Ballscrew or rack?

[dhaddox]

I am building a 4x8 router and I havent whether to go with ballscrews or rack gears on the long axis. I like ballscrews but dont know how much trouble the dust will cause with ballscrews. Is the dust a real big problem or not? Any suggestions appreciated.

[johnmac]

Have you priced an 8 FT. ball screw?

John

[dhaddox]

lol Yeah I know they are a bit pricey

[johnmac]

I would look at what the commercial machines have. Regardless of price, I would think an 8 Ft. ball screw would having a problem with sag, and whip when spinning.

John

[jsheerin]

You can go with a rotating nut to combat screw whip, but from what I've seen it's still tough to get a combination that gives you a good force vs speed curve if you're using stepper motors with commonly available screws and reasonable gearing.

[ger21]

You'll probably need much more power to spin a large diameter, long screw than you'll need to run R & P. Most mid price 4x8 machines use R&P.

[Hdale85]

Not to get off topic, but what do most expensive CNC machines use that are bigger then 4x4?

[Pplug]

My Techno lc4896 uses a ballscrew with the rotating nut. It can't be beat for accuracy or durability. The nut is mounted under the table so the dust is not a problem. On the x axis the ballscrew is rotating and are completely covered inside the gantry and NEVER get dust on them.

Check out ebay user Linearmotionbearings2008. I bet you could get the screw you want for a great price. He is sells from china but he has been reputable. I have purchased a spindle from him and it works great!

[Hdale85]

Originally Posted by Pplug My Techno lc4896 uses a ballscrew with the rotating nut. It can't be beat for accuracy or durability. The nut is mounted under the table so the dust is not a problem. On the x axis the ballscrew is rotating and are completely covered inside the gantry and NEVER get dust on them. Check out ebay user Linearmotionbearings2008. I bet you could get the screw you want for a great price. He is sells from china but he has been reputable. I have purchased a spindle from him and it works great!

How big is your machine? The problem is that at 8' you start getting slack in the screw and it vibrates quite a bit. The way around this is to use bigger screws but then you need bigger steppers and well all of that starts to get expensive.

[jsheerin]

Slack? Do you mean sag? In any case, with a rotating nut you can tension the screw axially which should help with any of those issues.

[wcarrothers1]

my long axis is rack driven through a planetary trans to either end of the gantry with 1/2" shaft then rack gears.

troll ebay for a while and you will find planetarys in ratios you will need to make things work (for pretty cheep.)

I've gotten 4 new planetarys to gear up to either convert my machine to be completely rack (right now the short 4' axis and z are ball screw) to rack..

will be a while untill I get to my next build or get to upgrading my current machine (it runs pretty good the way it is)..

Thing about spinning nut vs rotating from 1 end is rotating from 1 end will wip if it's a small screw. But if you rotate the nut then you have more weight (another drive motor) out on the gantry moving back and forth all the time.. If you don't have a solid gantry it can cause it to sag a little more then if it were lighter

I do wish I would have done both x and y in rack. Z is ok with the end driven screw..

I can do 170ipm rapids on my 4' screw axis vs 400-800 rapids on my rack axis.. You could argue that the 4' axis is half the 8 and half the rapid speed but that only works out if you are going diagonal..

Also if you have a rapid that includes a move of the slower axis with the faster (least in my case with mach) the slower speed is around where your max move speed will be.

All a trade off. If you don't expect to be in the same room when your machine is working it does not really matterheehe

Anywho.

b.
'

[scotta]

I've just gone through all of this with my new machine - the end result is rack and pinion drive for both the X and Y (long axis) and ball screw on the Z. I started out intending to use ball screws for all axes and then went through the process of costing out the build. I then did some research on rack and pinion drive and the costs associated with the need for gearboxes / belt reductions.

The end result is that for a small machine (one where the screw length is short enough to avoid whip) ballscrews a more straight forward, but as you start to go for longer axis travels then things change quite a bit. With rack and pinon the incremental cost to make a larger machine is very small as the dollards are invested in the drive (motor gearbox / belt reduction) already. So my machine grew from a 4x4 to a 4x8 because the cost was not that much more.

For my current build each axis drive cost about $120 in components (Stepper motor, timing pulleys, pinion gear etc) and I just received my delivery of Module 1 Rack which set me back AU$35 per meter. The rack and pinion is a very "scaleable" solution so to say....

Scott