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jacampb2
03-23-2009, 04:00 AM
Last year when I initially brought my CNC B'port S2 back from the dead, it was suggested by John Stevenson and others that I spend the money and replace my factory steppers. The reasoning being that the old steppers were likely never adequate for the job, and being about 30 years old only made the matter worse as the permanent magnets degrade with time.

Anyhow, they have served me well for over a year. I have recently been doing a lot of high(er) speed machining of plastic, and am starting to see problems with my Y axis. I had the motors originally tuned with a acceleration of 8 in/sec/sec(I think this is how Mach lists the units?) and a top speed of 120 IPM. It did fine with normal machining, and rapids were fine, no real issues. Over the last year, it began to very rarely cog the steppers, most of the time while manually jogging it into position. I Slowed the rapid speed to 100IPM, and it never had another hicup. When I started doing a lot of this acrylic engraving, I started to have problems again. The machine would make it though a ~9x4 piece, about 30 minutes of machine time with no issues. The problems showed up when I started running multiple parts on a single sheet of acrylic. After running for 45 minutes or so, it will occasionally loose steps in the Y axis. Normally when it is traversing at rapid speeds in an X/Y direction simultaneously. It makes it to the X destination, but not the the Y. This of course screws up the whole sheet.

So, I slowed the acceleration in the Y axis down to 6 or 7, I can't remember now off the top of my head, but since I slowed the Accel down, it hasn't given me any trouble in the last five or six runs of engravings. I still feel the need to babysit it though, to make sure I am there to stop the program is it looses steps.

Since it seems to be more related to how long the mill has run, and presumably how warm the motors are getting, my question is, is it time to replace the steppers with new units? The engravings have a lot of small moves, so a lot of the time the mill never hits the target feedrates because the acceleration is set so low. It is typically within 10% or so of the target feed rates, but I would love to be able to get it machining even faster. Will new 1200oz or so steppers help at all with that? My feed rates in the acrylic are typically around 50 Ipm. My budget would currently allow me to replace the X and Y steppers, I could swing the Z too, but it has not had any problems. I am just wondering if I might be pissing money away on them. Should I save longer and go servos?

TIA,
Jason

Rustybolt
03-23-2009, 07:30 AM
In the interim can you mount a fan on the steppers?

motorworks
03-23-2009, 04:11 PM
Hi
I run the same type of BP machine with the old steppers.
Mine runs with turbo, and I too can not set the speed or accel too high.
Milling parts is no problem, but using a program that was written in V-carve
I was loosing steps over time.
Slow things down to 75 ipm and a slowed the accl down and it stopped loosing steps.
Slow but gets the job done...but I may look to replace the
steppers and switch to Mach3.
good luck
eddie

jacampb2
03-23-2009, 08:29 PM
In the interim can you mount a fan on the steppers?

The old Boss Steppers have a huge aluminum finned heat sink case, they don't get that warm. They are slightly higher than room temp, but by no means uncomfortable to touch. I am only presuming that the issue is temperature dependent as it happens over time, not immediately.


Hi
I run the same type of BP machine with the old steppers.
Mine runs with turbo, and I too can not set the speed or accel too high.
Milling parts is no problem, but using a program that was written in V-carve
I was loosing steps over time.
Slow things down to 75 ipm and a slowed the accl down and it stopped loosing steps.
Slow but gets the job done...but I may look to replace the
steppers and switch to Mach3.
good luck
eddie

This is pretty much exactly it. The only real question I have, is, are new steppers in a similar size to the reported factory rating going to help... Should I maybe consider new beefy 2800 oz nema 42s. They are 3xs the money, but maybe necessary.

Thanks,
Jason

John Stevenson
03-24-2009, 04:47 AM
No don't go the 2800 route.

Steppers because they are magnetic motors have a detent torque, that's the power needed to overcome the parked resistance of the rotor.

The bigger they are the more torque is needed to free them. Large motors suffer from this enough so that smaller motors with faster acceleration in real world applications are actually more powerful.

Those Superior / Sigma motors fitted to the Bridgy were the same as fitted to a lot of other machines of the same era, Beavers, Matchmakers, etc although they were minus the big finned sleeve.
Bridgies had the big sleeve for cooling as their drivers were crap, even for the day.

I have seen various ratings for these motors 900 oz/in up to 1400 oz/in but when you take the end cover off all the ones I have, off various machines, they are rated at 850 oz /in and 9 amps.

I still have one Beaver running on it's original 42 series motors on a daily basis with no problems but it's running on the 7.8 amp Leadshine drives, shortly to be uprated to 9.04 amp drives when the container lands.
Originally it was on gecko 210's but these drives and the motors didn't get on.

I have also had good results fitting type 34 920 oz motors to another Bridgy MDI machine, again with 7.8 amp Leadshine drives.
This machine can accelerate much faster than the Beaver but in all fairness it's a smaller bed.

.

jacampb2
03-24-2009, 06:17 AM
I have the 7.8A leadshine drives, no current limiting, and running the power supply at ~78Vdc, as close to the drive limit as I dared take it. I am pretty sure I went from 1/8th micro stepping to 1/4 to to get some torque back. This still gives me plenty of resolution, so I am happy with it, but I would still like a higher accell to speed things up.

John, is your opinion still that I may be better off swapping out to the nema 34 kelling motors? I think they were 1280oz/in or something like that??? About $119 a piece if memory serves. Will I see an improvement in acceleration, or have I reached the limit of steppers on my machine?

BTW, The table on my machine is 38x18 overall dimensions. I suspect that the Y shows issues and not the X is due to the fact that there is the weight of the saddle as well as the table...

Thanks,
Jason

John Stevenson
03-24-2009, 07:12 AM
Yes you should see an improvement, Does your machine have pressure oiling and is it getting round ?

.

jacampb2
03-24-2009, 11:09 AM
Yes, it has a Bijur auto oiler. The oiler has sheared a drive pin somewhere in it's gear train, as the timing motor works, but it never lifts the plunger. I manually pull the plunger about every 20 minutes, which is what the factory timing was set at. I always have lots of oil on the ways. My manual mentions gib adjustments, but as of yet, I have found nothing that appears to be a gib strip. It is odd-- it is supposed to have a y axis gib lock too, as it had the rotary table option, but I can't find that either. I think I am going to spring for the motors. It apears that folks are still buying the old boss ones on teh bay for over a hundred each. I will probably just put the pair of mine I replace up for $100 to help offset the cost.

Thanks,
Jason

j king
03-29-2009, 06:20 PM
Hi Jacin.

Bought the new steppers for mine.Boy they are small.Kinda nervous about how they will perform. Time will tell.