How much power do you use?

I upgraded the VFD for the spindle on my old Denford Triac CNC Mill. While setting up the VFD I noticed it had the option to display the %age power used so I set this up to feed back the number into Mach 3.

I've found it really interesting how little power is often required. Motor is 0.5hp and I was flycutting some cast iron at the weekend 0.5mm deep cut 2.5" wide and it was only using 12% at 500rpm

The biggest surprise for me has been the difference between a 'cold' machine and one that is warmed up. The same cut as above with a cold machine needed 64% power.

Even more surprising was that in the real cold weather earlier in the year (workshop about 8C!) - just turning the spindle over 2000rpm, needed over 95% power when cold and dropped to 40% when warm.

I've now got into the habit of warming up the machines for 10 minutes before I use them.

Has anyone else done the same with their VFD's?

On cold days in the winter I can't run my lathe in the higher speeds until it has warmed up, about 1/2 an hour running time. The lathe is single phase 1.5hp.

Ian S C

Does the same thing apply to rotary converters?

CNC machines have warm up cycle to get everything to optimal temperature, ensures that lubricants are where they need to be. Quite normal to see 70% on the power meter when cutting at optimal speeds and feeds, time is money. Industrial machines are meant to run 24/7, where as hobby machines have a limited duty so when you start to get toasty with the drive it’s best to have a cuppa.

I measured my lathe a while back and reported the results in another thread:

'My lathe is a WM-280 fitted with a 1500W 3-phase motor, VFD and two continually running cooling fans. (One on the motor, one on the VFD). How much power does it actually consume in operation I asked myself?

So with belts set to the high-speed range; banjo gears set for fine feed; auto-traverse engaged; and the built-in gearbox set to the finest feed-rate:

• Lathe switched on (ie. with only the fans and electronics running) - 47W
• Motor idling at 150rpm - 210W
• Motor spinning at 1500rpm - 360W

Using 13.7mm o/d steel welded gas pipe as a test piece with carbide inserts and no coolant/lubrication:

• 0.3mm deep cut @ 1500rpm - 542W (A 0.3mm cut removes 0.6mm from the diameter)
• 0.5mm deep cut @ 1500rpm - 540W
• 0.7mm deep cut @ 1500rpm - 565W
• 1.0mm deep cut @ 2400rpm - 960W ( A 1mm deep cut removes about 80 thou from the diameter.)
• Parting off with a 3mm wide carbide blade @ 1000rpm - 600W, lower than expected.

At the finest feed rate (0.07mm per revolution), a 23mm long cut takes about 20 seconds at 1000rpm. I'm too befuddled this morning to calculate how much steel I can remove with 1kWHr's worth of electricity costing me about about 11p.

Taking 2mm off the diameter of a steel rod in one cut is pretty much the hardest I ever drive my lathe in practice. In this test doing so produced a string of smoking blue swarf from the gas-pipe rather than chips. At that rate the lathe pulls less than 1kW, which suggests that I need to drive the lathe significantly harder to stress the motor, and thus have the opportunity to make the carbide inserts work better. On the other hand an unstressed hobby lathe will last longer! (I've no idea what this lathes operational sweet spot is. My car is best if I drive it at a steady 60mph. Lots of stop-start slow speed city motoring is bad for it, as is hammering up and down a motorway at 90.)

Interesting that the belts, banjo and gearbox in fine feed eat a couple of hundred watts before any useful work is done.

Normally I guess I have the lathe burning about 550W during cuts. That's only a couple of amps from a domestic mains socket. I use more electricity lighting the workshop and making tea than I do cutting metal!'

So yes, quite likely a hobby lathe won't use much power.

Fair amount of energy is wasted in my lathes gears. Apart from the screw-cutting banjo there is also a 3-speed gearbox driving the leadscrew or power-traverse. Make sense to disconnect these from the spindle when not needed. The amount of power wasted in the transmission while warming up will be more obvious in a lathe fitted with plain rather than roller bearings. Being a weakling I don't normally use my workshop when it's very hot or very cold and rarely bother to warm my machines up. 'Very cold' in South West England is rarely the same as 'very cold' in Toronto!

Once power has reached the cutter some typical figures for metal removal in Watt-Seconds per cubic mm are:

• Aluminium Alloys - 0.3 to 1.1

• Cast Irons - 1.0 to 6

• Steel - 1.9 to 9

Dave

I have a variable speed lathe with a constant speed drive. I simply speed it up, if I am time constrained, until I know the motor is working hard - provided the finish, etc is satisfactory.

The mills are treated likewise, but kept at a reasonable load to lengthen end mill life.

One would not run up to full speed in the highest gear until the lube warmed up, during times of single figure temperatures (Celsius), when it was fitted with a single phase motor, but that was likely a speed/power/torque issue combined with the start windings not providing sufficient ‘kick’ at start u, so the motor could not reach full power.

The diameter of your fly cutter may well affect the average current drawn - the fly cutter would not be cutting for half the time, at least.

Lubricating oils have a working temperature range. It is likely that 10 minutes running time is unnecesssary at this time of the year - it only needing a little time to properly lube everything in the box.

My old tractors required only SAE 90 gear oil in winter conditions, but SAE 140 was required during the warmer months. Modern oils with wider viscosity/temperature ranges can be substituted these days, of course, in many cases.

I was just checking out the warmup chart on the Kitamura CNC at work. It has different warmup schedules depending on how long it's been off. If it's been off for a week, the warmup routine is 4 hours, starting with 20 minutes at 10% max spindle speed! This is as much for lubrication as it is for accuracy due to thermal expansion, I suppose.

I recall trying to kick over an old 45 cu in Harley with straight 30 weight oil at 15°F. I was down to my t-shirt before I got it started.