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Starting work on a 4' x 8' going to try out a rotating nut for the long axis due to whip and inertia concerns. Hopefully can get some feedback on the design. Wondering if anyone else has implemented this successfully. I do have access to a cnc bridgeport and decent lathe as well which i plan to use.
Rotating nuts are a good idea if you don't mind the mass of the motor involved becoming part of the gantry's moving mass..
(ie I'd have done a rotating nut maybe if I could have for my axis if I could have had that extra weight out on the gantry how ever I could not see supportnig another 20+ lbs with out seeing to much sag. So I got a rotating shaft (which does limit speed) and the motor out on the end.
It is generally easier to drive the shaft because, as wcarrothers1 pointed out, stationary motors reduce the driven mass and also the need for complex umbilicals.
Performance wise, you should see no difference between driving the nut or the shaft. A driven nut puts the motor right at the driven carriage and thus consumes valuable tool space so it is usually used only for single-end supported systems (like extension arms) and compact coaxial drives where the drive nut is the center of a planetary gear motor system which rides on a shaft.
Either method is perfectly acceptable. I just recommend you include the wiring issues in your decision.
Gerber uses it on there 4x8 Saber router and it seems to work great.
On a 4' X 8' router why not just go with rack and pinion like most of the other machines?
Rotating nut Either nut or shaft rotation will move the gantry. Seems easier to use rotating shaft from my limited perspective and gears on the end with some give on gantry threads-this may be a no no. You can then change end grear ratio-but guess you could use a variable speed drive too..... Seems each drive is fine. jmho,pw
This is the 1st rotating nut design i have seen that is a good visual view, and can see this should work well. It is very similar on the lines of what i have been thinking about doing.
Joe
I don't know how often it would come into play, but changing a belt wouldn't be very easy. You could perhaps tape a spare to the inside of the left bearing plate. Then if you did need it, well there it is.
Here is what my line of thought was... If i go rack and pinion i still have the mass of the motor as part of the driven mass, however; i have reduced the driven mass by taking the lead screw inertia out of the equation. But the rack and pinion is i think more difficult to attain with the same level of precision and repeatability as the ball screw. If i go with the rotating ball screw then i have serious whip concerns especially at this length. So with the rotating nut i have a few more components to make but in turn eliminate these other issues.
How about timing belt then?
Andy
Drat, imperfection has finally stopped working!!
Im not sure what you mean, if your refering to how the timing belt will be changed i think its reall just a matter of a few screws. You can see from this view what i was going for a bit better. So I think all that is needed is to unfix one end of the lead screw, and take out a few screws to remove the assembly shown exploded. The exploded assembly on the right can be kept together for the most part, that outer flange is just to apply preload to the bearings so it may need to be loosened and reloaded after belt replacement.
Sorry, I didn't make myself clear.
Instead of a rack, you can use an open timing belt stretched and held at both ends.
The timing belt is looped over a pulley wheel that is driven by a motor.
The motor turns the pulley and the motor moves because the belt is held at both ends.
Very easy to build and minimal precision required for manufacturing.
Andy
Drat, imperfection has finally stopped working!!
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