Starmill Wandering Y Axis

[NylonAdmiral]

I seem to be having yet another problem with my mill

Its a Starmill and running the newest version of VR Milling 2.

I first noticed the problem when I found that while finishing surfaces in a mold I was machining, the Y axis almost seemed to be wandering.

The attached picture shows an example of what I mean. Each of the black circles is the intended operation, each of the red circles is an approximation of the actual operation. Cutting from left to right, the problem seems to get worse and as far as I can tell it seems to only be along the Y axis. However I repeated the same job again and the amount by which the actual operation seems to "wander" is different so I'm sure its nothing to do with the .fnc file.

The only clues I have as to what may be the cause of this is that when I repeated the job, it paused part way through the program and displayed the error message "Invalid Pointer Operation", however when I clicked the play icon again the program carried on as usual.

I had recently swapped round the PC's I use to operate the mill so I thought I would try the old PC again which had always worked perfectly. When I tried to connect to the machine in VR milling it couldnt find it, however when I swapped the PC's back again there was no problem.

I considered the fact that it could be a faulty RS232 cable, however I don't have one to swap over. Does anyone know if Denford still sell these?

As always, any help is much appreciated

Thanks

[Denford Admin]

I think the Y axis is stalling slightly...
The quickest way to tell would be to run the program at very low feeds so you have the most torque out of the motor, then again at higher feed - which should 'wander' a lot more.

[NylonAdmiral]

Thanks for the quick reply. The program I am trying to run takes ages to complete (probably circa 20 hours) so I ran a test program this morning which was basically the same as the 4 circle example I attached to my first post. In my new test program each of the circles was a half ball shaped cut out in a piece of stock and mysteriously this seemed to work OK.

Now that you mention it, the last couple of parts I have machined have been out of tooling block so I have been using higher feed rates than I previously would have done.

I will run the program again over night at a slower feed rate and see what happens, hopefully I will be able to post about a succesfully machined part tomorrow!

If the Y axis is stalling slightly as you have suggested what would be the cause of this? A tired motor or something?

Thanks again!

[Denford Admin]

It could well be down to mechanical friction caused by lack of lubrication or the gib strips not adjusted properly

Because of the age of the machine, the stepper motors are probably not quite as good as current day ones and performance may be improved just by replacing with a new motor.The ones you have now will probably lose torque at lower speeds than newer ones, so you may be pushing the limit of their performance.
We have certainly seen less machine stalling over the years as the motors we fitted got more better

The slower speed test results should tell us something in the morning...

[NylonAdmiral]

It could well be down to mechanical friction caused by lack of lubrication or the gib strips not adjusted properly

I had also considered this. Before re-running the program I carefully cleaned and lubricated everything. Thankfully someone has been kind enough to post the Starmill manual on this forum somewhere so I used that to check the gib strips and they seem to be OK.

Unfortunately I had forgotten how long this program would take to complete at a slower speed and its still going, although (touch wood) slowing it down does seem to have cured the problem. Hopefully it will have finished by this time tomorrow!

[Denford Admin]

I'm guessing then, that the machine is loosing position/stalling because of the high number of small direction moves (if you're cutting moulds?)
Are you also running V2 milling in Turbo mode ? You may also find that not in turbo, the part will come out ok, but again, will take ages because each and every small move has to accelerate and stop

The older control cards weren't very good at blending moves together (in turbo) and often lost position when contouring...the latest V5 milling and USB control cards solved this problem with higher buffer/information download speed (USB) and clever look ahead control which can scan over a large number of blocks to determine the overall vector speed required to avoid stalling.
If you imagine a toolpath made up of very tiny moves and change in direction, but which overall end up reversing the direction of an axis, then the new card will spot this and slow down around the corner, whereas the old card would carry on at full speed and stall because it was happy that each small change in direction could be handled (ie, without looking at the bigger picture)

[NylonAdmiral]

The program completed during the night and this morning I found that everything had worked fine. As you guessed, I was trying to run the program in turbo mode when I was having problems, and yes there are a high number of small direction moves. Are you suggesting then that this stalling is likely to be a direct result of using turbo mode? Does that in turn mean that if I were to set higher feedrates in my program code, but not use turbo mode, that its possible to reduce run time with out having this problem or have I just misinterpreted you?

Thank you so much for all your help again, I'm always so impressed with how helpfull and quick to reply everyone on this forum is, not to mention how knowledgeable! Although long run times can get tedious I'm just really pleased this wasn't another expensive/time consuming hardware fault!

[Denford Admin]

We found that the only way to avoid stalling was to either lower the feedrate, or not run in turbo.
However, with so many small moves, running out of Turbo would probably take the longest time, as each move has to accelerate, feed and decelerate to a stop
This is why we worked closely with Baldor to get out latest machines 3D contouring as fast as possible without stalling the motors.

If you had the money, it would definately be worth updating to the new USB control card and v5 milling

[NylonAdmiral]

I would love to upgrade but I can't really justify the expenditure at this point

However I think at some point I will probably start looking for a Denford machine capable of machining larger parts and sell my Starmill in exchange. Maybe that would be the best time to upgrade.

Thanks again.

[Denford Admin]

The VMC1300 would be ideal then