Retrofitting a 1986 Maho MH400E

22 Mar 2024 03:43 #296530 by D Jensen
A few more things to add that may be of help for the Linux programing.I've finally got my Maho running faultlessly. The 2 final hurdles were some timing issues with the handshake between the gear-change PLC and the Heidenhain controller. I had to change some parameters on the Heidenhain controller and invoke the coded outputs for machine M calls, then change some of the strobe time durations because of the gear-change PLC was sending back function finished before the Heidenhain controller was listening for it. That must have caused years of anguish for the previous owners as for more than a decade it would have stopped machining intermittently, waiting for a "auxiliary function finished" strobe that had already been and gone. Lovely when it works, but I had to draw accurate timing graphs to be sure the various clocks step increments always overlapped. Not much interest to you guys, but just wonderful when irritating problems disappear.But the final issue may be of interest. My machine has an 18 speed gearbox, but the top 3 speeds had been inhibited by the machine parameters in the Heidenhain TNC 155 controller. So limited to 1250 rev/min maximum. 1600, 2000 and 2500 were not available. These are important when using smaller tool sizes. particularly when I've been routing wood frame members for my vintage Fiat Topolino. 10 times that would be nice.It appears the reason is that the star delta start on the spindle motor was on a simple adjustable stand alone timer originally. So only one star time available for all speeds.The gear-change PLC has plenty of spare capacity, so I added a whole section of ladder code which decides which gear has just been selected, then invokes a timer appropriate for that particular start inertia etc. At low speeds it seems the star time is roughly the same for several gear selections, probably because its mainly the motor armature inertia to be overcome. But around 1600 rev/min things change and the star time gets rapidly longer. To get to 2500 rev/min the star time needs to be 4 seconds, which is agonizingly long. I initially timed these with a stop watch, but once I had it programmed I went through and set them on the ammeter response with a bit of safety margin. I also added the to gear-change PLC an inhibition to the "auxiliary function finished" reply so that the Heidenhain doesn't start machining again until the spindle has switched to delta mode. The previous system didn't know about that and could mill down from the safety distance and start cutting before the old timer went to delta. Nasty.Another reason for inhibiting the speed could be spindle temperature. My Maho has a grinder spindle with taper roller bearings. I recently gave it it's 4000 hour disassembly and regrease. There is a run in procedure where you fit a thermocouple to the spindle and run it up one speed at a time, checking it doesn't overheat. If it does, you simply stop, let it cool down, and try again. So I needed to run in those 3 new higher speeds. If I run it for extended periods at 2500 rev/min it does exceed the recommendation a bit. But I can keep it in spec by using my shop vacuum. The spindle fan is crude as is the way the cooling air controlled around the spindle assembly casing. I'm looking a how to improve that. It's also been fairly hot this summer in Sydney and it's been operating in 28 C ambient.For those of you who are interested I'll add the ladder code from the gear-change Izumi FA-1J PLC. The code at the end is simple in case you need to do something similar in LinuxCNC. It's still a bit open loop. Monitoring the motor current might be better.BTW. I think I've seen comment asking how the cooling fan is driven. Mine seems to have bits missing and the fan was free to slip on the spindle. The section view in the manual is difficult to decipher. In my case I've added a spacer (the white piece) and a thick "O" ring that applies sufficient pressure to drive the fan. The "O" ring sits in an existing groove, but I don't know what the groove was really for. Does anyone know what is really meant to drive the fan? My "O" ring seems okay, but usually a nitrile "O" ring under tension like that gets attacked by ozone in the air and starts to crack. EPDM doesn't, but I couldn't source one. So far so good, but I think it will fail.Cheers,David
The following user(s) said Thank You: tommylight

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