First Build 5 x 12 Steel Frame CNC router

[routercnc]

Hi Scott,

Just to scale point #3 from Boyan - 3Nm Nema 23 at 70VDC could be used up to gantry weight of 50-70kg tops. As yours is 5 times this weight the stepper / driver combo needs careful thought.

I can sort of see the machine you are trying to make by piecing all the photo together - and it looks like a commercial wood router with raised gantry.

If you want to cut aluminium as well I would suggest you chop the legs off the gantry, and go for a raised X axis instead. Lift the X axis members up on supports away from the bed until they meet the bottom of the gantry. This makes a much stiffer machine for little effort. Your span is quite wide so you will need all the help you can get for aluminium.

Can you post up a picture of the overall machine design?

[Scott Damman]

Ok, lets start from the beginning. I will try to resume some things, you just need to further research the topics:

1. You should absolutely know what weight your gantry is with all things mounted. Knowing the mass of the gantry will be decisive on what motors, what size ball screw, rotating nut, Rack and pinion, etc..

2. Be realistic about the feeds and speeds. And what you really need the machine for. All in one would be usually a disaster in some area.

3. Motor sizing. People drive 3nm nema 23 motors at 70VDC. And we are talking low inductance motors. Expected reliable performance from such motors coupled with xx10 ball screws is 10000mm/min. The motors you have are high inductance motors. I am not a specialist here but there are a lot of knowledgeable people here that will tell you at what voltage, but i am sure that it must be more than 110VDC. Do your research and find at what voltage these motors perform well. And if these drives will be ok.

4.Without knowing the total weight of the gantry... but i see you start ambitious, so yes, you will need servos to drive the gantry reliably fast if you aim at 25000mm/min. And you will need rotating ball nut or rack and pinion

5. There is a very well made calculator at the forum here, search for it, and i am sure that the gantry legs deflection will not be as you say...but may be i am wrong




I dont want to discourage you, but i am having the feeling that if you continue without thinking it too much, there will be disappointments on the way. WHY JUST DONT YOU SLOW A BIT AND READ MORE BUILD LOGS. All the info is there.

And by the way it seems the american thing is to desire rapids of 1000ipm. And make rack and pinion machines. We here in EU prefer precise ball screw machines and rotating ball nuts for big machines and heavy gantries.

The gantry will weight around 550lbs, this number comes from the 3d drawing which worked out to 494 (this included the Z axis and the spindle) and I added 50 lbs for cables and cable routing. This was my bassis for sizing the motors. Sorry I thought I had been clear on it earlier in my thread about the weight.

Feeds I am happy to be realistic and dont care much about super fast rapids but I want to be able to drive the machine at proper cutting speeds.

Im sorry I came across as I jumped into this without any research when the opposite is true, I have done lots and lots of research both on forums and by inspecting commercial machines. I would not have been able to get to the point I am without lots of learning from all sources. I find there a very different views on most subjects associated with the gantry style cnc router so its hard to nail down whats the best design. And thats where some of my questions came from. The stepper route was more of a cost thing and I should have done more research on servos before going down that road.

I will find the calculator you speak of and give it a try. I just started out by doing the math, the hard part was actually finding out how much force would be applied at the tool so I could back calculate the flex at the gantry.

**I want to say machining Aluminum is a bonus for this machine as it will mainly be used for wood**

Hi Scott,

Just to scale point #3 from Boyan - 3Nm Nema 23 at 70VDC could be used up to gantry weight of 50-70kg tops. As yours is 5 times this weight the stepper / driver combo needs careful thought.

I can sort of see the machine you are trying to make by piecing all the photo together - and it looks like a commercial wood router with raised gantry.

If you want to cut aluminium as well I would suggest you chop the legs off the gantry, and go for a raised X axis instead. Lift the X axis members up on supports away from the bed until they meet the bottom of the gantry. This makes a much stiffer machine for little effort. Your span is quite wide so you will need all the help you can get for aluminium.

Can you post up a picture of the overall machine design?

I looked carefully at doing a bridge style machine with high sides, the problem that kept running into this design was not easily being able to side load the machine. I do alot of work with large wood slabs which this machine will be used to flatten and I need to be able to side loads these as I dont have an overhead crane. (they weigh in excess of 1200 lbs)

Here is the Frame starting, I have blocked up the frame to level and used the gantry to set the exact distance.


Initially checking square with a precision square, final squaring will need to be done once its all welded up and before pour epoxy.

[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.

[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/.

[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...