Limitations of cheap 4th axis components off eBay?


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Thread: Limitations of cheap 4th axis components off eBay?

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    Default 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?

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    Community Moderator ger21's Avatar
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    Default Re: Limitations of cheap 4th axis components off eBay?

    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.

    Gerry

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    http://www.thecncwoodworker.com/2017.html

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    JointCAM - CNC Dovetails & Box Joints
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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Member dharmic's Avatar
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    Default Re: Limitations of cheap 4th axis components off eBay?

    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.



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    Member KH0UJ's Avatar
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    Default Re: Limitations of cheap 4th axis components off eBay?

    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.



    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.







    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.



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    Last edited by KH0UJ; 07-21-2017 at 07:45 PM.


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    Default Re: Limitations of cheap 4th axis components off eBay?

    Quote Originally Posted by Goemon View Post
    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.
    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.
    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.
    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.
    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.



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    Default Re: Limitations of cheap 4th axis components off eBay?

    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?



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    Default Re: Limitations of cheap 4th axis components off eBay?

    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?



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    Default Re: Limitations of cheap 4th axis components off eBay?

    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?



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    Default Re: Limitations of cheap 4th axis components off eBay?

    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.



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    Default Re: Limitations of cheap 4th axis components off eBay?

    Quote Originally Posted by handlewanker View Post
    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



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    Default Re: Limitations of cheap 4th axis components off eBay?

    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.



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    Default Re: Limitations of cheap 4th axis components off eBay?

    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.



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Limitations of cheap 4th axis components off eBay?

Limitations of cheap 4th axis components off eBay?

Limitations of cheap 4th axis components off eBay?