CNC ball screw upgrade

[EL34]

I am replacing my X axis drive screw. It is currently a 36" long x 1/2" diameter 10 pitch acme screw with two Delrin nuts.

I grabbed a nice 900mm x 16mm Ball screw off of Flea bay and am in the process of fitting it to my machine.
 ;D ;D ;D ;D ;D ;D ;D

Once the X axis upgrade is done the whole machine will be all bearings, no acme screws or Delrin nuts.

The Y and Z axis are THK actuator assemblies.
The X axis already had two THK ball bearing rails for travel, which made that part very precise.
The X axis motor drive screw was the weakest link of the whole machine

See this page for more info on the ball,screw upgrade.
http://www.el34world.com/Misc/Cnc/CNC0.htm

[DummyLoad]

 8)

you'll see better repeatability and smoother, more accurate cuts with the ball screw setup.

have you given any thought to making your stepper drive system a closed loop system w/ shaft encoders?

i'm thinking about doing that when i upgrade my puny 220oz/in steppers to 400oz/in steppers.

http://www.encoder.com/literature/datasheet-755a-nema.pdf

[EL34]

The acme screw was actually pretty good since I ran two delrin bearings that could be screwed away from each other and tightened in place to take up any play.
But, as you can see on my CNC page, I have a big heavy gantry style machine.
the Y and Z axis parts, plus all the framework moves along those big THK bearing assemblies.
The delrin bearings and the 1/2" shaft would vibrate and chatter from all the stress.

The new ball screww is much larger diameter and with ball bearing instead of delrin, it should move back and forth way nicer.

I upgraded my X axis motor to that big green mother in the pics a while back, which is a 400 oz if I remember correctly.

As for the encoders, it works great now with a very high accuracy.
I am willing to keep it simple and leave well enough alone for now.

I'll post some more pics of the parts I machined today when I get time

[G._Hoffman]

So, how exactly do they ship a 30 foot long ball screw? 

Or is that 900mm?


Gabriel

* EL34 - ooops, yes 900mm - 90cm

[tubesornothing]

This is interesting,  for the machine I am building http://www.data-cut.com/router.html  the designer suggests **not** to use ball screws.  His design calls for 4 start precision ground acme with delrin anti backlash nuts.

[DummyLoad]

The acme screw was actually pretty good since I ran two delrin bearings that could be screwed away from each other and tightened in place to take up any play.
But, as you can see on my CNC page, I have a big heavy gantry style machine.
the Y and Z axis parts, plus all the framework moves along those big THK bearing assemblies.
The delrin bearings and the 1/2" shaft would vibrate and chatter from all the stress.

The new ball screww is much larger diameter and with ball bearing instead of delrin, it should move back and forth way nicer.

I upgraded my X axis motor to that big green mother in the pics a while back, which is a 400 oz if I remember correctly.

As for the encoders, it works great now with a very high accuracy.
I am willing to keep it simple and leave well enough alone for now.

I'll post some more pics of the parts I machined today when I get time

the 400oz/in motors are definitely happening soon on my machine.   :)

my rationale for the encoders is for absolute measurement and positioning. as the tool(s) wears they can drag and the steppers loose steps. on larger jobs that error can add up to making junk. if you accidentally bump a jig or fixture that means finding zero all over again... ugh!  :(

[EL34]

Gabe,
900cm is about 36"

Ball screws, (as long as they are good quality) are a step above acme acews.
I would guess precision ground acme is  pretty good and probably better than some crap ball screws.

There are rolled screws and ground screws.

ground screws are way more expensive and precise.
Not really needed for what we do.
If you are making quad core chips, then you need that kind of screw.

Those encoders look like they mount on the end of the screws???
I don't have any exposed screw ends on my machine.
they all terminate with a ball bearing and block assembly.
you can see a good picture of the X screw on this page.
http://www.el34world.com/Misc/Cnc/CNC10.htm

Y and z are both THK precision actuators, no screw ends showing there either and no place to mount anything.

[DummyLoad]

Gabe,
900cm is about 36"

Ball screws, (as long as they are good quality) are a step above acme acews.
I would guess precision ground acme is  pretty good and probably better than some crap ball screws.

There are rolled screws and ground screws.

ground screws are way more expensive and precise.
Not really needed for what we do.
If you are making quad core chips, then you need that kind of screw.

Those encoders look like they mount on the end of the screws???
I don't have any exposed screw ends on my machine.
they all terminate with a ball bearing and block assembly.
you can see a good picture of the X screw on this page.
http://www.el34world.com/Misc/Cnc/CNC10.htm

Y and z are both THK precision actuators, no screw ends showing there either and no place to mount anything.

errr... no disrespect intended doug, but unless you bought a 354 inch screw.

1 inch = 2.54cm. so then 900cm/2.54cm = 354.33 inches 

1mm = .03937 inch so then 900mm x .03937 = 35.433 inches - is that more like what you bought?  :)

the encoders mount on the end of a NEMA23/34 motor. if your stepper motors have dual shafts (i can't tell from the pics you linked to) then those will work for your application - they use a coupling collar. based on the info on the probotix URL, the 400oz/in steppers are dual shaft.

in our application, it helps with the math if you get encoders that are built with base10 output resolution. you want 2 phase (quadrature, A&B) output, weather or not you need open collector or pull-up outputs depends on the type of parallel I/O interface in your controller. if yours is looking for 5V pulses, then the pull-up output is likely what you'll need.

our steppers are 1.8 deg./step, or 200 steps/revolution; so with that info., it's easier to scale our encoders output if we used something like 1000pulses/revolution. the degree of accuracy improves with the number of pulses/revolution.

lastly, at low speeds steppers can be a bit "jerky" that can cause the occasional clock jitter error(s) by the encoder; scaling will help minimize the error resolution in that regard. our software (mach3) sets the limitation on how much we can scale - need to check on that.

[G._Hoffman]

Gabe,
900cm is about 36"

No, 900mm is about 36" (actually, 35.4331").  1 meter is about one yard.  1 meter contains 100 cm or 1000mm, and 1 yard is 36".  Don't believe me, CHECK IT OUT (I use this site all the time because I can't deal with metric to save my life!).  Just ragging on you though, don't worry about it.




Gabriel

[EL34]

I got 0 key stucky disease  ;D
I think it was supposed to be 90cm or 900mm, whichever you prefer

anywho,
My 260 oz Y and Z motors have smooth ends like this
http://www.el34world.com/Misc/Cnc/CNC13.htm

my X axis 400oz motor has dual shafts

[EL34]

Here's some pics of the ball screw nut support I machined
more info here
http://www.el34world.com/Misc/Cnc/CNC15.htm

[tubesornothing]

So a related question.  I presume you made your bearing plate with your exisitng CNC.  I noticed on the this plate you have end holes.  Did you just do those manually on the mill?  I need to do the same, with .003" accuracy and my manual mill skills leave a bit to be desired...

[EL34]

Yeah, I made all these pieces before I tore down the cnc machine this morning.
I'm doing the upgrade right now.

It would really suck to tear down the cnc and need to do more machining.   ;D

Had to think everything out for a few days to make sure I had it all right the first time.

The 3 big holes in the end of that bottom piece were drilled with my drill press.
The 6 small holes in the vertical piece were done by the cnc with a .125" endmill bit.

I used a .25" endmill to do all the other cuts.

You could do the end holes on a cnc if you had a accurate vise of some sort to hold the piece up vertically.
It was faster for me to just do those 3 holes manually.

[DummyLoad]

I got 0 key stucky disease  ;D
I think it was supposed to be 90cm or 900mm, whichever you prefer

anywho,
My 260 oz Y and Z motors have smooth ends like this
http://www.el34world.com/Misc/Cnc/CNC13.htm

my X axis 400oz motor has dual shafts

the encoder i linked to will work only on your X axis then...  :-\ the only other options are the through shaft type for your Y & Z axis which are a PITA to install, or upgrade your motors to dual shaft.

[EL34]

ok, the screw, she is installed mon.

I can't believe how smooth this screw is compared to the acme screw and delrin nut.
I was able to crank up the X axis speed in Mach3 with this ball screw.
The old acme screw chattered and made a huge racket if I ran it too fast.

I still have to calibrate the X axis travel with my digital gauge.

More biggerer pics and info here.
http://www.el34world.com/Misc/Cnc/CNC15.htm

[PRR]

> 1 meter is about one yard.  ....  Don't believe me, CHECK IT OUT

I knew what he meant.

When I was young, for some reason the best yardstick in the house was a METER stick. So I know a meter is a long Yard. Long to the tune of not-quite 40 inches.

It was numbered in mm. I didn't even know there were cm. I did know that there are exactly 1,000mm in a meter-stick. So "900mm" is a short meter. Maybe about a 3-foot Yard, though ISOtone did math and showed they cheated down a finger-width.

A 35-FOOT thread can be trucked same as telegraph poles. However it would not fit Doug's projects, workroom, or budget. I don't wanna think what 35 feet of good thread costs. Also, at finger-fat diameter, 35 feet of rod would be too whippy, even at stepper RPM. You don't want to imagine the cost of a telegraph-pole length/diameter threaded rod.

BTW: 2.54 or 39.39 inch/metric is EXACT. Since one is an English king's foot and the other is a sub-fraction of the distance from Paris to the north pole, you'd think they would never line up and you'd need an infinite decimal to convert. Turned out that the three Official Yard Sticks (UK, US, Canada) disagreed with each other! Not so they noticed in 1800, but by 1890 the difference could be proven with better comparators. And it happened that 39.39/25.4(exact) to the Meter was more accurate than one yardstick to another. So the king-foot Yard was redefined against the French Meter Stick, and the old Imperial Yard Sticks became museum curiosities. (As did the Meter Stick in the mid-1900s.)

> There are rolled screws and ground screws.

There should be Milled Screws. If demand is high, a 3-foot hobb will cut a screw in one turn of the screw. A 2-hobb machine can rough and finish a screw near as good as grinding at lower price. Or did rolling get much better in the last few decades? Used to be a cheap brutal process. Though a well-rolled screw sure could be more accurate than you need here.

> Here's some pics of the ball screw

Ah, I see. You don't need or want good thread-tips, just good thread roots. The flaw in rolled threads is that seam in the tip. They can press a couple large rounded hardened wheels against a soft rod, moosh-in the root, throw-up the excess like a plow, and harden. May not even need great hardness.

[EL34]

Ah, I see. You don't need or want good thread-tips, just good thread roots.

Yes, the ball bearings roll along in those valleys.
The smoother and more precise the valley, the better the screw.
Looks like the tops, between the valleys is where the excess metal goes when they are rolling a blank into a screw?

I saw how they roll semi molten rods into a screw somewhere on tv, pretty cool.

A ground screw is at the high end of precision.
They must start with a rolled screw and then do precision grinding to get the exact tolerance they are looking for?

Here's a diagram of how the ball bearings roll in the valleys inside the ball nut

[Dynaflow]

The smoother and more precise the valley, the better the screw.

 A musicians credo I lived by for years... :D


Regards,

Dyna

[EL34]

 ;D ;D ;D ;D ;D

[DummyLoad]

hey doug, requesting a small favor please... on your CNC machine, on the 400oz/in motor, on the rear housing where the shaft pokes out, are there any tapped screw holes? if yes, would you kindly measure the center-center distance.  :)

[EL34]

no holes.
The rear aluminum plate is smooth but looks thick enough (Maybe .25" ??) to drill and tap.

I got the motor at probotix.
link on my cnc page.

[DummyLoad]

no holes.
The rear aluminum plate is smooth but looks thick enough (Maybe .25" ??) to drill and tap.

I got the motor at probotix.
link on my cnc page.

thanks for doing that!  :)  i visited the probotix site and took a look at the drawings, i needed confirmation. i found some encoders that will work with my existing motors at the link below. i have 1.812" holes on my existing motors, so option G (on the part in the link below) would work, however it looks like option T if i upgrade to probotix motors, as i don't really want to disassemble the motor to machine/modify.

http://www.usdigital.com/products/encoders/incremental/rotary/kit/e7p/

[EL34]

How about just taking some 1/8" thick epoxy board or aluminum
Make a plate that fits the motor end
Mounting the encoder to that.
Then epoxy that to the back of the motor?
Doesn't look like it needs to be secured all that much to me.

[tubesornothing]

ISOTone

I am curious about the encoders.  If they are attached to the end of the motor I can see how they can compensate for steppers missing steps, but will that achieve your other goal of compensating for screw wear?

[EL34]

Don't know,

Mach3 has a screw mapping function so you can take care of things like that.

I don't worry about that so much since mine is just a hobby machine.
If you are doing very precise stuff, you would need to worry about that.

[DummyLoad]

ISOTone

I am curious about the encoders.  If they are attached to the end of the motor I can see how they can compensate for steppers missing steps, but will that achieve your other goal of compensating for screw wear?

goal of compensating for screw wear

encoder(s) will not help in that respect. i only want mach3 to know about where the spindle is with some degree of accuracy greater than that of the stepper's accuracy. IMO, counting steps alone to determine spindle position is not reliable means to do so. where it does help is when you have tool wear/drag. when the tool drags, then you will miss steps. the encoders compensate for that b/c they only increment when there is actual shaft movement. mach3 uses the encoder data and compares that to the step pulse data and compensates accordingly.

backlash compensation needs to be done in s/w. mach3 has that ability. currently my machine has an acceptable backlash and it's not a problem with the projects i'm currently milling/drilling. if i installed encoders, i would then enable the feature.

[tubesornothing]

Gotcha - I misread your original note - thanks.

[DummyLoad]

How about just taking some 1/8" thick epoxy board or aluminum
Make a plate that fits the motor end
Mounting the encoder to that.
Then epoxy that to the back of the motor?
Doesn't look like it needs to be secured all that much to me.

i suppose that would work with the centering tool that they provide however, the T option has an adhesive backing to just simply stick it on.

since i'm in planning to upgrade, at this time, i would rather seek out motors that would allow me to mount the encoder(s) with fasteners. 

[DummyLoad]

found em!!!  :)

http://www.anaheimautomation.com/hightorquesteppermotors.aspx

specs:

http://www.anaheimautomation.com/manuals/L010280%20-%2024Y%20Series%20Spec%20Sheet.pdf

[tubesornothing]

What CAM and controller software do you guys use?  I see ISO uses Mach3 for controller...

[DummyLoad]

What CAM and controller software do you guys use?  I see ISO uses Mach3 for controller...

i use sheetcam... looking at cambam in the future.  :)

[EL34]

AutoCad for CAD
Cambam for CAM
Mach3

more info here
http://www.el34world.com/Misc/Cnc/CNC0.htm

[tubesornothing]

Thanks!  Totally missed cambam on your web page, I'll try harder next time.