[Zumba]

This is a seriously old thread.

Anyway, scraping is part of the exception rather than the rule.

If a good craftsman were given some high quality profile tooling (for ballscrews) to use on a large 20x80 Chinese lathe (very rigid but questionable in quality), the resulting ballscrew could not and would not be more accurate than the leadscrew on the lathe itself.

On the otherhand, if the craftsman were given a decent grinder (ABEC-7 spindle bearings for good surface finish, everything else mediocre), he could possibly grind a high precision screw by carefully monitoring screw rotation and screw traversal through rotary and linear encoders, respectively.

a mill with a warped bed can still grinda nice flat.

You mean like a 1/2" flat on the end of a rod? Sure. Slap on a face mill and flatten an 8" x 16" piece of steel? I think not.

[ironDigit]

mind boggling.

anyone know were the motherscrew came from ?

[MrWild]

The mothier screw. I'm laughing hard. Okay, I keep reading the general consensus of many here stating you can't make precision more precise than the machine doing the work, and due to the machine's many falicies, you will end up with a crap(pier) product than the mother machine. Yeah, I guess you're right. If. (and here is the real ringer) you accept the product that comes out of the machine and say, "It is done and as perfect as I can make it." Well, I guess if you're not a craftsman, then I guess you accept the crap(pier) product that came from your machine. That's it in a nutshell really, and it is the divide between the craftsmen and the Engineers. <sorry for the dig guys>

Now when you get the best a machine can do from the machine and you go no further with it, you pretty much are stuck with the dilemma of what NC Cams speaks. Crap(pier) parts than the oringinal machine. But way back when all folks had were FILES and those were hand made at that, machines were born. Somewhere along the line we went from files to ultra precision. Sorry, god didn't do it. No, what craftsmen did was made the part as well as they could, then made a mating part that fit and they lapped the irregularities out. Or they used a profilometer and a stone, or they found other ways to HAND work the crap into excellence. Hand work.

It always gets me when I read it is IMPOSSIBLE to make precision. Imagine needing to make a special device that measured angstroms with the crappy acme screws and slide ways of olden days. Folks nowadays would tell you impossible without air bearings and other fancy machine accesories. Sorry, it was done long ago with machines many would shudder at. Back then, they used dynomite to blow broken drills out of train crankshafts. Forget an EDM, it wasn't invented. Break a drill off in a locomotive crankshaft? Hell, take it out in the back 40, dribble in some dynomite past the flutes, and blow that sucker out. Go to the library and get a book on old machine practices. You will be amazed and no, you don't need a golden grail of a machine to produce precision. You just need to learn craftsmenship.

[HuFlungDung]

I read, or saw a picture somewhere, of a device for making an accurate helix through kinematic methods. The method involved using something similar to a lathe taper attachment, where the guide bar was set parallel to the hypotenuse of the right triangle representing the helix angle of the screw. A system of cables and weights kept the travelling shoe in contact with the guide bar.

Such a device would be as precise as the flat and straight guide bar could be made. There are proven methods to improve flats by manual methods, reducing the errors and improving on the machine that machined the original.

I have not really thought out whether such a contraption could be made to work, but it seems feasible.

[Wade]

You can make a more accurate thing, with a less accurate thing. You just need accurate position feedback, to "correct" the error. You need more precise feedback, and steadier tools, not more accurate tools.

[ironDigit]

And probably : scrapping a lot of parts is -part- of making ultra precision parts.

thx for digging in guys.

[Zumba]

I think the best example of low precision parts being pieced together to create a high precision assembly would be your typical rotary table.

A phase II rotary table, made in China, is pretty cheaply made, but all the internal gearing, manages to be precise within several minutes of a degree. The same principles could probably be applied to grinding a high precision ballscrew using nothing other than budget gears and acme screws.

[ironDigit]

hmm this could make a nice feedback system when time is not an aspect of your succes.

a feedback for indexing a "high"precision screw that is.

see as long as you got quality men you can make quality tools.
wich again is quite comforting(and i might even be able to sleep again)

cheers Zumba

[Andre' B]

I read, or saw a picture somewhere, of a device for making an accurate helix through kinematic methods. The method involved using something similar to a lathe taper attachment, where the guide bar was set parallel to the hypotenuse of the right triangle representing the helix angle of the screw. A system of cables and weights kept the travelling shoe in contact with the guide bar.

Such a device would be as precise as the flat and straight guide bar could be made. There are proven methods to improve flats by manual methods, reducing the errors and improving on the machine that machined the original.

I have not really thought out whether such a contraption could be made to work, but it seems feasible.

Older gun barrel rifling machines also the common method used in home build rifling machines.

http://www.firearmsid.com/Feature%20...anufacture.htm
http://www.benchrest.com/forums/show...?t=25813&pp=15
http://www.practicalmachinist.com/ub...ic/16/631.html

[toastydeath]

Master screws were (and sometimes still are) made, first by a fairly accurate "roughing" generation method, either on a lathe, or a grinder, or from a special purpose machine geared to produce a screw without any pre-existing model.

The screws are then hand-worked on one side of the thread face with laps using feedback from special-purpose measuring machines that can continually analyze pitch error. After several iterations, you now have a screw that is more accurate than laboratory gauge blocks.

Ironically, these all have a form similar to an Acme thread or a square thread, because you can't lap a ballscrew.

Then, these master screws are used as length references and go into master thread grinders. The master thread grinders are ultraprecision machines that require a whole building of infrastructure to support their operation. These master screw grinding machines can produce very, very accurate leadscrews fresh off the grinder of any form you desire. The first ones were completely manual machines, and after a period of time, slowly became analog (not cnc) corrected machines. You can go buy a CNC thread grinder that has been profiled and relies on the masters generated by manual methods for its accuracy.

Now, this "artifact" method of using master screws for accurate grinding has been eclipsed by machines that are guided by metrology frames, and use no form of screw whatsoever. Rather, they use capstan drives for linear axis positioning, interferometers for angular and linear feedback, and piezoelectric fast tool servos for final correction at the tool interface.

[MrWild]

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[DaveP]

Rail Extension Kit for Screw Drive Garage Door Opener


I posted this in another thread, but maybe folks here would be interested too.

I was wondering if anyone had ever tried using a screw drive from a garage door opener to drive an axis on a homebrew machine? I wonder how the acuracy compares to threaded rod or belt drive home systems. Not the highest, I am sure, but for woodworking machines it might be ok.

I was searching the sears.com web site looking for a part and saw this item -

Craftsman 8 ft. Rail Extension Kit for Craftsman Screw Drive Garage Door Openers $19.99