nm so a little headroom on torque.

I also picked that same 0.75kW motor for the Z, my inertia ratios get closer to 1.5 here and if I find issues I could change the gear ratio

If anyone has thoughts on these options for the motors Im all ears.

One thing, that I cant yet confirm, but it looks like the drives from Fred show a limit on the inertia ratio of 5. This was based on data I found online from who I think is the supplier of the drives.

Resonance
I think I will cross this bridge if/when I need to. In my day job I am a noise and vibe engineer (although automotive and nothing to do with machine design) so I may be able to figure out the correct countermeasures. I will say, as the frame sits it rings when hit, so I may borrow some accelerometers and do some impact testing (frequency response...) on the machine and just see where things are if I get ambitious.

Thanks for all the help this far!!!

[Scott Damman]

I think this will be my final Z axis design. 20 inches of travel with the ballscrew inside a 3inch x 5inch x1/4 wall box tubing. I have added supports to the internal of the box tubing to add strength and support the tubing when I cut the large slot on the backside for the ballnut.

I think this will keep it pretty compact and also plenty strong. As always comments are appreciated.

Thanks
Scott

[Scott Damman]

I got a bit done on the Z axis over the weekend. When I was laying everything out for the Z I decided that 4.5 inches between linear rails was not wide enough. So I welded on some 3/8inch plate in order to widen it to 8 inches center to center on the rails.

Starting the milled slot for the ballnut to travel


Slot cut through and now cleaning up the edges



Internal view showing 2 internal supports. I figured these were needed to help keep movement to a minimum when I cut the large slot in the tube. They had to be driven in with a sledge as they were a little tight but I got them in. They are welded in place by drilling 3/8inch holes in the box tubing and then spot welding through those holes.


Begining the face milling process. I milled a linear reference for one of the rails. The curves are just the tool paths I had to take to make it all work as my mills travel isnt enough to do the whole thing in 1 pass.


Here you can see the milled linear reference for the rail


And a blurry picture of one of the rails test fitted