[BanduraMaker]

I'm in the market for a 4th axis and in my search, I came across this one:

eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

I imagine it's probably a knock-off harmonic drive but maybe that's not such a big deal for a medium duty basement shop machine.

I'm wondering if anyone's tried it or anything. If it's of decent quality, $380 would be a fantastic price!

[cyclestart]

It looks as if mini-lathe pieces like a 4-jaw chuck would bolt right on.
The thru-hole probably only extends about 3" but that's only another guess, the listing is a bit short on details.
Great price for sure.

[JerryBurks]

Looks very interesting. If I had not already bought a rotary table from Sherline, I would probably give that one a try since it is a turnkey solution and rather less expensive.

I would prefer a good wood turning chuck like the Oneway Talon because it holds on to lumber better but that could probably be mounted as well.

[BanduraMaker]

I was planning on going the Sherline route as well but heard some rumors that the quality was down a bit and that backlash could be significant on the workpieces of the size I'm planning.

I started investigating DIY harmonic drive setups when I stumbled across this. The vendor told me via email that he's willing to remove the chuck and even the stepper to drop price and shipping cost. I need a four jaw since I'll be unsung 4" x 3" stock but I might buy with the 3 jaw just to have if I actually go for it.

[JerryBurks]

I noticed some backlash on the Sherline rotary (have not measured it) but I think that is as good as it gets with a worm drive. Since it is reduced by 1:72 it should not be substantial (I hope).

But now I got the itch of using the Sherline for something else and buy the gadget that you found. Can you please explain this to my wife?

JB

[harryn]

Jerry, just tell her that you want to make something for her and it won't come out right using the one you have.

All I can say, is that I hope your sherline stuff is more rigid than an old lathe that I was given. The main benefit of it has been the training - I can readily see every deflection with my eyes so that I won't be fooled (as easily ) by other tools.

[BanduraMaker]

Jerry,

I'll tell her why you need it next time I talk to her

As to the backlash, from the Sherline site, backlash should not be adjusted to better than 0.1 degrees to protect the worm gear from excessive wear. At 2" out, .1 degrees works out to be .0035" so, with 4 inch wide stock, that would be .007" difference from one side to the other.

I've never used a 4th axis so I don't know how this works practically but, apparently the worry is that the backlash will cause the part to move depending on whether you're plunging, climb cutting, conventional cutting and whether or not you're using upcut or downcut bits etc. Because I'll be using this 4th primarily for indexing, a swing of .007" across the part would not be acceptable to what I'm doing.

I'm currently using a 4 sided vacuum fixture to machine some parts and I'm probably getting a max error of about .004" and for my purposes, that's not good enough (which is one of the reasons I'm looking at a 4th axis).

I know full well that over time wood can't hold a tolerance better than .004 but, for the process of machining, .004 is significant enough to cause major ridges, non flat surfaces etc.

Harmonic drive isn't perfectly accurate when it comes to positioning but, that can be compensated for in Mach. It is zero backlash which could be a very big deal in terms of surface finish and squareness between sides.

Thoughts?

[JerryBurks]

Thanks...appreciate your help But it is not that bad. It is bonus time anyway and I guess I have some money socked away to spend on such indulgences.

Actually, since I am a kind of impulse buyer, I ordered it already.

If you have the time to wait until it arrives and (more problematic) until I have it hooked up I can tell you if it is any good. On the to-do list before I get to that is:
- finish my dust boot
- make some hand wheels for the stepper axis sticking out
- Improved dust cyclone
- upgrade the control box with higher voltage stepper supply and 4th axis drive

Since I have it now anyway, I may make the Sherline a 5th axis sometimes (many moons from now). No real idea what I would use it for but a perfect toy for me. I have the DeskProto software that will support 4-axis and limited 5-axis CAM.

Anyway, thanks for the tip!!!

[HorridHenry]

You vould always buy these items.....
LittleMachineShop.com - Spindle, Mini Lathe 7x14
LittleMachineShop.com - Headstock Casting, 7x14
LittleMachineShop.com - Lathe Chuck, 3-Jaw 3"
Some bearings and a stepper and you're away.

[BanduraMaker]

Wow Jerry, you move fast! I don't really need my 4th axis until late this year so I'll definitely be waiting to hear your review of the product. My fingers are crossed!

Henry, something like you proposed is my plan "B"

[HorridHenry]

Henry, something like you proposed is my plan "B"

Ok,maybe this might be plan "C"

[ame=http://www.youtube.com/watch?v=DwB2FmsjByM&list=PLEF16F2DE81B7BCD2&index=10&feature=plpp_video]RoboCNC X1 Router 10 : Making a 4th Axis / Indexer / Rotary Axis / A-Axis - YouTube[/ame]

[JerryBurks]

Wow Jerry, you move fast! I don't really need my 4th axis until late this year so I'll definitely be waiting to hear your review of the product....

I know, I am a horribly impatient person with little restraint when I find interesting stuff. And a pack rat too, which is why my garage is so cluttered and I am not allowed to go to a surplus store.

Give me a month or two and I think I will have it set up. I will post the results here.

[cyclestart]

You vould always buy these items.....
Some bearings and a stepper and you're away.

Littlemachineshop also sells the headstock as a complete assembly.

I went that route to have a combination of lathe and 4th axis capability. All the examples I've seen are stepper/belt driven and have whatever disadvantages go with that type of set-up but I don't claim to have seen them all. This ebay unit should have superior holding power.

[JerryBurks]

In the meantime I received the Chinese harmonic drive clone (I picked 1:50) and started playing. Actually, it looks pretty nice, with a milled aluminum gearbox housing and a smaller (but likely very sufficient) NEMA23 stepper attached. It must be quite efficient, since it is not self-locking. Even with the stepper attached, it can be rotated from the chuck side. This would be impossible with a worm drive. Compared to the Sherline rotary table that I intended to use as 4th axis this gadget is easier to integrate (just need to strap onto the router table).

First I removed the 3-jaw metalworking chuck that comes with it (Banduramaker told me later that the seller would ship without chuck for less money but I did no know or ask, duh) and attached a Oneway Talon 4-jaw chuck. Not trivial but possible with some welding and lathe work on the chuck adapter.

I am in the process of fabricating a mounting base and a tailstock but in the meantime I did some rigidity measurements. This is quite interesting. Up to 2 or 3 ft-lbs of torque there is no backlash to speak of and the stiffness is approx. 0.045 degrees per ft/lbs of torque. As an example: For a work piece of 4" diameter and a machining force of 10 pounds circumferentially, the displacement would be about 2.5/1000". I think that is quite adequate for a wood router.
I noticed when applying higher torque, there is a kind of "variable backlash" that may be related to the harmonic drives splines beginning to ride up on the internal gear. But I am not really familiar with harmonic drives to say and I suppose with our hobby equipment that load is unlikely.

Here some pics. I will keep building the hardware and need to modify my electronics box but once I get it all going I will post again with results.

[JerryBurks]

I installed the harmonic drive and a tailstock and machined my first part (pic below). I can definitely say this drive does the job, has no backlash and (what I did not realize before) allows for easy zeroing of the bit to the rotation axis due to its square body. More pictures here. Obviously I don't know how durable this is, e.g. compared with a worm drive rotary table.

[Spinnetti]

I installed the harmonic drive and a tailstock and machined my first part (pic below). I can definitely say this drive does the job, has no backlash and (what I did not realize before) allows for easy zeroing of the bit to the rotation axis due to its square body. More pictures here. Obviously I don't know how durable this is, e.g. compared with a worm drive rotary table.

Cool.. I may have to pick one of these up...

[asuratman]

JerryBurks,

It must be quite efficient, since it is not self-locking. Even with the stepper attached, it can be rotated from the chuck side. This would be impossible with a worm drive. Compared to the Sherline rotary table that I intended to use as 4th axis this gadget is easier to integrate (just need to strap onto the router table).

What about when rotary stop, then there is force from top due to starting milling, the material will move down a bit. So machining is not precise! What you do? Can you show me pic of the harmonic drive. Will you put a brake mechanism on it to better lock, do you think so?

[JerryBurks]

JerryBurks,


What about when rotary stop, then there is force from top due to starting milling, the material will move down a bit. So machining is not precise! What you do? Can you show me pic of the harmonic drive. Will you put a brake mechanism on it to better lock, do you think so?

Sorry if that was misleading...I meant the gearbox is not self locking while the stepper motor is not connected. When the machine is running and the stepper is under power it is impossible to turn the chuck. Therefore no brake is required.

Here is another picture of the harmonic drive gearbox (or knock-off). It is actually pretty small.

[JerryBurks]

I've never used a 4th axis so I don't know how this works practically but, apparently the worry is that the backlash will cause the part to move depending on whether you're plunging, climb cutting, conventional cutting and whether or not you're using upcut or downcut bits etc. Because I'll be using this 4th primarily for indexing, a swing of .007" across the part would not be acceptable to what I'm doing.

If you go back to the beginning of this thread, you see this concern was behind the thread starter's (BanduraMaker) interest in this gearbox. Worm drive rotaries have significant backlash and the question was if this gadget would be better.

A few posts down you can see what I actually measured as backlash and I found when the machining forces are moderate (up to 3 ft-lbs torque on the chuck) the backlash is not really measurable. The equivalent force on my blanks (up to 6" wide) would be over 10 pounds which is a bit more than you normally will see in 3D mode with a 1/4" or 1/8" bit. If you exceed such forces I would not worry about accuracy but if the blank would rip out of the chuck and center or if protruding details will be snapped off.

Now, if you have very large parts hanging out 12 inches from the axis and use a 1/2" bit at 200ipm that little gearbox would probably not be up to the job.

[BanduraMaker]

Lookin' good Jerry! I think I'll just have to go ahead and order one.