Thread: Metal gear / worm gear needed

G'day Colin,
www.jaycar.com.au have a robotics section on their on line cattledog which has gears etc.
Don't know if they'd be big enough for what you want.

Originally Posted by colinshannon

Hi,
Hoping someone can point me in the right direction. I'm adding a stepper motor driven indexing head to my lathe and am trying to track down a suitable gear and worm drive set / pair.
Does anyone know anywhere I may be able to buy such things? few web s
Thanks.Colin

I have in the past picked up electric garage door openers.Also broken motors from computer speed controlled conveyor belts.
Let us know if you find any of the shelf .Last time i looked-4 years ago the off the shelf worm and gear were dear .Perhaps from overseas maybe the go now that the A$ is up. http://www.sew-eurodrive.com.au/produkt/A14.htm
[url ]http://www.gearcutting.com.au/wormw.htm
< OR >
http://www.turbofast.com.au/astrotel/odds&ends1.html[/url]

Your choice will depend on how accurate you want your set up.
I think there will be too much backlash in a conventional gear drive for CNC. You are probably going to address 1600 steps (with 1/8 microstep mode and 1.8 degree stepper) for each revolution. Conventional mechanical gearing will have backlash greater than one step so you loose accuracy. Have you thought about tooth belts and pulleys? Cheap way is to use overhead cam pulleys and belts or you can buy commercial products from here.
http://www.bsc.com.au/html-bsc04/indexBSC.htm
If you want mechanical then a harmonic drive gearbox is the choice but expect to pay heaps even second hand.

You could check out the range of Gearboxes for G Scale Model Trains, These have
some very fine Brass Gears and Pinions.

Regards,
John.

G'day Colin, try www.hobbymechanics.com.au, they have a rotary table kit to build and they sell the worm gear set, I think the have 4,6 and 8 inch tables, they are in Qld., the last time I looked they where about $140 or so dollars,
Cheers, Richard.

Thanks fellas,

Definately some ideas there. I also just recalled seeing a long worm gear in an old car wiper motor gearbox as well (these are cheap-as at the wreckers).

I 'll check some of the websites given as well. Backlash is going to be somthing to consider (can be compensated for in the cnc program but obviously the less backlash in the mechanics of it all the better), so I am also open to the belt / pulley idea. I've just seen it done in an English engineering mag twice using the worm set up, but a lot of time would have been spent getting it all to 'mesh' together perfectly.

My only hesitation with the pulley / belt idea was would the motor have enough holding torque to move the lathe spindle and then maintain position while making a cut, I use steppers on the lathe as part of the cnc conversion (x & y axis) but have not really had to rely on them to 'hold' any position as such.

I'll try a few things out, I guess this is why trial and error was invented eh?

Thanks again for the input.

Colin

Backlash is unavoidable in any mechanical system. IMHO, the simplest way to accommodate it is to provide a preload on one side of the output element; as long as it isn't overdone, wear shouldn't be seriously compromised. A biasing spring would provide increasing preload, so likely isn't best. For constant preload, wrap a cable around the output wheel with a pulley or two to a counterweight. All depends on the overall configuration, of course.

Joe

Colin to ease your mind steppers have their greatest torque at standstill. It takes a while to get your head around how they work but the faster they go the less torque they produce. The torque curve drops off rapidly and in my experience I only expect steppers to rev to 400 rpm max under load.

Of course torque can be increased by using a small to a large pulleys and on an indexing head the drop in speeed should not be a problem.

Software compensation for backlash does work while the components are moving however if the indexing is stationary while your other axes are making a cut then the backlash comes back into play.

As Joe has pointed out preloading to either side of the backlash is a solution. I have seen pnuematic rams used for linear movement. Rotary backlash is much more difficult to overcome because the indexing might only be in the one direction therfore it will eventually wind itself up.

At the end of the day I think it is best to eliminate the backlash in your mechanical components.

Thanks rodm and Joe,

So would you reckon the worm drive may not be the best way to go and better to look at a toothed belt / pulley type system? The belt and pulley would certainly be easier to rig up.

My configuration will be:

Using the spindle to hold round or square stock (funny that), however I am also building a cross slide mounted powered milling / drilling head (will kinda be a horizontal milling machine), which will do all the cutting.. Behind the spidle will be the mechanism with the stepper motor (as well as that for the normal lathe motor). When I want to use the stepper for indexing I will just 'swing' it into position / OR slip on the belt, another question to mind...if I left the belt to the normal lathe motor attached while doing the milling operation, would that provide pre-load to the mechanism? I guess a good fitting toothed belt and pulley setup would / should eliminate pretty much all backlash in the mechanism?

What are you guys doing to be so technically minded at such unworldly hours of the morning?

Colin

You could try cgb precision products in 94 voltri st mentone ph 9584 5311
They carry all types of gears belts toothed ,linear motion systems etc hope this is of some help...

Hi Colin

The toothed belts and pulleys in the link I gave earlier are used by industry and CNC builders for zero backlash applications. That is the path I would follow with your design. These pulleys and belts are used for mill conversions so they are proven for high loads.

Have you visited CNCZone for information?
http://www.cnczone.com/forums/
You will find all the answers you need there. Indexing heads are usually the forth axis so search on that and any other terms you can think of.

Leaving the lathe motor attached while indexing will only add load to the stepper and not help at all with backlash. As soon as the indexer is stationary or changes direction the backlash will come into play.

I am not an expert and will listen to anybody who knows what they are talking about. I have built 3 CNC gantry routers myself and nearly finished converting a mill to CNC. I have also helped convert a 9 X 20 lathe (AL50GB) and 5 other gantry routers so I have made just about all the mistakes one can with CNC building.

I am in Perth so we are a couple of hours behind EST.

Originally Posted by colinshannon

Hi,

Hoping someone can point me in the right direction. I'm adding a stepper motor driven indexing head to my lathe and am trying to track down a suitable gear and worm drive set / pair.

Does anyone know anywhere I may be able to buy such things? Have done a few web searches under the obvious stuff but cannot find anything. Would like an online shop or something if possible.

Thanks.

Colin

Hi Colin,

For what its worth I have available some brand new cast iron worm drive boxes ratio 40:1 there seems to be very little backlash in them.

They are pretty heavy but may suit your need.

There are a couple of others in the mix which I have but they are quite big like suitable for a truck winch.



PM me if you need more info.

Peter

Contrary to my earlier assertion about "unavoidable" backlash, cog belts can in fact have zero backlash. The tricky part will be to select appropriate pulley sizes for your desired reduction ratio. Dunno about cog belts, but for v-belts and flat belts, power transmission factor drops toward about 60% as the arc of contact on the smaller pulley drops from 180 degrees to 90 degrees.

Joe

Hmmmm,

Now I'm getting somehwere, thanks for all the info!

I think I will piece together something with a toothed belt for starters (I think I may have all the bits laying around here to make that conversion possible, also seems the best option at the moment for minimal backlash), (I can see this being one of those...'I'll just hook this up like this for the time being'.... years later .... 'when I get some time I'll fix that up properly'!). Will also disconnect the lathe motor belt when using the stepper if it has no advantage.

Thanks for the offer Peter, will definately keep it in mind as well.

I will let you guys know how it all turns out.

Thanks again.

Colin