Adjusting the horizontal mill

First, you might try fitting a 6-pole 750rpm motor. This example from Bearing Boys is 0.75kW, other powers available.

Second, control it with a VFD. The 6-pole/750rpm motor is better placed to deliver lower RPM whilst maintaining power output than a 1500rpm motor.

Torque and Power make my head hurt, but I'll have a go at explaining it/

• Power is the rate at which work can be done, note time and a unit of work are both involved. An example might help to put the two in perspective. 1HP (746W) allows approximately 1 cubic inch of steel to be removed in one minute. Therefore a ½HP motor will take twice as long (2 minutes) to remove a cubic inch of steel, and a 2HP motor will remove one in 30 seconds. As about 1 cubic inch per minute is a rate of work that suits most ordinary workshops, 'our' machines typically come with motors in the ¼HP to 2HP range. I guess Model Engineering tools average about 1HP, which is always a good guestimate - not too weedy and not over the top bonkers.
• Torque is twisting force, measured in Newton Metres. Imagine a stopped motor at the moment it's switched on. As RPM is zero, the motor cannot be doing any work - it's power output is also zero. Torque is the force that accelerates the motor up to speed. When the motor is delivering power, applying a braking force greater than the Torque will stall the motor.

The amount of Torque produced depends much on the design of the motor. Single-phase motors and petrol engines both suffer low torque at low RPM, and need to be up to speed before the load is applied. Three-phase motors have much higher starting torque, and stepper motors deliver maximum torque when stopped. All motors lose torque when over sped.

Even though it's vital I tend not to worry about torque much. Machine tools aren't heavily loaded from the get go. Therefore, provided the motor is of reasonable power, it should have enough torque to start and accelerate up to operating speed. If the operator stalls a machine running at design speed, he's working it too hard. Bad driver: back off. Unfortunately, overheating, mangled brushes, sheared gears and other damage are likely before a stall finally issues a Red Card.

The graph means the motor delivers best power and torque at 60Hz. No surprise in a motor designed to run at US mains frequency. The motor maintains Torque as the VFD frequency and hence RPM drop - it won't stall. But the rate at which the motor can do work does fall with frequency. It's power output is halved at 30Hz, so it would take twice as long to finish the same work done at 60Hz. The graph also shows that speeding the motor up maintains the power output (it doesn't increase), but the torque falls. It's twice as easy to stall the motor at 120Hz than at 60Hz. (A VFD is likely to reduce this effect electronically.)

Loss of torque and power might make the VFD sound a dicey proposition, but speed control is a major advantage that usually outweighs the disadvantages. For example, surface speed is arguably more important when cutting metal than power or torque. Certainly true of my lathe which I measured with a Wattmeter: even though the motor is good for 1.5kW out, I never use anything like that during normal work. Usually power input is between 200W and 1000W, with most cuts taking about 800W. The VFD, helped by a simple two speed belt runs the spindle at the RPM I want without any practical effect on power or torque.

When speed is changed with a gearbox or belt drive the ratio effects speed and torque inversely. The relationship is simple: decreasing speed increases torque, whilst increasing speed decreases torque, both pro-rata.

Dave

VFDs are a convenient way of using an existing 3-phase motor, and make changing speed simple. They also have other useful features such as soft start and current limiting. But consider the following:

1. Some people don't understand how VFDs work, and the tradeoffs involved

2. Some modellers don't run their machines to the limit, so a VFD is often fine

3. Some people don't know the following equation: Power (W) = torque (Nm) x angular velocity (rad/sec)

Andrew

At least my machines are ex-industrial so they don't issue red cards when you stall them.

Andrew

Ok, finding one that is rated for full power at 750rpm makes sense.

I found this one that says it runs at up to 1400 rpm, albeit with less torque.

**LINK**

Would this be a suitable VFD?

**LINK**

Steve

The data implies the motor will only run at 1400rpm (100Hz) intermittently, ie, not continuously. I can't see anywhere the duty cycle for intermittent operation. It will only run continuously at 1000rpm (70Hz). Otherwise it seems fairly standard and does what one would expect below 50Hz. The only caveat being that at very low speeds the power output is negligible.

The VFD is basic but seems to be fine. The easy guide gives clear diagrams for fitting external speed and direction control switches. In standard form it is not possible to fit a potentiometer for controlling the speed. One would need to buy the optional I/O expander.

Andrew

Back to this project after more than a year! Project status: Burke #4 horizontal mill works, however, when using slitting saws to cut steel etc, the minimum 250 RPM is way too high, resulting in burnt chips, lots of heat, worn out cutters etc. my understanding is that it needs to be more around 70 RPM to get good results.

I really want to use the mill for other projects now and don’t really want to embark on another engineering project - so the simplest and quickest, not necessarily the cheapest solution fits the bill.

current rpms:

Options

option 1 - extra countershaft. Another project, probably won’t be cheap once finished, lots of fabrication etc. not much room for it anyway. Don’t fancy it if it can be avoided.

option 2 - new/existing motor with reducing gearbox

looks like quite a good option. a 3:1 reducing gearbox would give the following approx rpms:

so now I have the minimum RPM I need but the max RPM is low! And such gearboxes don’t really seem to exist.

Option 3 VFD as proposed previously.

Start to drop off in power below 750 RPM. I really need to run it at about 450rpm. So maybe no good. Also, 3 phase Is a problem.

Option 4: DC treadmill motor

lots of US people seem to use these, swear by them, but I don’t really know where to start.

Any brilliant thoughts or inventive ideas very welcome!

Steve

Option 5: lower speed motor. 1400 is iirc a 4 pole motor. 6 pole and 8 pole are available.
8 pole would halve the input speed for no other changes.

Might be enough?

Steve,

You have three separate problems in reality.

1 you are starting to go around the circle again

2 spindle speed

3 table feed speed

2 can be fixed with the 12" countershaft pulley solution from page 2 that you suggested and from Jasons post gives you 80, 250, 750 spindle speeds that you seem to have rejected for some reason. 90 is fine for slitting saws on my Centec 2A. Keep the smaller pulley and belt in the drawer for when you want to use end mills ( assuming you can find /make a B&S taper cutter holder.)

3 is based on feed speeds for a 12 tooth cutter. Buy cutters that have a tooth count matching the machine.

That motor is an 8 pole which is quite a lot more expensive than one of their 6 pole motors. I bought a 6 pole TEC motor with both feet and flange mounting, and the feet can be removed if not required. It is 0.75 Kw and can run at 1400 all day long. I also bought a VFD and have it programmed for 25 to 75 Hz, it can run at 100 Hz if needed. Your choice of inverter is good because the Inverter Drive Supermarket has the truely excellent "quick start guide" which is printable as a pdf and makes wiring including remote controls plus the most used programming parameters so easy, even I could do it. Having the motor speed control should be not be seen as a substitute for the speed changes by belt, but as extra versatility.

Edited By old mart on 15/04/2023 21:30:47

Hi Dave

It is true that I’m going around in circles again to some extent

The reason for rejecting the pulley based solution was that it didn’t work quite as Jason anticipated. A 12” pulley would give around 250 rpm at the countershaft 119, 244, 325 at the spindle, given the ratios of the existing pulley. I actually need around 150 at the countershaft, which would give 71, 146, 195 at the spindle.

This means I need a motor that runs at about 450rpm, to get 150 at the countershaft- which is too low for VFDs without losing significant power, in theory anyway.

Now, I could go for a motor which runs at 750 rpm AND a bigger countershaft input pulley. That might be the way to do it, and by changing countershaft pulley, I’d have access to higher speeds if I need them.

Steve

Try the pulley first -- 71 is too slow if 119 is too fast, I don't think 119 too far off to worry about.

Any reason not to go for a 12 pole, 3 phase, 1 hp, 450 rpm motor that popped up on eBay?

12 pole motor

it’s cheaper than the 12” pulley and would provide the “correct” speeds.

Steve

That advert is strange. Many photos, but none of the data plate. Maybe it is 440V only.

it is a little strange but the guy selling it seemed sensible. It is a 3 phase motor which isn’t totally ideal but I have a converter I use for my surface grinder so it’s feasible. At £55 it’s a cheap solution which only needs wiring and mounting, so I’ve ordered it and we’ll see how it goes. Hopefully I’ll have a usable horizontal mill next weekend.