Spindle Torque (Lathe)

[dksoba]

If I want to cut a 6" diameter (3" radius) workpiece in 303 stainless steel on a lathe, how much torque would I need? The reason I ask is because this is the biggest piece I could imagine cutting on my lathe and I'm debating whether I need to build a reduction for the 12 nm motor I just bought. It's a fanuc AC servo motor. It's max rated speed is 2000 rpm, so I was thinking if I gear it 2:1 so I can go to 4000 RPM I'll be good. However, for cutting at large diameters and low speeds this might be an issue, although torque increase as speed decreases. I'm wondering if I need to have multiple speeds.

Thanks,
Matt

[jsheerin]

From a quick google search I found this calculator:
TaeguTec Machining Power - V5.0

This says that for AISI 304, 0.1mm feed/rev, 1mm doc, you'd need 341N of tangential force, or about 25.6N*m for a 150mm diameter workpiece. Cut the depth of cut in half and you would cut your torque required in half it looks like. That would get you close to what the motor could deliver without gearing up the speed.

Also, if you have an AC servo, the 12N*m torque is probably the continuous stall torque. That is, the motor will produce 12N*m of torque at 0rpm continuously (without overheating). As rpm increases up to 2000, the continuous torque typically falls off down to around 10N*m in the Fanuc data sheets I have. However intermittent torque can be much higher if your drive and power supply can supply the power.

[dksoba]

Thanks jsheerin,

I don't know why I didn't stumble across that before. I will be using field oriented control with ~200volt DC bus. I'll design the board to be able to run 30 amps or so but limit the current to 20 amps peak, 11amps continuous (in line with the motor cont. stall current). The motor is rated for 144 volts, but if I run it at ~200 volts it won't matter as long as I limit the current to 11amps cont via closed loop control. Also, the motor is rated at 2000 RPM, but would it be possible to run it at 3000 or 4000 rpm? I think the limitation here is that at 144 volts 3-phase excitation (without field oriented control) the motor's back emf will cancel out the torque produce with 144 volt 3-phase. If I use ~200 volt I should be able to go a little faster. At those speeds, the current going through should be minimal, but I wonder if the bearings can handle it. It's an 8-pole motor.

Where did you get the Fanuc data sheets that you have? Do you have pinnout information on the pulse coder?

Matt

[jsheerin]

Hi Matt,
What motor do you have (what's the part number)? I'll see if I have anything that would help you. I got my data sheets partially from people here and partially from finding them on the web.

John

[dksoba]

Here are the part numbers:

A06B-0501-B751 - 2000 RPM
A06B-0235-B605 - 4000 RPM

Matt

[jsheerin]

Sorry, don't have any data for those.

[Algirdas]

1. It will run on higher rpm, no problem. Less current, sure, hence reduction of torque. In order to keep torque constant, you need rise voltage on higher rpm's. Insulation resistance capacity is important at this point.
2. if stall torque is 12N*m, you can get 60 at least with 3000 rpm.
3. bearings must be correct up to 8000. Balance could be an issue.