http://www.ebay.co.uk/itm/1523042469...&ul_noapp=true
i'm looking to machine wood only on it, no metal and maybe some plastics. do you think this will be ok. Tolerance is not too important as it will be all arty type stuff. opinions please
Printable View
http://www.ebay.co.uk/itm/1523042469...&ul_noapp=true
i'm looking to machine wood only on it, no metal and maybe some plastics. do you think this will be ok. Tolerance is not too important as it will be all arty type stuff. opinions please
It *should*. Accuracy will likely be the issue however if this is not too much of an issue for you, might be fine.
There are many of these around, I'm sure ive seen them cheaper.
hi chaz. any idea where you have seen them cheaper. the only place I have seen is china and then you get vat and import on them so doesn't work out
cheaper. shipped from Germany as well.
what about this one? harmonic drive they say.
I cannot see any accuracy problem, maybe only in the axis alignment. the issues are resolution and torque. with the 6:1 reduction, at 100 mm diameter one full motor step will translate in 0.27 mm of movement.
harmonic drives are in a different league, if they are for real inside..
When I ordered my ballscrews the import tax should have been £68 when they arrived they were marked as 2nd hand ballscrews with a value of $10 each, customs didn't buy this but only put £15 in import tax on.Quote:
Originally Posted by johngoodrich
Import tax is luck of the draw but personally from China I'm pretty lucky anything from the US I get screwed on to the full value.
I thought these bolt to the bed rotating chuck things usually came with a worm drive and an index wheel/pin.
Sticking a stepper in place of the index handle is a CNC conversion.
Replacing the worm wheel with a toothed pulley sounds like a Chinese CNC conversion. More for selling than using.
Maybe you want to engrave toothpicks and this is exactly what you need.
Alternatively, maybe if you bought it as an incomplete conversion, played with it, decided what resolution your usage required, replaced their timing pulley with a nice anti-backlash worm from HPC gears, tried it with the stepper then replaced the stepper with one of those singleton servo drives doing the rounds on eBay.
That could work.
I have one of the type in the first link in the OP, I chose it for the decent through capacity which you don't get with harmonic drive versions, it worked well enough as standard but I added a link bar and bearings outboard at the back behind the belt to take bending load off the stepper shaft and was then able to add a belt tensioning pulley bearing.
A few hours work and a few quid in bearings yielded something which is up to 4-axis machining in Ally and Steel using tooling up to 12mm and 7000rpm.
I had a good indexing head but I believe that a converted/ home brew worm/wheel job is more of a bodge than pulleys, belts & bearings because long term under CNC use gears and plain bearings will require more maintenance, more expensive maintenance and more complex maintenance.
- Nick
Double Post, Sorry guys, gremlins on the interface!
What is a harmonic drive? Do they mean epicyclic?
Harmonic is a brand name and people recognise it just like hiwin,but yes its epicyclic.
Quote:
Originally Posted by Robin Hewitt
Er, no they don't and no it certainly is NOT!
https://en.wikipedia.org/wiki/Harmonic_drive
https://en.wikipedia.org/wiki/Epicyclic_gearing
Both have teeth, that's about it for similarity.
I stand corrected.
So its a Strain wave gearing.
Quote:
Originally Posted by magicniner
Harmonic drives are the perfect solution for high gear ratios, high torque, lightweight, and the most important, they have no backlash. Widely used in multi-axis cnc, articulated robots, etc.
There are a lot of them on ebay, SH from Korea, the only ones at an affordable price...
I was surprised that the Chinese claim to have 4th axis with harmonic drive... possibly used ones fitted inside..
I have been considering something similar for quite some time now, but can't make up my mind about it. In theory it would be very useful for me, it would save time because it would turn around the work piece automatically, but maybe the work piece would vibrate too much so it would be useless for what I am doing, so I am not sure. Never the less, there are plenty on eBay as well, shipping from Germany most of the ones I found.Quote:
Originally Posted by johngoodrich
Anyway, I am interested in the subject, as well as the use of it and the experience people have.
Has anyone bought one of these harmonic drive index chuck thingies? Are they any good? Is the lack of a through shaft putting you off? Who will Magicniner contradict next? So many questions, life is too short for all these questions.
As you clearly prefer your information to be of the non-factual variety I'll leave you to it :DQuote:
Originally Posted by Robin Hewitt
I am sure they are pretty good otherwise they would not be sellable.Quote:
Originally Posted by Robin Hewitt
Yes. That and the fact that it is considerably more expensive.Quote:
Originally Posted by Robin Hewitt
I found his comments very informative and valuable. I don't know what you see was wrong with his comments or choice of words, but I did not find them contradicting, quite the opposite. I would actually be interested in hearing more about his experience and the modifications he did, as well as how he is using it.Quote:
Originally Posted by Robin Hewitt
...so why did you ask the last one then? It was pretty rude and unnecessary. His comments were pretty valuable, as opposed to your last question... :disturbed:Quote:
Originally Posted by Robin Hewitt
Don't worry, he's a Yorkshireman, he's already retaliated.Quote:
Originally Posted by A_Camera
magicniner, do you have any photos of the mods you have done to your 4th axis. I would like to see it. from your experience with these, do you thinkit would be ok for 4th axis wood milling as is, or a bit of modding
John,
I have an early pic of the "proof of concept" I knocked up with what I had lying around,
Attachment 22607
I've subsequently re-worked the spacer support with alternate bearings to reinstate the through capacity of the spindle, it does what I need in brass, bronze, aluminium and steel.
I don't think it would have been up to much without the tensioner to remove backlash, without the support the stepper shaft bends if you tension the belt,
Regards,
Nick
Picked up this harmonic drive some time ago for a 4th axis project, not yet got round tuit. 50 -1 with 25mm through and pre-loaded cross roller bearing on the output ringQuote:
Originally Posted by magicniner
Attachment 22615
I just found these things existed but I already feel a deep need to own one, was this an e-bay find?
Yeah,Quote:
Originally Posted by johnsattuk
Harmonics with through capacity exist but are not fitted to the Chinese eBay 4th axis models under discussion here, nice thing to have in the parts drawer, please post some photos when you've completed a project with that,
Regards,
Nick.
Yes, came across it after I had bought a 5phase stepper with 50-1 harmonic drive ( 25000 steps /rev ) which I was going to belt drive a head 2-1 so that I could incorporate a through hole. I also have some very low backlash 5 phase 10-1 (5000 steps/rev ) epicyclic drives that I thought I might use with perhaps ~4-1 belt drive.Quote:
Originally Posted by Robin Hewitt
Attachment 22616Attachment 22617
I have now bought a 50:1 harmonic to play with, eBay item number: 291926642351
I thought maybe connect it to a shaft using my favorite Myford change wheels. Not easy to cut the middle out 100% concentric so I would have to spring two together, that seems to do the trick if you want low backlash with Myford wheels. I think I could enjoy this. Somewhere in my junk boxes I have a new pair of 40mm self-aligning, flange bearings, featuring grub screws on the extended inner race. They could easily become a part of this.
They come in various sizes, 11 is quite a small one, do you know shaft sizes
I use lots of harmonic drives for my motion control rigs. I buy them second hand from ebay, but I have also bought a couple of modern brand new ones from the Harmonic Drive rep in the UK. They are very expensive new. HQ is in Germany but usually made in Japan, they own the monopoly, so the cheapo Chinese contraptions may not live up to expectations re. "zero" backlash, quality of bearings and so on. A brand new size 25 HD will usually cost a couple of thousand. I paid £1500 for each size 11 for a camera rig. They are beautifully made.
The one above from Johnattuk is a proper one with very good angular bearings. These usually come from small robotic machines that are decommissioned, a lot from S Korea, and a bit old, but perfectly usable. Some work perfectly, some have seen better days and will work, although they may be a bit bumpy at certain speeds, which you wouldn't probably notice on cnc rigs, but you do notice in motion control where perfect smoothness throughout the range is paramount.
The one you bought, Robin is too small, being size 11, with small bearings. You need at least a size 25, with size 14 being a bit marginal. Typical price for a good decent second hand size 25 with hollow shaft is around £450.
Here is a render of one of mine showing both ends
Edward
Attachment 22623
And this will give you a perspective on sizes. Size 11, size 14 and size 25
Attachment 22624
Surely you'll be using Anti-Backlash gears from HPC Gears as it would be a strange choice to add backlash in the primary drive (regardless of how small an error this would introduce) of a system designed specifically to avoid backlash?Quote:
Originally Posted by Robin Hewitt
Lightweight ;), here's a size 50:Quote:
Originally Posted by Edward
Attachment 22644
I had two of these sitting in my shed for 5-6 years - like Robin I found out about them and immediately decided I must have two. Then I got very sidetracked, so I sold one recently and held on to the other. I think I'm more interested now in coaxing a high torque motor into a direct drive axis, as the cost of encoders is decreasing. Seems more elegant.
One interesting point with harmonic drives is that although they don't exhibit backlash (if well made), they still do have some hysteresis. Still, beats buying a chuck driven from a stepper motor via a rubber band.
It could, but I've been making parts with my "rubber band" for a couple of years, how's it going with your "Doorstop" ?Quote:
Originally Posted by Jonathan
I think it is 6mm in, 12mm out.Quote:
Originally Posted by johnsattuk
Well enough:Quote:
Originally Posted by magicniner
https://www.youtube.com/watch?v=lVuBEEceWJ4
http://www.mycncuk.com/threads/3341-...illing-machine
http://www.mycncuk.com/threads/11220...-as-a-4th-Axis
Unfortunately I ran the stepper motor on the rotary table into my milling machine column (wasn't looking) last year and bent the worm gear, so I do have another doorstop! I'm now wondering if these are better than the price suggests:
http://www.ebay.co.uk/itm/Precision-...sAAOSwZ4dZHZkC
The problem with the direct belt drive method is the stiffness of the belt is quite poor compared to a harmonic or worm drive. Of course, it can be useful but that depends somewhat on the diameter of the material, your accuracy requirement and other factors. If the rotary table above is any good - i.e. "most" of the backlash can be adjusted out, then that plus a 1Nm stepper motor seems like an attractive option.
Wow, it almost designs itself.Quote:
Originally Posted by magicniner
Robin if it's like mine in size 11, then it's probably 6mm in, 10mm out.
And Jonathan, I also have a size 50 monster that I use on a crane rig, weighs a bit that one:)
Edward
That's extremely fortuitous then ;-)Quote:
Originally Posted by Robin Hewitt
I just took that harmonic drive apart, one screw head decided to snap off, one had to be drilled off.
Much as expected, only remarkable thing is how fine the gear pitch is.
These gears are about 28mm diameter...
Attachment 22685Attachment 22686
Not just the fine pitch but also the fact that they are flexible to mesh and I am not totally sure about the inner workings, but I think they have one more tooth than the gears they mesh with, or vice versa. Incredible concept and I guess very difficult to machine such precision.
Edward
Need 2 teeth difference and because of the fine pitch, need a bit of care when reasemmbling as it is easy to be one tooth out, do not force. :eek:Quote:
Originally Posted by Edward
I have to get those headless screws out before I can reassemble.
M2.5 and darned close to the bendy cog. I don't have anything suitable to grab them with.
Any ideas?
Have you tried two or three nuts locked together perhaps with heavy duty locktite retainer as well