[robrmcc]

HI all:

I am embarking on a CNC router build and have a few specific questions. If there are threads elsewhere that answer some of these just point me there.

This will be a machine primarily used for wood working as I already have a mill and lathe (no CNC). Working size will be approximately 30"(X), 20"(Y), 6"(Z), I am planning to use Misumi series 6 extrusions for the base, 425 oz-in steppers, ball screw drive (5mm pitch), supported rails for all three axis, 20mm for x and 16mm for Y and Z. I have the rails and ball screws on order from Linearbearings2008 (chai). The spindle will initially be a Bosch Colt router with option to update to a Kress or similar perhaps at a later date. Speed is not a big issue with the system. I am looking at basically a system very similar to the Fireball Comet system with a single drive for X and some redesign on the Z axis. I intend to use 1/2" aluminum plate for the gantry as they do, as well as for the Z axis. My background is primarily electronics but I have a lot of woodworking experience, a lot of exposure to mechanical engineering, some CAD skills and some self taught rudimentary machining skills (beyond just drilling holes).

So my questions are as follows:

--I am a bit unsure how to spec the size of the Misumi extrusions. I will be using the 30mm series to match the supported rails and they will help strengthen the assembly but what would people suggest as the basic size for rigidity and strength for this size router.

--I am considering reversing the motors and bringing them inboard of the assembly so that they are parallel to the ball screws and drive the ball screws with toothed belt and pulley drives. Several reasons, first to get them out of the way and reduce the footprint of the overall machine, second so I can change the ratio of the pulleys to get a finer step or more torque if needed, thirdly to help balance the load on the Z axis assembly. Are there any issues with this approach that people are aware of?

--I chose the 425 oz-in motors as kind of the sweet spot for steppers and what other systems this size seem to be using. Will they be adequate for this size system? Any thoughts there?

--I noticed a lot of the moving gantry type systems have the gantry sloped back (as in the Comet). Is there some pracical purpose for this or is it just for the cool factor. I can't see how it changes the travel one way or the other, but maybe it changes the angle of the forces on the bearings or moves the spindle over the bearings to move the center of gravity?

All help and comments are much appreciated.

Rob McC

[jckstrthmghty]

HI all:

--I chose the 425 oz-in motors as kind of the sweet spot for steppers and what other systems this size seem to be using. Will they be adequate for this size system? Any thoughts there?

--I noticed a lot of the moving gantry type systems have the gantry sloped back (as in the Comet). Is there some pracical purpose for this or is it just for the cool factor. I can't see how it changes the travel one way or the other, but maybe it changes the angle of the forces on the bearings or moves the spindle over the bearings to move the center of gravity?

Hard to say which motor is best without knowing the of the electronics. What driver are you going to use and what power supply? Which 425 oz motors?

Sloped? Do you mean offset? Offset gantry's optimize the cutting table area for the x axis.

[RicknBeachcrest]

Here is a link that may give you some insight.

Do it yourself CNC router: Design Considerations, the Gantry

[ger21]

Speed is not a big issue with the system

Everyone who's never used a CNC machine says that, and everyone who has will tell you your wrong. You'll find very quickly that speed is very important.

And that leads to this. 425oz motors are about the worst ones you can choose for 5mm pitch ballscrews. In order to get high speeds out of them, you'll need to use Gecko G201 or G203 drives and a 72V power supply. For half the price, a Gecko G540 with 381oz motors will give the same, if not better performance with those ballscrews. You're going to want to spin those screws at least 1000rpm.

There's nothing wrong with driving the screws with belts. You may actually want to spin the screws faster than the motors, to get more speed. As long as the screws don't start whipping.

I noticed a lot of the moving gantry type systems have the gantry sloped back (as in the Comet). Is there some pracical purpose for this or is it just for the cool factor.

It allows the spindle to reach more of the table. The alternative has it hanging over the end on one side, and not able to reach under the gantry on the other side.

[Mauser]

And that leads to this. 425oz motors are about the worst ones you can choose for 5mm pitch ballscrews. In order to get high speeds out of them, you'll need to use Gecko G201 or G203 drives and a 72V power supply. For half the price, a Gecko G540 with 381oz motors will give the same, if not better performance with those ballscrews. You're going to want to spin those screws at least 1000rpm.

How did you calculate these requirements/results? That sounds like something useful to know.

[P.Passuello]

How did you calculate these requirements/results? That sounds like something useful to know.

Have a look at the Gecko website. They have some great articles on how to calculate the motor you need for your application and also how to calculate the driver and PSU to match it.

The biggest mistake a newbie makes is starting with the motor instead of the machine. The second mistake is finding the biggest motor they can and thinking that will suit whatever they build.

Bigger is better right?
Wrong! Not when it comes to steppers.

In general bigger motors have larger losses, run slower and require more expensive drivers and PSU's to make them run right.
As Ger said a 381ozin on a Gecko G540 will probably out perform a 425 oz and it will be much cheaper.
If you put a 425 on a G540 it will definately run poorly. It is very important to match your motor to your driver.
Drive it with a cheap Chinese drive and it will be a dog.

My machine rapids is R&P drive, 700mm x 1000mm cutting area, rapids at 600ipm, can cut at over 200ipm and is driven by a Gecko G540 with 269ozin motors. I have seen an 8 x 4 foot version of my machine using the 381ozin motors and a G540 surface it's table at 1000ipm. The limiting factor in your build will be how fast you can turn the ball screws without them whipping, how fast your spindle can cut and how rigid your machine is.

Sorry if this sounds a but preachy but it is important to get your motors and matching driver right first time. It will probably account 1/3rd the cost of your build and will be the one component you will want to take to your next machine. (There is always a next machine)

Cheers
Peter

[ger21]

How did you calculate these requirements/results? That sounds like something useful to know.

The simple answer is experience. The complicated answer, is, well, complicated.

The proper way to determine motor size requires answering some questions, and doing some math.
Basically:
How much weight are you moving?
How fast do you want to move it?
How fast do you want to accelerate? (this is the most critical, imo but few really know what they want)
What method will you use for movement? (screws, rack and pinion, ...)
What resolution do you want?

These will tell you how much torque you need at a given rpm. Two important factors here.
Steppers lose torque as rpm increases.
Most hobby controllers use linear acceleration.

What this means, is that whatever torque needed to accelerate to your target rate, needs to be available at the top speed you want to be using.

Now, stepper motors are rated for holding torque. Which is when they are not spinning.
What you need is a torque curve showing you how much torque the motor will have at a given rpm. The problem here is that torque curves for any motor will change based on several factors, including specific drive used, voltage used, and microstepping amount. And the torque curves available are usually only for a specific drive, at a specific voltage, and most seem to be half stepping, which noone actually uses.

So once you have all this info, you choose your components to work together as a team.

Your choose your drive system to give you the resolution and speed that you need, and choose a motor/drive combo that will give you the required torque at the required rpm.

Since a steppers resolution is finite, at 200 steps/rev, and they're speed is usually limited to 1000-1500 rpm (best case), you'll find that if you want very fast speeds, you need to trade resolution for speed. If you need both high speed and high resolution, then you should probably use servos, where you can change resolution by changing encoders.



Now, for the simple answer. In most cases, the maximum usable rpm from a stepper will be 800-1200 rpm. this is achieved using the maximum power supply voltage, calculated by the formula 32*(square root of inductance).

The 425oz motors are rather high inductance, requiring over 80V for best performance. Since most people will not want to spend $500 on Gecko G201's or G203's, they tend to use them on cheaper drives that can only supply 24V-48V. At best case, you'll only get 60% of the max rpm the motor is capable of. Worst case, maybe only 30%.
(Speed is proportional to voltage, btw)

The 381oz motors, on the other hand, have a maximum voltage of right around 50V. Which means you can use them with a G540 and still get maximum rpm.
Since both motors are basically the same size, they're potential maximum speed should be pretty close (this is not always the case as motors get larger, but should be fairly true for Nema 23 motors).
Now, the maximum performance from a stepper is right before it stalls. At this point, both motors will have the same torque.

Now, we have a rough idea that the 381oz motor will stall somewhere between 800-1200 rpm. This rpm depends on a lot of factors, and assumes the motor is sized correctly for the application.
When the 425oz motor stalls, it'll have the same torque that the 381oz motor stalled at, but it will only be spinning at ~60% of the rpm the 381 oz motor stalled at.

So, we can assume that the 381oz motor will possibly outperform the 425oz motor by 40%.

Now, if your application only requires 200rpm, then the 425oz will be a better choice.
If you want to run at the 425oz motors maximum voltage, then it will be a better choice.

Now, add the fact that the OP is using 5mm pitch ballscrews. Let's just call them 5 tpi for convenience.
To move at 100ipm requires 500 rpm. If it was my machine, I'd be looking for 300ipm, which would be 1500rpm. Realistically, that's probably not an option with steppers.
So, based on the ballscrews he's purchased, he's going to want to spin the ballscrews as fast as possible.

The most economical way to do that, is with a G540 and 381oz motors. Provided that they'll provide the needed torque for the application.

You can get equivalent performance from the 425's, but the cost of power supply and drives needed to achieve this will be at least double.

[crane550]

Hey, welcome to the fourm.

+1 on what everyone has said about the motors so far. Could not agree more. You might have mentioned it, but are you using 1 or 2 motors on your X axis? You might consider running 2 if your design permits and use a screw on each side of the gantry. This was by far the best improvement I have ever made to my 24x24 Solsylva.

This is a tutorial on leadscrews and pitch, it should be helpful to you:

Leadscrew Pitch Tutorial

As far as turning your steppers and driving them with a belt- I love this design. I do it with 3 of my motors currently and it has never been a problem ever.

As far as sloping the gantry back, this is to make full use of the cutting area of the table. If your risers are simply upright (with most table designs anyways) the router will sit 5-8 inches forward of the center of your X bearings. This is all lost space, as the router will hang over the front of the machine if you run it all the way forward, and not be able to get close to the back of the machine if you run it back. In my last design I gained another 6 inches of travel in my machine just by accounting for this offset. Plus you can make it look really cool.

If I were to give any other advice...go heavy. Weight is good. Weight will help you. Good luck!

[louieatienza]

Hey, welcome to the fourm.

+1 on what everyone has said about the motors so far. Could not agree more. You might have mentioned it, but are you using 1 or 2 motors on your X axis? You might consider running 2 if your design permits and use a screw on each side of the gantry. This was by far the best improvement I have ever made to my 24x24 Solsylva.

This is a tutorial on leadscrews and pitch, it should be helpful to you:

Leadscrew Pitch Tutorial

As far as turning your steppers and driving them with a belt- I love this design. I do it with 3 of my motors currently and it has never been a problem ever.

As far as sloping the gantry back, this is to make full use of the cutting area of the table. If your risers are simply upright (with most table designs anyways) the router will sit 5-8 inches forward of the center of your X bearings. This is all lost space, as the router will hang over the front of the machine if you run it all the way forward, and not be able to get close to the back of the machine if you run it back. In my last design I gained another 6 inches of travel in my machine just by accounting for this offset. Plus you can make it look really cool.

If I were to give any other advice...go heavy. Weight is good. Weight will help you. Good luck!

Another reason for sloping the gantry back or such is that it puts the weight of the gantry more in the center of the gantry carriages, which makes them do more equal work and wear more evevnly. Of course there could be reasons why you might want the spindle to reach over the
bearings, for example, machining a board on edge at the end of a table.

Belts can also act as a damper between screw and motor, is a bit more forgiving of misalignment, and as others have said, can allow for changes in drive reduction. For example, if you HAD to use 425in*oz steppers, and 5mm screws, you could theoretically gear them UP, though this is typically not done. I actually use them on my machine, but with 8 start leadscrews (1" lead.) Even with my puny 24v power supply I'm still capable of doing some productive cutting.

[Mauser]

Thanks Ger! Fascinating info, and it underscores the difference between Steppers and the PMDC motors I'm used to.

[robrmcc]

Thanks everybody. Lots to think about and read.

No advice on the extrusions though?

Rob McC

[crane550]

What info are you looking for on the extrusions?

I have to say, I have been avoiding extrusion for a long time in my builds. I finally broke down and decided to try one. It has been going really well. I'm not a fan of using extrusions for every little piece, I think that that is an over all weak solution, but using them for key components has been paying off in my last build.

I probably would not use any extrusions on the Z, but thats just me.

Alex

[louieatienza]

If you know what kind of cutting forces and weight you're dealing with, you can contact Misumi directly and they will be able to assist. You can even send them a sketch. I really like the GFS series, very heavy duty and substantial-feeling...

[crane550]

robrmcc,

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