4th Axis Round-up...

[BarryB]

Or in my case, 6th axis...

I'm looking at adding a rotary axis to my table and am looking for suggestions.  I thought about building one of these, but my small machine shop and my limited knowledge leads me to the answer of buying it instead.  The two that I've seen so far are:

K2CNC:
http://www.k2cnc.com/shop/products.asp?cat=29

Tormach:
http://www.tormach.com/Product_RotaryProducts.html

One of the main pieces I'd like to do, is mill instruments, both sides without using a jig, using the rotary axis to flip over the stock.  The size of the instruments would be about guitar body size, so I'd need enough room to rotate that kind of stock.  The material would be mainly hardwood, but I think I'd do metal every now and then.

What do you guys think.  Would either of these companies fit the bill?  What would you do, if you were in my boat?

Barry

[Dan13]

Hi Barry,

The one in the first link looks very small for your need...? It doesn't mention the size, but judging by the chuck, looks like it's something like 3 or 4 inch in diameter at most.

In the second link there are the 8" rotaries that might do. You have to consider that with the part sizes you're going to cut there will be significant moments generated by the tool cutting a foot away from the axis centre line. Looks like the tables in the above link have a worm gear drive so should be no problem with holding torque, but you will still have to work out the way you're going to fix the workpiece to the rotary.

I would make my own rotary table. Those in the second link, are nothing more than a standard rotary table that is fitted with a stepper motor. You can get a Phase II (or the like) rotary table for less than $400 and fit a stepper to it for another $50 and you have a CNC rotary table and save $400! Since you only need the rotary to flip over a work, you don't need to worry about backlash and these manual rotary tables have locks, so you should be fine. Besides there should be an adjustment for the backlash.

Here are the manual rotary tables I am speaking about:

http://www.emachinetool.com/accessories/catalog/large.cfm?OptionFamilyID=244

You can probably find a place with better prices too.

Dan

[BarryB]

Yeah, the K2 one is 3.1 inches.  I told them what my need was and they gave me a quote.  I thought it was small too!  Hmm, I'm not sure I have the experience to make a rotary table.

[Dan13]

Don't think it should get complicated. You will have to remove the manual handle and see how you can fit a stepper instead. You will use a shaft coupler to connect the motor's shaft to the table's one and you will need to do some machining to make the mounting plate for the motor. Easy said ha... Never disassembled these rotaries myself neither.

I think if you could find one to play with and figure what can be done there before you buy it, it would be best.

Dan

[JHChoppers]

Backlash is a big problem with these kinds of tables.   However, they might be OK depending on what your cutting.

JH

[BarryB]

Yes, I see that there is backlash on these tables, but don't see a way around it either.  I just got an e-mail from Tormach, that they don't have "dimensionalized drawings" (his words) for the rotary tables.  So it is up to me to measure the offsets, mounting holes etc.  This is the first company I've heard of that didn't include those from the outset.  How else can you design your machine with precision?

Barry

[Dan13]

Barry, can you possibly consider not CNCing the 4th axis?

Dan

[BarryB]

yeah, I can.  My machine as it stands now is 5 axis.  This would be a 6th.  I wired everything from the outset for that 6th axis, so it was kind of the completion of the machine.  For all jobs I have currently scheduled, I can do 5 axis no problem.

However, I could use the rotary axis for some personal art.  Perhaps I should just design my own, like you said.

Barry

[Dan13]

Yes, designing your own one would be best. Your best bet in terms of backlash and simplicity of design would be using a direct timing belt drive from a stepper to the 4th axis spindle.

You will need to establish the holding torque required. For this you need to know the most far point away from the 4th axis centre line that you will ever cut. And you'll need to consider the tool load at this point - tool diameter and material you'll be cutting. then we'll need to estimate the plunge down force produced by that tool. I can figure how to claculate longitudinal cutting forces and moments at the spindle, but have never calculated a plunging force. Best thing to do is to experiment, but I don't know how could you possibly arrange to put a weight scale under a workpiece and see how much force does the tool produce when you plunge...  If you find a way to do it safely it will be the most invaluable and important information you will need for this project... or at least I think so

Dan

[BarryB]

Here is a 6 inch one from Shoptask:

http://www.shoptask.com/

It's only $499 too.  Still trying to decide.  I'd prefer to not create one myself;)

Barry