Thanks for your opinions, BTW. Cost is really a motivator for me... First, because I don't want to spend a lot. Second, because more people would be interested in what I'm doing if they can afford it too...
I didn't think about front panel cutting, that's actually a useful application (if it would work). Even plastic panels would be interesting.
Given what you hope to achieve I think it is a reasonable bet, have a look around at other options and you will have an even better idea about what is available and how good a deal this is.
Engraving is even more doable because generally the forces will be lower, especially if you are thinking about front panels etc where it is quite light. Black anodized ali looks particularly nice.
I have some experience with machine tools and a can not imagine, that a PVC based machine of this size will not be floppy. Machining with a mill and stepper motors will excite every possible resonance and the material is soft, light weighted and not damping. But ok, to make this experience, it is great. A CNC machine has do be stiff, as the chips are very small and every little vibration will change the chip size, the cutting force changing orders of value and the result will be very poor. But it depends on your expectations. But here we discuss, propose and exchange info, so every contribution is valuable.
My plans are now: I ordered a "Mendel" and hope it's running end of march. Than: replace the stepper drivers by a prop control microstepping software solution. Than replace the controller by a propeller. So building up the functionalities step by step, always taking care to have a running system. An then, lets see how to change the other components and create a great solution.
You asked for my opinion so I will provide it, but please keep in mind I am not a big fan of buying tools that "look like a tool".
The price is reasonable at somewhere between $300 and $600 depending on options.
The machine is made out of plastic - while I am a fan of plastic, it has no place as a machine tool frame - nor does MDF (glued together sawdust). If all you are cutting is plastics and perhaps other soft materials likes soft-woods, okay. Your references include cutting metal (aluminum).
The DC motor/spindle kit is a neat kit, if all you want to do is drilling. Any machinist anywhere in the world will tell you not to use a drill chuck to hold an end mill and to mill with it. It would work but the performance would be terrible and the precision would be terrible. If all you want to do is drill, it might be good enough, but that depends on the run-out at the tool. If drilling with carbide drills (PCB drilling), those tools have to have very minimal runout to prevent breakage.
Someone else quoted .004" resolution which I suspect is very doable in a static test (eliminating machine deflection due to cutting forces).
Maximum feedrate without stalling relates to two different situations. Maximum velocity for rapid moves or maximum velocity for cutting moves. Rapid velocity is not critical for non production machines so lets exclude that concern. Maximum velocity while cutting is the more important spec. I looked over the web site briefly to see if I could find any test cut data provided and didn't find anything. I suspect they have not published any performance data which seems strange because the website is very nice.
You mentioned you would like to mill aluminum with a 10MM (3/8") tool bit. You would need to have a pretty big and solid spindle for that. I would agree with your estimated assesment of what you would have to do with the cutting conditions to cut aluminum or even brass - very light cuts, very low feed rates. This would be driven by all factors of the machine :spindle, drive system, and frame materials.
Your comment, quality has its price is certainly relevant. To me, I look at a lot of these DIY or hobbiest machines as things that look like their real counter part. I place many Chinese made tools in that same category, it looks like a tool but it doesn't perform like a tool. So, if a person is happy to spend their money on something that looks like what they want and if they don't know the difference between a real one and a toy one, then it could be a good value.
This machine in particular would be fine for some machining of very soft materials and I suspect it would have resonable accuracy. As a 3D printer, I don't see why it wouldn't work, there are no stresses on the mechanics to perform that function. Personally I am a designer builder kind of guy so I naturally am not drawn to a kit, furthermore, if I was to purchase a kit I would want it to be better than anything I could make and this certainly doesn't fit those parameters for me.
If I understand right the $79 is just replacing some parts of the 7x7 CNC kit for a bigger axle distances.
A mechanical hardware kit for three axis including three stepper-motors for $330 is still a low price.
Now the most interesting question are:
- What is the maximum resolution?
- How big is the play in the axles if the direction is changed? (I assume that the axles are NOT build from play-FREE ballscrews at this price)
- What is the maximum feedrate without stalling the steppermotors?
- How does this feedrate go down if the infeed-motion goes up?
(stepper-motors with size NEMA 17 are quite small and the torque is limitied. Even if it is a 1.3A motor which is quite big for the size NEMA 17
- How does the precision of the mechanic go down (caused through bending of the frame )if bigger forces are applied when using bigger steppermotors?
I would estimate if I would like to mill aluminium with teh following parameters:
- milling-tool diameter 10mm
- a infeed-motion of 2mm
- feed-speed of 100 mm per minute the whole thing will bend that the
precision will go down to 0,5 mm. Which will be useless in most cases.
So what you have to to is using parameters like this:
- milling-tool diameter 3mm
- a infeed-motion of 0,5mm
- feed-speed of 20 mm per minute
to keep the forces low that the frame will not bend to much to keep a precision of 0,08 mm to 0,1 mm.
If you compare the two parametersets it is easy to see that the productivity goes down a lot with this machine.
I want to emphasise that this a RAW estimations from me as a guy who has only few experience with CNC-machines
Maybe the things are much better than I estimate. So this is why I'm really highly interested to get hard information.
The website of ZEN Toolworks does not mention anything about precision. I INTERPRET this that the precision is
quite low compared to CNC-machines at a pricelevel of $1500 to $2000.
So an old saying is still true here: Quality has its price
best regards
Stefan
Maybe Chris would like to tell his opinion about that.
In order to have action, you have to have a plan. So far there has been no consenses on what this thing should do, how big it should be, what the budget should be. So far, ErNa has about the best plan with buying the existing machine and working from that baseline. You sort of offered your mechanical design for your machine but nothing more was provided on that. My design won't work because it requires a lot of machined components and would be too expensive to build.
Well, I'm going for the Zen kit and will try to mount the Makerbot extruder head to it. I showed my wife the Thing-O-Matic and she wasn't impressed by the overall appearence. At least the Zen kit has a serious look to it, even if it is plastic...
If I were to later remake the Zen kit parts out of aluminum, would that make it better?
Or, does it need to be steel?
Generally I'm with Chris and ErNa, I don't see it as a great machine when compared to a lot of commercial CNC machines or even better home builds. However when I think back to making model aircraft from balsa wood, I would transfer designs on to wood and cut out with a knife, if I had had this machine my parts would have been far more accurate. This is why you start with the part required and work back to the machine rather than just be a perfectionist.
For 3D printing it is probably fine, for machining it will depend on what you expect, to have fun with light materials and learn a lot it will be fine too.
And yes it can be improved, even keeping with plastic it would be improved, remember a U section is stiffer than a flat and a box section even better.
I'd also agree with Chris about MDF if it were not for the results I have seen from various machines, it is actually a very stable material and easy to machine.
Well, I'm going for the Zen kit and will try to mount the Makerbot extruder head to it. I showed my wife the Thing-O-Matic and she wasn't impressed by the overall appearence. At least the Zen kit has a serious look to it, even if it is plastic...
If I were to later remake the Zen kit parts out of aluminum, would that make it better?
Or, does it need to be steel?
'
I just ordered the little Zen.It looks perfect for the novice to experiment with.Small, lite, hopefully quiet, This I can use on my computer desk.
'
I started to build a little guy like this to use on my desk,The steppers alone would have cost me $80 bucks.
'
Great Find!
...so far absolutely nothing has happened except for a rise in air temperature ... just a bit frustrated.
Every minute spent in the planning phase saves (roughly) an hour of re-work in the later phases.
Requirements definition is my thing, we we still have more divergence than concensus, so I don't think its time (for me) to start building yet.
It can be costly to underestimate the task at hand.
Every minute spent in the planning phase saves (roughly) an hour of re-work in the later phases.
Requirements definition is my thing, we we still have more divergence than concensus, so I don't think its time (for me) to start building yet.
It can be costly to underestimate the task at hand.
'
I agree
'
I can write code all day long But with out testing it on a Hardware platform,What good is it.
'
Like I tell my Bosses at work, Its poo (parts on order)
One think I don't get: The motors appear to be rated for 2.8 VDC. So, why do they power it with a 12 VDC power supply? I don't get it...
Usually a motor controller uses switching. With this technic they control the motor current which results in motor torque.
See http://de.wikipedia.org/wiki/Vierquadrantensteller
( unfortunatelly in Geman, the English explanation is not detailed enough)
2.8V is meant for DC, which usually does not occur during switching. Steady-ON will destroy the motor or the driver due to the fact that the current would become to high.
Well, again: DC Voltage for stepping motors is meaningless as long as you don't want to drive them with DC.
More important is the motor current. As Saphia wrote: 1.3A
Rated stepper voltage is the minimum voltage, not the maximum voltage. As stated above, the main rating is the current rating. Stepper motors operate best when operated at 5 - 10X the rated voltage, but current must be limited to the rated current.
Sorry Guys, I was just joking, didn't mean to ruffle a bunch of feathers
Improper planning would be costly. And I agree that in order to build a machine as a team, there must be an agreement on a base design.
@prof_braino - I changed my mind, you'll know what I am talking about.
Okay guys, have any of you considered a design similar to the photos of the 3 axis machine I posted? If so, it would be inexpensive to build. Additionally, I could help out with some of the design charactersitics.
The current rating and voltage rating on a stepper motor are for the "stopped" case. Steppers by design are intended to move at various speeds, including stand still. When standing still, they do not generate any back-EMF. In that case ohms law takes over, and the current in the energized winding(s) simply becomes the alppledr voltage, divided by the winding resistance.
When the motor is moving however, back-EMF generated in the winding due to the motion becomes a factor, counteracting the applied voltage; like a regular motor. Effectively the winding sees a lowered voltage, and hence a lowered current. The faster it moves, the larger the back-EMF; hence while turning, the voltage applied may be raised to counteract the back-EMF.
Another thing also takes place. When applying voltage to a winding in order to step it to a new position, that winding initially has no current flowing in it.... at least if it was not being "microstepped" by a controller. The self inductance of the winding resists build-up of the current which rises linearly..... at least while the core does not saturate. In order to get fast stepping speeds the current must be made to rise from zero very quickly, and so the reactance of the inductance must be overcome, Normally this is achieved by applying a much higher voltage to the winding than what it could tolerate when standing still. At stand-still you have resistance, and when switching you have the inductive reactance added to (and much larger than) the DC winding resistance.
As a point of reference, I apply up to 30 Volts to a 2..6 Volt rated stepped to drive my CNC axis WHILE IT IS SWITCHING. I keep an eye on the peak current, and don't let that rise too much over the rated 1.9A average spec. Remember the initial current is zero, and rises linearly that is if you don't have resistors in the drive lines. At varying stepping speeds I use PDM (Pulse Density Modulation) to control the current I want at the particular step rate. This gets more complex as the motor accelerates and decelerates, requiring varying currents according to speed. Commercial drivers would usually take care of all that.
But I want to build my own using simple inexpensive power mosfets, so I'm doing a LOT of investigating. And interestingly have discovered that at SLOW speeds, when using standard PWM to control the winding current, the stepper will move forward as expected, but then bounce right back when the the current is shut off for the idle period of the PWM cycle. My theory is that during this idle period, the machnetic field collapes and introduces a current flow through the "back diode" inside the mosfet of the other half of the winding of the unipolar motor. That is while not using any half-stepping technique, energizing more than one winding at at time. At speeds faster than very slow, the inertia of the rotor and attached mechanics tends to pull the armature through this "reversing bump".
So, lots more to learn, as it appears steppers are not as easy to drive as one would have thought. At least that is to drive them well.
I would think that the commercial drive manufacturers have all this well under control, but if I bought those, then I would not learn as much.
I think it would be worth establishing if many people want a commonly designed machine or would prefer to just go their own way with lots of support and some common goals and interests (the propeller). It seems a few have voted with their wallets already.
it appears steppers are not as easy to drive as one would have thought. At least that to drive them well.
After searching for a long time for a good h-bridge design which uses mosfets, I finally gave up. Before settling on the G251 drives from Gecko, I was seriously considering the L6208 from STMicroelectronics. I fear, that unless you are one of those electronic design kind of guys, you may end up with the same search results as me. Unless you are bound and determined to make it from mosfets, take a peek at the L6208.
Peter, I think I'll use the Toshiba TB6560 chip to do the microstepping, just like the controller that Zen sells.
But, controlling over parallel port is too 1980's for me, so I'll definitely use a Prop and USB to talk to a Windows interface.
I think you're right about the ratings, and if the motors really are 2.8 volts then driving with a 12 V supply is just rediculous...
So, I'll try a 5V supply. I've got the Zen in my Amazon shopping cart...
Perhaps I did not explain myself well enough...... to get any reasonable speed, I believe you will need to end up driving with a much higher voltage like 24 or 36 Volts.. But you must limit the maximum average current to what the motor is rated for. Another point, if you are microstepping you need many micro steps per full step, and more than one winding will be energized at the same time, so the total current will be higher. But I believe that the specified current is for a single winding as, really, in the final analysis, it is the current carrying capacity of the wire that determines the limit.
Peter, ok maybe I can see where you want higher voltage when moving the motor.
I'll have to look at the datasheet of the driver IC some more to see what makes the most sense...
What Bruce says is a good guide, 5 to 10X the rated voltage is good. The rated voltage just lets you get away with the most basic electronics (no current limiting of any kind) but performance is lack lustre or worse.
I think Peter has explained why it is OK to drive at higher voltages rather well, he probably feels he knows a little too much about this subject now
But I REPEAT, unless you want to do it as an exercise consider buying the stepper drivers, they are dirt cheap these days and work really well.
There is a really good discussion on the reprap forums about stepper motors. For a small CNC or 3D printer, smaller Nema17 can be used which require less than 2A. An allegro chip controlls this nicely. Polulu (forget their exact name) have a nice little pcb but there is a better Allegro chip that I am going to use (has better inbuilt protection). IIRC I cannot use the same footpront pcb because I am not using the QFN part.
Comments
I didn't think about front panel cutting, that's actually a useful application (if it would work). Even plastic panels would be interesting.
Any chance it could do engraving?
Given what you hope to achieve I think it is a reasonable bet, have a look around at other options and you will have an even better idea about what is available and how good a deal this is.
Engraving is even more doable because generally the forces will be lower, especially if you are thinking about front panels etc where it is quite light. Black anodized ali looks particularly nice.
Graham
My plans are now: I ordered a "Mendel" and hope it's running end of march. Than: replace the stepper drivers by a prop control microstepping software solution. Than replace the controller by a propeller. So building up the functionalities step by step, always taking care to have a running system. An then, lets see how to change the other components and create a great solution.
You asked for my opinion so I will provide it, but please keep in mind I am not a big fan of buying tools that "look like a tool".
The price is reasonable at somewhere between $300 and $600 depending on options.
The machine is made out of plastic - while I am a fan of plastic, it has no place as a machine tool frame - nor does MDF (glued together sawdust). If all you are cutting is plastics and perhaps other soft materials likes soft-woods, okay. Your references include cutting metal (aluminum).
The DC motor/spindle kit is a neat kit, if all you want to do is drilling. Any machinist anywhere in the world will tell you not to use a drill chuck to hold an end mill and to mill with it. It would work but the performance would be terrible and the precision would be terrible. If all you want to do is drill, it might be good enough, but that depends on the run-out at the tool. If drilling with carbide drills (PCB drilling), those tools have to have very minimal runout to prevent breakage.
Someone else quoted .004" resolution which I suspect is very doable in a static test (eliminating machine deflection due to cutting forces).
Maximum feedrate without stalling relates to two different situations. Maximum velocity for rapid moves or maximum velocity for cutting moves. Rapid velocity is not critical for non production machines so lets exclude that concern. Maximum velocity while cutting is the more important spec. I looked over the web site briefly to see if I could find any test cut data provided and didn't find anything. I suspect they have not published any performance data which seems strange because the website is very nice.
You mentioned you would like to mill aluminum with a 10MM (3/8") tool bit. You would need to have a pretty big and solid spindle for that. I would agree with your estimated assesment of what you would have to do with the cutting conditions to cut aluminum or even brass - very light cuts, very low feed rates. This would be driven by all factors of the machine :spindle, drive system, and frame materials.
Your comment, quality has its price is certainly relevant. To me, I look at a lot of these DIY or hobbiest machines as things that look like their real counter part. I place many Chinese made tools in that same category, it looks like a tool but it doesn't perform like a tool. So, if a person is happy to spend their money on something that looks like what they want and if they don't know the difference between a real one and a toy one, then it could be a good value.
This machine in particular would be fine for some machining of very soft materials and I suspect it would have resonable accuracy. As a 3D printer, I don't see why it wouldn't work, there are no stresses on the mechanics to perform that function. Personally I am a designer builder kind of guy so I naturally am not drawn to a kit, furthermore, if I was to purchase a kit I would want it to be better than anything I could make and this certainly doesn't fit those parameters for me.
Just my 2 cents worth
Chris
And a long departed friend of mine, occassionally told me:
Let's see some action in this thread!
Chris
If I were to later remake the Zen kit parts out of aluminum, would that make it better?
Or, does it need to be steel?
Generally I'm with Chris and ErNa, I don't see it as a great machine when compared to a lot of commercial CNC machines or even better home builds. However when I think back to making model aircraft from balsa wood, I would transfer designs on to wood and cut out with a knife, if I had had this machine my parts would have been far more accurate. This is why you start with the part required and work back to the machine rather than just be a perfectionist.
For 3D printing it is probably fine, for machining it will depend on what you expect, to have fun with light materials and learn a lot it will be fine too.
And yes it can be improved, even keeping with plastic it would be improved, remember a U section is stiffer than a flat and a box section even better.
I'd also agree with Chris about MDF if it were not for the results I have seen from various machines, it is actually a very stable material and easy to machine.
Graham
I just ordered the little Zen.It looks perfect for the novice to experiment with.Small, lite, hopefully quiet, This I can use on my computer desk.
'
I started to build a little guy like this to use on my desk,The steppers alone would have cost me $80 bucks.
'
Great Find!
Every minute spent in the planning phase saves (roughly) an hour of re-work in the later phases.
Requirements definition is my thing, we we still have more divergence than concensus, so I don't think its time (for me) to start building yet.
It can be costly to underestimate the task at hand.
'
I agree
'
I can write code all day long But with out testing it on a Hardware platform,What good is it.
'
Like I tell my Bosses at work, Its poo (parts on order)
One think I don't get: The motors appear to be rated for 2.8 VDC. So, why do they power it with a 12 VDC power supply? I don't get it...
Usually a motor controller uses switching. With this technic they control the motor current which results in motor torque.
See http://de.wikipedia.org/wiki/Vierquadrantensteller
( unfortunatelly in Geman, the English explanation is not detailed enough)
2.8V is meant for DC, which usually does not occur during switching. Steady-ON will destroy the motor or the driver due to the fact that the current would become to high.
This is Page with spec of Steeper Motors --- I can't se in any place 2.8 VDC spec
On ZEN page: New Nema 17 Stepper Motor included, 2 Phase, 4 Wires, 1.3A = 4518S-02 - On LIN Engineering Page
http://wiki.zentoolworks.com/index.php/Nema_17_Stepping_Motor
It is NOT same Stepper Motor them use. now.
Look on "LIN Engineering" I posted before -- It is that one them use now.
More important is the motor current. As Saphia wrote: 1.3A
Rated stepper voltage is the minimum voltage, not the maximum voltage. As stated above, the main rating is the current rating. Stepper motors operate best when operated at 5 - 10X the rated voltage, but current must be limited to the rated current.
Bruce
http://www.devilmaster.org/sections.php?op=viewarticle&artid=40
It's in spanish and translation from spanish to english with babelfish is a mess, but the article has tons of good photos.
I think it's a good design and allows some modifications as needed, for example a bigger table or a higher portal ....
Did not find infos on price or accuracy ... but I love it ... ;o)
I agree, that machine in Post 46 looks pretty sweet.
Bruce
Improper planning would be costly. And I agree that in order to build a machine as a team, there must be an agreement on a base design.
@prof_braino - I changed my mind, you'll know what I am talking about.
Okay guys, have any of you considered a design similar to the photos of the 3 axis machine I posted? If so, it would be inexpensive to build. Additionally, I could help out with some of the design charactersitics.
Bruce
The current rating and voltage rating on a stepper motor are for the "stopped" case. Steppers by design are intended to move at various speeds, including stand still. When standing still, they do not generate any back-EMF. In that case ohms law takes over, and the current in the energized winding(s) simply becomes the alppledr voltage, divided by the winding resistance.
When the motor is moving however, back-EMF generated in the winding due to the motion becomes a factor, counteracting the applied voltage; like a regular motor. Effectively the winding sees a lowered voltage, and hence a lowered current. The faster it moves, the larger the back-EMF; hence while turning, the voltage applied may be raised to counteract the back-EMF.
Another thing also takes place. When applying voltage to a winding in order to step it to a new position, that winding initially has no current flowing in it.... at least if it was not being "microstepped" by a controller. The self inductance of the winding resists build-up of the current which rises linearly..... at least while the core does not saturate. In order to get fast stepping speeds the current must be made to rise from zero very quickly, and so the reactance of the inductance must be overcome, Normally this is achieved by applying a much higher voltage to the winding than what it could tolerate when standing still. At stand-still you have resistance, and when switching you have the inductive reactance added to (and much larger than) the DC winding resistance.
As a point of reference, I apply up to 30 Volts to a 2..6 Volt rated stepped to drive my CNC axis WHILE IT IS SWITCHING. I keep an eye on the peak current, and don't let that rise too much over the rated 1.9A average spec. Remember the initial current is zero, and rises linearly that is if you don't have resistors in the drive lines. At varying stepping speeds I use PDM (Pulse Density Modulation) to control the current I want at the particular step rate. This gets more complex as the motor accelerates and decelerates, requiring varying currents according to speed. Commercial drivers would usually take care of all that.
But I want to build my own using simple inexpensive power mosfets, so I'm doing a LOT of investigating. And interestingly have discovered that at SLOW speeds, when using standard PWM to control the winding current, the stepper will move forward as expected, but then bounce right back when the the current is shut off for the idle period of the PWM cycle. My theory is that during this idle period, the machnetic field collapes and introduces a current flow through the "back diode" inside the mosfet of the other half of the winding of the unipolar motor. That is while not using any half-stepping technique, energizing more than one winding at at time. At speeds faster than very slow, the inertia of the rotor and attached mechanics tends to pull the armature through this "reversing bump".
So, lots more to learn, as it appears steppers are not as easy to drive as one would have thought. At least that is to drive them well.
I would think that the commercial drive manufacturers have all this well under control, but if I bought those, then I would not learn as much.
Have fun with your Zen !!
Cheers,
Peter (pjv)
Graham
After searching for a long time for a good h-bridge design which uses mosfets, I finally gave up. Before settling on the G251 drives from Gecko, I was seriously considering the L6208 from STMicroelectronics. I fear, that unless you are one of those electronic design kind of guys, you may end up with the same search results as me. Unless you are bound and determined to make it from mosfets, take a peek at the L6208.
Bruce
But, controlling over parallel port is too 1980's for me, so I'll definitely use a Prop and USB to talk to a Windows interface.
I think you're right about the ratings, and if the motors really are 2.8 volts then driving with a 12 V supply is just rediculous...
So, I'll try a 5V supply. I've got the Zen in my Amazon shopping cart...
Perhaps I did not explain myself well enough...... to get any reasonable speed, I believe you will need to end up driving with a much higher voltage like 24 or 36 Volts.. But you must limit the maximum average current to what the motor is rated for. Another point, if you are microstepping you need many micro steps per full step, and more than one winding will be energized at the same time, so the total current will be higher. But I believe that the specified current is for a single winding as, really, in the final analysis, it is the current carrying capacity of the wire that determines the limit.
Cheers,
Peter (pjv)
I'll have to look at the datasheet of the driver IC some more to see what makes the most sense...
I think Peter has explained why it is OK to drive at higher voltages rather well, he probably feels he knows a little too much about this subject now
But I REPEAT, unless you want to do it as an exercise consider buying the stepper drivers, they are dirt cheap these days and work really well.
Graham
However, for a real machine, 2A is not enough.