Z axis troubles

[DSL PWR]

Well I finally got my z axis together, and thought I'd run it up and down with a 12 v battery. I ran it up and down a few times and then it jammed up. I took off the servo and pulled the lead screw and nut out. Once I got it into the vice I tryed to unscrew the rod and it was galled into the nut so bad I twisted off the screw.

So I made a new nut out of some different steel and ran a screw through it a few times with a cordless drill, then bang it also galled up! The original galled up with silicone grease on it, the second had some engine oil on it.

I'm using grade 5 3/8 16 allthread, and my block (nut) is 1" thich mild steel, with threads cut into it with a tap. The tap is good, and every thing works great... for a short while!

Will a grade 8 rod help, or will I have to upgrade to acme or a ballscrew?

[DSL PWR]

I just tryed pinning the allthread in some 3/8 nuts. I got 15 cycles dry before the nut and allthread galled, and 6 running grease on the thread!

Maybe I just have some bad rod?

[NC Cams]

I recall from an old machine design class that 90% of the torque applied to a fastener is used to overcome underhead and thread friction. Having said that, you have to consider what the TRUE design/operation intent of threaded rod is - and it is not necessarily power transmission.

Given that your biggest problem is friction (galling is from friction), you first would want to look into the lubricants used. Silicon grease is slipper BUT it is not a grease suitable for extreme pressure (EP) applications like that which you are putting it. Motor oil is designed for force feeding into a clearance space - it lubricates via hydrodynamic lube film theory, you are asking for BOUNDARY LAYER lube film creation and maintenance.

IT would seem that the type and quantity of lube that you are applying is not helping. Moreover, although they might work as a power transmission device, the threaded rod you're using may simply be too soft or posess the wrong surface finish/characterisitcs/coating/platings to frictionally slide without galling.

Even though the nut threads "look good", that does not mean that they will survive relative high speed relative motion between the nut and screw without galling. It may take more than what you're doing to generate a nut thread successfully - at least to serve as a ball nut.

You may have some cheap grade five or less metal instead of some decent stuff. Cut threads are typically not as well finished/smooth as rolled thread. If the budget allows, even inexpensive rolled thread rod is better than hardware store cut thread rod. A rolled thread ball screw is far superior to any screw thread.

Finally, steel nut against steel screw might not be the best for wear/scuff resistance. There is good reason why acme ball nuts are often bearing bronze of some type whereas the screws are steel - think about it for a while and you should be able to understand why.

With the above info, you can either redesign or shop until you can find a screw thread that will survive. If it were me, I'd look into moving up to a ball screw.

[DSL PWR]

The buget isn't a huge problem, my location is. I live in the middle of no where. The closeset decent supply house is a 6 hour drive, and one to get most of the cnc goodies is 11-12 hrs. So trying to do this build on evenings and weekends is starting to wear on me. For example it took over a week to get pinion gears, I had to order in 1/4 20 screws, heck I can't even get a love joy rubber insert, let alone a coupling in this town! (not that I would use this for cnc mind you...)

I would love to get a ballscrew but that will put this project back about 2 weeks.


Sorry guys I just needed to vent my frustrations, and am wondering why all the "hardware store builds" can get away with all thread.

[NC Cams]

You may have inadvertantly violated a basic axiom of triboloty - namely, you do NOT have two identical metals of essentially the same hardness rubbing together under marginal/boundary layer lube film conditions.

Reason: siezure/galling is probably going to happen no matter what you do. You want/need one of the parts, either or, to be harder than the other.

THis is why bearing material is soft and shafting materials comparatively hard. Even in ball bearings, the balls are made HARDER than the raceways even though the parts are made of the same material.

Hardware store builds work based upon luck, skill and sometimes simply due to the perverse nature of inanimate objects. God only knows why some stuff lives and in other case, in spite of ever effort to the contrary, stuff still fails.

First thing to try is bronze for the ball nut. THis has MUCH better lubricity than mild steel - which is gummy and prone to siezure anyway. If you have rolled all thread (usually you can look at it and see if the threads are cut or rolled), the rod thread will be smoother and a bit harder as a result of the rolling operation.

The threaded rod may NOT be the issue. The cut thread on your nut might be the culprit. Cut threads might be adeqate for fastenting but more siezure prone under power transmission - especially if the pointed nose of the thread is digging into the root of the threaded rod. Perhaps lapping the threads might help until the next trip to civilization.

Can you see where the galling is starting? Root, flank, where??? If you can see the initiation site, perhaps some lapping or TLC can remove the offending areas that are initiating the galling.

Make sure the nut threads are not bottoming out ANYWHERE on the all thread. A light siezure that goes and come will NOT get better.

Better yet, go to bronze nut ....

[DSL PWR]

Well I went out and got some gr8 allthread, and tried it in a cut thread block. It lasted a little better than the gr5, but not much better. I also tried it in a gr8 nut, with the same result. Then I tried a piece of gr8 rod in a gr5 nut, with some grease. I ran it through the nut at 1800rpm for a while, ang it seemed to work. Then I ran it with a fair amount of side load. I kept doing this and soon the grease started smoking on the allthread as started getting hot. I kept going and the smoke really started to pour off the shaft, and a while later it galled up. As far as where the galling occurs I can't tell you, since when it happens 1- I twist off the rod trying to remove it or 2-the threads are so mangled when I do get it apart I can't tell.

I'm going to make a new block with a gr5 insert nut and run gr8 rod. If I can get a hold of some lapping compound I'll run the allthread in a nut a few times before the final install.

Ultimately I will probably order a ballscrew and do this right.

[NC Cams]

I think you just MAY be asking too much from your device - especially with simply smearing grease on it and then running it continuously at 1600rpm and surely under oscillatory operation which surely can't be helped eventuall.

The key give away is the "smoking".

The smoke point of grease is essentially at or above the failing point of the lubricant. The purpose of grease is to lube but, in some respects, also to hellp transfer some heat between the rubbing members. When the grease is smoking, it is literally telling you that it is getting too hot to lube anymore - the grease has turned from grease to watery slime and lubrication is probably less than even marginal.

I doubt seriously that one could/should expect all thread rod to transmit power at 1600 rpm continuously - especially without continous lube replenishment.

Moeover, it is one thing to oscillate back and forth rationally and then encounter a siezure - it is another thing to take fastern rod, mis-use it to transmit power and then do it under boundary/marginal lube conditions, ESPECIALLY at 1000+ rpm at a continuous duty cycle and be surprised when/if it fails.

Even a ground ball screw, oontinuously operated at that speed and under oscillation without lube replenishment would run into problems!!!

If however, this is how your plan to run your machine, I seriously suggest then that you figure out a way to better and continously replenish the lube on whatever screw you use on the axis. The way you're doing it seems to be a bit too harsh for your selected lubrication regime.

[DSL PWR]

NC Cams, I don't thing you understood what I was doing. This was just a bench test to failure to see what combination would work best! This is for my z axis on my plasma table, so it will see short high rpm bursts for initial positioning and small slow tweaks for operation. It should never see smoke rolling off the screw!

I got a hold of a short piece of brass that I will try and fashion into a nut for a final test.

[DSL PWR]

Muh hahaha ...

I think I found the right combo with the help of you NC Cams.

I just made a brass nut, and lapped the gr8 rod to the nut. I ran it through about 10 times. Once I cleaned off the coumpound the allthread looked great. Then I greased every thing up and proceded to my bench test. Just running the allthread through with little to no side load it didn't get warm, so I wanted to see what it would take to fail it. I put enough side load on that the drill was begining to stall out and the rod was starting to flex. I ran it like this until all the grease burned off (at 1800rpm) and my drill battery died. The fit was a little sloppy but both parts lived. Backlash is not a concern so I gues this is the solution.

Thanks NC Cams!

[NC Cams]

Your culprit is, was and always will be friction.

All thread is designed for FASTENING. The pressure angle of the threads is conducive to locking - this is partly why nuts don't work loose, they essentially "wedge" themsleves into each other.

Moreover and more importantly, the first thread of engagement in the direction of loading sees a predominance of the load. When/as/if he thread deflects, the remaining threads will see/share the load.

Hence, in spit of your efforts, all your load is concentrated to a very small spot on your nut. Thus, it is going to be difficult to first generated an adequate lube film and then to maintain it. Hence, concentrated friction will readily occur. THis friction is what was raising the temp of the grease to the smoke point and inducing your scuffing/scoring/siezure problem.

Honing/lapping the threads did several things. It probably smoothed out the cut/torn surface induced by threading. It also increased the thread to thread clearance - this forced the nut to share more of the load over more threads insted of concentrating it to the initial engagement point. It also added more clearance for grease/lube to penetrate and remain in the clearance space.

WHen you get a chance to do some research, look into the contact angles of rolled or cut BOLT thread as opposed to the same angle of acme thread - you'll note that there IS a difference. That's because acme's are/were desiged for power transmission as opposed to FASTENING.

Yes, you can craft up all thread to get it to act like acme but, for the long term success of your plasma cutter, you might want to use stuff a bit better suited to the intended use. After all, there is a reason why even the cheapest, low buck import lathes use acme's (and brass nuts) on the lead screws.

In the mean time, I'm glad to have been of help to your short term development efforts