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VMC vs CNC Knee Mills

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
6 hp on single phase might be outside Vfd range. If they exist they will be expensive. 5 hp is generally the cut off in single phase.

I wonder if the universe would explode if you ran a 6HP motor on a 5HP VFD and current limited it in the settings to think it was a 5HP motor?
 

Bakerprecision

Plastic
Joined
Aug 20, 2022
I wonder if the universe would explode if you ran a 6HP motor on a 5HP VFD and current limited it in the settings to think it was a 5HP motor?
I'm certainly sure this is possible. Problem is, I'm not educated enough to figure the ins and outs of wiring a VFD properly. It seems like options and wiring configurations are endless. And the fwd reverse is gear driven on my machine (not sure if that matters)
 

LOTT

Hot Rolled
Joined
Nov 28, 2016
That's my worry. I'm leaning towards an American Rotary 30hp unit to run both my lathe and TM1. I know the TM1 will run single phase, but if I'm doing a phase converter, then why not.
I would consider a RPC, but not for the reasons you mention.

There's plenty of 6 HP VFD available for $200-$250. And wiring them isn't a big deal, I forget and have to relearn every time. It doesn't matter that FWD/REV is gear driven, just don't reverse the VFD.

DON'T set up your TM-1 to run three phase off a rotary. There's no point in having a rotary spinning away sucking up juice and making noise when you mill finishes the program and you walked away. Unless you can run it faster on 3P there's just no reason.

The reason TO get a big RPC (or PP, whichever) is that it's easier to get other 3P machines. Band saws, surface grinder, cold saws, all the other stuff you'll want.
 

memphisjed

Stainless
Joined
Jan 21, 2019
Location
Memphis
I'm certainly sure this is possible. Problem is, I'm not educated enough to figure the ins and outs of wiring a VFD properly. It seems like options and wiring configurations are endless. And the fwd reverse is gear driven on my machine (not sure if that matters)
They are not voodoo to wire or program for on/off. Wall wires in, motor wires out. You want them to respond to inputs and boost torque and tell you what time it is- that gets a little more in depth.

As long as motor doesn’t need all amps then the universe will not implode. The drive is just outputting chopped and screwed dc voltage- it does not know what brand or even if a motor is on the other side. Current limiting device on motor side is outside my pay grade.
 

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
As long as motor doesn’t need all amps then the universe will not implode. The drive is just outputting chopped and screwed dc voltage- it does not know what brand or even if a motor is on the other side. Current limiting device on motor side is outside my pay grade.

My point was just that the motor and drive do not have to match. If you power a 6HP motor with a 5HP drive you can't do 6HP of work with it, only 5.

If you overload a nice VFD it just shuts down.

I have had terrible experiences/luck with low cost VFD's. I would only buy one from a reputable brand. I highly recommend Invertek optidrives.

I wouldn't avoid the rpc route, but the VFD option is very simple for small 3 phase equipment.
 

memphisjed

Stainless
Joined
Jan 21, 2019
Location
Memphis
I have had terrible experiences/luck with low cost VFD's. I would only buy one from a reputable brand. I highly recommend Invertek optidrive.
I like the lesson smv for simple drive, dang near bullet proof. Ultimate simplistic, a little more cost is the kB “ac speed control”. Just two toggles and a dial on the front.
Agree, brands make a big difference on drives.

True, overcurrent error will shut it down.
 

Overland

Hot Rolled
Joined
Nov 19, 2017
Location
Greenville, SC
I have a large (21"x80") lathe with a 15 hp motor from memory.
I run it with a 15 hp VFD, which in theory is nowhere enough for this machine.
I can read the current output on the VFD, and I know the max output spec.
I can push the lathe, but as long as I watch the amps, no problem.
Bob
 

Bakerprecision

Plastic
Joined
Aug 20, 2022
My point was just that the motor and drive do not have to match. If you power a 6HP motor with a 5HP drive you can't do 6HP of work with it, only 5.

If you overload a nice VFD it just shuts down.

I have had terrible experiences/luck with low cost VFD's. I would only buy one from a reputable brand. I highly recommend Invertek optidrives.

I wouldn't avoid the rpc route, but the VFD option is very simple for small 3 phase equipment.
Is there a distinct advantage going with a VFD over RPC (other than cost)? I assume it's a "simpler setup" with the VFD once you get it wired, and they are significantly cheaper. I suppose if I can wire a RPC, I can wire a VFD.

How do you size a VFD? Just double the specs like a RPC?
 
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LOTT

Hot Rolled
Joined
Nov 28, 2016
Is there a distinct advantage going with a VFD over RPC (other than cost)? I assume it's a "simpler setup" with the VFD once you get it wired, and they are significantly cheaper. I suppose if I can wire a RPC, I can wire a VFD.
VFDs can have a soft start and variable speeds (which is nice on a step pulley mill). RPCs are nice in that you can have one run the whole shop, rather than another VFD for every machine.
 

Cole2534

Diamond
Joined
Sep 10, 2010
Location
Oklahoma City, OK
I'm certainly sure this is possible. Problem is, I'm not educated enough to figure the ins and outs of wiring a VFD properly. It seems like options and wiring configurations are endless. And the fwd reverse is gear driven on my machine (not sure if that matters)
It's no more complicated than a relay, at this level. As for sizing, Ideally you'd have the drive matched to the motor but that's really not the case.

The drive is there to supply current to the motor, as long as your motor doesn't demand more than the drive can deliver you're good. Most drives also have the ability to deliver 150% output for some amount of time. What this means is that drive sizing has a lot of fudge factor involved and is really forgiving...in this application. If you find a small unit cheap, beware aware you may not be able to max out the machine's available HP and may have to reset it occasionally. Of course you can plan around that too- use lower gears to get torque, slow the spindle ramp to limit current, resist the urge to buy a 6" 12 flute face mill, don't try to imitate an Okuma's ability to peel steel.
 
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hanermo

Titanium
Joined
Sep 28, 2009
Location
barcelona, spain
VFDs are much better than std power options.
Even better are AC brushless servo drives.
3x better.

Wiring a VFD is quite simple, endless examples on the web.

When I got my 12x24 light-industrial lathe with power feeds I thought it was fantastic.
1.5 kW motor originally, belt drives.

Now, with a 2.5 kW AC servo, it is vastly better and no need for belt drives.
0-1000 rpm with huge torque and servo stopping in ms in case of crash.
Happened several times.

Lathes really, really, want high torque and servo drives are ideal for this.
My 2.5 kW lathe has nearly the same torque == 90 Nm as a HAAS st10 (102 Nm iirc, 1200 rpm).
That costs about 10x more == 60.000€ these days delivered.

The servo motor to spindle transmission should be really robust.
Im using HTD 8 - 30 mm wide, with a 50 kg mass mounting.
2 cm thick steel plates.
It´s theoretically capable of several hundred hp.
Thats what You want and need.
The servo or vfd should not hunt due to vibration and oscillation.
 

Zach1800

Plastic
Joined
Feb 12, 2021
No need for everyone to get worked up. I didn't mean to start the "VMC vs manual" war. I asked for opinions, and I have gotten many (thank you). We all have opinions, and what works best for one individual isn't necessarily the best, nor the worst, but I think it's important to look at everything as a whole to make an educated decision. After all, even if we disagree, we've learned something (right?).

Either way, I've got a lot to consider. I don't know if I should buy one quality machine, or try to get as many different (old/used) machines as possible. On top of which type of machine to buy, I have to decide if I want single or three phase. And if I pick three phase, do I go with a VFD, or a PRC. Then I have what I consider a "minimum tooling list", which I have no idea about, but I'll figure it out as I go I suppose.

Thanks again for everyone's feedback and input. I know this is a very subjective topic, but again, I think it helps us learn.
I’m experienced with both cnc and manual machines. I also work at a mostly manual job shop that has an extremely diverse workload. We have a good VMC with cad/cam software and a cnc lathe but I still spend the majority of my time in the manual shop because it’s just more convenient for what we do. The cncs are great and necessary for certain types of jobs but most of the time not worth the hassle of setup to me. I am looking into Prototrak type machines to kind of bridge the gap. The cons of the vmc to me are just the setup process and having to deal with enclosures and doors plus I want to be able to crank handles sometimes and see what I’m doing. Also have had the cncs for 8 years now and if there’s a problem it is generally involving the tool changers so I think adding some manual cnc hybrids to the shop would probably be the best of both worlds even with their limitations.
 

garyhlucas

Stainless
Joined
Oct 17, 2013
Location
New Jersey
Having used a VMC, a CNC knee mill, and CNC Bed mill with a 21" Z no way I'd ever get another knee mill. Tool changing isn't an issue because o the spindle, because you are SO limited in tool length and have to constantly move out of the way for tool changes. Bed mills often have quills so you can use them like Bridgeport to your hearts content.
The one I bought was $6000, with about 100 hours real use, included 23 Nikken CAT 40 tool holders, 7 with Jacobs ball bearing super chucks. Oh, and a 4th Axis with a 10" 6 jaw chuck that had never been installed on the machine. I bought multiton dollies and hired a roll bed truck to move the 6000 lb machine 200 miles. That cost $1200 more.
There are deals out there, I don't consider a knee mill a deal.
 

markz528

Hot Rolled
Joined
Sep 25, 2012
Location
Cincinnati
VFDs are much better than std power options.
Even better are AC brushless servo drives.
3x better.

Wiring a VFD is quite simple, endless examples on the web.

When I got my 12x24 light-industrial lathe with power feeds I thought it was fantastic.
1.5 kW motor originally, belt drives.

Now, with a 2.5 kW AC servo, it is vastly better and no need for belt drives.
0-1000 rpm with huge torque and servo stopping in ms in case of crash.
Happened several times.

Lathes really, really, want high torque and servo drives are ideal for this.
My 2.5 kW lathe has nearly the same torque == 90 Nm as a HAAS st10 (102 Nm iirc, 1200 rpm).
That costs about 10x more == 60.000€ these days delivered.

The servo motor to spindle transmission should be really robust.
Im using HTD 8 - 30 mm wide, with a 50 kg mass mounting.
2 cm thick steel plates.
It´s theoretically capable of several hundred hp.
Thats what You want and need.
The servo or vfd should not hunt due to vibration and oscillation.
What makes you think that a servo drive has a torque advantage over a good vector drive running an induction motor? You can't violate physics - hp = torque x speed/5252.

Yes a servo drive has very fast response, but a good drive on a small rotor induction machine can approach 100 rad/s response time. Is that really want for a lathe? I think not.

A quality drive with an induction motor is just fine for a lathe. If you want higher performance put an encoder on the motor.
 

jhov

Aluminum
Joined
Jun 5, 2020
Location
SW Ohio
I was in your position 3 years ago. If you have the time, I'd recommend taking it and acquiring the equipment slowly as deals make themselves available. This gives you time to learn, which in turn will give you a better idea of what you really need. Good deals are somewhat rare, but they are out there. I've managed to acquire a 4 axis Brother tapping center, a Charmilles Roboform sinker EDM, Chevalier Smart-818 surface grinder, Milltronics toolroom lathe and a Bridgeport EZ path, and all the tooling and support equipment for less than your $50k budget. To really take advantage of good deals, it really helps to be able to do your own rigging and hauling. I'd highly recommend buying whatever equipment you need for the largest machine you plan to buy. This will be a significant investment, but if you buy this equipment used then when you are done you can sell it for what you paid for it or maybe even more and the cost of moving machines becomes your time plus fuel (vs many thousands of $$ you'll never get back).
 

hanermo

Titanium
Joined
Sep 28, 2009
Location
barcelona, spain

What makes you think that a servo drive has a torque advantage over a good vector drive running an induction motor? You can't violate physics - hp = torque x speed/5252.

Yes a servo drive has very fast response, but a good drive on a small rotor induction machine can approach 100 rad/s response time. Is that really want for a lathe? I think not.

A quality drive with an induction motor is just fine for a lathe. If you want higher performance put an encoder on the motor.
1.
Experience.

I have a 1940-50x Bridgeport M head that is fantastic, with the original 1/2 Hp 3-phase motor.
It runs extremely well off a Hitachi 200x VFD, 12-60 Hz, 20-nnn rpm.
Either 2-phase 220V (EU) or US power.

The motor is excellent, probably about 60-70 years old, masses 18 kg.

My servo motor driving the cnc lathe is 2.5 kW continuous, 220V, 1:3 belt drive,HTD 8/30 mm.
Masses about 8 kg.
The servo drive is about 12x better than the original very good ac 220V motor.

My new excellent VFD driven 2.5 kW spindle drive on my VMC (ISO 30 spindle), masses about 12 kg.
A new AC servo drive at 2.5 kW / 8 kg will be about 6x better, although only 3x more torque where it counts.
 








 
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