Acme screw "whipping" Please Help.

[Ranscon]

I have a homebuilt router. My X axis leadscrew is an Acme 1/2-10. 48 inches long. It has recently started "whipping" a little bit while running at 48ipm. What would cause this? Is it mouned too tightly maybe? (squeezed)
Thanks for any help on this issue, Jeff

[vladdy]

lots of things will make a low cost acme threaded rod whip...
If you can tension the rod in 'extension' it may help a little, compression will make it worse..
alignment of the nut is pretty important, any twist will cause some flex and wiggle in the rod..
if the rod wiggles 'a little', a higher rotational speed will usually make it worse..
flex coupling on the drive ends do help... but not usually a 'cure'

what will help the most is replacing them with a better quality rod, a harder material will be stiffer, and resist flex better..

I have whip in mine as well, 5/8 at speeds over 150ipm,
they were about $10/foot, better [supposedly] rods run about $30/ foot here, close to the cost of used 'proper' ballscrews on ebay [some days... ]

[Andre' B]

You are into the realm of harmonic vibrations and dymamic balance.
Any spinning shaft, be it a drive shaft, ball screw or lead screw has a critical speed that depends on the unsupported length and the diameter of the shaft (which is how stiff the shaft is). If everything is perfect it may be possible to run a faster speeds but as soon as somthing gets out of balance or alinment all hell will break loose.

Down near the bottom is an explanation of critical speeds.
http://www.roton.com/application_engineering.aspx

[Inventthis]

Had the same issue with my CNC Plasma cutter some few years ago. The issue was the unsupported length for a specific diameter at a critical speed. I had to get a larger diameter ACME screw and the problem went away.

[ahren]

The best thing you can do is replace your screw with a new, multi-start ACME screw. Whip is a function of screw length, screw straightness, and rotational speed. With a multi-start ACME, you can get the increased linear speed with the same rotational speed, and these screws are usually straighter than economy ACME. As an added benefit, they are far more efficient, and keep your steppers (if you're using steppers) running at lower speed and in their higher torque range, which makes up for the difference in mechanical advantage -- it's really a win win proposition. On my machine, I was able to go from ~120 IPM up to near 400 just by changing screws from 1/2-10 single start to 1/2-10 5 start. Now that I have better motors and a G540, I'm over 600 IPM (for a 3' travel).

5 start ACME rods can be purchased from McMaster Carr or Roton, and nuts can be purchased either from myself or Dumpster CNC. Good luck!

Ahren
www.cncrouterparts.com

[Ranscon]

I am getting ready to replace my existing leadscrews. My X axis screw is 57 1/2" long. What diameter would you recommend to avoid "whipping"? I am looking at either 5/8-6 or 3/4-6 Acme's. The 3/4" one's are cheaper for some reason. Also, My motors are Keling 425 oz/in. Will they turn the larger screws ok? I was also worried about the larger heavier screws sagging, I know they should be stiffer than my current 1/2-10 screws but I am no engineer. Any other suggestions or problems I may have overlooked please? What kind of nuts? steel or delrin?
Thank you very much for any help on this, Jeff

[LeeWay]

Something else that will help a little is to add a second nut a little distance away from the first. I found this out by accident on my router using ball screws. I have a spring between the two nuts on it and after that, at 300 IPM I had no whip, where I had noticed some on a single nut.

If it is spring loaded, it will give better accuracy (anti-backlash) to the screw as well as reduce the whip.
It added about 3" covered on the screw now for a total of maybe 4.5". 5/8" ball screw.
There is typically enough room on routers to accomplish this with little modification.

[Ranscon]

Yes, That is something else I was considering. Do you have a picture or a diagram? I would like to see what type of spring to use and how it is configured. I have read about this but have not actually seen one. Thanks again, Jeff

[LeeWay]

Here is a link to my router build log, but it doesn't have the double nut pictures.
http://www.cnczone.com/forums/showth...p?t=8632&pp=40

The pictures are here on my lathe build.
http://www.cnczone.com/forums/showth...t=59421&page=2

Look specifically at post 43.

Most guys use belville washers to get the load they need. I had plenty of room and figured the spring would serve as a good place to oil between the nuts too.

These are just typical compression springs. I think they are 75 or 80 pounds.
The first nut that is held fast to the machine is locked onto the U channel. The second nut is just a snug, but slip fit in the channel. This allows it to keep pressure between the nuts over any screw errors.
Works fine on this lathe, but did help eliminate whip on the router.

[Ranscon]

Lee,
Thanks for the links. Very nice lathe and router by the way. So, does the spring just ride unattached between the nuts with a bit of compression? The U channel keeps the second nut from turning on it's own? Watched your vid also, I had thought about the 72v power supply as well but my 4030 drivers are only rated for 24 to 40 volts. I am running the same steppers with a 24v power supply. Does the voltage increase make a really big difference in performance. What about the geckos, are they worth the expense of changing to them from my 4030's? I am currently running at 48ipm and would love to see 150. That along with the whipping is the main reason I am wanting to change screws. I have 1/2-10 acme's. Swapping to 5/8-6 acme's should get me to 90 or more correct? Are the 5/8 screws large enough for a 58 inch span or should I look at 3/4 acme's? My 1/2 acme's have the machined ends so there is alittle compression on my screw. I beleive this is contributing to my whip. I am considering attaching the new screws via Joe's method with nuts on both sides of the mount so I can put a little tension on the screw. Is this better? Should I double (jam) nut the screws? Sorry for so many questions but, I'm trying to solve several issues with my next update on my machine. Thanks again, Jeff

[ViperTX]

Well just get a 5/8 inch rod and support the ends and you will see some sag in the middle, do the same with a 3/4 inch and a 1 inch rod and the sag will be much less.

Paul

[LeeWay]

Lee,
Thanks for the links. Very nice lathe and router by the way. So, does the spring just ride unattached between the nuts with a bit of compression? The U channel keeps the second nut from turning on it's own? Watched your vid also, I had thought about the 72v power supply as well but my 4030 drivers are only rated for 24 to 40 volts. I am running the same steppers with a 24v power supply. Does the voltage increase make a really big difference in performance.

Thanks. Yes. Yes. Heck yeah!



What about the geckos, are they worth the expense of changing to them from my 4030's?

I don't know about those drives. I tried Xylotex first. It works fine. Then I tried Gecko's. It only took me two tries to find the best. I won't be using anything else. There is little or no comparison between the gecko's and it's possible hardware additions and the Xylotex, which is a lot like the rating you are using.

I am currently running at 48ipm and would love to see 150. That along with the whipping is the main reason I am wanting to change screws. I have 1/2-10 acme's. Swapping to 5/8-6 acme's should get me to 90 or more correct? Are the 5/8 screws large enough for a 58 inch span or should I look at 3/4 acme's?

If you are still going to stay with acme screws when you change over, then I would definitely go for the higher start kind as Ahren suggests. I have never installed anything but ballscrews. Started to install acme on my router, but switched to ball screws mid build. You would get higher speeds with slower turning and therefore less whipping. Larger acme screws may indeed solve some of the whip, but it puts an increased load on the motors. With such small a supply, you may actually loose speed with larger screws.


My 1/2 acme's have the machined ends so there is alittle compression on my screw. I beleive this is contributing to my whip. I am considering attaching the new screws via Joe's method with nuts on both sides of the mount so I can put a little tension on the screw. Is this better?

I would say no. One end of the screw is really all that needs securing. The other end just needs a bearing. Sometimes not even that. On my Y axis I just use a bronze oilite bushing on the lazy end. On my X I only use flange bearings, but the collar is only lock down on the motor side. It just spins in the bearing on the lazy end. I have no bearings on the ends of my lathe screws. With both ends essentially locked down, there is no provision for such a long screw to change with temperature, and they certainly do. No way not to put it in a bind or compression at some point with that method. It needs to be able to slip on one end.


Should I double (jam) nut the screws? Sorry for so many questions but, I'm trying to solve several issues with my next update on my machine. Thanks again, Jeff


If that is how Joe does it, he must have some advice to go along with it. I have seen a lot of Joe's build's being done, but never got into the plan parts. Just an avid cnc machine build observer.