Machining metal parts

   / Machining metal parts #61  
There is a term for 3ph motors that are "inverter rated". What does that mean? If I'm understanding 3ph motors correctly, they all will work off an inverter. The only concern that I see is that the inverter changes frequency to control the motor speed and cooling could be an issue at lower speeds.
You understand right. Most VFD have a setting for 2 phase in and 3 phases out.

Basically "inverter rated" means that the interior parts of the motor are epoxy impregnated to keep them more rigid. That is done so that the various motor parts won't begin vibrating at self-resonance when the drive frequency is varied.

You probably want that rating if buying a new motor for use with a VFD. It has become the signature of a better built motor for a slightly higher price.

But if you have a good old motor and want to use it with a VFD you can do so for years. Just pay attention to any motor vibration you sense. If you hear it/feel that the motor doesn't like a certain speed then change the VFD speed until it quiets down.

rScotty
3 phase 240 volt Rockwell mill with VFD, power feed, rotary table, R8. & a WWII vintage motor.
 
   / Machining metal parts #62  
You understand right. Most VFD have a setting for 2 phase in and 3 phases out.

Basically "inverter rated" means that the interior parts of the motor are epoxy impregnated to keep them more rigid. That is done so that the various motor parts won't begin vibrating at self-resonance when the drive frequency is varied.

You probably want that rating if buying a new motor for use with a VFD. It has become the signature of a better built motor for a slightly higher price.

But if you have a good old motor and want to use it with a VFD you can do so for years. Just pay attention to any motor vibration you sense. If you hear it/feel that the motor doesn't like a certain speed then change the VFD speed until it quiets down.

rScotty
3 phase 240 volt Rockwell mill with VFD, power feed, rotary table, R8. & a WWII vintage motor.
Inverter rated also means ability to handle higher heat and higher RPM. If a motor is run substantially below its rated speed it runs the real risk of overheating. Adding another fan to blow air through the motor cures the heat problem. Running a non inverter rated motor no more than 150% of rated speed is usually OK. But be careful.
Eric
 
   / Machining metal parts #63  
Inverter rated also means ability to handle higher heat and higher RPM. If a motor is run substantially below its rated speed it runs the real risk of overheating. Adding another fan to blow air through the motor cures the heat problem. Running a non inverter rated motor no more than 150% of rated speed is usually OK. But be careful.
Eric

Eric, you are absolutely right and I forgot to mention both of those things. Heat and high RPM are both things that you get more of with Variable Frequency Drive controllers. And yes, the motor is built with those things in mind as well.
An interesting aside is that in large motors you can pay a premium and buy the exact same motor as being"inverter rated". I just ordered one for our water plant that is a couple of thousand dollars. The same motor in inverter duty is only a few hundred extra - although it is a special build by the manufacturer and takes another month to get.
rScotty
 
   / Machining metal parts #64  
As some on here have likely found, if you are constantly in search of the holy grail of the perfect used tool deal, you'll likely spend all your time on the computer (or phone) looking, and none of it learning to use the tools. I'd rather have any machine than no machine. I started with a used round column Rong-Fu and a more than well used South Bend 9" lathe. If you start out used you can likely upgrade at some point and not take much of a loss when you re-sell to that next new user looking to learn. I wouldn't recommend new unless you already have experience and know what you want, or if you live in a black hole without used machinery.
 
   / Machining metal parts #65  
Yep - started with the Rong Fu also, as previously noted. The problem was that everything I had done in my career and schooling in engineering was on Bridgeports or clones. When I tried doing stuff on the Rong Fu it was a nightmare as it just would not work like I was used to. It was better than nothing. Perhaps I was just spoiled but now that i have a used BP, it is like heaven as it will do the things i want without a huge fight or wasting tons of time like the RF did. Can you make good parts on the RF? Sure. it requires patience and time. If you can come up with a couple $k more, you can probably find a BP/clone and be in a much better place to start. If you know nothing about machining, then perhaps fins the cheapest way to start and decide and go from there.
 
   / Machining metal parts #66  
Yeh I had one of those (w)rong fu round column mills years ago, inherited from my dad. You could do some stuff with it obviously, but good luck if you need to move the z beyond what the quill travel allows, what a pita! I do NOT recommend getting a round column mill unless you get it dirt cheap. On the upside, you could get one dirt cheap (lol) in pretty good condition, coz everybody who's got one ends up wanting to upgrade to a knee type of square column mill for the above-mentioned reason....or jump into a cnc milling machine altogether.
 
   / Machining metal parts #67  
I've decided to give myself a christmas gift in the form of an X,Y,Z readout for the Rockwell little mill... It's half the size of a Bridgeport. And also something similar for the Atlas lathe.

Anyone have any suggestions or things I should consider? So far I've only looked on Amazon.
rScotty
 
   / Machining metal parts #68  
Be sure to get magnetic scales, not glass (optical). Mag scales can be cut to length and coolant doesnt affect readings. Get as fine a scale as you can afford.
 
   / Machining metal parts #69  
I've decided to give myself a christmas gift in the form of an X,Y,Z readout for the Rockwell little mill... It's half the size of a Bridgeport. And also something similar for the Atlas lathe.

Anyone have any suggestions or things I should consider? So far I've only looked on Amazon.
rScotty
It depends on the resolution desired. But DROs have come way down in price since I bought my last one so price isn't really a big concern. Magnetic and capacitive scales can be cut to size, glass scales cannot. Capacitive scales are the same type found in cheap digital calipers. They tend to be the cheapest. They do not tolerate moisture well. Magnetic scales don't care about moisture and are usually the most expensive. Glass scales don't tolerate contaminants well because they rely on light. For this reason the scales are mounted in nice and robust protective shells. Glass scales tend to be slightly more money than capacitive scales. Because capacitive scales don't need the same protection against dirt they are often not as well protected against damage from big stuff. I myself like glass scales the best. In over 20 years of hard use on machines in my shop I have never had any problems with my scales. I would look for .0002 resolution. Most folks in your position will be fine with .0005 resolution, but if the price differential is small I would go for the .0002. I think now for a Bridgeport size mill a .0005 DRO can be had for about 250 bucks. When I bought my DROs they were more like 1800 bucks. But they were the .0002 ones, which cost more.
Eric
 
   / Machining metal parts #70  
More about scales for a DRO. Magnetic and capacitive scales are analog while the glass scales are digital. This means that the position of the readhead is done differently depending on the scale. The magnetic and capacitive scales have a signal that changes gradually as the readhead passes by. Because of the way the signal changes the resolution can theoretically be very fine, much finer than what the display shows. But because the signal is also subject to electrical noise, which must be filtered, the practical resolution is much lower than theoretical resolution. Capacitive scales are so noisy they are limited to a practical resolution of .0005. Magnetic scales are way less noisy so the practical resolution can be much finer, even ten times finer. When I say the scale is noisy what I really mean is that the way the scale reading system works is noisy. Also, because of this electrical noise capacitive scales tend to not be as repeatable as other scale types. Magnetic and capacitive scales require some pretty complex electronics to be read, part of the reason for the noise in the electronics. On the other hand, glass scales are truly digital. They have discrete lines on the scales that are read by passing light through the scale. The light is either blocked or not. So the signal is either on or off. This means the scales are simple to read. It also means they have a certain fineness of resolution that cannot be made finer. They are typically read in quadrature because they have two LEDs and two photo receptors to do the reading. The LED and photo receptor pairs are staggered so that as the readhead is moved pair A senses the leading edge of a line on the scale, then pair B senses the leading edge, then pair A senses the trailing edge, then pair B senses the trailing edge. So, 4 simple on or off signals per line. This divides the spacing of the lines by 4, meaning that the scale can be read to a resolution 4 times finer than the marks on the scale. So even though the scale resolution does not change a finer resolution can be had by how the scale is read. The accuracy of the scale depends on the accuracy of the lines on the scale. These days it is possible to make very accurately placed lines on the scales for very little cost. Because the position of the lines on the scale cannot change, because no interpolation is done, glass scales repeat. You can always count on the readout showing the same position when the readhead is moved a certain amount. Even if the scale is inaccurate the inaccuracy will always be the same. Because the output from a glass scale is purely digital, just on or off, the signal is easily used for other things, like adding another display in another location so that you can have two readouts at once. Or the signal from the scale can be used to position a stepper motor.
Eric
 
 
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