Random stuff, reduction, servo motors, ideas and thoughts

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29 Apr 2020 10:12 #166068 by rodw

thefabricator03 wrote: I would like to be able to machine linear rails flat after they have been welded.
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We were actually discussing this today. I know the big guys machine the linear mounting surfaces and ART (Advanced Robotic Technology) here in Brisbane have devised a method to do the machining on the table after its built. But they are selling $150k high def plasma cutters as their entry level machine. It would be nice to have a machined slot to mount the rails into so you could register the rails to one edge as the Hiwin install instructions mandate. BUt for non high def air plasma cutting table, Is that level of accuracy required for our blunt axe plasma machines?
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29 Apr 2020 10:29 #166074 by thefabricator03

rodw wrote: BUt for non high def air plasma cutting table, Is that level of accuracy required for our blunt axe plasma machines?


Not really. Would be handy to be able to do that for hi def machines. But if you had a milling machine capable of doing that, it would make sense to process all welded plasma machines the same way regardless of the plasma power unit.

I would be using the machine mostly for robotic linear tracks. In robotic precision is king. Although I am working on some sensors to try and lower the high precision barrier, it would be nice to be able to skim the surface of a linear guide mounting surface to get it a bit flatter.

When I built my robot linear track, where I welded I needed to straighten the main beams a little. I then had to grind a section flat near where I welded. I just ground a little then checked with a straight edge. Its pretty flat now but not 0.01 accurate.
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29 Apr 2020 10:58 #166082 by rodw
Stefan, I think for parts like that, I'd get Laser Central in Brisbane to cut the beams and the holes on their tube laser cutter. Then everything would be in perfect position and You just need to tap the holes. I think you could put some sort of fence on a bridgeport sized machine table and slide the rails along to machine the channels. But it would be a long way down an 8' table on my little Seig SX3 mill! with 400 mm of travel. I drilled all the holes for my gantry on it like that using a piece of linear rail as a drilling guide.and it was a pain in the butt!

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29 Apr 2020 14:46 #166094 by machinedude
it is not easy on a Bridgeport size mill either. the problem is the stock is never straight to begin with so that cause problems working on 2 foot sections at a time. story of my life the past 10 months. any kind of linear rail needs machined surfaces to get the most out of them. structural steel tube is any thing but flat especially on a long section of the stuff. so as you mount the rails to it you follow the bow and twist of the tube when it gets bolted down. then you get into a set of rails which is worse yet they need to work together so they play nice with one another. it would not be hard to for things to get sideways on you honestly.

then you get into squareness between each axis. your not going to have precision with a pistol drill and tapping the way most go about it. with plasma you can get away with it but it's not what you need to do for more precision applications.

you could use shim stock to correct some stuff but that's a pain and a lot of work fighting with stuff to get in the ballpark.
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29 Apr 2020 21:52 #166150 by thefabricator03

rodw wrote: Stefan, I think for parts like that, I'd get Laser Central in Brisbane to cut the beams and the holes on their tube laser cutter.


You do know Rod that you are a bad influence on me! ;)

Laser tube cutter sounds like it would come in handy Might have to put it on my list of projects.

I would imagine that transporting lengths of 200x100x9 around the country side would get pricey. My goal is to keep everything in house.
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01 May 2020 06:59 #166279 by machinedude
anybody have any thoughts on the best way to get your encoder feedback from the load side of a machine rather than the screw? i think linear encoders are the way to go with what i want out of my machine but i am a bit lost as to how you do it.i found some information on dual loop feed back with PID but it's a bit over my head at the moment. is there a simpler way to go about it or is the PID the solution? i'm pretty sure i need to ditch the steppers and go with the DMM servo's i have to do something like this because you need the closed loop drives to make it work but beyond that i'm still trying to wrap my head around it.

obviously the electrical side of things is not my strongest point :) i few bread crumbs of information to get me pointed in the right direction would be appreciated :)

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01 May 2020 16:01 #166336 by machinedude
This was something i came across and broke it down into something easy to understand. i was wondering why not replace the rotary feed back with linear feedback and this was the first thing that i came across that explained why you need both for something like this.

would this be a basic outline of what needs to be done?

granitedevices.com/wiki/Dual-loop_feedback_position_control

i know the DYN4 drives can be set up for the velocity mode. and since they are newer i think the drives tune themselves on that end of it. i think older drives were not as easy because this was not something that was done?

so from what i can tell the error and the rotary encoder make up the inner loop which are the P & D values and the linear encoder makes up the outer loop which is the I value of the control loop? and this is all done on the Linixcnc side and linuxcnc is the PID?

so the next question would be what kind of Mesa hardware is needed? breakout board?

and yes Linux is very new to me :)

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02 May 2020 20:40 #166426 by rodw
Nice diagrams. The last one even mentions Linuxcnc. From what i understand, the linear encoder's output is electrically identical to a rotary encoder. So with a mesa card, you'd probably need to add an additional board with encoder inputs to receive the linear encoder (Probably a 7i85s). With the right servo controller, I think you could still continue step and direction signals but you send the linear encoder feedback signal to the stepgen. Have a look at the hal pins for your stepgens now and you should start to see the inputs.

I think it was Hakken who did a video of connecting a linear encoder rail.
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02 May 2020 21:35 - 02 May 2020 21:37 #166432 by machinedude
i was looking at mesa boards and i do believe the 7i85s is what i need to add to another board like the one you used on the space ship build i can't remember the number off hand. but i did get a chance to pull a stepper and measure the bolt circle and it does match the bolt circle on the servo's so just the boss would needed opened up to fit the servo's i have. the stuff on line said they were different but i did buy the steppers between 10 and 15 years ago so something could have changed over the years? i'm just happy the bolt circle diameter is the same for the mounting. now it is just reworking some odds and ends rather than making them over again :)

i seen the videos of the member here that did the same thing with glass scales on a mill i just have not had time to look at them all the process was over like 3 video's. i watched them all but will have to probably watch them several times before it sink's in completely.

i'm sure there are some pitfalls to watch out for. i think the resolution of your scale is one of them. 1 micron would be nice but 5 micron or 10 even might be more realistic for what the machine is. need to look into all this stuff when i get time.
Last edit: 02 May 2020 21:37 by machinedude.

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02 May 2020 23:47 #166452 by rodw
I used the 7i76e and it only has one encoder (for spindle) whihc is used for torch voltage and ohmic sensing so it would also require a 7i85s

When I put scales on my manual lathe, I went with 1 micron scales that cost about $10 more each. But that gives resolution to 0.001 mm (1 micron) or 0.00004 inches. You can't work to those tolerances. Some people put a bit of tape over the last digit on their DRO. I don't know what impact the scale resolution has on the encoder frequency. But you would think that the Mesa boards would handle them.

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