Official Luthiers Forum!

I stumbled across this in a couple of places the other day and was just wondering if anyone had any thoughts on something like this.

http://www.ebay.com/itm/250977194397?ss ... 499wt_1180

I came across it because I'm leaning toward building my own 4th axis using a surplus harmonic drive. The one in the link above claims to use a harmonic drive but for $400, it must be a knockoff rather than an "official" harmonic drive.

I'd need to replace the chuck with a 4 jaw which adds about another $100 and would need to add a tailstock along with a few other items but if the drive is any good, it would still be a good value - assuming the flex spline doesn't break in no time flat.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Handy, but I would be leery of buying anything from China on eBay.

_________________
"Act your age, not your shoe size" - Prince

I share the same skepticism.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

I'm a skeptic too, but I think it really comes down to the dealer you're working with and the speed of China's mail system. I've actually had good luck with the few things I've bought from China via Ebay--mostly it's been little, cheap things like rare earth magnets by the 100. But I have bought some pretty expensive audio triodes from Chinese vendors as well. China is actually making some extremely good direct heated triode tubes in the style of the 30's American and British tubes...

At any rate, so far I haven't gotten burned. Shipping time seems to usually run a couple of weeks. Like everything on Ebay, check the vendor's feedback for comments from your own continent before you commit....

Dave

Looking at this, 55m from bed to center is just a hair over 4" diameter. So,if you plan on doing angled headstock type of necks you will need to add a spacer to raise it from the bed. With that being said I would bet you could get it made at a local machine shop for the same price and to your specs as well based on a tail stock you plan to use. A large stepper would not really be required as rotational axis would be used more for indexing most of the time. At least that has been my experience. I have looked at building one for myself, and having it machined to myspecs. as far as the spindle for the chuck I can get replacement parts from grizzly for less than $25. then have the head stock machined to fit my stepper and tail stock and bed.

Just my02.cents worth.

Mike

_________________
I'm outside looking in, just farther from the window than most.

I'm not a fan of that tiny output shaft between the drive and the chuck, mainly from a vibration standpoint. It's marketed as an engraving spindle, so it's not an issue in the field it's meant for.

I haven't had any more issues with stuff from China than stuff from anywhere else, but I do my due diligence on anything I buy that matters. I'm also a big fan of 'buying cheap and souping up'- reboring and repacing bearings can turn a cheapie into a real steal.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Speaking of which...I looked and the same seller sells a 'CNC Router' 4th axis with a 1" output shaft on the gearbox for $400.

http://www.ebay.com/itm/CNC-Router-Rotational-Axis-4th-Axis-axis-engraving-machine-1650-/260938753053?pt=LH_DefaultDomain_0&hash=item3cc129641d#ht_3710wt_952

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Well if you buy new, you can't even get a size 14 (probably the size they're using) harmonic drive for $400. Hence my skepticism at the quality.

I've found surplus Harmonic drives in sizes 14 to 20 (sizes useful for a 4th axis like I'm looking for) ranging from about $200 to $400 but there are so many different types depending on the application that you really need to know what you're doing to make sure you get one that can be made to work.

Unfortunately, I know only enough to be dangerous!

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

It is a nice looking design.
I would find out more about what the "gapless" design means. I assume it means zero backless which is nearly impossible to achieve as there has to be some working clearance between mating parts. There is an interesting thread on CNCZONE on this very topic in reference to 4th axis headstocks.

There actually is a way to get zero backlash which is to use something called a "harmonic Drive". You can learn more here: http://en.wikipedia.org/wiki/Harmonic_drive

It is a super clever design. Apparently there's only one company, Harmonic Drive LLC, that makes "real" harmonic drives and everything else is a knockoff. Genuine Harmonic drives are very expensive (unless you buy used). This manufacturer claims to be using a harmonic drive (and it's got a 100:1 ratio which would be hard to achieve otherwise) but, it's almost certainly a knockoff.

There's a few guys on the zone who've built 4ths with harmonic drives but, they had full metal working capability and, it seemed that they knew a bit about harmonic drives as well. There's like 8 different "families" of drives designed for different purposes and even though they're fundamentally the same, it's much more difficult to force fit one into a design it wasn't meant for.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

That is a way cool animation of the harmonic, Andy!

This is the discussion on 4th axis designs and mentions harmonic drives in post #8.
http://www.cnczone.com/forums/linear_rotary_motion/72261-backlash_free_rotary_table.html
It's a long read but some interesting stuff.

Yep, I came across that in my searches. Here's one where a guy makes his own drive with a surplus drive:

http://www.cnczone.com/forums/benchtop_ ... is_x2.html

And here's another using a different style of drive (but still from Harmonic Drive LLC) : http://www.cnczone.com/forums/benchtop_ ... monic.html

Surplus Harmonic drive guts are available on Ebay and they're not even crazy expensive but, you have to either know what you're getting or have enough metal working equipment to adapt what you're getting to what you want. They all have an input side (wave generator), a flex spline and a circular spline. Some come with an output bearing and some do not. If I had just a little more experience with this and a bit metal working equipment, I'd go for it and buy one off e-bay but, it's at least a $200 experiment with what's available on ebay at the moment!

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Those are interesting threads, Andy.
http://www.use-enco.com/CGI/INSRIT?PARTPG=INLMKD&PMPXNO=947172&PMAKA=418-4320
This would be a good start to making the mounting for the Hdrive.
The webs could be removed if necessary on a bandsaw.
I noticed that the guy in the thread used a solid chunk of aluminum which would end up a little lighter than the cast iron angle plate of course.
One thing that I would do different with the aluminum block is drill a thru hole before bandsawing out the excess aluminum.
This would leave the radius that he generated with a round nose endmill. No big deal either way, I guess.
This kind of stuff gets the old man excited!
Nelson

I didn't like the idea of machining it out of a solid chunk of Al either. Seems expensive. If you've got that sort of equipment, it seems it wouldn't be too difficult to simply bolt two plates together at 90 degrees.

Truth be told though, I was thinking that a cast iron angle plate would be a better and easier idea as well. If there's room between the webs, I'd just leave them in.

I'm slowly caving in to temptation and might eventually order one of the chinese ones. My secret plan is to ask about this on as many forums as possible until someone else buys one and tests the water for me

I'm having visions of spending $400 in parts to make one myself and ending up spending $800 on all the re-dos from not being able to get things square and such the first try. I have a 3-in-one machine for metal work but I just doubt that I have the experience to get it right the first try.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

But for under $400 you can get a known quantity with one of the Sherline CNC rotary tables. Sure, there's going to be backlash, but you can compensate for that in Mach 3. It's at least a path that's well traveled and well documented. I've been on the "bleeding edge" too many times in my day job. I don't mind blazing a new trail now and then, but CNC is a new trail for me already and there's something to be said for following the well trodden path...

Dave

Well, that's the thing though is that you can't really compensate for backlash in terms of table movement when you're in a static position. i.e. with the table in an indexed position, it will rock by the amount of the backlash depending on which direction you're cutting, whether you're climb cutting etc.

There is a lock on the table but, now you have to be present to lock and unlock the table as it's indexing.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

True...

Hence my temptation to buy a "holy grail" harmonic drive at prices left brain tell me it's Chinese junk.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Would it be possible and cost effective to use an extra large stepper motor coupled directly to the chuck with no reduction?
I would think that the holding power of a large stepper would be sufficient for wood routing on a 4th axis.

Without reduction, I don't know. Dave's 4th axis has a step coupled to a chuck spindle with a toothed belt but he said that he can move the chuck with the motors active but not moving.

Kelling has a 495 oz " motor that would run with my drive (g540). Perhaps that combined with a wide belt and a decent reduction.....

Dave, can you tell us more about your 4th?

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Here are a couple of pictures of mine (minus the tailstock--nothing special there...). It's basically a Taig micro mill headstock with a toothed gear driven by a 380 oz stepper with a hunk of aluminum holding it all together. 2:1 ratio. The Taig headstock is nice because of its 3/4-16 thread--easy to get faceplates and chucks that fit.

If I get the axis turning, I can't slow it down or stall it by grabbing the edge of the chuck. But, as Andy mentioned, if it's sitting at idle, the I can grab the chuck and turn it. Not easily, but I can make it turn.

The stuff I'm doing with it is continuous 4 axis stuff rather than indexed so far--no problems there. The A axis takes the place of motion on the Y axis the way I have it set up, so there's never any movement of the cutter that isn't directly on center for the rotation, so there aren't any forces trying to rotate the table other than the stepper itself...

Dave

Dave, did you do the motor mounts for this setup?

Nah--I didn't do them. There was a guy I found on the Internet who sells Taig accessories who does a nice job of machining them at not too high a price--about a hundred bucks for all the mount parts, the pulleys and a dovetail mount for the headstock to mount to. A 2:1 ratio on a stepper is fine for the kind of stuff I'm doing. There's a similar unit already assembled out on Ebay now that has a 6:1 ratio (Hong Kong listing), but it costs about the same as the one Andy posted at the beginning of this thread.. In either case it has no backlash worth speaking of, and it's simple....

Dave