Worm Gear

Poulos Cues

Poulos Cues

AzB Silver Member
Silver Member
Anyone,
I am building a CNC machine and currently the 52" of travel-"y" axis has a 1-turn, 1- inch coarse acme screw. At 200 steps, (non-micro controller- I picked up a 4-axis MaxNC CL controller for cheap!) the smallest movement that I can do is .005 IF my math is right??
(By the way, the other screws are 1/2" x 20 tpi, v-groove NOT ball.)

For just doing od turning this would be fine I think? I eventually want to do inlays.

So the second question is getting smaller steps. I could put a timing pully / reducer on the screw but I am being told that the "little" 200oz motor I am using would probably not be able to hold the Z axis in place (right now in manual mode you can move the Z axis slide by hand without turning the crank 'cuz the screw is SO coarse.) Plus the gear on the end of the screw would be about 4" in diameter and the gear on the motor would be less tha 5/8" I think.

Short of going thru the expense of buying a new screw, I am being told that I could use a WORM GEAR and that this would allow the motor to hold the Z axis in place when plunging downward.

Anyone here know about this stuff and might could help me out? If this is the way to go, I would like to "buy" an assembly and not have to fabricate one.

I can get into more details if needed.

Thanks in advance for any help on this.
Chris
 
I'm no expert, and much of what I know is based on material from Boston Gear and Stock Drive Products, but they are very cool gears and sound reasonable for your application. Check out these two companies, they have some excellent reference material on their sites that should answer any and all questions you have. In general with any reasonable gear ratio (say 25:1 or more) it is very difficult for the worm gear to turn the worm (run 'backwards') which is why the holding torque is increased using a worm gear assembly.

Here are a couple of suppliers :

http://bostongear.com/products/open/worms.html

https://sdp-si.com/eStore/Direct.asp?CP=Index.htm

If it were me I would be looking at the surplus joints for recycled worms and gears and adapting them for the application by making hubs/reducers/etc. On the other hand, worm gears can be made fairly easily using a standard tap as a hob. The resulting gear will mesh perfectly with worms made from the same screw pitch as the tap used to make the gear. Here is a brief discussion on how to hob a worm gear on a small lathe :

http://www.dragonworks.info/Metalworking/Gearmaking/Gear Making.htm

If this is something that you may want to pursue let me know and I'll dig out a much better pdf file that describes the process including the critical part about determining the OD of the gear blank.

Dave
 
DaveK said:
Here are a couple of suppliers :

http://bostongear.com/products/open/worms.html

https://sdp-si.com/eStore/Direct.asp?CP=Index.htm

Thank you for the help.
I contacted Boston and the gentleman told me that they do not produce precision gears and that he thought that I should contact SDP. I contacted them and they put me over to Techno-Isel who SDP is a division of.
He said that we were right in thinking that the timing belt system would NOT hold the Y in place to do Z milling due to the coarseness of the screw.
He furthered by saying that worms gears can be very tricky to deal with in machining and aligning them for backlash reasons, and that buying another screw for that axis would be the best way to go in his opinion. I can get an anti- backlash screw and have it machined with bearing blocks for less than 250.00.
I guess this is what I should do????????
I did see some pics of a guy who converted his taig head to a 4th axis relativley easy and made it for under $100.00. It was pretty cool!
Any other suggestions?
Chris
Click to expand...
 
I'm also no expert but have done a lot of reading on the subject...

I would get new screws. If you go to McMastercar (spelling) .com or Reid tools .com they both have ball screws that are very economical. The 5/8 diameter sized is "economically priced" and is significantly cheaper than both the smaller diameter screws and the larger diameter screws. I think it is something like a dollar an inch. The ball nuts in that diameter are about $25. I bought enough screws and nuts a couple of years ago to build a machine for all 3 axes and the total was only a couple of hundred or so. (I never built the machine...)

Ball screws have two major advantages over standard screws:
1. They are extremely efficient and require very low torque to turn them.
2. They can be fitted with anti-backlash nuts and still have very low turning torque.

If you add a worm and worm gear setup, you are introducing lots of backlash to the system in the x and y axis.

However, in the Z axis, it might not be a bad idea because the weight of the Z components always takes up the backlash in the screw. However, if you have a heavy z axis, I think it is best to counter weight it. That is how our cnc bridgeport mill is setup at work.

As far as your step resolution, you should be way under .005 per step. You need a new screw for that. Just imagine lathe work where you smallest increment would be .005. You could never make a precise part.

You can get cheap and good controllers from gecko.com and other places. I would definitely use a 8 microstep per step controller and have each micro step in the tenths or smaller. Or use servomotors with encoders.

As stated earier, read up on cncznone.com. Also, an excellent site is this one, where the guy built a nice cnc and chronicles the whole project.

http://www.5bears.com/cnc.htm

Anyone planning on building a cnc for the first time should definitely read the entire above link.

Finally, just my personal experience, I spent a couple years off and on reading up on cnc, buying slides, screws, stepper motors, servo motors, stepper controllers, servo controllers, you name it. I probably spend a couple thousand dollars, and countless hours on ebay and all the cnc sites. If you count the time spent learning about it, buying parts, then designing a machine around the parts you bought, then building a machine, debugging it, etc., you are probably much better off just buying a machine.

By the way, I lost interest and never built it...

Good luck.
 
Thanks for your help.
I could not tell from McMaster-Carr if they sell anti-backlash nuts for the ball screws though.

I will do some more research (as you did) and make a decision. This machine may be only good for OD turning and tapering (1 spindle but 4 lathe stations- stationary gantry system)...we'll see:eek:
Chris
 
Poulos Cues said:
Thanks for your help.
I could not tell from McMaster-Carr if they sell anti-backlash nuts for the ball screws though.

I will do some more research (as you did) and make a decision. This machine may be only good for OD turning and tapering (1 spindle but 4 lathe stations- stationary gantry system)...we'll see:eek:
Chris
Click to expand...

Ball screws use ball nuts. You can use anti-backlash nuts on precision thread screws. Anti-backlash screws/nuts are much cheaper than ball screws/nuts but have much more friction and aren't as durable.

Dick
 
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