Why are CNC rotary tables so expensive?

Even the small ones seem to sell for the price of a modest manual lathe that has a lot more parts and material in it. I'm talking about just the table/servo/brake combo, not the ones that add electronics by having a dedicated controller on them.

Is it because of high precision worm gears to avoid backlash?

If timing belts are fine for driving the ball screws on my Tree CNC mill, why aren't there (or are there?) rotary tables with a timing belt drive from the servo? It seems like there could be a big pulley on the spindle of the rotary table and a small one on the servo to gear things down to get small increments of rotation at the spindle and magnify the torque of the servo.

The one photo of a cutaway CNC RT I could find

http://www.workholding.com/RT160%20Section.jpg

shows a couple of big ball bearings behind the platen, a worm gear in the middle of the spindle, and more big bearings at the far end.

You could almost just get a sturdy spindle/headstock assy off of a lathe with junk ways and power it via a belt and have a vertical (horizontal spindle) rotary chuck/table. Of course that might end up pretty heavy and bulky if you used the headstock from a big lathe. Or just get a spindle (like this one from a Mori Seiki)

http://www.eurospares.com/graphics/metalwork/morispindle.jpg

and shorten it and make a housing.

I presume there are lots of details to the design of a CNC RT that I'm not aware of, but I'm definitely curious.

cheers,
Michael

I had a disaddembled Haas. I sorta remember that it had a cogged timming belt and pulleys. No worm. But it was a while ago and other makes are probably different.

Michael,
If you think rotary tables are high, I just bought an NC tilting rotary table from Troyke. Final price about $16K. Tsudokoma wanted $30K.

Look here http://www.machinetools.com/mt/machines/Listview.tmpl?lvt=Group&MachineID=2580&MachineClass=used

There were some tables starting in the hundreds. Get a manual table then a compatable motor for your machne. You will have to know how your control works so that you know how to "gear" the motor to the table. Cogged belt is the norm. You should be able to buy a table, new motor, conectors and stuff for $1500 or less, depending on size.
JR

I have wondered the same thing. I think it has mostly to do with the fact that people need them so they are not afraid to charge for them.
The Haas units are somewhat reasonble compared to some others.

We have a Haas HRT210 at work. Decently made and works well. The construction entails a short spindle supported by large tapered roller bearings. The drive is a worm type. I believe the worm is driven through a timing belt from a brushless motor.

In the end, if you have one it lets you work that those without can't.


Later,

ARB

Michael;

When a linear axis ball screw is driven by a belt, the motor force transmitted by the belt goes through the gear reduction of the ball screw and multiplies the power of the motor to produce a useable force at the output end of the system. The ball screw acts as a "transmission" (for lack of a better word) to multiply the force that the belt transmits. If you operated a rotary table by belt without a worm gear, it would be like coupling a belt drive directly between a servo motor and a linear axis on a machine and putting all of the strain on the belt instead of on the ball screw. You would need a very strong belt and large servo to produce any useable forces without breaking the system. Because of that it is probably more feasible to go with the worm gear to produce the accuracy, force and small size required. That's my take on it anyway.

Michael,

If you check in with some of the CNC groups on Yahoo, you will find lots of people who have modified rotary tables for CNC use. It's fairly easy.

Since the table already has a 90:1 reduction ratio, the stepper motor or servo typically drives the worm directly, without a timing belt.

If you want to retain the ability to use the table manually, then a dual-shaft motor is used and the handwheel is moved to the outboard shaft.

Lots of info on the DIY-CNC forum. You should be able to find someone there who has actually done this using the Centroid controller.

Glenn Minch

Economics 101:

They are so expensive because only a small number are made and sold.

Only a small number are made and sold because they are so expensive.

They are so expensive because ..... Well, you get it.


Paul A.

Glenn, searching the archives at DIY-CNC and CCED was one of the first things I did, and not finding much info surprised me. Maybe I stopped the search before finding the right posts - there are a large number of posts and the Yahoo search doesn't serve up many posts/page in the search.

The Google searches for the general net (I should go back and try running Google against DIY-CNC - nope, I couldn't get that to work) didn't turn up much, and what they did show was largely small inexpensive tables converted by hobbyists.

I'm going to be wanting more of an industrial quality RT when I get the money together for it. I am getting the 4th axis drive and software with my control, so I don't need an RT with its own controller, just one that works with the Centroid control/drive.

Mebfab, I could see the 90:1 gear reduction was going to need to be at least somewhat matched with a belt drive which sounded like it could get bulky because of big sprockets. Maybe if I could get a 9 tooth sprocket for the spindle and a .1 tooth sprocket for the servo . . .

It looks like the brakes on many of the commercial units are pneumatic, and not electric. Ajax has a 40 inch/pound servo with a 24v DC brake for about $700. Any ideas of how powerful of a servo motor the smaller commercial units tend to use?

The big rotating element bearings would seem to be there because of the higher rotation speeds that are available. A manual table appears to generally use a bronze bush or thrust washers, but I presume they can take similar cutting loads. For commercial use the rolling element bearings also may provide greater longevity.

I've got a 10" Yuasa H/V table, but I was thinking that selling that to my friend who bought my old mill would make more sense then trying to cobble it into a CNC unit. It also gets him a rotary table.

Plus, the Yuasa does have some backlash, and I'd guess that a CNC table would need to run with pretty much zero backlash so it doesn't chatter around. Zero backlash without wearing out quickly sounds like it could mean more precision and smaller clearances in the worm and therefore more money. Does that sound reasonable?

Paul, some of the tables that Troyke sells look a lot like they've motorized their manual table, though they do mention dual-worm lead screws (backlash?) and rolling element spindle bearings and pneumatic brakes. I was thinking that the basic rotary table technology wasn't going to be the big part of the price as they ought to be able to crank out a spindle/platter/housing pretty readily.

JR, I've seen the prices on those trunnion tables and stuff, and they do get pretty impressive even when used. Did you buy a new one from Troyke? Their smaller RTs seem to be among the more reasonably priced units, though I haven't seen a lot of prices listed on the net.

A previous thread I did in the CNC forum mentioned a lot of the high-end RT makers. Without understanding all the factors involved (which I don't claim to do) it just looks like there isn't "a lot of there there" in the CNC tables to justify the prices they are getting.

cheers,
Michael

Michael,

One thing that is sorta misleading about the CNC tables unless you're seen one first hand is just how big they are. I've got a Tsudakoma THNC-301, which is a 320mm (12"+) table thats rotary powered and manually tiltable from horizontal to a bit past vertical. It weighs somewhere between 400 and 450 lbs, and I guess the new cost now is somewhere north of $16,000. They have positioning accuracy within a few arc seconds, and are capable of holding accuracies like this with a part that weighs a couple hundred pounds and cutting forces that can generate several hundred ft-lbs of torque.

Pricewise, assuming my 16K current estimate is accurate, that would work out to a bit less than $40/lb, because it definitely weighs over 400#. For comparison, a 10" Kitagawa power chuck for a lathe retails now for about $4000, and although I've never weighed one, I guess they weigh less than a hundred lbs from picking one up a "few" times. So that puts the chuck at over $40/lb. If you ever have cause to take one of these chucks apart, they are surprisingly simple. They're accurate and repeatable, and everything is hardened and ground, but they're still simple, especially when compared to a rotary table. A good sized collet chuck from Royal to fit an A-8 spindle nose on a lathe weighs barely over 40 lbs and costs close to $2500. Once again, its all hardened and ground, but this is the price of a pullback type chuck which is just one single piece of steel with no moving parts whatsoever, yet it sells for over $60/lb. When you compare the rotary tables and their size and complexity to other machine tool parts of similar complexity and quality, the price begins to look not so far out of line, even though its still not cheap by a long shot.

you get what you pay for.... Nikken 4ths and 5ths have carbide worm gears and nitrided screws for an exceptionalyy long wearing and accurate device. think of the tolerances cnc is expected to maintain compared to the ole rotary bridgeport table. no backlash, extremely rigid, hermetically sealed,,, it all adds up.

I inquired about a Suda CNC rotary table that is for sale and was told that it has hydraulic lines running to it for the brake. My impression is that most RTs would have an air line hookup and then a solenoid to feed air to a brake, sort of the way my Tree has a solenoid to control the pneumatic draw bar.

Is anyone familiar with what might be needed on an hydraulic brake system? A remote pump would seem to add more complexity/expense to this table.

I couldn't find out much about Suda except they are a precision gear maker in Japan.

Thanks,
Michael

suda is an acronymn for Tsudakoma.... some of the higher end tables have hydraulic clamps. I have some 20 inch nikkens in horizontals that are all hydraulic. activated with electric solenoid valves for clamp and unclamp via relays in the control. The control circuitry is the same for air. the remote pump comes on and powers other machine functions in the spindles and tool changers as well.

Actually, Suda International Gear Works

http://www.gleason.com/new/GlobalSolutions/245_pdf_english.pdf

appears to be a different company from Tsudakoma Corporation

http://www.tsudakoma.co.jp/

Adding some additional mechanism in the garage to supply hydraulic pressure only to a rotary table is probably going to be a reason for scratching off any RT that doesn't use electrical or compressed air for braking.

thanks,
Michael

cheers,
Michael

Here is a pic of my Troyke that was converted to cnc, I also have a Yuasa tilting rotary that is cnc.

Hi Cliff,

Did you make a new clutch basket or are you just cleaning up the dents from the clutch plates in that one?

Did you rig up a brake, or is the servo powerful enough to hold things steady? Any backlash issues?

I'd be interested in any comments you care to make on the conversion process and what, if any, problems you have had with the conversion.

cheers,
Michael

I was just cleaning the dents on the basket.
That is a stepper and holds pretty good without the brake, my Yuasa has an air cylinder for brake actuation and dc servos for motion. I didn't do the conversion on either table, I bought them surplus already done, but the motor mounts directly to where the handwheels would be, the one pictured has the 90 deg drive which also reduces the table 2:1 which works out to 100 steps per deg in my setup and the stepper has 940 oz of torque. Let me know if you need any other specific info.

Mebfab said:

I had a disaddembled Haas. I sorta remember that it had a cogged timming belt and pulleys. No worm. But it was a while ago and other makes are probably different.

Click to expand...

Haha, nice to know that there is no magic, even behind something like that.

Most of the stuff we have to make on a 4th axis isn't that big. For the majority (>95%) of our 4th axis work, our workpiece is under 2" in diameter and I would guess that I am not alone. I wonder why doesn't anyone just use smaller CNC rotary tables for their work? These look very affordable:
CNC 4th Axis Rotary Table (100mm / 4 inch)

You can probably buy one from saving up your beer change.

for small stuff, I have been looking at this one. Actually made here in the US, and will accept 5C collets aswell.

CNC Rotary Table Sherline Maxnc Taig CNC Router CNC Milling Machine Pool Cue | eBay

Let me also ask a question in this thread.

I'm searching for an old 8..10" Rotary table for my Fanuc Robodrill. I choose between nikken and tsudakomas. Tsudakoma's are a bit cheaper. they can be found for around 2-3k while nikkens are 4-5k

But the main question is the type of a brake they use. Nikken use pneumatic brake, so it can be easily connected to the machine. What about tsudakoma? for example there are some decent tsudakoma RNCV-201R tables on ebay, but if they have hydraulic brake it will be a big problem for me.

if somebody know about them, please....

Best regards.