View Full Version : How to pre load screws
13-08-2012, 01:23 PM
Hi i've been reading up a lot , and i've seen people say you need to pree-load the ballscrew and nut,, now me just been new to cnc and i'm planing to convert my viceroy tds11gb lathe to cnc, i do not no how to do this?
someone on an other forum said to get the c5 gear as its 0 back lash but on here and other places people say you have to pree-load the c7 to get 0 back lash?
could anyone tell me how to do this and if you have any photos could you put them up?
13-08-2012, 06:06 PM
Using C5 screws doesn't mean or guarantee zero backlash it's just the quality of the screw and accuracy over a given distance is better.!
Ball Screws are usually measured for accuracy over a set distance IE: 0.001mm over 300mm. The higher the accuracy over longer the distance the more they tend to cost.
The better quality C5 and above screws are nearly always ground screws has where C7 and low quality C5 tend to be rolled screws.
C5 and above ground screws are at best expensive, upto the point of obscene for C1 high accuracy. £1000's not £100's.!!
Notice I never mention zero backlash because true zero C5 & C7 rolled screws don't really exist off the shelf.? People make them or force them to be near has damn zero by trickery using using 2 ballnuts back to back with Belleville spring washers forcing them to preload against the screw.! . . This works but can if over done wear the screw and nut's prematurely.!
Double nut preloaded ballscrews can be bought of the shelf and use various ways like adjustable collars or shims, tough these are not always true zero backlash just very close.!
Robin and Jonathan have played with Belleville washers and C7 double nuts so search or check there posts but I think you'll find the Belleville washer the easiest method.?
If not wanting to DIY then be prepared to spend BIG BIG money for true Zero backlash C5 screws.!!
13-08-2012, 06:11 PM
C5 / C7 is just the grade of the screw which specifies the lead error. Lower the number the better, so C0 is the best. The common C7 grade screws people get are 0.05mm per 300mm. You don't just add the error for longer screws though, so it's not going to be 0.1mm over a 600mm screw for instance - it will be less. Getting higher grade screws doesn't immediately help with eliminating backlash since that depends on the nut. Double or preloaded (by using larger balls which has limitations) nuts are available for the higher grade screws, but they cost an arm and a least one leg...Zapp Automation Ltd - Stepper motors & Stepper Motor Drivers - Servo motors and Servo Motor Drivers - Ballscrews & linear motion products. : R1605T3-FDID-P1 Ballnut [R1605T3-FDID-P1] - £120.00 (http://www.zappautomation.co.uk/r1605t3fdidp1-ballnut-p-574.html?cPath=1_59_66_196)
Instead of that, as you've read, you can get two cheap ballnuts and preload them against each other to eliminate the backlash. Bearing in mind the RM1605 (or whatever size you choose) ballscrew from China is much cheaper and the ballnuts are about $20 each this works out a lot cheaper even with the added cost of springs and the mount. I just got three RM1605 ballnuts for £44.70 including postage.
Preloading in principle works by applying a constant force between the two nuts trying to push them apart. One nut is fixed to the machine as normal, but the other is only rotationally contrained so that it is free to move axially. The easiest way to do this is using a spring, but normal coil springs are bulky so we use Belleville washers, otherwise known as disc springs (http://www.leespring.com/uk_browse_catalog.asp?springType=W). So long as this force is not exceeded the backlash is eliminated. Therefore to choose the amount of preload you need to consider the maximum force the axis will experience (the sum of peak forces due to cutting, friction and acceleration) and set the preload a little above this to make sure. Since setting too high a preload will shorten the life of the ballscrew significantly, aim not to exceed about 10-20% of the ballscrew static load rating.
The only worthwhile pictures I have of doing this are in my micro lathe build log (http://www.mycncuk.com/forums/lathe-build-logs/4497-micro-lathe-zero-backlash%2A-conversion.html).
Having said that, this all depends on what you're expecting from the machine and the machine itself. If the lathe itself is wobbly and not particularly accurate then pre-loading the nuts isn't going to sort that. Equally if you're only turning quite basic profiles then backlash can be compensated for in the same manner as you would when using a manual machine.
13-08-2012, 07:33 PM
The problem with backlash is you can get anal about it PDQ. How much is too much? Removing it 100% means springing the nuts and the axial bearings. You can get it down 20 microns with a shimmed double nut, that's better than .001", is that enough?
Hassle is you can still put your thumb against the bed and turn the screw a smidge without feeling it move. If you want to get rid of that it will cost you time more than money.
13-08-2012, 09:53 PM
Right i get it now,only thing i'm struggling with is how much load i'd need to put on mine,
My lathe runs real good, but theirs no way now i'd get 001" , i'm looking for as best i can get it but i'm not spending a bomb, can input the back lash in LinuxCNC just the same as mach? the work i will do with it just don't warrant that much, so its not worth spending a ton of cash now! maybe later on when i fully understand what i'm doing i'l happy plug a few ££££ into it, my main reason is i want to be-able to cut inch/mm threads , where is i can only cut mm at the minute! plus i can speed the threading time up on cnc..
I new i'd always get back lash but its just the way a member from another forum said get Class 5 and you'l have zero back lash, and he sent me to a uk site where it was going to work out 500 easy just for the x axis!
can i ask on my lathes cross slide its 370mm from the handle to the very back of the saddle
so i'm planing on doing it the way you have done it Jonathan by having the ballnut at the end as it wont fit under the cross slide, so how long would i long do i need for the ballscrew ?
same go's for the z axis , from the gear box on my lathe to the end of the bed its 900mm
Oh and i forgot to ask what sort of total cost do you thinks it will run upto, as best guess you can give me, i.m just going for 2 nema 34 900oz motors.. can any one put a link up to the best drivers and broad to use for them motors .. i've a total price but i just want to be sure!
13-08-2012, 11:20 PM
i.m just going for 2 nema 34 900oz motors.. can any one put a link up to the best drivers and broad to use for them motors
Those motors are massive and you will get away with far smaller motors.
Seen has most lathe conversions I've seen are geared 2:1 then 3Nm Nema 23 and 75V PM752 or 50V Pm542's will easily do what you need far better than those big motors. Even 1:1 they will be fine.!
Don't fall into the bigger is better trap.!! . . . 3Nm 23's when connected to ballscrews are are very powerful and run with decent drives and decent amount of voltage will be fine for your needs.
I can put you in touch with someone who is selling some New PM752 drives if your ready to buy now.? . . . . PM quick if your interested.
14-08-2012, 11:07 AM
I'd spend MY MONEY right now and buy them mate.. put the keeper of my money wont have it!
But when i'm aloud to buy , i'd be ok with this 3Nm CNC Stepper driver Kit - 3 Axis for CNC. | eBay (http://www.ebay.co.uk/itm/3Nm-CNC-Stepper-driver-Kit-3-Axis-for-CNC-/220992304733?pt=UK_BOI_Industrial_Automation_Contr ol_ET&hash=item33742af65d) but ask them to take one motor and driver out, or get the kit then have one spare , or one extra for when i buy a new mill then do that!
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