Retrofitting a 1986 Maho MH400E

29 Dec 2018 10:11 #123061 by db1981

i am back at home and have no tour job for the next two months....
I have started to work on the Maho again removing the dust from a 2,5 year pause, mounting the new ballscrews (X,Y) and attached the servos again.

I have an question:

After switching power on, the servos are loud humming and have some oscilations(no turning, fast vibrations). This is also with control input connected to 0Vm and adjusted drift to stand still.
I can't remember if this has been souch load 3 years ago. I think it is not normal.
The humming is about 100hz (switching rate of the thyristors, but so heavy that you can feel it on the machine).

I have measured the whole control electronics from the Indramat 3trm2, all signals are clean and look like the pictures in the manual. Have done all adjustments descripted in the manual. Checked Brushes and clean Motors, no changes.

Only the 24V Volt in the indramt is about 31.3V. Ok, line Voltage has increased over the years, but theoretical this should be 26-27 Volts. But there is no regulation in the Indramat for this, only rectifier -> capacitor, so i think this is ok.

Do your servos are humming to?

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29 Dec 2018 10:24 #123063 by RotarySMP
Cool that you are back working on the MAHO.

Yes my servos also hum. Gets pretty annoying over time. NOt loud enough to hurt the ears, but annoying.

If you are interested, a friend of mine programmed a gearbox.comp. My programmer didn't want to start with reverse engineering yours, as it isn't really commented, so he started from scratch. If you wonden't mind taking a look at it:

I would be interested to hear what things you did differently. We are still working on a solution for LinuxCNC resetting motion.spindle.speed.out-ABS to S0 whenever a spindle stop is ordered. We are tending to give complete spindle control to our gearbox.comp, and just have LinuxCNC's output spindle commands as gearbox.comp inputs.

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29 Dec 2018 11:00 - 29 Dec 2018 11:00 #123065 by db1981
Ok, and do they also vibrate when you held your hand on them?

I have the complete Spindle Control in the gearbox/Spindle comp, because i want to reuse the old control panel with the manual controls.
I will take a look at your component, i imagine there have been a few differences with the relay control from the Spindle and Brake between your E and our 700-C. The gearbox is the same.
Last edit: 29 Dec 2018 11:00 by db1981.

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29 Dec 2018 12:07 #123067 by Glemigobles
Hi John,

I can also confirm that the Indramat servos hum. In fact, the shafts also spin a little back and forth which you can see if you power on the machine and remove the Y axis cover.

Honestly, for a while I was fantasizing about changing the servos just to get rid of the noise, but that's pretty ridiculous once you start working since the cutting and spindle are far louder. Overall, the Maho is much quiter, even when cutting, than my el cheapo shop vac and air compressor. I have to wear ear protection when I have those on.

When I have more money, I want to buy some nice noise-cancelling, in-ear phones from Emyotic. They claim their completely passive system reduces outside noise by up to 42 Db and the earphones weigh almost nothing. My normal ear protectors get the job done, but they're uncomfortable to wear for any longer period of time and get really hot in the summer. Plus, I can't play music on them.

Anyway, I don't want to knock on the calculator you linked Mark, but it won't substitute for doing the math yourself using the manufacturer's product catalog. There's so many tool materials, carbide grains and coatings, you've got to know what you're cutting with before setting any feeds and speeds. Usually, you can find the tool catalog number and manufacturer name on the shaft somewhere and it's very easy to find the necessary info online.

Good tool salesmen will give you a catalog for free, and reputable tool makers have all the info online. For example, the guy who sold me my machine also bundled up some old accessories for me at a nice price. All the endmills were made by Garant and had numbers on them.

Using those numbers, I was able to find all the info (Vc, feed per tooth, cutting depth, cutting width for all supported workpiece materials) online. Then I simply used the appropriate formulas:


n - spindle speed (rev/minute)
Vc - surface speed (m/min)
D - endmill diameter
F - feed (table)
fz - feed per tooth
fn - number of teeth

The catalogs say when to use coolant and what materials to cut. Tool salesmen will sometimes provide you with extra information, such as expected tool life depending on the job. It's virtually impossible to do any jobs profitably without all this info.

Don't bother with no-name tools from companies that don't provide you with all the necessary info/support, it's a waste of time and money. The beauty of CNC machining is all the math and numbers that allow you to be successful with your projects, even when you're doing something for the first time. Afterwards you can play with the parameters to get a result you want, but you should never be forced to start from scratch, at least IMO.

Then there's the Sandvik site where they also have handy formulas for calculating the required power and torque depending on workpiece material resistance (abbreviated to kc, measured in N/mm2; not equivalent to hardness, but related). That's also great to know when you're using large endmills or cutting difficult materials.

Mahos with gearboxes are best suited to HPC (high performance cutting) strategies, where the high torque and rigidity of the machine allows you to put the biggest tool you have and cut at full allowable depth for roughing (provided proper workholding!), as opposed to HSC (high speed cutting). And, you can do this very accurately (if your machine isn't as worn as mine and doesn't need reconditioning as soon as the retrofit is complete).

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31 Dec 2018 02:48 #123185 by Mccartymachine1
My 1987 mh600c that I am currently retrofitting is almost silent when the 3trm2 is holding the servos still. No hum. The machine only has 82 hours on it.
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31 Dec 2018 07:34 #123191 by cgroth
Mark, unfortunately no mentionable progress yet. Regarding my old Kunzmann with the same servo amplifier I also came across the anoying hum. At first glance I thought about the capacitors as you changed yours. Did not make a difference (and from the electrical perspective it also shouldn't make a diff because there is no DC circuit involved). Finally two things turned out to be the reason: Sticking oil on the bedways, bearings and ballscrews that kept the slides a few microns offset and thus the controller engaged. And since this type of motor regulations does not provide high torque at acurate low speeds it kept its inacurate position, humming. And second: Indramat writes about "Z├╝ndwinkel├╝berdeckung" which primarily means that even at zero RPM the motor will be powered in both directions equally with each sine wave. This was a trade of between stiffness at low RPM and accuracy as the Thyristors can't provide high currents from a DC supply. If you take of the belts from the drives, there shall be only very little noise. Make sure drives are forced to stand still e.g. Sending 0 V to the Indramat when doing so.
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31 Dec 2018 08:57 #123192 by Glemigobles
Hi Cgroth,

So, you're suggesting that if I tear down the machine and clean all the parts the hum might lessen significantly?

I'm asking since that's been my intention for after the retrofit. There's something wrong with my alignments but I can't check for sure before I buy a crane hoist. The Y axis is perpendicular to the Z column to within 0,02 mm. The Z column is perpendicular to the table (up-down) to within 0,03 mm. Yet sweeping the table (with a Haimer 3d taster mounted in the vertical spindle head) results in 0,13 mm of error, and after fitting a special aluminum plate and facing it on the machine, the alignment is off by 0,05 mm. I don't know whether the table itself is so worn, or if the ways are, or if there was some other reason (like a missing shim), but it was always my intention to tear down the machine and see if I can fix it. That would necessarily imply cleaning all the ways, slides and ballscrews.

For now, my parts don't need much accuracy and I have a lot of work, so that's something I've been putting off, slowly buying some gear (like a matching angular straight edge). I'm looking for a surface plate, and I will also need an engine hoist that can handle 2000 kg to disassemble everything. I know that if there's something wrong and I keep using the machine, it's only going to get worse. Luckily, there's a company nearby with fantastic grinding gear and they can grind the table and all the spindle tapers if need be (and even the ways if I'm not too cheap, I'm leaning towards scraping because there doesn't seem to be that much work and they can't grind the main casting anyway).


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31 Dec 2018 12:22 #123197 by db1981
My humming can't be normal.
Of course there must be a 100hz hum, but not like this.
I have to refresh my knowledge about magnetism and DC Motors, this was an era before my electronics education.

After cross changing everything several times, i turned out that there must be an problem with the X-Servo (Tach is ok). When the X-Servo is attached the problem exist equal on which channel of the TRM it is connected. Without X-Servo both other Axis are ~4 times quiter and stand still. Also when you turn the engaged X-Servo a little by hand all Servos start there big vibrations.

Maybe theres something broken with the magnets or the rotor.
I have unmount it, and will send it for testing.

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01 Jan 2019 10:37 #123258 by cgroth
Marcin, I can't promis that this will omit all humming but when I changed the toothbelts I came across the fact that the screws where showing massive stick-slip when turning the wheels by hand. I then removed the cross-slide taper and found very thick, nearly polymerised oil in the oil grooves. I decided to take of the entire slide and found even more oil grooves that were blocked by that oil. After thorrow cleaning and new Oil (it might have also been the wrong oil, bedways have Moglice surfaces which can swell if you use synthetic lubes) everything is running smooth now and numming is much quiter.

A possible yet not approved solution instead of dismanteling everything could be to loosen the tapers etc. and then press new oil through the central lube inlets so that any old oil is rinsed. But first of all I would check if there really is significant stick slip when turning the screws by hand. And check the central lube system for blocked valves!

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02 Jan 2019 10:33 #123312 by Glemigobles
Thanks Christian, I'll be sure to check all the components of each axis for old grime.

Does anybody who has rebuilt a German-style milling machine such as the Maho, Deckel, Mikron, Kunzmann, etc. have any tips on what lifting equipment would be good to get for the purpose?

I was thinking of a crane style engine hoist, but I don't know if the base wouldn't bump against the base of the machine. I'm working in a garage, albeit a spacious one, but I don't have a lot of room for large equipment and the garage door is too low for a standard forklift.

An engine hoist would be good for putting heavy stuff on the table as well, right?

The mill has the spindle motor on top that needs some sort of crane to remove to check the Y axis, and some heavy lifting is required to take off the table to check the X axis (the table weighs 200 kg).

I'm pretty sure a whole bunch of German machinists have done this many times with their machines but I don't speak German and can't ask on CNC Ecke :(

What I can find on Youtube in English is only guys who use forklifts or can take the parts they need off by hand with a partner. The gear on the Maho is way too heavy for that.

Maybe one of you guys has a link to some German-language thread that has pictures of a teardown of a similar style mill?

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