[Markllar]

Has anyone tried mounting a motorized 6" rotary table to a motorized 8" rotary to create a somewhat cheap 4th and 5th axis? Any feedback as to the feasibility of such an endeavor? I want to cut 3D shapes in PVC on my Tormach 1100 and I need full motion 5 axis to make it work.

[jeffrey001]

Are you talking about some thing like in this video?

New CNC Trunnion Table can be 4th or 5th Axis made by HTS - YouTube

Jeff

[Markllar]

Hi Jeff,

That is very much like what I need. I was hoping that I would not need the trunion "saddle" type setup. Though I suppose the two point support of the trunion would be needed for stiffness. I have tried the Sherline rotaries and they are not powerful enough for smooth operation while cutting. I know the Tormach rotaries are plenty powerful but they are heavy as heck and the added weight of the trunion puts the table weight over the limit. If I could "just" bolt up the 6" rotary to the 8" without a trunion, that would be great.

[SCzEngrgGroup]

I would think your two biggest problems would be:

1) The cost of CAM software to generate code. Decent 5-axis CAM does not come cheap, and is not easy to "drive".
2) The amount of backlash. With two low-cost rotary tables, you'd have a pretty large positional uncertaintly, and a significant amount of backlash, at least at some positions. The worm gear sets in those cheap rotary tables are not precision affairs. A high-quality 4th and 5th axis would use harmonic drives, or some other zero-backlash arrangement, and also no doubt much better bearings.

Regards,
Ray L.

[MMMWWWPPP]

Now wait a sec--doesn't the 1100 have a 500lb table weight limit? The tormach rotaries are about ~100lb each. That leaves a pretty healthy weight budget for additional support structure and workpiece. BTW, what you're taking about has actually been done, and by tormach no less--see here: 5-Axis Prototype Sneak Peek- Tormach CNC - YouTube .

I think their comments were along the lines of Ray's on the backlash & rigidity though (might be more workable for wax and plastic than metal). Tormach does quote a number for backlash on their rotary tables at the equivalent of .0006" at the outside edge of the 8" table (4" radius). You can see where if you start getting a fair distance from the pivot, the backlash begins to approach (or exceed) the chipload for a typical cutter...can't be good, particularly for climb milling.

BTW, a while back I was looking at motorizing the tilt axis on one of the tilting rotary tables that they sell. Seems like that would get you away from the trunnion.

So far as reasonable 5th axis CAM software, sprutcam would seem like the natural option given that it is part of the tormach "ecosystem" and includes 5th axis support in the tormach version that they sell. I own it and like it, though haven't yet used for 5th axis work...would like to if I can get my hands on a machine.

[Don Clement]

The amount of backlash. With two low-cost rotary tables, you'd have a pretty large positional uncertaintly, and a significant amount of backlash, at least at some positions. The worm gear sets in those cheap rotary tables are not precision affairs. A high-quality 4th and 5th axis would use harmonic drives, or some other zero-backlash arrangement, and also no doubt much better bearings.

I have experimented with alternate drive systems for a rotary table using a backlash-free cable drive. In this example I also used a SINGLE 8” bore Kaydon X bearing (1/2” x 1/2” cross section) http://www.kaydonbearings.com/RealiS...s_KD_typeX.htm to support the “rotary table” equatorial telescope drive. I got sub arc second repeatability and no measurable periodic error, something very difficult to achieve with a worm. http://www.clementfocuser.com/images...rive_Print.pdf Nice features of the cable drive are complete immunity to flood coolant and requires no lubrication.
I built the cable drive years ago using only a 13" lathe and drill press. The parts of the cable drive are easily machined round surfaces. The play for the single Kaydon X bearing was taken up by using a thermal differential between the aluminum housing and aluminum bore and adding red Loctite 271 while the housing and bore thermally stabilized. One can build their own high precision rotary table using only ordinary shop tools.


Don

[Steve Seebold]

Are you talking about some thing like in this video?

New CNC Trunnion Table can be 4th or 5th Axis made by HTS - YouTube

Jeff

That's cool, but that stepper sticking off the front severely limits your range of motion forward motion.

[philbur]

That's cool, but that stepper sticking off the front severely limits your range of motion forward motion.

I can't see how the position of the stepper limits the forward motion? What am I missing?

Phil

[kstrauss]

I think that he means that the stepper position prevents tilting the cradle very much. Someone (I can't recall the name) makes an adaption for the Tormach rotary to reposition the stepper.

[rowbare]

Scott has done some 5 axis stuff: http://www.cnczone.com/forums/tormac...s_running.html

http://www.cnczone.com/forums/tormac..._pictures.html

bob

[Markllar]

Thanks for all the great information! I like the way Don thinks. I may just "roll my own". I will need to learn more about cable drives though.