The motors on this mill are rates at 15 amps, 135 volts DC continuous, and 32 amps intermittent.
I have three AMC drives that run 15 amps continuous, but only 80 volts DC. I was planning on installing them, however, some local guy is offering me servo drives that go up to 15 amps, 135 VDC, so it made me think.
Superficially, more is better, and with more volts I could get more motion (force/speed) out of the drives, but is this really a good idea? Would the mill, say, be more dangerous if I could get some very rapid table movement? Is such power really necessary?
Ignoramus27711 fired this volley in news:6eOdnQ9L9PnDb4XRnZ2dnUVZ snipped-for-privacy@giganews.com:
Iggy, the mill IS dangerous, even if you put 130 in-oz steppers on it. It is a machine tool, not only with flying cutters going, but with pinch- points, crush zones, etc.
Use what will drive the mill at the forces and speeds you want. You have to be careful around the machine no matter what you use.
Ignoramus27711 fired this volley in news:6eOdnQ9L9PnDb4XRnZ2dnUVZ snipped-for-privacy@giganews.com:
Also... you can set the over-current limits on the servos wherever you want them to be with properly designed drivers. So you can get the performance of the lesser servo out of the larger ones, but not the other way around.
The rapid motion is only used while NOT cutting. So it doesn't typically do much good to make it more rapid, if you're spending most of the time cutting. Now if you're hurrying over to an auto tool changer all the time, that would be different.
Acceleration is more of a performance limitation than speed, since most moves are short distances, assuming your parts are inches and not feet long.
I suppose, but there may be some circumstances where you want to force feed the cutter a little harder. Maybe for drilling or roughing off bulk material with a large mill? I don't know as I am not a machinist
No real difference in danger. If you crash the mill due to bad code, the danger is from flying end mill parts, where the "power" comes from the spindle rotation, not the axis movement. The danger of under powered servos is not having enough axis power available to take a sufficiently heavy cut and in turn dulling the cutter, work hardening the part, etc.
The original servos were spec'd by engineers who calculated the force required to overcome the axis slide friction and provide the required axis forces for the rigidity and spindle HP of the machine. Don't second guess them.
My suggestion if you are concerned would be to build an enclosure for the machine table out of some square tube stock and Lexan, with sliding doors at the front. These enclosures are/were fairly common on knee mills, and contain coolant splash as well as flying parts. They can get in the way for larger parts, so build them so the panels can be readily removed for long parts.
Ditto. Treat that thing as potentially lethal -- meaning keep your cute little kid away from it, too. When the curious fingers in my house were little the lathe stayed unplugged when not in use -- in fact, I'm going to go back to doing that. With yours, I'd consider putting a cutout high on the wall and away from the mill so that any adult can reach it to turn it on or off, but so that the kids can't get to it. Better, put a key switch in, and put a big red stop knob down low where the kids _can_ rescue Daddy if he gets caught in the mill.
(Some guy made national news a few days ago -- he got his arm trapped in his furnace and spent three days trying to free himself before he cut it off. Don't be like him!)
An SUV is a dangerous beast too. But if one drives one properly..it is pretty safe.
Gunner
One could not be a successful Leftwinger without realizing that, in contrast to the popular conception supported by newspapers and mothers of Leftwingers, a goodly number of Leftwingers are not only narrow-minded and dull, but also just stupid. Gunner Asch
I don't know if I'd want it running unattended anywhere that a curious kid could get to it -- there's "not at the work station" unattended, and there's "not in the room" unattended. The first would be fine were it my setup, the second would not, at least not until my 11 year old grows into a bit more sense.
Remote monitoring and E-stop would be fine. Have a camera pointed at the mill connected to a monitor in your office across the house along with a remote E-stop button. Use a camera with audio so you can listen to the sound of the mill while your in the office.
"Harder" as in more linear force translates to more torque from the motor, not more speed. Torque in a motor comes from current, not voltage. You must analyze on a physical basis using all the factors that count.
The power to CNC move through a cut differs from the power to move while not cutting, not just obviously due to the cutter feed forces, but in a complex way since the spindle power shares the load. Indeed, the feed power can be negative if the cutter pulls itself into the cut.
The old analysis of horsepower required for a given material removal rate did not count the power of the meaty arms moving the handwheels.
You really should stay near the thing. Stuff happens like inserts blow out, your stock pulls out of the vise, ect. I've repaired too many machine crashes when an insert blew and the machine kept going while the operator was in the chitter or chatting with the pretty girl on the next machine over. Most of my crashes are lathe ones but mills have some of the same issues.
Lights out/unattended stuff has good load monitoring built in so if something gets out of range, it stops. Depending on the load level it either finishes the parts or stops in middle of cycle.
That isn't your mill given the technology you are currently using.
You need to calculate the back EMF at the maximum speed you plan to move to know what power supply voltage you need.
So, I think I remember a 2:1 belt ratio. 100 IPM with a 5 TPI leadscrew needs 500 RPM. With a 2:1 reduction, the motor would be turning 1000 RPM. If your motors do 2000 RPM at full voltage, you only need 1000 RPM, so half voltage will do fine. I am throwing out some typical numbers, you need to check the actual rating points on your motors to be sure. You want at least 10 - 15% headroom on the voltage to cover motor resistance and servo amp resistance and duty cycle limits. More voltage does NOT give more torque/force, it only increases speed.
15 A x 135 V is 2025 W or 2.7 Hp, a SERIOUS motor! I wouldn't be overly concerned about the hazards of such a machine, but it certainly could mash a finger or maybe even knock you over if you were standing too close. If you are really leaning into the works when changing tools or setting up workpieces, you can hit the E-stop. With a servo system, that will not cause a loss of coordinate alignment, so you can do that any time you are in close or putting your hand in pinch points.
Commercial machines all do 400 IPM rapids or more today, but I have limited my system to 100 IPM capability, and 60 IPM commanded velocity, as "the hand is quicker than the eye", meaning I tend to cause collisions when moving faster than that during setup.
Well, the motors seem a bit overpowered to me, but then I run a SERIOUSLY underpowered mill. A Series I Bridgeport with 1/8 Hp (continuous) servo motors. It does suffer just a little from rounding off of sharp corners, due to limited acceleration. if I turn the acceleration up, then it breaks timing belts, and the setup was made such that wider belts would be difficult to install.
But, I STILL get 750+ Lbs of linear force on the table, and the machine can snap a 3/8" HSS end mill off clean in the spindle, without causing a following error. So, unless you are hogging out mold cavities in inconel with 2" end mills, I think your 15 A servo amps will be quite sufficient. You can convert motor torque to linear force on the table. My guess is that it will be an awesome number even at 15 A, probably approaching 3-4 tons. You need some for acceleration, but you surely don't want to subject your spindle to several tons of radial force.
These are awfully good to contain coolant spray and a shower of chips. I have a couple different plexiglas shields, one fits over the vise, the other is a wide shield for larger parts. You want to make sure the shield drains back into the table slots to return coolant to the sump.
So is a flatbed trailer with a backhoe, as long as some idiot isn't tailgating. I saw a car up under the back of one tonigt, on the way home from Harbor Freight. :(
On Wed, 16 Jun 2010 14:02:33 -0500, "Pete C." wrote the following:
Yes, the sound of machinery is music to the ear of the techie. We can tell what's going on with it by its various hums, roars, & harmonics.
Pete, would any of these books be a good intro for me to CNC?
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Hood-Daniel/Kelly, _Build Your Own CNC Machine_
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E. Hess, _The CNC Cookbook: An Introduction to the Creation and Operation of Computer Controlled Mills, Router Tables, Lathes, and More_. (brief, catchy title, wot?)
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Alan Overby, _CNC Machining Handbook: Building, Programming, and Implementation_ (not yet released)
I think the prerequisite is a regular machining book where you will find details on the actual machining process, speeds and feeds, fixturing, etc. so you know *what* to tell the CNC machine to do, since ultimately the metal cutting is the same with the computer moving the machine for you. The actual CNC end is just G code which is nothing more than simple movement commands.
Where it gets complicated is when you want to do more complex parts and have to get into the CAD / CAM end of things. For simple parts, you can readily hand code the G code and be done with it. For 2D work the CAD / CAM isn't too bad and you can use any number of CAD programs (I use TurboCAD) to produce the design to export the DXF to bring into a CAM program (I use SheetCAM) to apply cutter compensation, place start points, cut orders, etc. and export the G code for the CNC machine to run (I use Mach3). Full 3D parts that aren't readily broken into 2D layers brings another level of complexity.
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