[PeterJK]

Hello!
I am totally new here. I have been given a box of parts and I am trying to build a heavy duty cnc router table.
Table all steel. Gantry weighs approx 60 kg.
4kw spindle
3mx2m cutting bed
linear guides (25mm) on all axis
rack and pinion system on all feeds
dual motors on x axis, single motors on all other axis
Motors- 4 pcs. Nema23 4.2A motors w/ 8mm shaft.

I was also given 2 1:30 planetary reduction gears to fit 2 of the motors.

I need to purchase 2 more planetary reduction gears.

The I need help part - should I use the 1:30 planetary gears that I have?
Do I keep those and get 2 more the same?
Do I get 2 more different?

I am totally stuck, - not much cnc experience. Any comments/help will be greatly appreciated!

[peteeng]

I'm not sure why they are 1:30 usually you use 1:2 or 1:3. 30 will be really slow but pull the Titanic up from the deep. Check its not 1:3.. Peter

[awerby]

I'm not sure why you want to run your router that slowly. Most steppers are run 1:1; they have a hard enough time getting up to speed even then. If you were running servos, reduction gearing would make sense, but in this case, not so much. What's the inductance on those motors? High inductance will further limit the top speed of stepper motors.

[dazp1976]

I'm not sure why you want to run your router that slowly. Most steppers are run 1:1; they have a hard enough time getting up to speed even then. If you were running servos, reduction gearing would make sense, but in this case, not so much. What's the inductance on those motors? High inductance will further limit the top speed of stepper motors.

DM860T (110dc/80vac) drivers and a 80-100vdc power supply will solve the inductance issue no problem ????
Those 4.2a type motors are generally around 3.5mh.
I have some of them around somewhere.

[PeterJK]

Do you think that I can set up the motors without using any reduction gears? Any drawbacks to this ?

[peteeng]

Hi Peter- you say that the drives are R&P so you need to figure out what speeds you want the machine to run at. Then figure out if gearing is necessary to achieve that speed. Then calculate the pinion size to the rack etc and get all that sorted. Yes you can go direct from motor pinion to rack but that will give you a very very fast machine usually.... Plus you need a mechanism that connects the motor to the rack that allows for alignment as it goes along. Usually a spring on lever deal, with the mechanics mounted on the lever. have a look at avidcnc to see what I mean they sell a R&P mech. Peter

I also suggest you go ballscrew for Z vs R&P better suited to the application...

https://www.avidcnc.com/standard-rac...a-23-p-50.html

[PeterJK]

Hi Peter- you say that the drives are R&P so you need to figure out what speeds you want the machine to run at. Then figure out if gearing is necessary to achieve that speed. Then calculate the pinion size to the rack etc and get all that sorted. Yes you can go direct from motor pinion to rack but that will give you a very very fast machine usually.... Plus you need a mechanism that connects the motor to the rack that allows for alignment as it goes along. Usually a spring on lever deal, with the mechanics mounted on the lever. have a look at avidcnc to see what I mean they sell a R&P mech. Peter

I also suggest you go ballscrew for Z vs R&P better suited to the application...

https://www.avidcnc.com/standard-rac...a-23-p-50.html

Pete- thanks for the advice. So I don't need the rear reduction for added torque- the 4.2A motors will handle the loads?
I plan on using the gear on the motor to directly engage the rack - without the use of a belt.
What speeds should I be running the machine at? Where can I find this info.
A spec sheet for the motors shows a rated speed of 800 rpm. The gear circumference is 69mm. If I know the appropriate required machine spees, I can calculate which planetary gears to order.
I already have the whole setup for the z axis w/R&P so I will stick with this for now. Maybe I can use one of the 1:30 reducers on the z axis?
As I mentioned- totally new to cnc routing and really appreciate all of the input.

[peteeng]

Hi Peter- You or someone has to do the numbers to understand the loads. Need to know what your cutting and what speeds you want to cut at, masses of parts being moved. R&P usually has a gear reduction. Or look at avid machines and use them as examples. A 69mm circumference pinion at 400rpm will be doing 86m/min at 100rpm its 22m/min way way too fast for most cutting you will need speed reduction... maybe the 1:30 is a good idea? anyone out there know about these I have no direct experience with R&P....Peter

[joeavaerage]

Hi,
I used, and still have, Vexta 5 phase steppers with an integrated 10:1 low lash planetary gearboxes. I think Vexta used Apex gearboxes and are eye-wateringly expensive, but really good, they have a lash of
less than 2 arc minutes.

With a 69mm circumference at a realistic speed of 500 rpm for those steppers a 10:1 reduction would result in rapids of:

500/10 x 69=3450mm/min, which is probably about right.

With a 30:1 reduction and the same 500 stepper rpm:

500/30x69=1150mm/min, which is bit slow.

I suspect that with 30:1 the machine will be slow but have great acceleration and great thrust. Without gear reduction your rapids could be potentially be 34500mm/min, but the steppers would
stall instantly, and you'd have crap acceleration and no thrust for cutting. You'll need some reduction at least.

Discounting cycloidal drives and harmonic drives, low lash planetary gearboxes are the next best thing.

Low lash planetary gearboxes like Apex, say 5 arc min or better are expensive. Cheap Chinese planetary gearboxes, say 10 to 20 arc min, are still pretty good and much cheaper.

About 10:1 is the max reduction for a single stage planetary, 30:1 will be two stage. A two stage will have more lash. If a single stage has 10 arc min, then a double stage will have closer to 30 arc min.

I would recommend OP investigate single stage, that is 10:1 gearboxes. If he wishes to use the existing 30:1 will result in a slower machine, but that may not be a huge problem, and of course may
save some money too.

Spur gearboxes are cheap but have several degrees of lash, not much cop for CNC.

Craig

[rodw]

With rack and pinion, regardless of machine type you have to use a reduction. I think Craig is about right with 10:1 reduction for that size pinion. For plasma we would use 3:1 but we want a lot faster rapid.
The other argument against 30:1 gearboxes as is stated is that they are likely to be multistage so the back lash will be higher (eg sum of the stages)

[PeterJK]

With rack and pinion, regardless of machine type you have to use a reduction. I think Craig is about right with 10:1 reduction for that size pinion. For plasma we would use 3:1 but we want a lot faster rapid.
The other argument against 30:1 gearboxes as is stated is that they are likely to be multistage so the back lash will be higher (eg sum of the stages)

Thaks for the input. Could I get away with using 5:1 reduction? I sourced some used 5:1 planetary gears locally at a good price.

[dazp1976]

Hi,
You could certainly try, but it could be a expensive mistake if it turns out less than optimal.
If you can gather some significant data the we could do a calculation which would help guide your choice.
We really need to know the specs of the motors, and really to make a realistic calculation we need to know the torque of the steppers at speed, the holding
torque is not helpful. We also need the rotational inertia of the armature. The rest we can guess at pretty well.
Craig

They'll be the ones from Longs.
They don't give much info away.
4.2a, 425oz/in hold, 3.5mh ind, 4 wire.

That's about all you get.
I have the same on a sieg X2.
Not much cop at 36v with a 442 driver on peak.
They respond well to higher voltage though.

[joeavaerage]

Hi,
we really need OP's input as to the specs of the motors.

If dazp1976 guess is correct then 425oz.in holding, but with 3.5mH inductance even at 60VDC or 70VDC they will still be down to 30% or so at 500rpm,
or 128 oz.in, and really that's pretty small. I've met rice puddings with tougher skins than that!

Come on OP tell us more about your motors and what driver you want to use.

Craig

[PeterJK]

Hi,
we really need OP's input as to the specs of the motors.

If dazp1976 guess is correct then 425oz.in holding, but with 3.5mH inductance even at 60VDC or 70VDC they will still be down to 30% or so at 500rpm,
or 128 oz.in, and really that's pretty small. I've met rice puddings with tougher skins than that!

Come on OP tell us more about your motors and what driver you want to use.

Craig

Here are the specifications on the motors:


1. The stepping motor adopts advanced technology bearing, has low self inductance and good response.
2. The internal rotor is coated with imported rotor glue, which can prevent the increase of motor noise caused by burrs after surface oxidation.
3. The rated speed of the motor is 800rpm, the step angle is 1.8°, and the operation is more stable.
4. This stepping motor has low resistance and good thermal conductivity, and is often used in CNC milling machines.
5. Made of die cast aluminum and stainless steel, which is hard and corrosion resistant, not easy to wear.


Specification:

Item Type: Stepper Motor
Material: Die cast aluminum, stainless steel, silicon steel sheet
Purpose: Stepper motor
Rated Power: 120W
Product Type: Hybrid Stepping Motor
Rated Voltage: 24V
Rated Current: 4.2A
Static Moment: 3N.cm
Positioning Moment: 12N.cm
Momentum of Inertia: 800g.cm²
Rated Rotating Speed: 800rpm
Rated Torsion: 3NM
Motor Length: Approx. 112mm / 4.4in
Shaft Diameter: Approx. 8mm / 0.3in
Step Angle: 1.8°
Lead Number: 4
Working Environment Temperature: -20-50?
Working Environment Humidity: 15%-90%

[PeterJK]

Hi,
we really need OP's input as to the specs of the motors.

If dazp1976 guess is correct then 425oz.in holding, but with 3.5mH inductance even at 60VDC or 70VDC they will still be down to 30% or so at 500rpm,
or 128 oz.in, and really that's pretty small. I've met rice puddings with tougher skins than that!

Come on OP tell us more about your motors and what driver you want to use.

Craig

For the drivers- I have a HY- TB3DV board. Here are the specifications:

Electrical properties (ambient temperature Tj = 25 ?):
Input power 12 - 48V DC power supply
Output current 4.5A (peak 5A)
Driven
approach
Bipolar constant-current PWM drive output
The drive
motor
42,57,86 stepper motor, two-phase -4 phase (4-wire, 6 wire, 8 wire
stepper motor)
Weight About 300 g.

Unfortunately this is only a 3 axis control board. Although I have only 3 axis, I will be using 4 motors (2 on the long axis). I was hoping to hook up 2 motors in parallel on one connection point on the board, but my motor amperage is too big, and I will burn out this connection. So- I am also looking for a suggestion how best to drive the motors.

[joeavaerage]

Hi,
all they say is low inductance but they don't specify what it is????

Inductance is a critical parameter to determine how drastically the torque degrades with speed. Even a low inductance stepper, say 2mH or less will still
have only about half its torque at 500 rpm.

Have another look and see if there is any number associated with inductance, or better yet a speed/torque curve.

Craig

[PeterJK]

Here is another specifications table that I found. My motors specs are circled in the red box.
I hope that his helps.

[dazp1976]

Here is another specifications table that I found. My motors specs are circled in the red box.
I hope that his helps.

That's not "low".
I've ended up using 80vdc on one of mine that's 3.8 to get a good rapid. My other 2 are 3.0 and are on 60v.
They stalled very early on 36v or less, upping the voltage doubled their speed, made them much smoother and get instant acceleration.


You're wasting your time with wanting to have 2 motors on one step/dir output. Many try, most fail.
You need a 4 axis board with slaving compliance.

[ger21]

I don't know if anyone mentioned it, but what most people do, when using rack and pinion with steppers, is to gear them to get between 10-25mm of travel per revolution of the stepper. The same goes for screw drive as well.

[PeterJK]

I don't know if anyone mentioned it, but what most people do, when using rack and pinion with steppers, is to gear them to get between 10-25mm of travel per revolution of the stepper. The same goes for screw drive as well.

Thanks, Gerry.
So in my case gear reduction options look like this:

1:1= 69 mm/revolution
1:3= 23 mm/revolution
1:5= 13.8 mm/revolution
1:10= 6.9 mm/revolution

1:5 reduction should be ok, and will give me ample torque. Am I understanding this correctly?