Need Help!- Changing my leadscrew...need some help deciding

[basskitcase]

I currently have a 1/2-10 single start lead screw with steel nuts, and i am going to change the nuts fore sure. I am also thinking of changing the screw and i need some help deciding how to proceed. Here are my options:

1. Lowest cost option: replace only the nuts with dumpster CNC nuts. due to driver and motor limitations, my steppers will only do about 1500 RPM unloaded, so my theoretical limit would be 150 IPM. Cost: about 60$

2. Middle of the road option: replace screws with 1/2-10 two start, dumperster nuts. Double my theoretical max speed. Cost: about 120$

3. High cost/performance option: Replace x and y with 5/8ths 2 start ball screws, which would require 4 new bearings @ 10$ each, 2 new sprockets @ 10 each, and new screws and nuts, for a total of around 250-300$

If it was your machine, which would you choose and why. I don't necissarly need rapid speeds. how much increase in performance am i going to see with ball screws vs dumpster nuts in terms of mechanical efficiency???

[jsheerin]

I don't have the exact numbers on hand, but changing to the dumpster nuts will help a bit as efficiency will probably go up a bit. Changing to the 2 start will similarly raise efficiency, as would 5 start which is what I have. Just be aware that force available will go down as will resolution. However you will be able to move faster before you run into screw whipping. Going to the ball screws should in theory give you the highest efficiency. If they're 5/8-10 2 start, you should be able to go faster than the 1/2-10 2 start due to the larger diameter screw whipping at a higher speed. You can calculate all this out if you want.

The one thing I'd watch out for with the ball screws is whether or not they have anti-backlash nuts and whether or not end machining is included in the price.

At the moment I use 1/2-10 5 start screws with the dumpster nuts on my machine. However I'm working on changing to rack and pinion for x and y and a ball screw for the z. Hope that helps.

[basskitcase]

If i get a bearing witha 5/8ths bore, I wouldn't need any end machining right? Isn't that only with bearing blocks? This is how i have my acme screw now and it seems to be doing fine (they're pretty hefty bearings)

[ger21]

How long are the screws, and what motors and drives are you using? What voltage are you running at?

[MrWild]

http://i46.tinypic.com/2vah4e9.jpg

How many in oz are the motors? How many amps do they take, and is it met by the drivers?

Dude, you welded the lead nuts on. The precision of that is debatable. The stiction a misaligned lead nut can have is huge, and it only takes a little to cause it.

Nice looking build.

[basskitcase]

I have 305 oz steppers from hobby cnc and a hobby cnc pro board, running at 36V, 3A per motor (the motors could really probably take a little more, since they only just get warm to the touch, but the board is rated for 3A so i haven't pushed it).

Yeah i welded the nuts on (tacked) to get it going so i could cut out other parts for the machine (i reaized the steel nuts weren't going to be acceptable after i ordered them, as i'm more of a builder than a planner, for better or worse). Some people build a junky first machine with plans to build another better one. I built a machine with decent potential, with plans to modify as i go along and eventually do other CNC projects with what i've learned from this one (I'm really more a metal person than a wood person, so a mill and lathe will come when i can convince the wife they're necissary and get a better melting furnace built to cast the parts to make them ) In fact, one of the reasons i built this was to cut foam for lost foam casting. Once i get the machine "finished" i plan on tearing it apart and realigning, cleaning and repainting all the parts.

Really, the nut alignment is ok because i had the screws and gantry done and just lined up the gantry over the nut and tacked it, since the weld puddle fills in the gap, you just have to be careful the nut doesn't get cockeyed and that the gantry is perfectly perpendicular. Not saying i would choose to do this again, but you live and learn...

[basskitcase]

Originally Posted by ger21 How long are the screws...?

44" and 36" for x and y, respectively.

[jsheerin]

First, you need bearing blocks if you don't have them. The motors are not designed to take the thrust of the screws. I'm not saying you have to buy the fancy ones, but you do need some thrust bearings in there.

Second, the ball screws very likely won't have a good surface for putting through a bearing like the acme screws do although it might be possible.

[basskitcase]

Originally Posted by jsheerin First, you need bearing blocks if you don't have them. The motors are not designed to take the thrust of the screws. I'm not saying you have to buy the fancy ones, but you do need some thrust bearings in there. Second, the ball screws very likely won't have a good surface for putting through a bearing like the acme screws do although it might be possible.

The motors aren't direct connected. I have bearings on both ends.

[ger21]

I'd go with 1/2-8 2 start and Dumpster nuts. Should give you similar performance to ballscrews for a lot less. I use them, and get up to 180ipm with 250 oz motors and a Xylotex @ 24v.

[basskitcase]

Thanks for the input Gerry! I ordered my screws and nuts last evening.