Newbie- STANDARD THREADED ROD

[eric5501]

Hi Guys, hopefully these questions are in the right place, FIRST, what would the accuracy be using standard threaded rod (galvanised or plain) still using anti baklash nut(s) over the precision rods (acme thread costs an arm and a leg and a few other neccessary body parts here in Australia). SECOND, today I heard that it is not a good idea to use steel bearings (v-groove or flat) on aluminium as the aluminium will "stick" to the steel (case hardened), what are your thoughts on this. Thanks, Eric

[dkirtley]

The precision is potentially better in that they have much finer threads. The required force to turn them would be lower because of mechanical advantage. The downsides are speed and durability. The angle of the bearing surface is not as strong and the smaller, and non-hardened thread will wear faster. It also cannot handle the forces as well (acme threads are pretty beefy.) Everything in life is a trade off.

As far as the bearings, any time you have a hardened steel rolling or rubbing against something as malleable as AL, you can have problems but overall, I wouldn't be worried about it on a hobby machine. All parts will eventually wear and need to be replaced. Just keep an eye on it and if it becomes too much of a problem, then think about replacing or upgrading.

[johnmac]

Speaking from experience, common threaded rods have too many turns per inch, and the machine will run way too slow. Steppers produce the most power (torque) at slow speeds. As they rev up, they loose power to the point they simply stall out. You will be much happier with Acme 8-10 TPI, 2 start screws. That will give 4-5 turns per inch. With half steps or finer, that will still give all the precision you will need.

John

[eric5501]

Thanks guys for your comments, sorry I have not got back earlier, your comments are pretty much what I had thought, just needed confirmation.
dkirtly, I read your blog, it is almost contradictory but made sense second time round. The cnc I plan to build will be about 1200mm x 1000mm (4' x 3') +- I have a 4 axis controller with 4 nema 23 motors, initially I will not be working the full size after all one has to learn to crawl before one can walk.
As more questions arise I will post them, Thanks once again, Eric

[dkirtley]

Eric:

I am planning out pretty much a wood and plastic cutting machine. It would be dead wrong for other materials. It just depends on what you want the machine to do and what you have available.

[Jason Marsha]

Originally Posted by eric5501 Hi Guys, hopefully these questions are in the right place, FIRST, what would the accuracy be using standard threaded rod (galvanised or plain) still using anti baklash nut(s) over the precision rods (acme thread costs an arm and a leg and a few other neccessary body parts here in Australia). SECOND, today I heard that it is not a good idea to use steel bearings (v-groove or flat) on aluminium as the aluminium will "stick" to the steel (case hardened), what are your thoughts on this. Thanks, Eric

Aluminum is too soft to run steel bearings on. It is better to use steel angle as
its cheap, hard and will last along time. As others stated the multi-start acme
screws are best as they allow the stepper to operate in its highest torque band
while allowing a decent speed on the machine.

If it is difficult to get acme screws in your area start with threaded rod and
switch when its convenient. My JGRO still uses 5/16-18 threaded rod and it
paid for itself on its very first job. I reduced my speed to get a better finish
cut, resulting in less sanding.

Jason

[eric5501]

Thanks guys, what i would ultimately like to do are lithophanes, on flat sheet and tube and also wood carving and maybe some aluminium "carving" depth 1mm.
One thing I forgot to mention/ask what about using reduction drives as in belt drives say 2 :1 or 3 :1 that way the motors won't lose torque and the thread will turn faster, yes there is the trade off, wear of the thread, bear in mind this is a hobby (initially).
Cheers Eric