[knightridar]

hi, sorry but i'm not sure to post this in the ground ball screw section or the help on buying a cnc section or here.
I have questions that may cover multiple topics.

I've searched the forums about ground ball screws and various websites.
However, i need a more definite answer.

The plan is to buy a CNC from K2CNC and the model is a KG3925 that will be modified with a 4th axis attached to it that will also have indexing capabilities. Attached to the 4th axis will be a pneumatic collet closer from dunham tools. I believe K2CNC uses gecko controllers/drives??

We need our parts to be within +/- .001" for tolerances. The plan is to use this machine for high production of parts. The parts are stainless steel 304 tubes that will have small features cut on them such as slots and circles.


We will be using a laser to cut the parts but it's the motion accuracy that I am worried about. From what I've been taught--> The cnc machine that produces your part should have tolerances that are greater by some magnitude than the tolerance of your parts. i.e. to account for wear and tear of machine, accurate long term production of parts, etc...
Is this correct?

This particular model and most of the other models have a backlash tolerance of .001". So they recommended to use a ground ball screw. They said this would reduce backlash tolerance to .0002".

1. However, will this be good enough for our part tolerances? The total linear travel range of the cnc is 39" by 25"

Based on what I read on another website:
Besides ground versus rolled, ballscrews come in different accuracy grades that you should be aware of:

C0 - 3um or 0.0001" per 300 mm / 12"

C3 - 7um or 0.00027" per 300 mm / 12"

C5 - 14um or 0.0005" per 300 mm / 12"


2. Also this machine will be using servo motors, will the GECKO drives (if that's what K2CNC will use) be good enough? I read somewhere that the quality of the drives/motion control equipment has an effect on the accuracy of the cnc motion.


3. Based on my experience with CNC's at school and industry so far (hasn't been much) I know that CNC motion is suppose to be accurate but how about at the micro level or small features on parts? Suppose my tube has an 1/8" diameter and 1/32" or 1/16" diameter hole will be cut on it using a laser. Will the motion control be good enough at these small increments to give me the part tolerances of +/-.001?

I will continue to search the internet for more answers.
Any input is appreciated. Please let me know if I should post this elsewhere. Thanks!

[ger21]

A bigger issue, is that I don't believe that machine is anywhere near rigid enough to cut stainless.

[knightridar]

Hi there is a 200 watt fiber laser cutting head that will be attached to the z axis. The vendor is gsi and they recommended a .0005 perpendicularity and parallelism tolerance for attaching the laser on a custom z axis table. Rigidity should still be okay in this case right?

The laser just needs to maintain a proper focal plane distance and so no tool piece shall be coming into contact with the parts.

Also a 12 foot bar feeder is being attached to this and the laser needs to move around to cut features on the parts as they are being fed which is why I didn't go with a regular mill setup cnc because the mills work tables move not the actual z axis tool holding mechanism.

Thanks!
I will post pictures soon to clarify.

[mactec54]

knightridar

I'm not sure about the K2 for the accuracy you want to get, it's take's more than just ballscrews to get were you want to be

[silyavski]

i was like you on the market for small cnc and and the end choose belt driven Romaxx HS1, they have also ball screw WD1. Till now i am pretty happy with the machine.
my thoughts on why i choose exactly this one and i believe most is valid for you:
- fully supported rails on all axis and definitely not made at home from Alu extrusions and in my case steel welded construction
- easy to clean V rails
- fast
- gecko 201x driven with good BOB
- stepper driven but has separate encoder board to simulate servo accuracy with closed loop control
-enclosed electronics at the base of the machine
-good support from manufacturer/ this believe me is very important/
-time proven design/ about mine i read guys are pushing it years 8h a day
-thick straight aluminum bead versus MDF or so, with holes for fixtures

well, after i have it for a while i see its not perfect / i am a perfectionist :violin: / but is well thought of, rigid and cheap for what it is. My point is: i dont have an idea what your actual knowledge is but don't get in the illusion of airspace technology, look at the various sides and decide what is best for you, because from a good manufacturer you can be pleasantly surprised by features you machine has and you did not know in the time of buying or vice versa . I dont have nothing against Al extrusions but if i go this way i will rather buy a good kit. i hope it helps.

[HuFlungDung]

Laser cut pretty nice....compared to a plasma torch or an oxyacetylene torch

But I have my doubts about the cutting accuracy being better than .001".....maybe it is that repeatable (with some program path tweaking). But the surface quality of a laser cut surface leads me to believe it is not so precise that you'd blame errors on a ballscrew.

You don't say whether or not the features need guaranteed accuracy as to placement over long distances, but that might be something to consider: the ballscrew error over 12" is perhaps insignificant over 1/16".

Ballscrew error is not the same thing as backlash. You want high precision thrust bearings and preloaded ballscrew nuts to get rid of all the backlash. That is easier than fussing around after the fact with backlash compensation, etc.