Happy new year from Mikini Mechatronics

[MIKINI MECH]

In a previous post, we offered to present spindle torque and output graphs to assist a user in understanding of a system. Attached is that documentation, which was prepared for, and will also be on our new website.

There are 3 graphs here and a chart.

- Load response

Load response details what happens to the actual operating RPM of a drive, when loaded on a machining center. Remember that axial drives will not compensate for spindle variation without a very advanced system, so any variance to load will affect thrust, axial load/power, surface finish, tooling life, and achieved precision.

You can feel this if you use a drill or saw and cut something with an AC, open loop motor. The motor changes tone, and the speed slows down. Same is true on an open loop spindle drive on a machining center. Servo drives "push back" by changing frequency, voltage, current, etc. If you fully overload an AC system, the drive, generally, does not know about the overload, and the motor stops and starts to melt ("locked rotor").

If you overload a servo drive, it simply shuts down (and in this case also tells our axial drives to shut down within about 1/100 of a second), protecting the machine from significant damage and loading from shoving a non-operating tool into "something".

- RPM vs Power

This is the actual difference between 3 different combinations of drives and motors. If you're wondering how a 1 HP AC motor with a 2 hp VFD drive (think bench top machine) compares to a 2 hp motor with a 3 hp drive (Think knee mill), to a 3 HP servo drive - here's the data.

This also is why AC machines with traditional drives often have narrower speed ranges, 2 speed controlled transmissions, or in manual machines - belts for multiple ratios. See where the torque starts higher up the RPM scale - this is what is being compensated for.

There are other graphs out there that compare BLDC servo and AC open loop motors that don't have data/details and appear theoretical.

It's true you could under-size a BLDC system and scale the axis to get the graph that has been presented. But take 2 systems that are around 20-25 lbs each, test, and you'll get this data. Otherwise the power density and constant torque characteristics make no sense (as a user has pointed out).

A linear power curve / constant torque, also allows the very simple programming (scaling RPM scales power) and is also ideal for machine, bearing, and precision/tooling life - as thrusts end up staying the same as intended.

You can run feed/speed 10/1000 20/2000 30/3000, etc (if the power demand of the application is linear) to prove out tooling, fixtures, etc. Basically less variables are changing at the same time as torque is constant.

These are the "power curves".

This should be well versed information, specific to a tool, for any machine tool operator and programmer, for any automated machine tool. Attempting to program a tool without comprehensive knowledge of a power curve is a hugely frustrating experience, as you're guessing at how much power/torque is available.

However, it's a common point of confusion in the industry. Scale things up, and things get more expensive fast.

Spindle Power Curves

Yes - there are systems that are [B]wildly[/B] expensive that will slow the machine down to compensate for spindle or axial loading. But it's not your typical machining center that will do so - think way exotic.

- Torque

Torque is how much "grunt" a system has at a given RPM ? Always the same for an advanced servo drive generally, and most at a "center" or "sweet" spot for most open loop systems. This also presents graphically what 5:1 or 20:1 or "linear" refers to (the flat top part).

If you had a transmission, you'd have multiple curves for each system at different ratios.

This is one of the strong suits of a brushless motor, as well as extreme life (no brushes to wear and replace all the time), and constant performance.

These are the "torque curves"

- Power density

More important than you might ever think. Who cares about 30 lbs on a device that weighs 2000 lbs ? Turns out it makes a world of difference to bearing life, selection for precision, and is a key driver of machine systemic design.

Helpful ? Confusing ? Indifferent ? Welcome any and all feedback.

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Mikini Mechatronics, llc

[Al_The_Man]

An important safety note before continuing. DC is far more dangerous than AC. If you make a mistake working around the DC high voltage bus, it likely will be lethal. No second chances here like ~200VAC.

ElectroMedical fact
It is not voltage that is lethal, it is current of the right level of any flavour.
100ma and over is considered lethal whatever the source.
Al.

[MIKINI MECH]

That's quite accurate, and for AC, the current level is on the money.

Many people, even well trained in AC safety, have little knowledge of the hazards of HVDC.

Here's a link to what currents at various levels are noted to cause.

Ohm's Law (again!) : ELECTRICAL SAFETY

And DC (direct current) presents dangers that AC (alternating current) does not, and affects the body in a different way. If you've ever gotten a low voltage/low current DC shock, you'll be very aware of how scary it is. Yes, a 12v car battery, can, and will shock you, if you have hands that are wet.

If you got a low voltage, high current shock, well, you would have nothing to say on the topic, ever again.

Safety manual from unm.edu

Note the fact that DC will discharge to your bodies capacitance. Our drives will dump an instantly lethal current, though dry skin, even when totally disconnected from AC if not discharged. Check out the big capacitor bank ....

What is the maximum DC voltage, that is not lethal?

read down a bit - focus on the post regarding DC voltages over 200V. System could be over 300V. Poor English in the post, but get the idea ?.

Very serious topic.

We fully understand it may come off as paranoia if you're not aware of the issues. We live around these systems day in and day out in production and R&D.

Anyone servicing our machine systems is required by our documentation and machine labeling to have proper knowledge of these topics, training, and safety equipment available, prior to attempting service. Attempting to do so without is beyond inadvisable. Information is often available for DC-HV safety surrounding issues of hybrid vehicle servicing and large scale solar installations for reference as well.

Be Safe.

Mikini Mechatronics, LLC

ElectroMedical fact
It is not voltage that is lethal, it is current of the right level of any flavour.
100ma and over is considered lethal whatever the source.
Al.

[mcphill]

VERY helpful, thanks. Not sure of the usefulness until I digest it longer.

I do have to say I don't believe the Torque Output curve presented. It just does not match with my "gut feel" experience. Can you suggest a way to challenge/test those values safely? My impression is the curve looks much more like the 1 HP AC Conventional shape shown in the graph (or maybe the 1.5 hp vector).

[mcphill]

Sounds like all the more reason to post ALL the pinouts... In the name of safety.

[Al_The_Man]

If you got a low voltage, high current shock, well, you would have nothing to say on the topic, ever again.

After 50+ years in the electrical/industrial electronic business I have lost track of the zaps I have had from just about every source there is.
Al.

[slowtwitch]

Looking at the new specs..Version 3. I see that you have changed the spindle board and motor.

[MIKINI MECH]

The only place that DC HV is present in the system is on the 3 output pins of the spindle drives. Worth documenting this in the machine schematic as well as warnings on the physical drive itself, which is a good suggestion

Mikini Mechatronics, LLC

Sounds like all the more reason to post ALL the pinouts... In the name of safety.

[MIKINI MECH]

We're not aware of a simple, low cost, safe, and accurate way to test what you are requesting.

We test our systems (and have done so of others) extensively to generate the data behind the charts we publish. We posted at length how the drive can be tested for input and output. It requires a huge investment in test equipment and setup, as documented. We're here to help, if you wish to do so.

If you wish to focus on the output side, testing could be done alone ignoring the input metrics. It will still require a constant torque brake, of less than 5 lbs inertia, with torque or ideally power (Rpm and torque) monitoring, capable of the amount of power dissipation at the rate (RPM), Power, and duty cycle you wish to test to. Essentially a low inertia, 5000 RPM, 3 HP+ dyno. The other issue is ensuring you don't damage spindle or axial bearing systems in doing so. Still not a simple thing to do.

We encourage you to verify the fundamental torque characteristics of a brushless DC drive, which will also confirm this "curve". Tons of information on the web, on the topic. It's possible to get a lower or higher torque for any system, but the "shape" of a torque curve is generated by the motor/drive physics in big picture.

"guessing" or "feeling" isn't a great measure here. Do be very aware that open and closed loop systems "feel" and "sound" very, very different under load. It's why we have not one, but two load meters on our control systems, each with important information (instant and average loads).

Imagine what would happen if your table saw had a servo drive, and never slowed down until you hit ultimate load. To take it to a little bit of an extreme, what if it even sped up as the stock was fed faster to keep the thrust into the saw the same (constant chip rate) ?. Other than if you had a load meter - you'd have - nearly - no idea of what the load applied to the drive was, as the operator. More you push, the more it would push back.

In an open loop, manual process, as an AC drive is loaded, it slows, makes different noises, the operator backs off, then allowing a drive to speed back up (operator is closing the loop). This is what we're used to on a drill press or a manual machine. Other than very, very complex CNC controls, this luxury is not afforded to the drive systems of an automated machine (unless the operator runs over and slaps feed hold or cuts the feed rate as he sees a load peg or hears an AC drive start to fail).

What it comes down to, is that the way to properly program and load a machining center is by the #, not by "feel" when it comes to spindle torque loads and axial drive thrusts. "Feel" is very important for other issues like harmonics, however.

The other thing to realize is the inertia and breakdown torque of various drive systems have very different "feels" and effects on downstream systems due to torsional vibration and stored energy. Whole different thing to have a 40 lb 6" armature spinning at 5000 RPM vs a 20 lb 4.75" one. Both systems could be rated at, and produce, the same 3 hp and torque.

In simple terms, all other metrics even, the energy from a larger (more massive, but same power) drive, would tear apart precision bearings rated for a smaller drive, even of the same power and RPM, in a very short matter of time.

Think of the torsional vibration gearbox requirements for a 100 HP diesel at 1800 RPM vs 100 HP turbine (just to book end things in extremes ...)

Helpful ?

Mikini Mechatronics, LLC

VERY helpful, thanks. Not sure of the usefulness until I digest it longer.

I do have to say I don't believe the Torque Output curve presented. It just does not match with my "gut feel" experience. Can you suggest a way to challenge/test those values safely? My impression is the curve looks much more like the 1 HP AC Conventional shape shown in the graph (or maybe the 1.5 hp vector).

[Spinnetti]

Man, the internet forums are a hostile audience!

I found the original post refreshing. Having worked as a engineer and formerly machinist at a smaller company, I think it was the right balance of feedback. You'll never "win" arguing with people on the internet! One that that would be helpful however is a FAQ of issues and solutions that are of the highest volume.

I really like the look of the machine, but posts about spindle motor and drive issues, company size and lack of product depth has kinda scared me off. This thread will have me take another look before I decide....

[MIKINI MECH]

Great feedback.

Would love more details on what more in "product depth" information you're looking for. Feel free to be in touch at any time.

Have a great weekend, to all

Mikini Mechatronics, LLC

Man, the internet forums (especially these sorts) are full of hostile people!

I found the original post refreshing. Having worked as a engineer and formerly machinist at a smaller company, I think it was the right balance of feedback. You'll never "win" arguing with people on the internet! One that that would be helpful however is a FAQ of issues and solutions that are of the highest volume.

I really like the look of the machine, but company size and lack of product depth has kinda scared me off. This thread will have me take another look before I decide....

[Spinnetti]

Great feedback.

Would love more details on what more in "product depth" information you're looking for. Feel free to be in touch at any time.

Have a great weekend, to all

Mikini Mechatronics, LLC

What I mean is that you have from a "retail" standpoint just the one machine. I know your site says you do lots of custom stuff, but no matter how good the product, its a lot of money to spend for what is (for me) a toy, and not having a fuller product line makes you more vulnerable to market fluctuations. Not trying to imply anything, I just want to be real confident that 5-10 years from now, I can still get parts and/or service....

That said, I really like a lot of what you have to offer, but that won't fit through my shop door. can the upper bit get unbolted and taken off to fit through a standard doorway?

By the way, your website really needs an update (price doesn't match page to page, future releases listed as 2009 etc...) - its your public face; a good website means a lot!