Problem blowing fuses after belt drive upgrade of spindle.

[maxspongebob]

So, here is the issue. I have this milling machine, it's a G0704. As I bought the mill, the spindle runs off a cheap Chinese DC, permanent magnet motor. The motor is supposed to be 1HP, 4300 RPM, 12 Amps 90 Volt DC. I have never been able to get more than 3800 RPM out of the motor and I have never been able to measure the HP but I suspect that it peaks at about (3/4).

Also, there is a two speed gearbox between the motor and the spindle. There are 3 gears in the gear train. In high gear the spindle is under driven with a ratio of 0.56:1 in low gear it is 0.32:1.

My desire was to increase the RPM of the spindle to about 4000. Also I wanted to get rid of the gears because they are made of plastic and I have stripped them out a couple times. To do this, I replaced the gears with a pulley and a belt. The pulley has a high and low speed where the high speed is a ratio of 1:1 and the low is 0.64:1. The spindle also got new bearings in the upgrade. I did a bearing break in procedure and didn't have any problem. The spindle spins relatively freely.

I have attached a couple photos to show you what the system looks like.



SO HERE IS THE PROBLEM:
If I run the motor on the low speed belt setting, it works. The RPM is up where I would expect it, about 2100. However, the amount of current being drawn by the motor, with zero load, is 8 Amps. If I put the belt on the hi range, I can't increase the RPM of the motor more than 1000 RPM or the fuse on the motor driver will blow. It is a 10 amp fuse.

If I remove the belt and run the motor, it draws about 4 Amps at max RPM.

What do I need to do?
Is this motor just a piece of crap that sucks too much power under load?
Is there too much resistance in the spindle and belt drive?
How much power does it take a just run the pulley and spindle?
Shouldn't I be able to get at least 1HP out of a motor with 90 volts DC at 10/12 amps?

Fun Fun Fun...

Bob Dixon.

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[mactec54]

It sounds like you have too much preload on the new Bearings, the belt drive is a bad choice also, should be using a poly-v-belt they have a lot less drag than those V belts, but the over all the motors on these mills have never been great performers

[maxspongebob]

I think the bearing preload is OK. The spindle is not hard to turn at all. I should mention that the actual spindle bearings were not changed, the one's that were changed are the quill drive bearings. Understood on the belt choice. But should it really take 8 amps to just spin the pulley?

[mactec54]

I think the bearing preload is OK. The spindle is not hard to turn at all. I should mention that the actual spindle bearings were not changed, the one's that were changed are the quill drive bearings. Understood on the belt choice. But should it really take 8 amps to just spin the pulley?

No it should not take 8 amps to just spin just the pulleys and spindle, there is a lot of drag somewhere, even with the belt off, the amps are too high, should be like 1/2 an amp or less with no load

[kawazuki]

90v @ 10A = 900W and from memory 1.5kw equals about 1hp so if correct you are about 0.6hp.
But something is wrong - if the motor has no torque like you describe i'd start looking at the brushes / commutator for damage / wear?

[sparkness]

746Watt =1HP. But at about 65-70% efficiency 1HP draws 1100 - 1200Watts , I agree motor should not draw 8A with no load or only pulley load. open the motor up and and have a look

[maxspongebob]

Ok, I have been working this problem. First step was to make it easier to replace the fuses that were mounted on the KBMM-125. It was a pain to lay on the ground and use a screwdriver to pry out the fried fuse and replace it. So, here is the solution for that. It kills me to do it this way as the inside of the controller was so clean before. Oh well.

[maxspongebob]

Second step is to order a Treadmill motor off Ebay. Here it is. The new motor needed a mounting plate and a new pulley. Scott at BenchtopPrecision is making a pulley with a 17mm hole for the new motor shaft. For the bracket, I will have to limp the mill along just long enough to make it.

New bracket.


Bracket mounted on the motor.


Bottom view.


All ready for the new pulley and we will be good to go.


A couple more notes.

I remembered that the fuses I was using were not the slow blow type. Made that switch and at least the mill is not blowing fuses right and left.
I also replaced the Horsepower resistor on the KBMM-125. The old resistor was for a 1 HP motor and was .01 Ohm. I installed a 1.5 HP resistor, .006 Ohms. Now the KBMM will send more current to the motor when it needs it.

[MiniSoCalCNC]

Looks like youve already sorted out part of your problems ... but

The recommended fuses and HP increasing resistors for my KBMM-125 were [ based on the Volts and HP ratings of my motor ] ... .006 Ohms resistor and 25A fuses ? Perhaps Im over amp'd on my fuses ?




output voltage with MAX trim pot adjusted to almost absolute maximum. When motor is connected and running - this voltage reads 134+ [ seemed odd to me ] RPM was well of 7500 at this setting and motor got warm Fast.



After adjusting the MAX trim pot down to around 90v - 100v - I backed into the trimpot setting based upon the rpm readings I was getting to make sure I was getting things into the motors listed Duty Cycle ranges whilst maintaining the High Speed rpms I wanted - getting over 6,000 rpms no load at about 90-95v ...

https://www.facebook.com/steve.aylor...3187320444205/

I really like your bracket! Need one :-) Need to talk to your belt kit mfg now to see about an X2 Sieg kit and a cutom pulley.

How'd ya cut the threaded end off the motor ? [ my mini mill and stuff is in hundreds of pieces atm ... ]

[MiniSoCalCNC]

Enclosure looks sweet .... working on mine as well - time consuming especially for a non electrical guy like myself.

maybe relocate that ground position - put the power supply where the vertical mounted KBMM was and put the KBMM on the case sidewall by where the gound was. That little KBMM-125 becomes a major space hog with that gawd awful heatsink bolted onto it ... gave me fits trying to incorporate it and all the other components into a std PC case so I had to make my own box ... [ mistake on making own box but ] . Almost gave up and was going to house the KBMM in its own 3d printed enclosure LOL - space pig.



work in progress - long ways to go ...

Ok, I have been working this problem. First step was to make it easier to replace the fuses that were mounted on the KBMM-125. It was a pain to lay on the ground and use a screwdriver to pry out the fried fuse and replace it. So, here is the solution for that. It kills me to do it this way as the inside of the controller was so clean before. Oh well.

[maxspongebob]

MiniSoCalCNC.
On that ground wire. What I found when I finished the enclosure is that the special DIN mounted ground blocks were not making very good contact to the DIN rail. The green terminal blocks are supposed to have a little metal foot that connects the incoming connection of ground to the metal of the rail. There must be something wrong with mine as there was no connection from the terminal to the box. So I tossed in there that ugly green wire as a quick fix. Someday I will figure out a better fix. Also, yes, it would be better to have that heatsink vertical and directly in the air stream. Putting the spindle controller inside the box was an afterthought and this was the only empty place on the panel.

Your enclosure looks like it will turn out well. I am always amazed how much time these things take to do the job right. Keep at it. Oh, one other thing. I put a digital amp meter on the front of my enclosure. Originally I thought that I would never use it, but it would look cool instead of just a blank panel. I was wrong. It is extremely useful. Just being able to see how much load you are putting on your machine is important and debugging issues is much easier.

[maxspongebob]

BTW.

Your post about the setup of the KBMM is extremely useful. I have looked high and low for information about using this controller and motor combo and found very little.

I did not cut the threaded section off the motor shaft. There is clearance for it. Initially I thought about having the pulley made so that I could use the nut to hold the pulley on by making a shoulder inside the pulley bore. Scott at BenchtopPrecision.com indicated that using the nut and a setscrew was overkill. I agreed, also the nut is left handed and should be right handed to work properly after changing the polarity of the motor.

If you need one of these brackets, I can make one for you.

Also, Scott makes an X2 belt drive kit.

Here is my build thread. http://www.cnczone.com/forums/bencht...are-posts.html

[maxspongebob]

Ok, I finally fished the installation of the treadmill motor. It looks good but I am still having issues drawing excessive current. I took several measurements from the old motor to compare to the new motor. I started by measuring the old motor with different loads. The first is no load (belt removed). The second is the old motor with the spindle engaged with the belt in the low speed position, for the third set of measurements I moved the belt to the high speed position. I also replaced the 10 amp fuses with 15 amp slow blow fuses.

Here are the measurements.

Old Motor, Load = Nothing, no belt

SetRPM	Volts	Amps
200	5.5	.6
500	17.7	.8
750	28.2	.9
1000	38.8	.9
1500	60.0	.9
2000	81.1	1.0

Old Motor, Load = Spindle, Low Speed

SetRPM	Volts	Amps	SpidleRPM
200	7.1	2.0	105
500	21.9	4.5	420
750	33.0	5.7	690
1000	43.5	6.4	950
1500	64.6	6.6	1500
2000	85.2	6.9	2020

Old Motor, Load = Spindle, High Speed

SetRPM	Volts	Amps	SpidleRPM
200	7.7	2.4	150
500	23.0	5.6	630
750	34.3	7.1	1060
1000	45.0	7.9	1480
1500	66.2	9.3	2320
2000	88.2	11.5	3120

So, you can see that at the highest speed the motor is hitting 11.5 amps current draw just to turn the spindle. Then I replaced the motor and re-took the measurements.

New Motor, Load = Nothing

SetRPM	Volts	Amps	SpidleRPM
200	5.1	.7
500	17.8	1.0
750	28.4	1.1
1000	39.0	1.1
1500	60.3	1.2
2000	81.5	2.3

New Motor, Load = Spindle, Low Speed

SetRPM	Volts	Amps	SpidleRPM
200	8.6	3.3	135
500	23.6	7.1	570
750	33.6	7.7	930
1000	45.1	8.2	1320
1500	66.1	9.3	2000
2000	88.0	12.3	2800

New Motor, Load = Spindle, High Speed

SetRPM	Volts	Amps	SpidleRPM
200	9.3	3.9	180
500	20.4	7.4	570
750	35.6	8.1	1370
1000	48.7	9.1	1480
1500	breaker blows
2000	breaker blows

What these numbers tell me is that the new motor draws more current for the same RPM output. The new motor also spins faster given the same DC voltage input. These are expected, and what I was looking for. I wanted more RPM and HP. However, it doesn't look like the efficiency of the new motor is any better than the old one. So, although it is very easy to turn, the spindle and quill drive are causing a significant drag on the motor. Unfortunately I have nothing to compare this to or experience in this area.

On the G0704, there are 4 bearings in the head. 1 pair is on the quill drive, and 1 pair on the spindle. The factory bearings in the quill drive are Deep Groove Ball Bearings, the bearings in the spindle are Taper Roller Bearings. I have replaced the bearings in the quill drive with new one's. The spindle still has the factory bearings. So, I decided to take another set of measurements with the spindle removed but the quill drive still running.

New Motor, Load = Quil Drive Only, Low Speed

SetRPM	Amps	SpidleRPM
200	2.4	150
500	5.1	590
750	6.1	940
1000	6.5	1300
1500	7.2	2040
2000	7.7	2750

New Motor, Load = Quil Drive Only, High Speed

SetRPM	Amps	SpidleRPM
200	2.7	250
500	5.4	920
750	6.8	1510
1000	7.7	2000
1500	9.5	3100
2000	10.5	4200

What I can deduce from these numbers is that without the spindle bearings turning, the motor draws a bit less current. At 1500 RPM, the draw is 2.1 less, and at 2000 it draws 1.8 amps less. Although this is a lot, It still seems like just turning the quill drive alone is taking too much effort.

So, not knowing if this is normal or not, for this kind of system, I can only try to make what I have as good as it can be. What I am going to do next is rework the wiring in the control box to run off a 20 Amp AC circuit, upgrade the wiring to the spindle motor to 12 AWG, and install an amp meter directly on the motor so I can take voltage and current measurements there. Also I have ordered a new set of bearings for the quill drive and the spindle.

Any other recommendations that others have are welcome.

[maxspongebob]

So after working this issue some more, I think I have it licked.

After replacing the bearings in the spindle, not the quill drive, the amount of power consumed has gone down considerably. From what I have learned, even though the preload of the bearings was ok, the spindle was too difficult to spin at higher speeds. It turns out the extra load on spindle was the amount of grease in the bearings. During disassemble I found that the bearings were so loaded with grease that there was no place for it to go when it was turning. This generated a lot of heat and resistance to rotation.

So, I think the problem is solved. The machine is actually better off for it. I upped all the fuses and breakers so it can draw more power without blowing fuses. I installed an ampmeter directly on the spindle so I can see just how much effort it is using, and the bearings are now ALL new with the proper amount of grease.