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Bill Gruby
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My bad Steve. I was answering a PM at just before I posted. I fixed it. Not waitin on anybody in particular. Jump righnt in there.
"Billy G" :thinking:
"Billy G" :thinking:
I guess I'll add a little onto 7HCs topic of couplers, including when you need them.
You want a coupler if you are using a motor to direct drive a screw of some sort (acme, ball, etc). They allow some miss-alignment between the screw and motor to exist without destroying the bearings in your motor or in the bearing blocks supporting your screw, and without introducing much backlash. If you use a rigid coupling, and the screw and motor shaft are not _exactly_ aligned (exactly), then when you bolt everything in place, the combined screw/shaft/coupler assembly is going to "dog leg" (somethings gotta give). When your motor spins you are gonna get a "cachunkachunkachunk", which probably won't be audible at low speeds, but you bearings will feel it. You can get a sense of it if you turn the shaft by hand, you will feel the resistance increase and decrease as you turn the screw. So, long story short, if you are going to direct drive, get some couplers.
Other than the 2 types that 7HC mentioned, there is also a type called a bellows coupler. As far as cost go, they tend to go (from cheapest to most expensive) lovejoy < helical-beam < bellows, which corresponding improvements in backlash with increased price.
In most cases, if you are using stepper motors, you are going to want to direct drive. This is because steppers have a lot of low end torque, and pretty poor high speed torque. If you are going with servos, they have a higher top speed, and a lower max torque, but the torque, but the torque is more even over the rpm range. That means that with servo motors you are likely going to want to gear down with a belt system, IE, no couplers.
One case you may have a stepper motor and not want to direct drive is if you have a really heavy load, like a z axis. In this case you can get a huge motor like Steve's NEMA42, or you can get a more standard motor and gear down. You will get slower rapids in your Z, but everything is a tradeoff.
Other cases where you may be using steppers, but don't want to direct drive are if you are using a direct belt drive, such as on some 3d printers (Prusa Reprap X/Y axis). These are typically really low load situations (low torque) and high speed. Incidentally, this is also how a lot of inkjet printers work. They have a head driven by a stepper motor and a toothed belt. They typically ride against an incremental encoder, and there are home and limit switches so the thing knows where to start. Pop one open some time, and you will see what I mean, lots in common with a CNC machine.
You want a coupler if you are using a motor to direct drive a screw of some sort (acme, ball, etc). They allow some miss-alignment between the screw and motor to exist without destroying the bearings in your motor or in the bearing blocks supporting your screw, and without introducing much backlash. If you use a rigid coupling, and the screw and motor shaft are not _exactly_ aligned (exactly), then when you bolt everything in place, the combined screw/shaft/coupler assembly is going to "dog leg" (somethings gotta give). When your motor spins you are gonna get a "cachunkachunkachunk", which probably won't be audible at low speeds, but you bearings will feel it. You can get a sense of it if you turn the shaft by hand, you will feel the resistance increase and decrease as you turn the screw. So, long story short, if you are going to direct drive, get some couplers.
Other than the 2 types that 7HC mentioned, there is also a type called a bellows coupler. As far as cost go, they tend to go (from cheapest to most expensive) lovejoy < helical-beam < bellows, which corresponding improvements in backlash with increased price.
In most cases, if you are using stepper motors, you are going to want to direct drive. This is because steppers have a lot of low end torque, and pretty poor high speed torque. If you are going with servos, they have a higher top speed, and a lower max torque, but the torque, but the torque is more even over the rpm range. That means that with servo motors you are likely going to want to gear down with a belt system, IE, no couplers.
One case you may have a stepper motor and not want to direct drive is if you have a really heavy load, like a z axis. In this case you can get a huge motor like Steve's NEMA42, or you can get a more standard motor and gear down. You will get slower rapids in your Z, but everything is a tradeoff.
Other cases where you may be using steppers, but don't want to direct drive are if you are using a direct belt drive, such as on some 3d printers (Prusa Reprap X/Y axis). These are typically really low load situations (low torque) and high speed. Incidentally, this is also how a lot of inkjet printers work. They have a head driven by a stepper motor and a toothed belt. They typically ride against an incremental encoder, and there are home and limit switches so the thing knows where to start. Pop one open some time, and you will see what I mean, lots in common with a CNC machine.
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Bill Gruby
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Gotta take us back for a moment or two. I just went to the Surplus Center link and they have Stepper Motors with high AC voltages and what I would consider minimal torque. What would they be used for?
"Billy G"
http://www.surpluscenter.com/
"Billy G"
http://www.surpluscenter.com/
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I'll try a little on linear slides, ballscrews and acme screws
To be honest I didnt do much reading on items I can't afford except to note the differences.
Most people think the main reason for a ballscrew is backlash, while this is a reason to use ball screws it is not a feature of a ball screw that most hobby machinists can afford. So lets talk about friction first. The motors used for cnc are small and required to move very heavy loads in exact amounts. Friction is the biggest enemy. If you have ever tried to turn the handle of a machine that has set for a while you will feel a snap before the axis begines to move. The torque has to overcome the friction or adhesion that is between the surfaces before moving. This is everywhere in most manual machines. The thread mating in the nuts the dovetails mating to each other and the slides. Once this adhesion is overcome the motion becomes easier. Its the theory of motion things that are still want to stay still, things that are in motion want to continue moving. This plays hell on accuracy and the size of motor to overcome the initial friction/adhesion and a change in direction multiplies this as the mass has to be stopped then started to change direction (this happens fast but at one point it does stop ).
We overcome this with ball bearings. Bushings are mated surfaces rubbing against each other and the lube holds them apart so they slide on the thin film of lube instead of each other. These bearing surfaces are everywhere in a mill or lathe the dovetails are bearing surfaces and require lube to keep the two surfaces from dragging on each other. Ball bearings work different they do not rub the other part they roll against it always in contact at two points. If i put a ball on a table and put a book on top and move the book the ball will roll as i move the book not slide. There is far less surface area touching each other so there is less friction. Now if all the balls are in a straight line and "exactly" the same size they will all roll together and not against each other. Here is where precision bearings come into this, if one ball is the smallest amount larger than the rest of the balls it will catch up with the others and eventually end up pushing the entire line of balls along. This has the balls now touching in 4 locations top bottom front and back. The front and back of each ball is turning the opposite direction of the ball it is mated against so the friction and heat doubles. So ball size is very important
The surfaces the balls roll on have to be perfect also, a high spot on the table and a book that cannot be moved up leaves a tight spot in the bearing surfaces. So the balls have to be no bigger than this point in their path. Everywhere else they will be a loose fit. This is the reason for precision grinding a ball screw and it's nuts internal route for the balls to roll in.
To make the ballscrew antibacklash all the parts must be in contact at all times. but not ball against ball and they continue to roll along the thread including a passage to return them back to where they started from in the thread.
Ball slides work exactly the same way and the balls roll along the two mated surfaces and are returned through a passage to start over.
The cost of real precision ballscrews and ball slides are to high for most hobby machinist but anything that reduces friction is an advantage for our machines so even less precision ballscrews are a big improvement leaving us to just deal with the friction of our slides. this requires constant lube.
I know there is a lot to add to this and probably corrections needed
Steve
To be honest I didnt do much reading on items I can't afford except to note the differences.
Most people think the main reason for a ballscrew is backlash, while this is a reason to use ball screws it is not a feature of a ball screw that most hobby machinists can afford. So lets talk about friction first. The motors used for cnc are small and required to move very heavy loads in exact amounts. Friction is the biggest enemy. If you have ever tried to turn the handle of a machine that has set for a while you will feel a snap before the axis begines to move. The torque has to overcome the friction or adhesion that is between the surfaces before moving. This is everywhere in most manual machines. The thread mating in the nuts the dovetails mating to each other and the slides. Once this adhesion is overcome the motion becomes easier. Its the theory of motion things that are still want to stay still, things that are in motion want to continue moving. This plays hell on accuracy and the size of motor to overcome the initial friction/adhesion and a change in direction multiplies this as the mass has to be stopped then started to change direction (this happens fast but at one point it does stop ).
We overcome this with ball bearings. Bushings are mated surfaces rubbing against each other and the lube holds them apart so they slide on the thin film of lube instead of each other. These bearing surfaces are everywhere in a mill or lathe the dovetails are bearing surfaces and require lube to keep the two surfaces from dragging on each other. Ball bearings work different they do not rub the other part they roll against it always in contact at two points. If i put a ball on a table and put a book on top and move the book the ball will roll as i move the book not slide. There is far less surface area touching each other so there is less friction. Now if all the balls are in a straight line and "exactly" the same size they will all roll together and not against each other. Here is where precision bearings come into this, if one ball is the smallest amount larger than the rest of the balls it will catch up with the others and eventually end up pushing the entire line of balls along. This has the balls now touching in 4 locations top bottom front and back. The front and back of each ball is turning the opposite direction of the ball it is mated against so the friction and heat doubles. So ball size is very important
The surfaces the balls roll on have to be perfect also, a high spot on the table and a book that cannot be moved up leaves a tight spot in the bearing surfaces. So the balls have to be no bigger than this point in their path. Everywhere else they will be a loose fit. This is the reason for precision grinding a ball screw and it's nuts internal route for the balls to roll in.
To make the ballscrew antibacklash all the parts must be in contact at all times. but not ball against ball and they continue to roll along the thread including a passage to return them back to where they started from in the thread.
Ball slides work exactly the same way and the balls roll along the two mated surfaces and are returned through a passage to start over.
The cost of real precision ballscrews and ball slides are to high for most hobby machinist but anything that reduces friction is an advantage for our machines so even less precision ballscrews are a big improvement leaving us to just deal with the friction of our slides. this requires constant lube.
I know there is a lot to add to this and probably corrections needed
Steve
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thats the same thing all steppers run on a series of electrical pulses and that could be concidered ac.
thats a problem i run into between manufacturers all the time. controller and driver are the same thing also pulse and step...
they are talking about the highest rated voltage we talked about driving the motor at higher voltages to get the amperage we need in the time allowed per step. they are giving the max.
steve
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Bill Gruby
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Question --- In a screw thread some backlash has to be present. Zero backlash on a screw thread equals zero movement. Is this the same with a ballscrew. I see many advertized as "Zero Backlash". Is it really "zero"
"Billy G" :thinking:
"Billy G" :thinking:
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Bill Gruby
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I still question it Steve. I'm a little apprehensive. If you tighten to get to zero the closer you get the slower it will rotate. I may be out in left field here with this oine.
"Billy G" :thinking:
"Billy G" :thinking:
Take a look at the listed RPM on these guys (3000RPM!!!). That's way fast for a stepper. You would use this on a low load system where you wanted higher speeds (3d printer, plasma cutter maybe, router).
I wouldn't use it for a mill though...