Annoying ACme screw vibration

[EL34]

My 36" acme screw goes into a bad vibration when the gantry is at one end or the other of the screw.
When the gantry is halfway down my 36" screw, the gantry itself divides the screw in half and it's quiet as a mouse.
As the gantry proceeds towards either end of the screw, the longer screw section at some point it starts vibrating.
I can reach my hand up under the machine and touch the screw and it stops and behaves itself.

I'm not running my X axis motor very fast, it's not whipping from high RPM's.

This is not motor induced vibration, it's more that my screw is not perfectly straight and the Delrin bearings are not as tight as say a ball screw is and so there is some slop.

My gantry is very heavy and the ACME screw is under a bunch of stress.
I really need a nice ball screw, but that's some big bucks.

The best method I can see is to somehow dampen the screw at it's longest sections. I came up with a design that I may try.
These swinging pulley dampners could be placed at the 9" and 27" mark on the 36" screw.
When the gantry contacts one of them, it pushes it over and the pulley eventually rolls under the gantry and then pops back up.
The gantry can push them over in both directions.
The cable at the bottom of each pulley dampner is attached at the ends by two identical springs that keep the dampner balanced when upright.
If you had two of these dampners, you could stagger the cable to one side on one dampner and the other side on the other one and the cables would bypass each other.
The pulley could have some sort of dampning material that contacts the screw.

Have to figure out a way to let the pulley come up slowly so it does not slam up and into the screw making a bit clunk noise.
That would be as annoying as a vibrating screw
All the lengths of the pulley stick, other parts and where to attach the cables would have to be figured out so the pulley dampner can tip over and the cables don't hang up on something.

Just an idea at this point. Any ideas are welcome.
Here's a pic of how my gantry,screw and bearings are set up and a diagram of my dampner idea.

[Oldmanandhistoy]

Any ideas are welcome.

Straighten the screw.

[jdell42]

Out of curiosity did you get your screw at Enco? They are terrible. McMaster has much better and straighter screws.

[EL34]

Can you describe to me how to go about getting the screw straight?

Jdell,
Neither places.

I got it from a machine shop that turns the ends for bearing blocks on both ends and 1/4 motor mount on one end.

[Oldmanandhistoy]

Can you describe to me how to go about getting the screw straight?

How I straighten screws

I place the screw on an aluminium spirit level and rotate it so the high point of the bend is facing up. I then place my hand firmly on the high point and lift the end of the screw to bend in the opposite direction. Try to only bend as much as is needed to get the screw straight (trial and error). As the screw gets straighter you can tap the screw to help find the high spot. It will make a thud sound if the screw is contacting the level and a tap sound if it is raised. If the screw is to stiff to bend by hand I do it in a vice with three wooden blocks to protect the screw. You need to be VERY careful not to put to much pressure on the screw and bend it too far (again trial and error). Using these methods carefully you will be able to get the screw very straight, probable more than is necessary.

[ger21]

I'm not running my X axis motor very fast, it's not whipping from high RPM's.

You need to define high. Because I'd guess that when you move very slowly, it doesn't whip, right? You have 3 options.

1. Straighten the screw.
2. Get a screw with a higher lead.
3. Get a bigger diameter screw.

Both 2 and 3 require that the screw is straight as well.

[monte55]

Out of curiosity did you get your screw at Enco? They are terrible. McMaster has much better and straighter screws.

My table is 60"x60" using ENCO 3/4 x 6tpi Acme screws.....2 screws and 2 495 steppers for the X and 1 screw and 495 stepper for Y axis. All screws are about 61" long and they run pretty smooth. The steppers are direct drive to the steppers. To straighten one is not that bad. If one has a slight bend then just pull slightly in the middle the opposite direction untill it's reasonably true. It will hardly every be perfect if it's long. Acme nuts are very sloppy on the screw and I wouldn't want to use them anyway(metal to metal). I got my screws from ENCO as I stated and also purchaced a 13" piece of 1.25" Bronze bearing stock, and a 3/4 x6tpi acme tap. Generally drill sizes for taps are based on 75% threads which will have a bit of slop which will let the screw whip at higer speeds and clap as they turn. I found a bit that would give me close to 100% thread eliminating most of the slop and run a lot smoother plus making my machine more accurate and helping to cut down on backlash. Tapping close to 100% threads takes a lot more torque but it's worth the effort in this application. My unit has been running since Nov of 2006 and all is still well. If you want to see it running, go to YOUTUBE and type in "Thunderbird by Sharpie" in the search box. Hope this helps some. BTW my machine runs 60 ipm but will go a little faster.
Nick

[Buzz9075]

I had the same problem, while maybe not as bad. This was on Joes 2006 model and what I did was place an addition bearing on the other side of the gantry so it would stablize the rod. So I ended up with a bearing at either end of the machine, then a bearing beside the lead screw nut and other one about 8" away. Did an amazing job. You can see what I did if you checout out the section titled "Torsion box adjustments" on my web page www.cncmachine.phillipsfamily.ca. I aslo had another idea but I did not need it you can see what I was thinking of in the section "Whip Suppression".

[EL34]

Thanks for the help guys.
Buzz, didn't see a torsion box section on your link?

I'll try and straigten the screw and see how that goes.

I would love to get rid of the 1/2-10 delrin nuts.

I have been cutting in the center of the Table/screw length.
The X axis gantry stays quiet when the screw is divided in half.

[Buzz9075]

Sorry you have to first select "Build out of the second machine" under "Machine Build Outs" on the main page. This will document the building of Joe's 2006 CNC machine. While you still need to get it as straight as possible, what the second bearing will do for you is act like a cantelever support. I installed this method for both my X and Y axis.