First Build 5 x 12 Steel Frame CNC router

[Scott Damman]

Very ambitious for a first build.

Some thoughts:

-the gantry legs dont seem adequate for the rest of the parts you will use on your machine

-the A may work for foam but i dont see it serious for anything more than wood

-the stepper motors , ok- the huge stepper motors, what drives will power them? 230VAC?

-that bracing may be was totally unnecessary. I dont see how that will strengthen anything. It could even make things worse, strengthening one point too much and not allowing the forces to spread uniformly and creating pressure points. So later all rings.


What will you be using this machine for? Remember- the weakest point is the criteria for how strong your machine would be. Again-the gantry legs.


PS:
That gantry legs will greatly benefit from bolting or welding on them 10mm steel plates in both directions, 2 per leg in total

Hi Scott,

This is how I see couple of not so little things.

Steppers:
Really Cannot comment if they will be good enough because you haven't said anything about what your using for linear motion on the X axis. I assume Rack n pinion but what ratio etc.? . . . But let me put it this way 550lbs or put anohter way 1/4 Ton is lot of Mass to Accel and stop so better get this correct or you'll seriously regret using steppers.


A Axis: You need to think again on this I'm afraid NO WAY will Stepper with such small ratio hold that Big spindle at angle without moving or vibrating.
Remember the Z axis is the MOST important area on machine so weaken this and all that OTT work you have done on the frame is pointless.

I think you are talking something along these lines for the sides of the gantry legs? I was wanting to do FEA on these legs but never got around to it, I did however do calculations (free body diagram) to try to calculate total deflection based on 1000lb force at the bit and keeping the total deflection ~3 thou at full load.

I could go higher with the bracing as well this is just a quick drawing.


The thinking behind the bracing was to connect the upper and lower gantry beams. I used box tubing between the beams. Now the rear bracing, the thought there was to reduce the twist in the gantry. Hopefully it doesnt cause issues...

So the A axis, this is exactly what I am worried about, holding it in place when at angle and also moving it. The A axis would really only be used when machining foam blanks for paddle boards. However since I want to be able to machine wood, and aluminum as well then I really need to make sure this thing is right.

Couple things I am thinking thoughts are greatly appreciated.
- Eliminate A Axis...
- greatly increase the belt ratio
- keep A Axis but have an air operated taper pin that holds it at 0 when machining wood/alum
- look into a harmonic drive rather than belt drive

X drive
I was planning on using rack and pinion with a pinion of 1 inch I was going for 4:1 ratio to the pinion.
so 3.14 inches per revolution of the pinion, 0.785 inches per motor revolution, works out to 0.0039 inches per step. I was thinking of micro stepping them x10 = 0.000395inch per micro step.

I am brand new to this and trying to learn what feed and rapid movements I need to shoot for to cut wood/aluminum properly. I know it depends on the number of flutes on the bit etc. Am I crazy to think I should shoot for 1000ipm?

The motors I have are chinese 1841 oz/in with MA860H drivers along with 60V drives. Based on the questions it sounds like these are not adequate?

Servos
So if scrap the steppers for X axis then im looking at servos. Something I am even more NEW to than steppers. I need to do more learning on how to size them please correct where I have errors!

Things to keep in mind when sizing?
Gear ratio - want to keep the servo motor spinning at higher speed to stay in its power range - unlike steppers which have the most power at 0 speed
Load to motor inertia, this one I have seen 10:1 ratio but I dont really understand how to use that

One thing I dont understand, when X is not moving what is holding if from moving? I had assumed that the steppers hold the load from moving at 0 speed (is this the case with steppers?). If that is the case what is doing the holding when using servos?

Can my router have both steppers and servo motors? I think the stepper would still be adequate for the Y Axis and could use one for the Z axis (as Im sure I can not send them back)... I plan to use ethernet smooth stepper with mach4.

Got the X axis steel drilled these needed to be through drilled as well since the linear rails are bolted from the back. 244 holes!!!!



Any help with servos for this size of gantry is greatly appreciated.

[mekanik]

Hi Scott
I don't have any experience of the 4/5 axis machines but i would imagine that the "A" axis you are trying to emulate would be a wheel & pinion mechanism.
Regards
Mike

[Boyan Silyavski]

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.

[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.