Face mill decission

[JasonBModerator @jasonb]

Buffer, here is something else you might want to try that i was thinking about a couple of weeks ago and this thread made me try it out as I was carving a cylinder out of some iron bar today.

The **GT non ferrous inserts seem to work well on smaller lighter lathes so why not try the same with milling inserts and use the APKT polished ones instead of the "blunter" APMT ones.

I took a few cuts with the APMT and then swapped out the inserts for APKT, machine sounded happier, speed and feed could be increased and the swarf looked a lot more like chips than what was being thrown off by the first inserts.

63mm 5 insert head. 750rpm which equates to just under 150m/min cutting speed. feed rate of 150mm/min which is 0.04mm chip load. DOC of 1mm and the cut is approx 30mm wide.

[BufferParticipant @buffer]

I had a look at my vfd this afternoon and it does indeed have a frequency display but it is on the wall around towards the back of the machine and I’ve never really looked at it. Anyway it seems from that I haven’t been running the machine anywhere near 50hz, I think I have been running it way too slow. Probably just been a bit wary of cranking the speed up as it just felt a bit more comfortable at slower speeds. So I’ll try it at this speed and try a few pulley changes and see how it goes. I also tried the shiny inserts and they did give a really nice finish on steel. I didn’t push it though as I didn’t want to break them but I’ll definitely use them on the next job when it comes up.
thanks for your help I’ve learnt a little bit more again.

[Anonymous]

The trouble with running a VFD below 50Hz is that they have a tendency to keep phase currents, and hence torque, constant. Since power is torque times velocity as the velocity decreases, but torque stays contant, the power also decreases. Given the motor is only 1/2hp it really needs to be putting out a 1/2hp.

Andrew

[JasonBModerator @jasonb]

Buffer, do you tend to just leave the belts in one position? If you are leaving it in the highest speed ratio and just slowing the motor to get the speed it is happy cutting at that is not ideal.

Better to shift the belts to get in the ball park of the required speed and then just use the VFD to go up or down a little as needed

We are not particularly suggesting that the spindle needs to run faster but that the motor will be at it's best around the 50hz frequency. for example if the cutter is happy at say 500rpm then select the slowest belt ratio and run the motor at 50hz rather than leaving the belt in the highest speed ratio (2780rpm) and run at 9hz to get the same 500rpm spindle speed.

Edited By JasonB on 27/09/2022 07:35:15

[BufferParticipant @buffer]

Yes I have left the belt in the same mid position since I got it years ago and have been slowing the motor to get get the speed I was happy at. I can see now how this is not ideal and I will try a pulley change when I next use it. I thought the vfd was great and had no idea I was only using about 1/4hp!

[Martin ConnellyParticipant @martinconnelly55370]

Buffer, the speed you are happy at may not be the speed the machine and set up is happy at. If you have not done it yet I suggest reading up on speeds and feeds to get some idea of making the best use of the machine and tooling. You do not need to follow the numbers exactly as they are generalisations but you might find they are a good starting point for experimenting from. Here is a previous thread regarding this Thread 95687

Martin C

[Neil LickfoldParticipant @neillickfold44316]

On my mill, I have the pulley ratio so that at 50hz is 700rpm. I can over hz to get 1500 rpm for small cutters at low loads, and can slow down below 700 rpm, again with low loads. If I go above 50hz, the power is actually less than at 50hz and if I go below 50hz the power is also less, as a percentage of the hz, for example, 10hz 20% of the 50hz at 2kw, so will be down to 0.2kw or 200 watts at 10hz.. It's true that the motor can be over current and get more power at the lower revs, but at the cost of shortening it's coil and insulation life. Using the nonferrous inserts on a home machine can often work very well, especially if the out hard skin is removed first, or if you are not trying to be cutting the harder materials above 30Rc

[Anonymous]

Posted by Neil Lickfold on 27/09/2022 19:09:22:

I…above 50hz, the power is actually less than at 50hz….

Above 50Hz the VFD/motor combination should be constant power. As the frequency increases above base frequency (50Hz) the phase currents, and torque, go down as there isn't enough voltage to maintain the currents. But the speed increases. The decreasing torque and increasing speed cancel out, giving constant power.

Andrew

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