Resurrection of a lobotomised Maho 600T

[Finngineering]

The new servos I got with the machine are:

X+Z (Maho): Leadshine ELM1000FM80H-SS, 3.18Nm @ 3000 rpm
Y (Maho): Leadshine ELM2000EM130F-H, 7.7Nm @ 2500 rpm

I got a (German) manual with the machine. The below pages are from that one. In the first you can see the "nominal" torque of the original motors, the transmission (gear) ratio and in the second one the adjustment instructions for the slip clutch.

So at least on paper, the torque of the new servos are on par with the original ones. Base on the original servo motor data sheet, I had a feeling that the original servos would have had more peak/dynamic torque. But based on the Y-axis (Maho) slip clutch setting, maybe not, . In fact, the Y-axis (Maho) transmission ratio is 1:3.27, so the 8 Nm "nominal" torque on the drawing would translate to 26 Nm, which would just barely cause the clutch to slip.

There are two things I don't like with the new servos. The rotational inertia is quite low for the X+Z (Maho) axes, which could lead to stability issues, and the drives for the same only accept step + dir inputs. The Y axis (Maho) drive accepts analog +-10V, which is what I would prefer. I have ordered a Mesa 7i95T card which uses step + dir, so that is what I will use. There is a risk that LinuxCNC would start trying to correct for a slightly incorrect position before the drive has even "commanded" the servo to move there. But others have apparently got it to work. I guess the closed control loop in LinuxCNC just needs to be slow enough that the servo/drive loop is faster.

 

[AlfaGTA]

Would expect this to be assembled dry. It only moves when there is a crash. Oil or grease will squeeze out over time changing the set breakaway.
Might consider dry graphite spray on the friction surfaces which will remain constant and potentially prevent corrosion on the friction faces.
Cheers Ross

 

[Finngineering]

Yes, I would also be inclined to think it should be assembled dry. What I did not write was that when I disassembled it, there were a significant amount of oil on all the surfaces. I believe this was because of the disintegrated ball screw seals. I fear that if I had assembled it dry, oil might again get there from the ball nut, which would tend to lower the breakaway torque. I did renew the seals, but the inner lips of the seals are running on a bushing, and between this bushing and the ball screw shaft, there is no seal. So oil might get past even with good seals.

I don't want the table to come crashing down because of the friction clutch slipping. This is in my opinion a bigger concern than crashing the machine by direct operator (=me) mistake. Now, most likely the friction clutch would start slipping noticeably before suddenly "dropping" the table. Still, if I crash the machine I have myself to blame, and have to accept that. If the table suddenly drops to the base and destroys something, there is not much I can do about that. Except make sure that the clutch tends to tighten over time rather than the reverse

 

[Nerv]

It was interesting watching RotarySMP's video. I've done similar retrofits in the past and I could see for this specific machine the value in going the route that he did, especially since this machine doesn't even have a toolchanger, much like retofitting a CNC grinder. What is really nice about Mahos and Deckel Mahos, as well as early DMG machines is the external E-Stop chain. That is significant safety that you don't even have to think about that is retained without having to recreate it via logic in the control itself. If you hit the E-stop button, the machine will stop no matter how messed up the control install may have been. Most controls aren't like this so this alone is one of the reasons why a Maho is a great candidate for retrofitting, from an electrical standpoint. Mechanicals and glass scales obviously set it several leagues beyond any of the cheaper stuff out there, I personally cannot imagine why anyone would start with a hobby machine but then again my MC800H weighs 40,000lb so obviously size and weight matter more to some than others.

Now, if you have a VME based control, something 532 or early Millplus, I strongly advise against a retrofit and reach out to a few of us here on PM and we might be able to help you repair what you have, even the 432 CNC5000 controls may be worth repairing. The VME controls have amazing capabilities with some pretty great trajectory planning that allows the machine to maintain cutting accuracy even if the G-code is calling for motion that will cause unwanted servo acceleration and deceleration. Try this with an older control, like Fanuc and you will hear the mechanical's taking a beating during an elaborate feedmilling operation or other high speed milling op, I wonder how well RotarySMP's retrofit handles this. There are somewhat more industrial PLC controls that are available too that might be worth considering that could make the machine more valuable if it were to be sold one day. I picked up a Siemens 802C control for low cost that is setup for driving analog amplifiers and still is an industrial control, when I have people looking at machines a DIY retrofit control can really kill the deal, after all, what has been done and who can support it? When we do these projects it's easy to think we'll keep the machine forever but this is a "hobby" that can turn serious pretty quick and next thing you know this machine is out and more and more bigger and better machines take it's place... and start making you money in the process.

Best of luck with your project!

Nerv

 

[Martin P]

Just a note on "table crashing down":
as I have worked on the servo of my FP4AT some, trying to figure out issues, I had the servo out several times.
I found that the "Z" (Deckel Z is table up/down) with servo removed will slowly run downwards by itself, but not accelerate.
Looking at it, it looks like a slow downfeed of constant speed. It does not accelerate. If it would, that could be bad.
In this state it can also be pumped up with a jack, which can be useful.
So my point being that both brake and clutch have to hold very little to just hold the Z in position.
Its not like there is a huge force constantly stressing things.
A clutch gummed up with old oil will be almost unreleasable, so any torque value will be pointless.

 

[Finngineering]

While there is really not much left of the original E-stop chain, I do plan to recreate a similar one. The only thing I'm not sure of at the moment is if I want to remove the power to the servos directly, or with a small delay. With the delay, they might brake for a bit (the documentation is not 100% clear) instead of free coasting. It appears that the Maho was short-circuiting the servo armatures in an E-stop situation. But I don't think the servo drives would like me if I did that here. I might do some tests later on to see if the delay is worth it.

Regarding the control system, I will stick with LinuxCNC. Like you Nerv say, there may be better ones out there, especially if taking the resale value into account. I have also heard about the Siemens ones being fairly affordable. However, I'm not much concerned about resale value in this case, even if it would go out as scrap one day. I also have some experience with LinuxCNC from a couple of other machines. And if I later want to add capability like a 4:th axis, rigid tapping, I know that LinuxCNC can handle "at no cost". LinuxCNC has the G64 Path Blending G-code, which may be similar to what you spoke about in the VME controls.

I may have to revisit the Y-axis (Maho) slip clutch at some point. If the table indeed falls down at more or less constant speed, maybe my fear of the clutch slipping is unwarranted. I might have to do some tests on this as well when I get the machine back together and the servos/brake powered.

 

[AlfaGTA]

Don’t think any delay in shutting off the servo’s is something you would want.
Need motion and power off immediately.
As to freewheeling on the vertical slide , think the rate of movement has much to do with the state of repair of the elevating screw. ( ball screw).
Will add that on my daily driver FP4NC I had a weak brake within the “Z” servo such that the vertical slide would creep down when power was off.
Movement would continue till the screw bottomed out( could damage the ball nut)
Was bad enough that it prevented use is the control graphics. When selected turning the graphics on the servos are turned off. When this happened the vertical drop would trigger an “E” stop
(Detected slide movement with no movement command)
With power off only way to stop the slide from dropping was to pull out the hand wheel. (mechanical handwheel friction device was enough additional friction to hold the slide)

Finally cured the problem by replacing the servo(e-bay) with one that had a good brake.
Cheers Ross

 

[Finngineering]

Delaying powering off the servos can lead to a shorter braking distance. But that requires that the servos support it, handle it fast enough, and handles it safely. It's difficult to be confident about at least the last part with the Leadshine ELP drives I have. So yeah, maybe it's best to just cut power immediately.

On my MH600T, I believe there is a mechanical stopper for the vertical slide that takes the load prior to the ball nut clashing with something. Anyway, I need to investigate this a bit more once I have the saddle and table back on.