Originally Posted by airbrush  hey mad.sculpture,
what sort of rotary table do you have? did you make it or buy it or have you not gotten that far yet? |
Hi airbrush
My whole setup is theoretical so far, well some of it's in the post to me..
I thought if i was making a rotary axis the difficult part was the positioning of it rather than the mounting and plate which i thought if i was lazy could just be sherline/taig type headstock/faceplate.
so after reading the responses about harmonic drives by other forum members i've been looking into that.
to correct my earlier post, yup they are quite fragile according to the data sheets.They make them like i said earlier as a complete robot arm joint so i thoght i'd easily just put a faceplate on that but even though i found one large one that wasn't too insanely priced the warnings in the data sheets i found on the web have pretty much put me off (suscebtible to impact don't hit it with hammer during installation warnings + vibration ratings which are beyond me)
of course we'd not hit it but if you imagine a robot arm clobbering something - all that leverage (or you crashing the cutting head into it)- don't know how they're robust enough for their intended app. and harmonic drive co. haven't replied to my email queery.
basically at the bottom of my A4 piece of paper regarding parts sources & orders it currently says
FOR ROTARY TABLE JUST BY SHERLINE PROBABLY! Honestly i mean that
of course you could mount the harmonic drive with a coupler like you use at the end of leadscrew/ballscrew (those that cope with slight missalignment)so
that you isolate the drive from certain loads) but vibration will still pass through it some.
I've got a harmonic sevo drive coming but it's almost definately going to be to weak as it's only rated at 21 oz/in continuous 215oz peak 46W - My FXXK up
however a guy on the net's got a sherline rotary table with a 75oz servo that's working(he took normal sherline servo apart to see what made it tick end weakened it lots so now just uses it on table) so if i gear the drive 3:1 via a wide toothed belt i'm still wondering if i may be able to do something with it ie.machine jewelers wax/foam wood at a strech but unlikely.
The belt is obviously going to be inferior to direct drive and introducing the very sort of inacuracys that i went harmonic in first place
but it's also the only thing i can think of (as a mech. engineering dunderhead)
that would provide the protection the drive seems to need.
short story long :
by the time you get a servo/harmonic all in 1 with output flange assy big enough to be definately usefull to us the servo requires an exotic amplifier
that you'll be lucky to find on ebay and if you do i'd guess it'll be $150 - upwards (think 600+ new) and then there's the shock/vibration issue
if you can address these issues though the accuracy is truly all you could want.
ebay just had a very beefy looking unused industrial H.drive on it for $100 so you could prob. google that and still make him offer as i don't think it sold (auction ended last night) it's a stand alone unit so you could put any servo on it (1 shaft in & 1 out 160:1) and just make sure you used a good encoder, then just drive a faceplate in a good set of bearings via a 1:1 belt purely to make sure your creation lasts.
as far as i can see at the moment with what i've learned so far the hard way this is your best bet if you realy want to make one,i reckon $50 motor +100 drive + $50 second hand cheap lathe headstock & then 2 pulley +belt so $200 ish versus sherline $390 i think (none yesterday ebay)
- if you read info when sherline rot.table released it wasn't just the std. one modified so be aware of that as quite a few manual ones on ebay.
'course by the time you've done the work coupled with the fact that you could get it wrong and sherline's guaranteed i'm not sure it's worth it.
I don't know how much a belt in this situation will screw accuracy up,it's going to be all that's holding that faceplate as you machine it but they have a reputation for being much tougher than they look.my dads HP printer shakes the whole shelf as carriage travel changes direction + several big motorbikes have belt drive)
anyhue, sorry it's a bit long winded and defeatest but don't want you/other newbies making my mistakes
final thoughts :
there was a 5" table for $175 i think it was on ebay yesterday,as in manufactured,so they keep postin new ones but are warning the material stock running out 35 left,listings are about 29 dollar at moment but 6 days to go so i think from meory last one went for above figure.
it looks thorough but could only find good reviews of seller not the product(didn't have much time though)it uses planetary set up and huge reduction ratio but don't know what the backlash is - possibly not great as i didn't see it mentioned.
You could just look into nema planetary gearboxes as someone else mentioned in other forum but at least if you're using a worm gear it's got that inherant backdrive resistance
There's always the things we'd like (insane resolution)and what 99% of us can get by with.
That doughty drive link is the unit that Rainnea graphics are using by the way that i mentioned in the previous post & is impressive.
Plus Rainnea Graphics(check spelling in previous post) have lot's of very educational tech stuff for anyone wanting to do 3+ axis particularly software methods of generating G code from any art/model cad files
You also get to see the quality of the work the machines are capable off by the shop/gallery sections
when i say that i mean machine + exellent craftsman of course!
Cheers dude, hope this helps someone
p.s. honestly i'm not the new sherline agent in your area !