Linear Actuators/Screws

I've been wanting to build an XYC CNC machine for a while for various projects(mainly drilling precise holes(at least 1mil accuracy).
I can't seem to find any prices for such machines but I imagine they are expensive(5k and up for the most basic).
In any case I feel it should be pretty simple to build one(the software and electronics are no problem).
I've been looking at different screw's but can't seem to get a price. (seems like every site wants you to call them but I'm not going to spend days calling everyone to find the best price)
I've seen some amazing precision linear actuuators(10um accuracy) but again, I imagine these are expensive.
I need a drive of about 1 to 5 feet(depends on the projects as some will be for wood working and others for drilling(such as pcb)).
In any case could someone give me an idea on the cost of getting the screws for this or some linear actuators that are not too expensive but have the desired accuracy I need.
In fact I guess it's best too build a large less precise and small more precise for the different projects.
I'm looking at something like a few hundred total for the 3 screws + motors or linear actuators(electric).
Any ideas?
Thanks, Jon
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On Thu, 27 Nov 2008 21:14:10 -0600, "Jon Slaughter"

Your local friendly hardware store carries threaded rod in various diameters to 1/2 inch at several lengths to five ft or so, and threaded couplers to suit which might well ride nicely giving a little lapping, when driven by stepping motors, for which drive modules at hobbyist prices are available off the shelf. This might be a rather low cost way of getting started. Not sure that holding one mil at 5 ft range is particularly doable this way though just by turns counting: thats 1:60000 - rather high reslolution, still, you could easily prove me wrong.
BrianW
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wrote:

This was my original idea actually. I remember taking a rather large lag bolt and messing with it. 10 threads per in means 1/10 of a inch per "thread"(per revolution) or (pi/180)*t/(20pi) = t/3600 where t is the angle in degrees. So If I turn it 1 degree it will go .3mils. The larger the diameter the easier it is to turn it 1 degree(or less even). In fact mounting a large gear on it can increase it by a large factor. I remember doing all the calculations a while back.
The idea was simply to attach a large gear to a long threaded rod and drive it with a DC motor(not stepper but could be. Then use external inferometery to determine the precise distance(I was thinking about acoustic methods at the time).
The only problem I ran up against is that there is a lot of play in the nut which will throw off the repeatability. Not sure how it would work in practice. Maybe with a load on the nut it wouldn't have too much play but I was looking for something a bit more professional. The rods I looked at(at lowes), while long(2 feet I think), had rather coarse threads(std 10/in) and hence using something actually designed for what I want would be a huge step in the right directionalthough maybe too costly?
If I can't find any specific stuff suitable for it I guess I should go ahead and try it. I haven't thought about it in a few months though and from what I remember there were a few issues. (although probably minor for any mechanical guy)
Thanks, Jon
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On Thu, 27 Nov 2008 22:49:47 -0600, "Jon Slaughter"

There are several home-brew CNC rigs detailed on the net. This CNC router from HobbyCNC gives you the flavor, though it it is not up to your range spec. http://www.hobbycnc.com/products/cnc-router-plans/
BrianW
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wrote:

Thanks, I didn't realize that it was such a popular DIYer. At least then I know it is doable. Problem seems to be doing it cheap yet proper. From what I found it costs a little over a dollar an inch for acme screws(1" steel). Might have to try and salvage some from something ;/
Thanks, Jon
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On Fri, 28 Nov 2008 21:23:35 -0600, "Jon Slaughter"

One inch diameter acme rod may be suitable for moving the heavy carriage on a machine lathe. But you are interested in precision drilling, I understood. Think of threaded rod for moving the carriage, and use smooth rods, angles etc for supporting it. More on getting into a CNC drill here.
http://www.engadget.com/2006/06/29/how-to-build-your-own-cnc-machine-part-1 /
BrianW
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wrote:

I was thinking about that too because the smaller screws are much cheaper. The thing I don't know is if one would be enough? I was thinking about using 4 screws and one solid rod for the support but this might be overkill and introduces it's own problems? (the smaller screws have more threads per inch which is a plus)
Standard 1.8 degree stepper motors give a linear displacement of about .5 mills for 10 Ths/in(.63 for 8 and .31 for 16).
http://www.roton.com/Screw.aspx?line me
has 3/8 in 20 Ths/in steel acme screws for about 10$ a foot. The mates seem outrageous though ;/
It would seem that I could use a standard threaded rod and get good precision of if I used a gear to increase the angular resolution. I'm not sure if it is that great of a method though(less torque and slower). I'm going to have to shop around and see what I can find.
There's a site I joined yesterday called cnczone that seems to be a great resource(haven't had time to look at it though).

Yes, I've seen this.. I need something a bit larger though ;/
Thanks, Jon
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