i am ready to purchace the parts to build an xyz machine, 24" and 30" shafts and thompson bearings ( 1" and 0.75" dia. which i have) but i am not sure what type of drive screw to use, acme or ball, there will be steppers on the ends, i would like the machine to be as veritile as possible.
acme or ball , dia. and tpi....... any sugestions ??????
thank you all in advance for your help
smarbaga

Is price a concern? What's the purpose of the machine? What size steppers?

i have no steppers as of yet, as a lead screw will be more fixed and dedicated to its souroundings, i figure i will match a motor to the screw, i was thinking of putting a die grinder on this thing perhaps for milling alum., brass, a router for wood, and a plasma cutter, as there is no fixed bottom in my design, work height (thickness) will not be a problem, also some kinda dremil tool for engraving copper pcb's., i realise that ball screws cost about 5 times as much as acme, but considering how much is invested in the unit, i think i will only be as good as its weakest link.
i am not sure which is the best screw for the job.
thanks

Ballscrews ar probably the best for most uses. But, you have to have a way to keep them clean. Especially with plasmas, or so I have heard. On the other hand, good quality acme will probably work just as well for a lot less money, for the examples you gave.

thank you very much .... gr21

Hi, I guess you have read the ballscrew basics in the cnc FAQ section? Good article that compares relative merit of different solutions from the hobbyist perspective. Anyway, if it were me, considering your other components and assuming you want to do reasonably accurate work, I might go for the thomson rolled .004"/ft accuracy 5/8" screws. But, depending on how you mount it, how fast you want your axis to move, and how long the axis on the 30" rail ends up, 5/8 size could be a little small. It is expensive compared to acme, nuts are around $22 each, and the screw material costs around $1.00/in depending on where you buy it+end machining. I've seen it available at reid tool and mcmaster-carr, and a slightly less accurate, .009"/ft(and cheaper) screw is available from roton(www.roton.com). A problem with all this stuff is minor backlash (up to .007 according to roton) and depending on your tolerance for this, you might have to build a preloaded nut out of two regular nuts preloaded by springs or attempt to reload your ball nut with slightly larger balls. The main advantage of ballscrews over acme is a much larger efficiency, say .9 to .2. This means more of your motor's power will get to the axis it is coupled to. Of course, if you are plasma cutting, this may not matter much, but for the other uses that you mentioned, I suspect that it might. It might also mean that smaller motors would work. I have little experience with steppers, but at first glance, I suspect that you might be able to directly drive the .200 in/rev stuff. I'm not sure though.

I am building a machine very similar to what you are describing to assist with cutting steel(oxy-acetylene) for my larger machine. I am using the 5/8 thomson ball screws I described above for this machine. I don't need the accuracy for what I am doing but got the screws really cheap....

Good luck!

Mitchell

The main advantage of ballscrews over acme is a much larger efficiency, say .9 to .2. This means more of your motor's power will get to the axis it is coupled to. Of course, if you are plasma cutting, this may not matter much, but for the other uses that you mentioned,

I believe most acme is closer to .35-.4. But you can get multiple start acme with as high as .75 efficiency. (1/2-10 5 start from Nook). And for plasme use, where higher speeds are important, multiple start acme might be a better choice.

Yep, looking at nook's website and comparing a 5/8-.200 acme screw with the ball screw I was talking about shows a .53 efficiency for a plastic nut and a .44 for a metal nut. Better than I thought! There are some in there less than 20% though.

Here is where I am getting this:

http://www.nookindustries.com/acme/AcmeInchAvailability.cfm

Seems like the trend is the more TPI, the less efficient. Kind of makes sense since the lack of efficiency is due to friction and the nut has to be turned more times for a given linear travel when there are more threads per inch.

As you say, given the need for high speed and not wanting to turn the screw a million rpm for a number of reasons, multiple start acme might be a better choice-- if anything for cost. Rolled ballscrews get really expensive fast if you stray from the 5/8-5tpi "standard".

Mitchell

It will cost less to purchase Nook ballscrews through McMaster-Carr then directly through Nook. McMaster-Carr is a volume discount buyer of Nook products and get a big price break that they pass onto their customers.

http://www.mcmaster.com/

I think you need to pin down what type of work you want to do. My buddy has a 4' x 8' CNC plasma cutter that he built for about $8K, and he makes about $30K a year with it. If I had the space I would make one too.

Drive: rack and pinion on all axis
Motors: Steppers with a 3:1
Controls: Flashcut and Torchmate
Hypertherm plasma torch

The thing is a mess. Plasma cutting is NASTY when its being run to its full potential (duty cycle), turned off for a few minutes and then run again...over and over. His walls and machine are covered in soot. He is looking into a water bed to negate some of the smoke created.

my point is; If he ran ball screws or lead screws he would be cleaning them more than running the machine.

I have done a lot of research into these things... at some point you will have to pin down the work you will "most likely" be doing the most of. You can design a machine to be an "all around" machine, but you will pay in the performance category in one or all of the different aspects you trying to cover.

This is what I have been told by my master toolmaker boss and I think it is sound opinion:

Multi-purpose tools are not excellent at anything...they do many things marginally.