Limitations of cheap 4th axis components off eBay?

If anyone here has any experience of using some of those $200 - $500 4th axis components off eBay or some of the diy CNC stores, I'd be interested to hear what you have been able to make with them and what their limitations are?

Most people seem to mount them on their short axis in the pics I have seen. I was wondering if there is a reason for that? My interest would be in using one to make wooden rifle stocks and possibly some aluminum chassis systems. This would mean work pieces that are 20" - 30" long by 6" tall and 1.5" thick. Is there any reason you can't buy a cheap 4th axis and just mount the head and tail stock 30" apart on the long axis? Or do you need to invest more in something capable of holding longer work pieces? Are the cheap 4th axis options capable of working with aluminum?

What (if any) are some of the other diy 4th axis options? For example, has anyone converted a mini benchtop lathe to turn work pieces on their CNC router? Some of the mini metal lathes that are capable of turning steel sell for less than $500 delivered these days. They seem to be made of superior components to your average 4th axis eBay kit. Could you simple replace the lathe bed with your CNC router table and run the motor from your Gecko?

Quote:

---

I was wondering if there is a reason for that? My interest would be in using one to make wooden rifle stocks and possibly some aluminum chassis systems. This would mean work pieces that are 20" - 30" long by 6" tall and 1.5" thick. Is there any reason you can't buy a cheap 4th axis and just mount the head and tail stock 30" apart on the long axis? Or do you need to invest more in something capable of holding longer work pieces?

---

You may find that as the workpiece gets longer, it becomes less stable, flexing and vibrating while being cut.

There's no reason at all you can't mount the assembly along the long axis instead of across the short. Across the short axis on a gantry router just makes it easier to access - you don't have the gantry in the way and usually on the smaller machines the short axis is the front of the machine closest to the operator. So it's convenience.

To the hardware itself: there's a reason these cost $200-$400 and a decent 4th axis can run well over ten times that.

I bought one as a part of a package with the router. It's ok for very light duty work, but you need to be aware of limitations:
- It's bendy. The frame holding the spindle flexes under load which means you really do need to use the tailstock rather than have the work supported only in the chuck.
- It's weak. The stepper doesn't have a great deal of holding torque even with the reduction so you need to be careful to plan your toolpaths so that cutting forces work directly into the axis rather than creating torque on it. Of course it will hold /some/ but you need to be a little careful. Not too bad for timber but it gets very noticeable with aluminium (dreaded sound of skipping steps on the axis, broken tools flying about, many curse and swear, much sad etc).
- It's slow. Stepper drive, 6:1 or similar ratio, means you're not going to be using it as a lathe.
- Usually the cheap ones only have a 60mm swing or so, ie the largest diameter you'll be able to work is 120mm / 4.5". If you have the gantry height you could put both head and tailstock on parallel riser blocks to increase the clearance but be aware of the increased torque etc of the wider parts.

The cheapest alternative with a bit of strength and holding power seems to be to get a cheapo chinese rotary table with a worm gear, futz with it to reduce backlash and add the stepper, bang it on the table side down with a tailstock either made or purchased and tailored to suit the centre height.

After that you can start chasing things down the rabbit hole with harmonic or hypocycloidal reduction drives or two punch combos that use a clutch to select between servo/stepper drive or a higher RPM spindle motor driving a more typical lathe head which is where you end up going for that mill/turn type setup.

It`s doable on a small 4 axis chuck but you need to construct a special mount to place it higher, I always use the 4 axis almost on a daily basis.

http://www.cnczone.com/forums/attach...d=368416&stc=1

But on my opinion if you want more rigid and attain more speed on finishing the project I recommend to just place the material on the bed itself, I made butt stocks too but I did not use the 4 axis because number one I got too lazy making custom brackets for the 4 axis chuck, number two, it`s too slow and you need to minimize the cuttings otherwise the chuck will miss steps and ruin the entire project, so what I did on a butt stock project is to cut it in two, the left side and the right side, im not dependent on autoCAM programs though, I can divide the CAM programs anyway I wanted, it does`nt matter where I begin, it will always be in one piece after it`s finished.

http://www.cnczone.com/forums/attach...d=368418&stc=1

http://www.cnczone.com/forums/attach...d=368420&stc=1

http://www.cnczone.com/forums/attach...d=368422&stc=1

This is an M4A1 But stock to be mounted on an air gun, The code is divided in two parts, the left and the right, same thing on our custom hand grips that we massed produced.

http://www.cnczone.com/forums/attach...d=368424&stc=1

Quote:

---

Originally Posted by Goemon

If anyone here has any experience of using some of those $200 - $500 4th axis components off eBay or some of the diy CNC stores, I'd be interested to hear what you have been able to make with them and what their limitations are?

---

You still get what you pay for. It would however help if we knew which net in particular you are looking at.

Quote:

---

Most people seem to mount them on their short axis in the pics I have seen. I was wondering if there is a reason for that?

---

There are probably dozens of reasons, many being individual preference.

Quote:

---

My interest would be in using one to make wooden rifle stocks and possibly some aluminum chassis systems. This would mean work pieces that are 20" - 30" long by 6" tall and 1.5" thick. Is there any reason you can't buy a cheap 4th axis and just mount the head and tail stock 30" apart on the long axis? Or do you need to invest more in something capable of holding longer work pieces? Are the cheap 4th axis options capable of working with aluminum?

---

You can mount them in any way you want, in this case work piece stiffness will likely limit capability before the axis assembly. This given a fourth axis assembly that is reasonably well built. Even a 1.5" thick piece of walnut will vibrate over that distance and it would only get worse as the stock is trimmed down and inletted. Obviously stocks are done this way but the point is the material itself will be an issue unsupported over that distance. An aluminum chassis system might even be worse depending upon how thin some of the cross sections become.

For an aluminum chassis system I would imagine you would need support for the material to get anything near decent removal rates. These guys Martin Machining Trunnions for Troyke and Haas Vertical Machines have examples of Trunnion tables that might give you some ideas. What you would need would highly depend upon your chassis system design, it's length, thickness & etc.

Quote:

---

What (if any) are some of the other diy 4th axis options? For example, has anyone converted a mini benchtop lathe to turn work pieces on their CNC router?

---

Actually I seem to remember seeing just that on the web someplace. Its been a while and frankly I'm not going to go looking for it at the moment but I believe he demounted the lathe head but putting a complete lather on a mill/router isn't impossible. You would need to allow for clearance. Considering the cost of a Mini lathe and the hardware to upgrade it, you could buy a pretty good fourth axis with a bit more money.

Quote:

---

Some of the mini metal lathes that are capable of turning steel sell for less than $500 delivered these days. They seem to be made of superior components to your average 4th axis eBay kit. Could you simple replace the lathe bed with your CNC router table and run the motor from your Gecko?

---

While you can do that do not underestimate the cost of doing such a conversion. For one you need a substantial stepper motor with pretty large holding torque. Even with a belt reduction you still have torque being applied directly to the stepper by the cutter. These forces can easily exceed the hold power of a small stepper. So this mean a big stepper, a big stepper drive and a heavy duty timing belt of some sort from stepper to spindle. This means a custom pulley for the spindle and a timing belt pulley for the stepper. Then of course all the custom bits and pieces to glue it together.

If your router/mill can handle it mounting a whole lathe does have some advantages. For one the tails stock remains in alignment with the spindle. Second most of these lates have long flat ways upon which you can rung a dial indicator for setup alignment. Third a DIY Trunnion can be easily built to operate between centers. Sadly the between centers distance would be a big limitation in your case. There are work arounds though for using just the headstock and tail stock, you can machine an alignment feature into the router bead to register both against which should lead to rapid setups.

In any event if you have access to a lathe and mill you could easily do a DIY 4th axis that could compete nicely with the Chinese stuff. You would need to get some 1 to 1-1/2" thick steel stock (two matched pieces) and some heavy wall rectangular tubing. The tubing would act like a spacer between the two pieces of steel which would end up bored for a spindle. The thickness of the endplates would depend upon the bearings you ultimately choose to support the spindle and may need to be even thicker if you choose to go the spindle bearing route. In a nut shell a fourth axis can be an extremely simple mechanical device and can be built very rigidly using nothing more than drops from your steel supplier.

In the end there are many ways to go about solving this problem in the end it is up to you and the quality demand you have.

Ok that answers the question. It sounds like I would need to invest in something more robust to do what I want reliably and with repeatable quality.

The question is how much more would I need to spend. It seems like there is usually a large gap between the cheap Chinese hobby options and the next level up.

For other areas, there is usually a diy option that allows us to achieve better results for similar money but I haven't seen many diy 4th axis projects.

What do you guys think about the possibility of repurposing parts from a lathe instead? Wood lathes are relatively inexpensive and mini metal lathes aren't too bad. I am assuming there is a problem with this idea or you guys would already being doing it?

I have seen Nema 34 gear motors with large amounts of holding torque for sale at reasonable prices. Why aren't people attaching themselves to lathe chucks instead?

This one seems to be the next level up in cheap Chinese 4th axis kits:

https://www.automationtechnologiesin...he-4th-axis-2/

It has an extra long bed and uses a Nema 34 gear motor but I can't really tell if the rest is any better in terms of how sturdy it is or if there is sufficient holding torque to work on aluminum.

It uses a small 3" chuck with 3 jaws. Does this limit the potential size of the work piece to 3" or can you add screw in bars on either size of a larger work piece to give it something to grip onto? E.g. If I wanted to work on a 2" x 6" x 18" slab of aluminum, could I screw 1" diameter tubes on either side for the chuck and tail stock to grip?

If I needed a 4th axis for my router, I would probably buy a wood lathe and hang a NEMA 34, 1200 oz/in stepper on it. Probably with a double reduction timing belt drive, at about 10:1, that would pretty much eliminate any possibility of backlash and provide plenty of holding torque for wood work. It would proabaly work OK for aluminum also.

The real question is what is your available working height, and can you set a small wood lathe low enough to clear?

Hi, the height problem is one reason many people work across the table instead of going under the gantry ........the cross table X axis travel does limit this option.

As far as holding power, a worm drive box does give you the braking force that the worm and worm wheel automatically have........using the belt drive option means you are working with only the holding power of the stepper and reduction to resist the force of the cutter.
Ian.

Quote:

---

Originally Posted by handlewanker

Hi, the height problem is one reason many people work across the table instead of going under the gantry ........the cross table X axis travel does limit this option.

As far as holding power, a worm drive box does give you the braking force that the worm and worm wheel automatically have........using the belt drive option means you are working with only the holding power of the stepper and reduction to resist the force of the cutter.
Ian.

---

Yeah sounds like a great idea, weak stepper motor + worm screw + gear = maximum holding torque, just like a wiper mechanism :rolleyes:

Hi, it's a trade off as to whether you want a fast rotation or a strong rotational holding force.

You can get a 1:1 ratio with a belt drive for fast rotation but limited holding power that is dependent on the stepper size or as much as 1:10 reduction....the limit is the drive pulley size that determines the driven pulley diam.

You can get a much higher reduction with belt drive if you go to a multi pulley compounded solution

On the other hand, a worm drive is limited to not less than 1:10 reduction due to the size of the worm wheel diam.....it can be as much as 1:100 reduction without making the worm too big.

In all cases the worm/worm wheel drive has the greatest holding force when it comes to resisting the forces of a cutter acting on rotation and the highest resolution capability for fine work.....even if it is slower.

The cheap belt drive models on EBAY are cheap because they can be made for less money than a worm drive model, but give you 1:6 reduction at most .....they compromise between rotational speed and resolution.
Ian.

Yep, as with most things there seems to be a compromise triangle (in this case a tetrahedron) of "pick two" between strong, fast, accurate (including minimal backlash) and cheap.

Khouj, if you want continuous 4th axis a worm drive isn't for you BUT most ops are more 3.5d ie using the 4th axis as an indexer and, for that kind of work, the worm drive is a brilliant choice because it gives great holding strength cheap. Speed and backlash suffer but speed doesn't matter for an indexer, really, and backlash can be worked around.

I plan on building my own by using a 400w servo and a planetary gearbox like this:
NEW IN BOX HARMONIC DRIVE SYSTEMS HPG-20A-21-J6GCK PLANETARY GEAR REDUCER | eBay
I have two of these gearboxes with zero backlash.
Just need to get an adapter made to mount a wood lathe chuck.
I bought a used wood lathe tailstock on Ebay to use.
Total cost will probably be about $1000.
Gearbox - $250
Servo and drive - $450
Tailstock - $50
Chuck - $100-$150

US$1000.....wow, that's expensive but probably par for the course if you want to get serious.

For a build I have in mind I need to get a 36 tooth bronze or plastic gear, about 80mm diam, to mesh with a worm I have, then I'll do a 4th axis build for a design that has no backlash.

I've got an 30 tooth white plastic gear, about 75mm diam, that came out of a garage door winder mechanism, so I know that the plastic material running in grease will be OK for a long time.

The problem is, practically all gears I've found are straight cut whereas I need one that has the teeth cut at the helix angle of the worm.......I suppose that means I'll have to cut one myself from a plastic material.......I could go to a straight cut gear and angle the worm over.
Ian.

Quote:

---

Originally Posted by Jim Dawson

If I needed a 4th axis for my router, I would probably buy a wood lathe and hang a NEMA 34, 1200 oz/in stepper on it. Probably with a double reduction timing belt drive, at about 10:1, that would pretty much eliminate any possibility of backlash and provide plenty of holding torque for wood work. It would proabaly work OK for aluminum also.

The real question is what is your available working height, and can you set a small wood lathe low enough to clear?

---

With my current components I have 10" of clearance under the gantry beam but I can easily increase that if needed. My rails on the long axis are mounted 6" above the table on a supported base so I have some room to make taller risers without making it too unstable.

The part that concerns me is choosing the the right motor, head stock, chuck and tail stock so I can work on longer hard wood and aluminum parts. I guess it's the indexing function that is most important. I can't see myself using it to add threads to cylinder shaped part or anything like that.

It would be nice to have the capability to turn and index parts but if I have to prioritize, indexing the part with stability is what matters. That and being able to hold work pieces that start off as 2" x 6" x 30" slabs of aluminum, walnut wood or maple.

This is why I was thinking that parts from a wood lathe might work. I can't see how I would be able to turn the chuck without a belt drive using a Nema 34 motor though. It is definitely possible that I am using the wrong search terms but I have never been able to find any coupling parts to direct drive a decent sized chuck. It sounds like this is the main limitation although many of the cheap 4th axis kits seem to be belt drive mechanisms.

What do do you guys think about the 4th axis kit from Automation Technologies from the link below?

A timing belt drive would be the simplest way to go. And they are very reliable and accurate. I see the Automation Technologies unit is using a 6:1 timing belt reduction on one of their units, and what looks like a 20:1 gear drive on the other. I think either would work. You could just design & build your own system pretty easy, if you have access to some metalworking machines. A wood lathe would not be a bad place to start, it already has a headstock and a tailstock. Hanging a stepper motor on it is not too much of an engineering challenge, but will take some metal work that probably can be done on a router if you don't have metalworking equipment. There are a lot of chucks available that will mount on a wood lathe spindle.

If you wanted to direct couple you might have to build a coupler, but I would use a timing belt reduction. Better resolution and the torque is multiplied by the reduction. You don't need a lot of rotational speed, but you do need accurate indexing and holding torque.

As far as I know, there are no direct coupled 4th axis units in the hobby price range. Some of the high end industrial units have a huge, hollow shaft stepper or servo motor drive, where the motor is the rotating plate, but these are really expensive.

Check out the Sherline rotary table: 3700-CNC CNC-Ready 4" Rotary Table — Sherline Products. It's a fairly small unit, but well-made (in the USA), with a mount for a directly-coupled NEMA 23 motor. With a 72-1 reduction, that provides plenty of mechanical advantage. It accepts 3 and 4 jaw chucks that Sherline also supplies, using a threaded adapter. Or you can mount a holding fixture directly to the faceplate. It's a lot better than the Chinese versions of a cheap rotary table, which either just mount a chuck directly on a stepper motor, or use a timing belt for a little reduction, at the expense of stability.

You can build a harmonic drive 4th axis for not much money. This was is based on a harmonic drive transmission that came off ebay for $100. The stepper motor was $25, and the old 4 jaw another $65. I built a split collar adapter to go from the transmission output shaft to the chuck. Around $200 plus my labor. 200 to 1 reduction ratio so my small stepper holds at 250 ft-lbs. No backlash. Seems to work pretty good.

Quote:

---

Originally Posted by awerby

Check out the Sherline rotary table: 3700-CNC CNC-Ready 4" Rotary Table — Sherline Products. It's a fairly small unit, but well-made (in the USA), with a mount for a directly-coupled NEMA 23 motor. With a 72-1 reduction, that provides plenty of mechanical advantage. It accepts 3 and 4 jaw chucks that Sherline also supplies, using a threaded adapter. Or you can mount a holding fixture directly to the faceplate. It's a lot better than the Chinese versions of a cheap rotary table, which either just mount a chuck directly on a stepper motor, or use a timing belt for a little reduction, at the expense of stability.

---

Thanks. That looks like a great option for me if the quality is right. It's both affordable and modular. I like the idea of being able to upgrade the mounting plate and the motor to fit my needs. I also like the idea of a made in America product from a company that would stand behind their product.

Do you own this unit yourself?

How would you mount a 2" x 6" x 12" slab of aluminum (or wood) as the work piece with a unit like this? Would you attach tubes on either side (of the blank stock) to fit the chuck and tail stock? Or would you mount something else (instead of a chuck) to the mounting plate? I have seen work holders on wood lathes that look like round plates with nail spikes sticking out to grip larger slabs of wood for turning.

I am trying to figure out if a 4" or 5" chuck is sufficient for what I intend to work on or if the chuck diameter has to (at least) match the diameter of the blank stock.

Quote:

---

Originally Posted by johncarpenter

You can build a harmonic drive 4th axis for not much money. This was is based on a harmonic drive transmission that came off ebay for $100. The stepper motor was $25, and the old 4 jaw another $65. I built a split collar adapter to go from the transmission output shaft to the chuck. Around $200 plus my labor. 200 to 1 reduction ratio so my small stepper holds at 250 ft-lbs. No backlash. Seems to work pretty good.

---

Now that's what I am talking about! I was wondering why we don't see more diy projects for 4th axis builds. The obvious question is what tools you needed to make the coupling components. My manual metal working skills are very limited and there is little to no obvious availability of off the shelf coupling components to go directly from stepper to decent sized lathe chuck.

I have a drill press and an angle grinder. I wonder if I could make a less professional-looking version of your coupler... or maybe the timing belt option is a better option for me....

Where did the blue headstock base come from? Is it something you build or repurposed from something else?