HH Roberts 5Hp CNC mill- torque issue- intermittent

[M. Moore]

Anyone run across an intermittent torque issue? The five horse 3 phase motor is speed controlled by a VFD.
I was cutting some 4140 for a few hours, light cuts, 3/4" end mill. Then I noticed (heard) it suddenly slow down in the cut but luckily I had my hand on the feed knob and slowed the feed right away and was able to keep cutting.
The slowing of the spindle speed was intermittent so I am wondering what could cause that to suddenly appear?

Any chance it is related to a crash? Just a coincidence? I made a mistake and broke an end mill (yes operator error) so I reset up everything and kept cutting for at least a 1/2 hour at normal feed and speed rate and then the intermittent slow downs began.

Machine is from 1998 with the control updated about ten years ago to an Anilam 3000 from a 1000. Max rpm is 4000. I have owned it for just over two years so still a beginner but I am starting to get the hang of it.

Today I was cutting a couple of pieces of aluminum with a 3/8" 3 flute at 4000 rpm, cutting a rectangle pocket at 1/8" depth of cut and heard the spindle slow briefly (when it got to full feed rate) and recover and it did not do it again. A bit unnerving.

Any ideas what is causing this issue? Should I post this in the electrical forum?

 

[Vancbiker]

Have you investigated anything? Is this a belt drive? Is the drive and motor open loop or closed. Checked for loose connections? More info here would help.

 

[007Rob]

FWIW HH Roberts is a machinery dealer, as far as I'm aware they don't manufacture any machines themselves. But I could be wrong.

The problem definitely sounds like what I've experienced with a drive belt going bad or losing tension on a similar machine in the past.

 

[CITIZEN F16]

Belt slipping when it gets hot, motor over heating, voltage drop?

 

[M. Moore]

Thanks for the responses. Starting at the top, I will post this in the electrical forum.
Vanc, I did investigate the motor to spindle belt drive and the cogged belt looked fine and everything seemed good. I hit the back gear lever and ran the spindle up to speed, all good there, and then back to high range and ran the spindle which sounds fine. I don’t know what you mean by open or closed loop? I have not checked for any loose wiring but that is on the list of things to do.

007, Hhroberts is the dealer and this is the fairly large open turret design with 40” X travel and 20” in Y and Z.
There is only 4” of Z spindle motion but manual up and down of the knee (powered). She weighs a pretty beefy 6200 lbs. There is no slipping of the cogged belt and the tension seems fine.

Citizen, the motor is cool as a cucumber and I will check the voltages when I get a chance. I don’t use the mill every day and when this happened I was just making a new bottom die for the press brake.

I did have the rpms down to 1150 with the 3/4 end mill but the fact it did the same thing ( briefly) at full rpm makes me think it might be the VFD or other electrical gremlin. Also the aluminum was cut the next day, machine powered up and then two parts run so no way the motor was hot.

 

[Vancbiker]

Open loop means there is no speed feedback signal from the spindle to the drive or control. Closed loop means there is a feedback signal. This can be a tachogenerator, encoder, pulse generator, or even a single proximity or hall sensor.

Since you mentioned back gear and cog belt that tells me this is a knee mill or similar so most likely an open loop. Rarely do those include niceties like closed loop systems. The negative to open loop is that the drive and control don’t know how the spindle is running. It could be stopped and yet the program still executes. Closed loop allows the drive to monitor the spindle and even adjust power to try to maintain commanded speed.

 

[M. Moore]

Vancbiker,
Thank you for the excellent explanation. It is an open loop as the control does not know if the spindle is turning at the correct speed. When using the back gear you have to tell it to run at 2000 rpm to get 250 rpm.
It is 4000 rpm max and 500 max at low gear.
It is not a super sophisticated machine but I am having fun learning and it has been very accurate so far as it was not run much by the last owner. Hopefully I won’t do any real damage while learning.....

 

[twr]

Hi give Ron a call at hh roberts he is very helpful 9056245536

 

[M. Moore]

Update,
I ran the mill yesterday and all was fine except just once it did the low torque problem however I was just using it for setting up and testing some tapped holes. Just a 3.2 mm hole with 4mm tap in the auto reverse head. All went well. Then I went through the machine and tightened all the connections in the main box. Taking care with the incoming voltage and all larger wires. There were some that were not tight enough but nothing terrible. I called the tech from HHRoberts and he thought it was a phase out so I double checked this morning and incoming power is fine.
Now the motor will not come on when I try to run a program or just turn on the spindle. The rest works fine as it will run the program it just won’t be cutting anything and the coolant pump will come on as well.
So either I disturbed something in there or it was a coincidence but as a rule it is usually something I did that causes the problem.
I ran it with the panel door open and the main power contactor for the VFD does not pull in when it should so now there is no power to the VFD. There is a click sound when I try to turn on the spindle but I can’t see in the panel at the same time.
I can do some basic work with guidance but I am not overly skilled at diagnosing electrical problems and this one has electronics in there as well so it is above my skill level to tackle this one but any help would be appreciated.
Thanks

 

[M. Moore]

As I suspected I had knocked a wire loose and it is now fixed. Motor works as it should via the control.
However I had to relocate the VFD so I could open the cover. Now the spindle pulses at certain rpms, it revs up and down maybe 40 to 80 rpm as seen on the VFD readout. The test was at 2200 rpm and at 3500 it is less noticable but still showing a pulse of 15-30 rpm.
The amp readings for the motor circuit showed a slight reduction in one leg, approx 10 vs 12 on the other two.
I have not yet tested the amp readings while making a cut.
Any ideas? VFD starting to die?