Re: [CAD_CAM_EDM_DRO] Re: Plasma table gear reduction

The best way round the cooling problem is to use a larger motor. With a
larger motor you will probably get more torque per amp. The armature
resistance will also be lower so the heat generated will be lower. It
will also be designed to dissipate more heat. The down side is the
increased cost and inertia. Realistically motor inertia is not really
much of an issue as we are not looking to shave every last millisecond
off of each job.

If you look really closely into stepper system design you will find that
there are also a large number of design decisions that should be made.
In practise most people just use an oversize motor and don't worry about
it. With servos it is the same. It helps to know all the design criteria
so you can make an informed decision but in practise you can overcome a
lot of problems by oversizing the motor. Luckily a plasma cutter needs
very little power in the first place so an oversized motor is still
pretty small.

The big problem with surplus motors is the huge variety of motors and
usually a lack of useful specs. Steppers have a big advantage as they
are much more standardised.

I have finally got my mill running all axes with servos and the
difference is amazing. The maximum rapid rate is limited by Mach2 to
about 180IPM. I have turned it down to about 100 IPM because 180 is just
too scary. Acceleration is about 4 times higher than the steppers.
However the really big improvement is the fact that I can be sure I am
not going to scrap a job because of lost steps.

I am in the process of testing some new features in SheetCam so several
times I have had to stop the code in a hurry when I have spotted
something going wrong. With my steppers stopping a program halfway
through was guaranteed to lose steps. The servos took everything I could
throw at them without missing a beat. Over the last couple of days I
have probably put in about 10 hours machine time. In that time I have
had no motor related problems at all. I have had plenty of software
issues but that is another matter :-)

I have spent days playing with the stepper tuning, running the steppers
slower and slower, trying to fix the lost step problems. I managed to
reduce them but I never managed to totally eliminate them. In total I
have probably spent about 3 hours setting the servo tuning. It isn't
perfect but even so the servos out perform the steppers in every way. I
can probably reduce overshoot and following error by a few tenths by
spending a bit more time fine tuning but the gains will probably not
make a measureable difference to the finished work.

Les

PS anyone want some monster steppers? They would make ideal boat anchors...


turbulatordude wrote:

>So, the armature (all the stuff spinning and the shaft also) have
>only the conductive heat paths of the ends of the shaft. Pully and
>encoder.
>
>The rest of the motor has a only the bolts holding the coils in place
>as the conductive heat path. The case gets hot from radiated heat.
>
>It would be a simple matter of drilling a few holes and adding a
>blower and filter to force circulate air.
>
>But.... what I am hearing is that Servos are considderably harder to
>use than Steppers although they do have some advatages.
>
>I don't mind making a work-around for this or that, but it seems that
>adapting servos to a gear rack gantry is mostly compromises.
>
>Dave
>
>
>
>
>
>
>
>
>
>
>Addresses:
>FAQ: http://www.ktmarketing.com/faq.html
>FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
>Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
>Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
>List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@..., timg@...
>Moderator: pentam@... indigo_red@... davemucha@... [Moderators]
>URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
>OFF Topic POSTS: General Machining
>If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
>http://www.metalworking.com/news_servers.html
>
>http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
>
>NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
>bill
>List Mom
>List Owner
>
>
>Yahoo! Groups Links
>
>
>
>
>
>
>
>
>
>
>