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-   Driving Mechanisms: Rack/pinion, gears, screws, belts & chains (http://www.mechmate.com/forums/forumdisplay.php?f=24)
-   -   Driving an "indexer" , or a ("4th") rotary axis (http://www.mechmate.com/forums/showthread.php?t=826)

edrowell Wed 11 June 2008 06:53

Driving an "indexer" , or a ("4th") rotary axis
 
2 Attachment(s)
I have this gearhead and I'm thinking of buying a new motor and using it for a 4th axis. I'm wondering, if I make a mounting plate similar to the plates for the gantry motor mounts and spring load it like the other motors, would it remove any potential backlash. I love the spring loaded design of the gantry motors and would like to incorporate it here. I've handled a few rotary tables and after some use they all seem to develop backlash in the gears.

Evan


J.R. Hatcher Wed 11 June 2008 07:13

Evan you can only use this type of apparatus if you have 2 and are willing to fix me up with 1:rolleyes:. This is a new rule I just made it up:eek::confused:.
Joking aside it looks good could you get more of these? Does it work with a pinion if so what size (grooves and bore)? What does the other side look like?

edrowell Wed 11 June 2008 07:40

J.R.,

I actually have two of them, but I have ambitious plans for them. They are gear heads from satellite actuators and you can see the pinion still on the motors. You can find them on ebay every now and again. As a matter of fact, I just searched and there are a couple on there now (just search for cnc rotary and you'll see them). Here is the link to the ones I bought and you can see the other side from there:

http://cgi.ebay.com/ws/eBayISAPI.dll...m=130217202508

The ones I have seem to be in really good shape. THe bearings turn smooth and there appears to be no wear on the gears or pinions. I have yet to determine the tooth count or ratio.

Evan

edrowell Wed 11 June 2008 09:21

The ratio for the gears are as follows:

Pinion - 20 tooth
Gear - 110 tooth
Ratio 5.5:1

Robert M Fri 13 June 2008 09:19

Since Im not a robotics & gear head inclined, can anyone advise / help me if this rotary table type is a good apparatus I should consider for an indexer on my future MM or should I aim at some home-made gear drive reduction w/belt, say like JR latest motors trany gear belt reduction set-up !
Thanks, Robert :confused:

javeria Sun 15 June 2008 13:06

There are also some sort of gears called harmonic gear - i bought one on ebay - have a reduction of 1:100, but i feel if using steppers - a home made timer pulley reduction will be sufficient for 4th axis.

Gerald D Sun 15 June 2008 13:25

A classic "turntable" indexer, used for decades by the metal-cutting shops on milling machines, has a worm gearbox drive. Very slow turning, locks solid, zero backlash.

Amateur woodworkers dabbling in CNC, have cottoned onto this " indexer" word and have distorted the concept beyond recognition. They are mostly thinking "lathe" and talking "indexer".

Some quick questions for all of you talking indexer . . . . .
1. What range of diameters will your workpieces be?
2. What move speeds (cutter speed relative to workpiece surface) do you want from the drive motor/system? Max and Min.
3. What force do you want to apply to the cutter while the indexer locks itself in a steady position?
4. How much backlash (slop between cutter and workpiece surface) could you tolerate?

A rough formula for discussion:

The gear ratio of an "indexer" = Ri :1
. . . where Ri = 3 to 8 times the diameter of the workpiece in inches.

ie. for a workpiece that is 4" in dameter, you need an indexer ratio of between 12:1 and 32:1

for workpiece 2 to 6" in diameter, you need a ratio 16:1 and above, or something like that.

At those big ratios, backlash becomes a big issue . . . . . unless you go back and look at the mechanical blokes who started off with worm-drives.....

http://images.google.co.za/images?q=...A_enZA279&sa=2

edrowell Mon 16 June 2008 06:48

I have used worm gear rotary tables in our machine shop at work and they would seem to be ideal for an indexer. However, we have several of them, and the ones that have been used for a while all have developed backlash.
The pics of the satellite gear drive I posted are currently have little to no backlash but I am expecting some develop over time. Therefore, my question is whether it might be possible to spring load the motor/pinion on the gear box, similar to the rack and pinion setup on the mechmate, in order to get rid of the backlash.

Gerald D Mon 16 June 2008 07:52

Quote:
Originally Posted by edrowell View Post
. . . my question is whether it might be possible to spring load the motor/pinion on the gear box, similar to the rack and pinion setup on the mechmate, in order to get rid of the backlash.
Of course you can - there is no reason why it will not work. In fact, the first spring-loaded anti-backlash gear system I saw was on a ship's fire control radar antenna in the early 80's where there was a double springloading of motor to primary gear as well as the final pinion to the ring gear.

Gerald D Mon 16 June 2008 08:14

Have a look at these for an anti-backlash worm design:
http://ralmtec.com/Descriptions.htm

And the "Constructions" links on that page. Very similar principle to what we do for our spring-loaded system.

Art Mon 16 June 2008 08:32

My indexer has a 4.5:1 ratio driven by a 960 oz stepper. The biggest problem is that occasionaly if I take too deep a cut or hit a hard part in the wood there is a kickback that will actually over ride the stepper and ruin the piece. Your 100:1 should prevent this problem and be a better solution. Assuming that the stepper runs at a max of 1500 RPM then you could get 150 RPM more than adaquate. I just ran a 6" column at 26 RPM which is 450 IPM. Presently I am running 2 different configurations. One is is a standard mill configuration with the A axis running in degrees and the other has the spindle set with the same port and pin as the A axis which is disabled. This allows me to have the A continiously rotating with speed controle with spindle speed.

Gerald D Mon 16 June 2008 09:34

Quote:
Originally Posted by Art View Post
Your 100:1 should prevent this problem and be a better solution. Assuming that the stepper runs at a max of 1500 RPM then you could get 150 RPM more than adaquate. I just ran a 6" column at 26 RPM which is 450 IPM.
Art, shouldn't that "150" you mentioned actually be 15?

edrowell Tue 17 June 2008 06:20

Gerald,

Thanks for the link. Those rotary tables look very, very nice.
I just wanted to check that I'm not doing something silly here, so I think I will try spring loading this gearbox similar to the mechmate design. If it works, I'm thinking an Oriental Motor stepper with a higher gear ratio to help with kickback, and larger pieces.

Art, are you using the indexer as a lathe and getting them up to those rpm's, or are you turning your pieces incrementally as I understand a true indexer?

Evan

edrowell Tue 17 June 2008 06:56

1 Attachment(s)
This is kind of what I am thinking of. Looks very similar to what we are doing with the rack and pinion setup.

isladelobos Sun 28 June 2015 17:41

Im thinking in a reduction like 30:1. for the indexer, with steppers.
My main board can't run more than 1000rpm the engines i receive errors.
1Ghz old.

with 30:1 at 1000rpm at full velocity it reduce to 33rpm or 1 revolutions in 2 second. (G0 moments)
At half velocity 500rpm it reduce to 16.6rpm or 1 revolution in 4 seconds. (milling)

If 100:1 at 1000rpm it have 10rpm or 1 rev in 6 seconds aprox.

200 steps per revolution at 30:1 reduction are 6000 steps in one revolution.
6000/360 are 0.06 in one step

A 200mm indexer plate= 2pi100mm= 628mm circunfernece lenght
This is 628mm/6000steps =0.1mm off movement in one step.

30:1 is good for steppers if the computer can handle this.
100:1 is very slow for big milling pieces?

your free to comment.


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