First Build 5 x 12 Steel Frame CNC router

[Scott Damman]

Alright so I ran the numbers through the excel spreadsheet.

Heres what I got


0.010mm deflection
0.00028 deg delfection
deflection at tool 0.002mm


0.00094mm deflection

Maybe I am entering the numbers wrong I will run through the numbers again but these look better than what I got when I just did the free body diagram. Thoughts on these?

These seem acceptable but im not sure what I should be shoot for as far as max deflection?

Thanks for any help. Want to make sure this is right before I get too far that I cant make changes.

[routercnc]

Hi Scott,

I wrote that spreadsheet some time ago to compare simple shapes. For the numbers you have entered it looks like it is giving the correct answers.

Things to note:
It assumes everything else (e.g. spindle, tool, bearings, bedplate, joints . . . .) is infinitely stiff - so it is best case
Have you entered just one of the two gantry beams? If so the real stiffness will be higher as you have 2
Or have you given the overall gantry dimensions as a single beam? If so the real stiffness will be lower
It makes no prediction for deflection due to cutting vibration and machine resonance

It terms of max deflection it is personal choice but I would say wood <0.05mm and alum <0.01mm as a starting point. Others opinions welcome!

I get the point about raised gantry vs raised X axis (in the US this can be Y axis) for material loading, so as long as you've thought about the options and made a choice that's fine. Didn't know what you knew.

In summary you have a very stiff machine that is stiff enough for wood, but that gantry is heavy (hence our concern over stepper/driver/voltage - if you know these are OK then that's good), and resonance is unknown (the sheet can't show this).

[Ger21]

Is this what you have?
http://www.ebay.com/itm/3-Axis-CNC-K...-/231424600113

If so, you're better off re-selling them and taking a loss.

A quality 300oz motor driver kit will typically cost more than that one does. What does that tell you? Do you really think that you're getting 6x the performance for the same price?

As Boyan mentioned, those are high inductance motors. Ideally, you want an inductance of around 2mH. Those motors really need about 100-120V to get the best performance from them. But even then, they won't spin nearly as fast as a much smaller motor.
You generally won't find large motors with low inductance, because they would require very high currents, and very expensive drives.

A few other things to consider. As steppers get larger, their maximum rpm's tend to decrease. I wouldn't be surprised if you can't get much more than 300rpm from those before they run out of torque. Which probably equals around 250ipm with your setup?

Also be aware that you often can't count on the microstep accuracy. Especially with large motors, where the microstep torque may not always be able to overcome the high detent torque those large motors have. When calculating resolution with steppers, 1/2 step is the best that you should hope to achieve.

Personally, I think a machine like that would really need something like 1Kw AC servos. Expect to spend at least $2000-$2500 for three.

As for cutting speeds. I run a Morbidelli at work, and we cut at 1000+ipm every day. Rapids much faster. But this is cutting sheet goods, plywood, MDF, particle board.
Hardwoods are not cut as fast. I typically will cut hardwoods at 300-500ipm with 1/4" and 3/8" bits.

If you're not cutting at these speeds, then you're not taking advantage of your machine and spindle.
One thing that most people don't consider, is that it can be very difficult to hold materials in place at high cutting speeds. My hardwood speeds are limited by my vacuum holding power. If the hardwoods are not held tightly enough, cut quality will suffer, and you'll need to cut slower to get acceptable quality. Slower = shorter tool life.

[JAZZCNC]

Ok the bottom line is that large steppers need Mains voltage Drives to get any speed from them. Like Ger says 110v will only get you limited speed.

If using R&P then you 100% will need Servos because they are so much less efficient than ballscrews so with heavy gantry like this you will need really high torque motors and Large steppers just won't give the speed you'll need.

Servos from China are now so low that it's No brainer to me that you need to look east.! Approx $400 per set drive/mootr/cables.

However be aware that you'll also need match the rest of the electronics to match so you'll need an external motion control card with high pulse frequency to get the full speed/smoothness of the servos.

[Scott Damman]

Gerry, those are the ones, I have come to that conclusion as well. I think I will re-list those and find some servos.

Now do I keep a stepper for the Z (a smaller stepper not these big honking ones)? I would lean toward 2 servos on the X and 1 on the Y. Then I am thinking a smooth stepper in between it looks like that is fast enough to run the servos...

I know that my table is still going to be vacuum limited even with 8hp of regenerative vacuum. I plan to have 4 zones and also thinking about ways to include T-slot for hold down of hardwood parts.

Jazz, Im coming up empty on sets that are $400 im finding ~$600 for 1kw AC servo with driver. If you have any links to what you are talking about that would be greatly appreciated.

Im really leaning toward running a powered ballnut/ballscrew setup for the X, like Boyan said most builds here in the states use R&P but the 0 backlash and less wear aspect of the ballscrew is really appealing. Learning as much as I can from this forum and how other people implemented it. Im sure I will have questions...

A few updates, I dont love the look of the legs they look out of proportion to the rest of the machine but its steel that I already had on hand from other projects and its plenty strong with bracing. It gets a 3rd leg just havent gotten that far yet. Lots of bracing. I am trying to keep the one end open so I can use some of the space under the machine for rough sawn lumber storage...




Thanks for all the help so far! opened my eyes to some issues.

Just for fun I took a quick vid of me manually moving the gantry, its amazing with the linear guides allow...~move 500lbs with just fingers.

[Ger21]

I'm building a 4x8 machine and using AC servos, and I'm using steppers and Leadshine AM882 drives for the Z axis (two, it's a dual Z machine) to save money.

I would think that a 5mm pitch ballscrew and a 600oz stepper would be OK for your machine? That's just a guess, though.

As for $400 servos. I've seen 400W servos on AliExpress for around $400, but not larger ones. And at that price, you may not know exactly what you're getting. I'm paying around $575 for 400w Leadshine motors and drives for my machine. 750W and 1Kw are closer to $800 a set. The price difference between 750W and 1Kw is very small, so I'd definitely go with larger ones for a little more money. http://americanmotiontech.com/motor-...tage-ac-input/

As for vacuum, you can never have enough. We have two 10HP Becker's on our Morbidelli with a 5x12 table, and could always use a bit more.

[Scott Damman]

I'm building a 4x8 machine and using AC servos, and I'm using steppers and Leadshine AM882 drives for the Z axis (two, it's a dual Z machine) to save money.

I would think that a 5mm pitch ballscrew and a 600oz stepper would be OK for your machine? That's just a guess, though.

As for $400 servos. I've seen 400W servos on AliExpress for around $400, but not larger ones. And at that price, you may not know exactly what you're getting. I'm paying around $575 for 400w Leadshine motors and drives for my machine. 750W and 1Kw are closer to $800 a set. The price difference between 750W and 1Kw is very small, so I'd definitely go with larger ones for a little more money. http://americanmotiontech.com/motor-...tage-ac-input/

As for vacuum, you can never have enough. We have two 10HP Becker's on our Morbidelli with a 5x12 table, and could always use a bit more.

Gerry do you have a forum for your build? I tried to do a search and didnt come up with anything.

The vacuum does worry me, I may have to add more or sell these I have an get a single larger unit... But we will see I think for now I will be able to work around the vacuum pumps that I have and see how things are.

Our friend Fred from BST Automation has 1kw servo motor + drive for 332euro +shipping which will cost you in total~400$ for the combo when you buy 4.


I am not sure you need exactly 1kw if you go with rotating ball nut. But as prices are similar. I think you will need 0.75kw geared like mine 20t:30t and that will give you speed of 20000mm/min on the long axis/ball screw with rotating nut/.

Ok I had came those on ali but like Gerry said I had no idea what I was looking at or what I would be getting. So having someone point them out is very helpful. For the cables, do you just make up longer cables or order specific lengths? 3m wont make it for the far side X motor. How have you found the shipping to be? I see it says 23 days... I wonder if thats pretty accurate or if they come faster than that.

The other motors/drives I came across was dynamic motor motion technology.

I need to dig deeper into your build Boyan and see how to built those ballnut drives. I found a few options online and on the forum here for how to build them.

Have your guys seen these
http://www.stober.com/motion_control/EZM_series/

I havent priced them and I know they cant be geared so that has to be taken into account but seems like a great way to run the ballnut.

I hope to have a plan here in the next couple days then I can run it by you all for advice before ordering everything up...

[Boyan Silyavski]

Our friend Fred from BST Automation has 1kw servo motor + drive for 332euro +shipping which will cost you in total~400$ for the combo when you buy 4.


I am not sure you need exactly 1kw if you go with rotating ball nut. But as prices are similar. I think you will need 0.75kw geared like mine 20t:30t and that will give you speed of 20000mm/min on the long axis/ball screw with rotating nut/.