Anti-backlash bearings for acme screws

[RichardPiper]

I have found a lot of information from Rockckliff and other sources but I can't find a simple description/drawing of an anti-backlash bearing for acme screwed rod. The Rockcliff plans seem to rely on the motor bearings themselves, unless I've missed something, which is always possible! I have a lathe so turning the rod end isn't a problem.
Some kind of bronze or brass flange tightened against the frame mounting? Or do I need bought-in thrust races?
All comments gratefully received!
Richard

[RichardPiper]

This is a follow up to my own previous post. I think 'backlash' is not what I really mean- I'm talknig about eliminating end float in the acme screwed rods (leadscrews).
Richard

[Lee Roberts]

There i was righting up a nice big explanation on "anti-backlash Nuts" lol, o well ill pop it in the FAQ's for next time anyway.

I think now i understand what it is your talking about, you called it "float" another term used is called "whip", so when the rod is spinning at high speeds or any speed the end that isn't supported has the possibility of movment as its unsupported.

Ok so how can we corroct this? or how can we eliminate this ?

I think the best way i have seen it done in the rockcliff machine is like CheekieMonkies has done on his build, he has simply used 608zz Skate Bearings mounted in the machine giving a supported option to the Threaded Rod. Please see the quoted text below, along with his pics.

Right i'v got a few ideas on things i'm going to change with the default rockcliff plans, you dont have todo what i'm about to show you, but you may as well as it will make for a better machine in the end.

i'v decided to added skate bearings to my threaded rod to support them, as i say the plans dont suggest to do this but when your machine is doing rapid movments you can get whats called "wip" in the threaded rod if its not supported. At first i was only going to support my threaded rod at the opposite end to the motor, but then i thought well why not do it both ends while your there it will only make it better anyway, so thats what i did.

You can see in this first pic i'v cut out a 22mm diameter whole to recive the skate bearing, the skate bearings are 8mm inside diameter by 22mm outside diameter and they are 7mm deep. I'v cut 10mm deep so the bearings sit 3mm deep from the surface. Befor i did this i drilled all the way through with a 14mm bit so the threaded rod and nut can pass through and spin, that would be the smaller hole you can see in the pic.

Click Images To Enlarge !

[img]http://www.mycncuk.com/forums/download/file.php?id=51&t=1[/img]

[img]http://www.mycncuk.com/forums/download/file.php?id=50&t=1[/img]

So the sun was still blazzing and the girls still shoping, so i pushed on and got some more done.

i decided befor i went on to the next stage, i would test fit the rods and rails and the bearings of course, you'll notice that i'v put my bearings on the out side of the machine. This isnt a must but i would like to put some tension on the rods so by putting them on the outside we can do just that.

To test fit the rails i had to make the 50mm x 50mm rail support plates, again i just marked the center and drilled out a 13mm diameter hole to recive the rails. You can see the rails are to long so ill cut them down later on in the build, i may not though as i could just rebuild the machine to a bigger size !

[img]http://www.mycncuk.com/forums/download/file.php?id=69&t=1[/img]

[img]http://www.mycncuk.com/forums/download/file.php?id=66&t=1[/img]

I have seen this done on a few diffrent machine builds and it has given good results, i would recommend you take a look at CheekieMonkies build log CLICK HERE !. He documents his build stage by stage and i hope you find it useful.

i'v got an ebay shop up selling things related to the forums as i stock them. i'v sold quite a few sets of the bearings if your still looking for some please take a look on this link: eBay

The bearings are £3.97 for a pack of 8 and i'v only got 2 sets of 8 left in stock, however i'v just imported the same again but only this time they will be abec7's so that little bit better.

Let me know if i can be of help !

CNCUK

[RichardPiper]

Thanks very much Lee, that is all very useful, I will go down that route for supporting the leadscrews. But that isn't what I was after! I was concerned about leadscrew movement in the direction of the axis- what you might call -fore and aft' movement rather than 'side to side'- as the leadscrew begins to turn, what stops it moving a short distance twoards the stepper motor (or away)? On my lathe there are thrust bearings to prevent that movement.
Thanks again
Richard

[NickW909]

Richard,

I was concerned about this also after reading several posts, mainly on cnczone about eliminating backlash.

So, I have made some supports for the leadsrcrews out of delrin that have a thrust bearing sat behind a 608 bearing. The 608 bearing will support the leadscrew and the thrust will eliminate any end float. The end closest the motor is fixed the other end is adjustable axially.

I also bought an extra piece of leadscrew and made a tap out of it to tap my own anti-backlash delrin nuts.

I haven't fitted these in yet as I'm still waiting for the shafts (that someone is 'aquiring' for me.. ;) ) but I can't see why they wont work??


Nick.

[Lee Roberts]

I was assuming that the 608's would take care of movements in "Spin" and "axis" directions?

There would be Nuts and Washers ether side of the 608's holding the rod in place ? would that not stop the movement in both directions? understanding the 608's would be pushing against the machine as everthing is in use?

I'm still very new myself, so i appreciate i could learn somthing here. I just cant see that the thrust bearings will do anything more then the 608's would be doing?

So i'v just had another look at your pic Nick, so we would have 608's supporting each end of the threaded rod, then we would have our anti-backlash nut inplace next to our "lead screw" and this would consist of our 608 with a "thrust bearings" also behind that.

Again i cant understand how the "thrust bearings" will do anything more to help, i wait your replys guys.

CNCUK

[NickW909]

Lee,


Errrr, sort of!!

I'm completely new to all this stuff also....very steep learing curve!!

The 2 round parts machined from delrin sit at each end of the leadscrew, one has a hole through to allow the leadscrew to connect to the motor, the other is closed. The 10mm dia leadscrew will have the ends turned down to 8mm. The leadscrew then butts up agains a 608 bearing at each end, behind the 608 is a thrust bearing.
Its completely over the top for a machine made out of MDF I guess, but for the cost of a few 608's and the thrust bearings, I thought I would see how it works (I already have visions of Build No.2 !! ) The 608's would probably have enough capacity to take the axial load that will be on them in a Rockcliff router, but at some point they could 'bind' as they are not designed to take an axial load.

The anti-backlash is just that rectangular hunk of delrin, I made a tap out of a lenght of leadscrew and tap a hole through it. It shoulf have 0 backlash and the basic tests I've done so far it seams to work ok.


Nick.

[KevinDamen]

I have a simple anti backlash nut for sale on my site http://www.damencnc.com. Anyway feel free to take the idea. If you are in a hurry you can buy it... There are also some design pictures online, i have included one here.
I have tested many setups. i have also tried the auto adjust spring loaded nut, this gave a lot of trouble, the machines was actually able to compress the spring. With increased spring stiffness the nut had a lot of drag..

A friend in the netherlands has also made a nut using compressed air to regulate it. Take a look at http://www.aanrip.nl

I have included a picture

grtz, Kevin

[NickW909]

Has anyone got any experiance of using delrin lead nuts that have been tapped using a tap made from a lenght of leadscrew?

Does it elimiate all backlash?

Thanks,


Nick.

[Lee Roberts]

I havnt but i'v just had this made as my lead nut, Dom from these forums made it for me as of a conversation we had over msn messenger one night. Its 1" Ali with an M8 hole.

i'v included a pic and a pic of what it is going to replace, i'm hoping i wont get any backlash and i'm thinking over time and use the thread will slowly get worse but i'v never used anything ali like THIS so i dont know.

Not much help but a reply never the less.