mill ballscrews

Hello.
I have an old mill about 3ton with 1m X axis
that has X dovetail ways and 32mm leadscrew that
is fixed with rotating bronze nut.
Motion to all 3 axes are given by one 2kw 1000rpm dc brushed motor
by engaging each axis to motor via gears train clutched by 3 electromangets
so each time only 1 axis can move.
I want to convert this machine to cnc with 3 ballscrews and 3 servomotors.
For X axis leadscrew length is 1400mm 32mm diameter.
because replacement 32mm ballscrew is a bit costy can i use 25mm ballscrew?
Also what pitch 5m or 10mm? Rotating ballnut design or rotating ballscrew?
I have a servomotor 500w 800rpm(is 8000rpm with 1:10 backlash free reducer) is this power enough or need something more powerfull ?

In the photos shown is how it is originally and the last machine
is the same machine retrofited to cnc by 3 ballscrews and 3 servomotors but i can't understand
from that photo how X axis ballscrew and motor mounted.. Is it rotating ballscrew or ballnut?
Can understand someone from photo how X axis ballscrew-servomotor is mounted?
Thank you.

Hi,

Quote:

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For X axis leadscrew length is 1400mm 32mm diameter.
because replacement 32mm ballscrew is a bit costy can i use 25mm ballscrew?

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Yes you could but it may reduce the rigidity of the axis also and that would be a crying shame to make the mill LESS capable than before you did the 'upgrade'.

5mm pitch ballscrews result in high accelerations and high thrust, both very desirable, but require high rotational speeds to get fast linear speeds.
10mm pitch ballscrews result in less thrust, often lower accelerations but the machine will be faster for a given servo speed.

My own design mill is about 800kg, so rather smaller than yours but I use 5mm pitch 32mm diameter ballscrews direct coupled to 750W servos. It can do 25m/min...which
is much faster than I am comfortable with, can do 0.27g accelerations at rated torque and 0.75g as a temporary overload, all in all, scary fast.

The trick here is not to rush in and buy this or that, but to do the momentum calculation so you know how its going to work out before you spend a cent. Real easy to make
a decision about servos say only to find they are not really suitable......and you've wasted countless dollars.

The bigger question is what grade of ballscrew do you want....cheap C7's or ground C5's or even better C3's?. Ground ballscrews are ten times the price of
rolled screws but will determine what sort of accuracy your machine can achieve.

Quote:

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Rotating ballnut design or rotating ballscrew?

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Rotating ball nut designs are for much MUCH bigger machines and are eyewateringly expensive, don't go there! Rotating screw is what you want.

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I have a servomotor 500w 800rpm(is 8000rpm with 1:10 backlash free reducer) is this power enough or need something more powerfull ?

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Yes it probably is powerful enough, depending on gearing and ballscrew pitch...but I think you risk painting yourself into a corner. If you are going to
convert a machine to CNC you will have to buy servos, get used to it. What you should not do is try to re-use a marginally suitable servo to save a few hundred dollars
only to find that you have compromised the overall result. A CNC conversion is expensive, but provided you don't make any missteps or bad compromises then your
machine will come to life and be worth every cent you invested in it. I would suggest that you should budget on three servos in the region of 750W to 1.5kW, which depending on the brand (does
not include el-cheapo Chinese made) are $500-$1000 each.

Cannot see any photos.

Craig

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Originally Posted by mrsa

Hello.
I have an old mill about 3ton with 1m X axis
that has X dovetail ways and 32mm leadscrew that
is fixed with rotating bronze nut.
Motion to all 3 axes are given by one 2kw 1000rpm dc brushed motor
by engaging each axis to motor via gears train clutched by 3 electromangets
so each time only 1 axis can move.
I want to convert this machine to cnc with 3 ballscrews and 3 servomotors.
For X axis leadscrew length is 1400mm 32mm diameter.
because replacement 32mm ballscrew is a bit costy can i use 25mm ballscrew?
Also what pitch 5m or 10mm? Rotating ballnut design or rotating ballscrew?
I have a servomotor 500w 800rpm(is 8000rpm with 1:10 backlash free reducer) is this power enough or need something more powerfull ?

In the photos shown is how it is originally and the last machine
is the same machine retrofited to cnc by 3 ballscrews and 3 servomotors but i can't understand
from that photo how X axis ballscrew and motor mounted.. Is it rotating ballscrew or ballnut?
Can understand someone from photo how X axis ballscrew-servomotor is mounted?
Thank you.

---

I would think because you have already a servo motor mounted to drive the gearbox and axis, you should use a similar rated motor for your application. You might get away with having lower powered axis motors for X/Y axis, whereas Z needs to lift the weight of the knee.

10mm pitch ballscrews are more robust than 5mm, I think 25mm dia would be ok, but refer to ballscrew manufacturer data. Remember cheap ballscrews are cheap for a reason. Rapid traverses on machines with dovetail and gib adjustment have much higher frictional loads than linear rails, so rapid traverses are between 5 to 8m/min.

Axis motors are usually mounted on a thick steel plates, to save space motors inward facing if space available. I would say a rotating ballscrew makes more sense in your application because driving a nut is complicated mechanically.

For Y axis surely i will go for 32mm ballscrew(heavier load than X) and for Z axis with more than 2kw servo
because knee has a lot of weight.Exists some counterweight mechanism or will be more problem?
For Z can i keep leadscrew because the weight will compensate backlash?
The retrofited machine has 5000mm/min rapids as description say so i suppose
i must be near that number for example max 10m/min.
Originaly this machine in X axis has rotating nut and i see more easy to
remove brass nut from rotating mechanism and mount ballnut-pulley this way i will keep
old alignment and because table for X has enough height to couple it with a 80-90mm wide servo
via htd5m belt-pulley under the table.
In machine of that size if i use linear rails instead of existing dovetail what will be good option?
hgr35-hgr40 or something bigger?

Hi,

Quote:

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and for Z axis with more than 2kw servo
because knee has a lot of weight.Exists some counterweight mechanism or will be more problem?

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I think you may be overestimating the power required of the servo. I would certainly recommend a fine pitch ballscrew, 5mm or even less, because of the mechanical advantage it gives you.

Just as a ballpark calculation; how about we work out the thrust available for a 5mm pitch 32 mm diameter ballscrew for a 1Nm applied torque:

1Nm applied at the external radius of the screw 1/0.016=62.5N
Mechanical advantage = (32 x PI)5=20.1
Thrust =62.5 x 20.1
=1256N or 125kg force.

How heavy is your Z axis? If you used a 750W servo with 2.4Nm (rated) torque it could support 3015N or 300kg.

Note this calculation does not account for acceleration which you will certainly want, but it does show that even a modest servo can support a lot of weight because of the mechanical advantage
of the ballscrew. The cost of high power servos goes up dramaticly, so be careful about over specifying an axis servo, it can cost a lot of extra dollars.

Craig

Hello.
In the photos shown the rotating nut leadscrew mechanism.
I can mount ballnut on this and also mount a htd5m pulley
like those shown in photo and motor below this mechanism by milling cast iron to pass the timing belt.
htd5m will be backlash free or only direct coupling with precision reducer is backlash free?
Z axis mass is more than 300kg of course i removed all geartrain cast iron pieces that was for manual operation and now is about 60kg less i remember the old 2kw-1000rpm brushed dc motor
moved easily x-y axes but not z.

Hello
What is this green-skyblue filler material that has the oil groovings and is glued on mill dovetail ways?
Mine mill has the same matterial and am looking to replace it
https://youtube.com/watch?v=Dt4f-FGwx_0

I have found c5 precision 3210 dfu(double) ballnut is this ok for Y axis
or to go with 5mm pitch which is better 5mm or 10mm ?
Standard servomotors are 3000rpm so with direct coupling 10mm will give rapids
30m/min and 5mm 15m/min which is too much in both cases.
So reduction will needed but is better to do reduction with precision reducers
that couple direct on servomotor or with timing belts-pulleys for example htd5m ?
Which of both options is more precise?

Hi,
I personally prefer 5mm as they allow for greater thrust with less torque. Of course they are slower, but as you point out even 15m/min exceeds what you want or need.

You don't have to reduce...unless you want to. Just because a servo is rated to 3000rpm does not mean you have to run it that fast. Reduction would be useful if you want to use a smaller
servo but get greater torque.

Low lash servo reducers are good, but when you get to the real low lash like <2 arc min they get expensive. There are plenty of Chinese made ones around the <8 arc min to <15 arc min, still is
pretty damned good, and for less than $100 each.

Belt and pulleys are also pretty good. With care you can get backlash free or nearly so. Belt reductions are limited to about 5:1 otherwise the smaller of the two pulleys gets that small that
the force on the few teeth in contact goes up and risks stripping the teeth off.

If you want to reduce the servo speed either approach will be fine. In-line planetary reducers are so easy to install, I know they will appeal!

Craig

5mm pitch doesn't need less torque from motor because for 10000mm/min rapids
needs 2:3 reduction after the 3000rpm motor shaft and ballscrew will rotate at 2000rpm.
10mm pitch for 10000mm/min rapids needs 2:3 reduction after the 3000rpm motor shaft and ballscrew will rotate at 1000rpm.
Rotating 2000rpm(5mm pitch) vs 1000rpm(10mm pitch) the ballscrew needs more torque for the 2000rpm(5mm pitch) because of inertia
also there is the ballscrew rotating critical speed limit before resonance so it better the 1krpm vs 2krpm..
That torque you will win in 5mm pitch is lost in previous step with the lower reduction.
Of course these calculations have done with 10000mm/min rapids and in reality this speed
is unreal and more realistic speed is 5000mm-7000mm/min

The green-sky blue filler in the ways am asking before is called turcite it is a ptfe based composite
that reduces friction protects the ways reduces vibrations and has autolubricant properties becsuse of ptfe nature.Does anyone knows where can i buy it in europe?

Hi,

Finer pitch means LESS torque is required to get the same thrust, that plain physics. This refers to the torque at the screw. If a servo is direct coupled then a 5mm pitch screw
will develop more thrust than the same motor direct coupled to a 10mm pitch screw

Does the complication of a reduction gain you anything...or at least anything useful? Your choice of either 2:3 or 3:2 could only really be achieved by belts and
pulleys, and the question is do you gain enough to make the extra trouble worth it? All geared servo reducers tend to be 3:1 or 5:1 or 10:1 etc.

For instance you could direct drive the screw, be it 5mm OR 10mm, and just chose to run the servo to the speed required to achieve your desired axis speed.
For instance if you want 10m/min then a 5mm pitch requires 2000rpm rotation.....well just run your 3000rpm servo to 2000 rpm, easy. No extra reducer or belts or anything.
Alternately if you chose a 10mm screw then just run it to 1000rpm, you'd get the same axis speed.

Higher power servos, say in the range of 2kW have quite high torque but often max servo speeds of 1500rpm or 2000rpm and some around 2500rpm. Thus it might be possible to chose
a higher torque servo and therefore require no reduction, a saving in simplicity and cost of the reduction, offsetting the extra cost of the servo, and still take advantage of nearly all the servos potential.

You may want to consider a lower power servo, say 750W, as 750W is often a very well priced option from many different manufacturers, and they are typically 3000rpm.
If you matched that to a 3:1 planetary gearbox, a Chinese made one for about $100, that would give an output shaft speed of 1000rpm, and direct couple that to either a 5mm
screw for 5m/min, or a 10mm screw for 10m/min. The 3:1 reduction gives you a handy multiplication in torque without having to pay big dollars for a 1.5kW or 2kW servo.

Craig

Quote:

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Originally Posted by mrsa

Hello
What is this green-skyblue filler material that has the oil groovings and is glued on mill dovetail ways?
Mine mill has the same matterial and am looking to replace it
https://youtube.com/watch?v=Dt4f-FGwx_0

I have found c5 precision 3210 dfu(double) ballnut is this ok for Y axis
or to go with 5mm pitch which is better 5mm or 10mm ?
Standard servomotors are 3000rpm so with direct coupling 10mm will give rapids
30m/min and 5mm 15m/min which is too much in both cases.
So reduction will needed but is better to do reduction with precision reducers
that couple direct on servomotor or with timing belts-pulleys for example htd5m ?
Which of both options is more precise?

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The material you see on the ways is Turcite this is used when there has been a lot of material remove in the rebuilding process, or to reduce the stick-slip (friction on the ways)

It is imposible to do direct coupling in y and z axis i will use timing belt pulley.
Pulley that will be mounted on y axis ballscrew can be maximum 100mm diameter to fit.
Which type pulley-belt is best for this case that has very low backlash?
htd5m htd3m gt3mm gt5mm at5 or something other better?

Hi,
mostly I direct couple servos or use servo reducers. On those few occasions where I've used pulleys and belts, and I doing exactly that combination right now, I have
used HTD5mm pulleys of 16mm width, appropriate by manufacturers recommendation for a 15mm wide belt. This is for a 750W servo. I think its adequate, but for
a higher torque servo maybe 25mm wide would be better.

The pulleys I've used are 30 teeth ie 150mm circumference (diametrical pitch), or about 50mm diameter. That would be as small as I would like to go for this power. The smaller the diameter the more force
is applied to fewer and fewer teeth. I have not encountered any problems with this arrangement....but then I've done no experimenting to establish the limits before failures occur.

Craig

Quote:

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Originally Posted by mrsa

It is imposible to do direct coupling in y and z axis i will use timing belt pulley.
Pulley that will be mounted on y axis ballscrew can be maximum 100mm diameter to fit.
Which type pulley-belt is best for this case that has very low backlash?
htd5m htd3m gt3mm gt5mm at5 or something other better?

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If you have the GT Profile available, then that is the best you can use, each Timing Belt Profile have a minimum pulley diameter you can use for the motor torque being used, once you decide on what Profile you want / can use, you can check the Gates Powergrip GT3 Design Manual and find your minimum pulley diameter.

To give you an idea of size a 24/ 48 2:1 the 24 is 37.1 Diameter and the 48 is 75.3 Diameter what plays a big part of your smallest Timing Pulley is the Motor Shaft that it is fitted on.

For y axis i can use upto 100mm diammeter pulleys for both motor and ballscrew(more doesn't fit)
servo motor will be be 1.8kw 3000rpm 6nm.
5mm gt with 60teeth on ballscrew and 30teeth on motor 25mm width will be ok?

The problem is that can't find 5mm gt profile only gt2mm htd3mm and htd5mm

Quote:

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Originally Posted by mrsa

For y axis i can use upto 100mm diammeter pulleys for both motor and ballscrew(more doesn't fit)
servo motor will be be 1.8kw 3000rpm 6nm.
5mm gt with 60teeth on ballscrew and 30teeth on motor 25mm width will be ok?

The problem is that can't find 5mm gt profile only gt2mm htd3mm and htd5mm

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GT2-5 is the same Profile as GT3-5 the only change was for Patents, so if you can find the GT2 Profile with 5mm pitch you will be fine.

Hi MRSA - Here's the design manuals for belts. maybe helpful. Peter

Here shown a cnc retrofit of an old manual mill 1/3 of size than mine.
He used cheap rolled chinese c7 ballscrews 2505 for x/z and 1605 for y
reduction 1:2 with htd5m pulleys-belts 750w cheap chinese servos with 2500lines rotary encoders
(i have those in 1.8kw version) and 2 nitrogen gas springs as counterweight for knee.
Very nice retrofit with cheap ballscrews-servos

https://youtube.com/watch?v=DxrFXhX9NjM