[rwskinner]

Hey moderator, get rid of this guy. His profession is "wannabe", he has 39 post and over 50% of those are smart Ars remarks and derrogatory.

Richard

excuse me but mind your own beeswax, i was asking simple questions of ray, you are the one trying to instigate something. If he's not going to answer I'll just assume the answers are no, no big deal, no insults, just asking
walt

[waltpermenter]

well that's totally uncalled for richard, i just ask questions, how many posts do i need before i can get into the club that will answer them.
walt

[SCzEngrgGroup]

Ray and others,
I am initially looking to build a machine with a 32x 48" travel and was considering using a 2525 precision rolled ballscrew for the X axis however, nook doesn't have one in the XPR accuracy. Perhaps I should settle on one with a smaller lead. What are your thoughts on this?

Cheers

What makes you think you NEED a 25mm lead? What specs (speed, thrust, accuracy, resolution) are you aiming for? The combination of high lead, and high accuracy is somewhat contradictory. Fast lead gives you high speed, at the cost of reduced accuracy. And you'd have to have a VERY powerful, VERY high-resolution (i.e. - servos, not steppers, with high-resolution encoders), VERY stiff drive to get the most out of it.

Regards,
Ray L.

[Padrino]

What makes you think you NEED a 25mm lead? What specs (speed, thrust, accuracy, resolution) are you aiming for? The combination of high lead, and high accuracy is somewhat contradictory. Fast lead gives you high speed, at the cost of reduced accuracy. And you'd have to have a VERY powerful, VERY high-resolution (i.e. - servos, not steppers, with high-resolution encoders), VERY stiff drive to get the most out of it.

Regards,
Ray L.

Thank you Ray,
I initially wanted the end product to have the capability of running close to 1000 ipm however, the more I read, the more I am leaning towards an accurate end product rather than speed. Forgive me with the newbie questions. I appreciate everyone's feedback. My motors are lin engineering 8718S-05S Nema 34 step motors 434 in-oz. I was planning to use high precision linear bearings with a heavy preload to minimize the fluctuations, but is that required as this machine only has a total travel of 48"

Again thank you for everyone support.

[SCzEngrgGroup]

Thank you Ray,
I initially wanted the end product to have the capability of running close to 1000 ipm however, the more I read, the more I am leaning towards an accurate end product rather than speed. Forgive me with the newbie questions. I appreciate everyone's feedback. My motors are lin engineering 8718S-05S Nema 34 step motors 434 in-oz. I was planning to use high precision linear bearings with a heavy preload to minimize the fluctuations, but is that required as this machine only has a total travel of 48"

Again thank you for everyone support.

I think it would be best to start your own thread to discuss this, rather than hijacking this one. Sounds like there should be a lot of questions....

Regards,
Ray L.

[waltpermenter]

Padrino, you'll be much better off starting a thread in the router forum, you'll find many experienced builders there that can answer any of your questions. you don't want to start a thread where it doesn't belong.
DIY-CNC Router Table Machines - CNCzone.com-The Largest Machinist Community on the net!
walt

[Padrino]

thank you guys.
I have just created a thread. Sorry for high jacking this thread.
Thanks
Rob

[bigspike]

I plan to use single ball nuts, doubled up with spring washers on the X & Y axes to reduce/eliminate backlash. With just a single nut on the Z. My thinking is that the weight of the head should compensate for backlash.

Any opinions on why this won't work for most operations?

[Padrino]

I plan to use single ball nuts, doubled up with spring washers on the X & Y axes to reduce/eliminate backlash. With just a single nut on the Z. My thinking is that the weight of the head should compensate for backlash.

Any opinions on why this won't work for most operations?

You mentioned you were considering the nook xpr line of ballscrews. Would using one ballnut produce significant backlash? Sorry for the noob questions
thanks

[691175002]

I plan to use single ball nuts, doubled up with spring washers on the X & Y axes to reduce/eliminate backlash. With just a single nut on the Z. My thinking is that the weight of the head should compensate for backlash.

Any opinions on why this won't work for most operations?

It is not always true that the weight of the head will pull down against the screw. Stick slip against tight gibs can hold the head in place. You also have issues where some operations push the head up while others pull it down.

Not that backlash on Z is a problem, in most cases z accuracy is far less important than x and y accuracy.

[bigspike]

I spoke with a rep for Nook and he saw no reason why I could not use the XPR ball screws with the SRT ball nuts. I am still waiting for quotes from various companies.

It feels like most companies just aren't interested in the one off market. Even for off the shelf stock w/o machining. Maybe it is the lack of machining that is the problem? No value added = less profit?

[amyers]

hossmachine has a chart with specs of a bunch of ballscrews on this page. Shop Info he doesn't have the chinese ebay screws listed probably cause it's an old list but he called them the best bang for the buck somewhere not too long ago on his monster g0704 thread.
am

[mdonovan]

....
4) Axis drive systems - Anything less than a double-close-loop servo with both position (via linear encoders on all axes) and velocity (via rotary encoders on all motors) is fundamentally incapable of achieving anything much better than a few thou accuracy. If, on a given day, the machine DOES deliver such accuracy, it due as much to luck as anything else.

absolutely untrue statement.

I've produced .0001" repeatability slides with a single rotary encoder attached to the end of a precision ballscrew many times.

Of course the bases were always hardinge CHNC platforms, which may be the the most precise mechanical lathe platform ever built.

at the factory they had to repeat within .00005" and produce a 3 micro finish on a radius before we were allowed to ship.

The servo loop was a single encoder on the end of the ballscrew with a dc servo drive running in torque mode through a timing belt!

yeah I understand if you don't believe it... nevertheless it's true!

[691175002]

I've produced .0001" repeatability slides with a single rotary encoder attached to the end of a precision ballscrew many times. I understand if you don't believe it... nevertheless it's true!

Repeating to 0.0001" is certainly believable. I can even imagine machining with sub thou accuracy given proper setup. Positional accuracy over an entire work envelope is a different matter entirely though.

If you have a meter long ball screw and turn up the thermostat just 5 degrees, it will be 0.003" longer. Total positional error is a combination of all three or more axes as well.

[rwskinner]

I checked into the Nook XPR screws and SRT nuts. The screws were very reasonable but the ball nuts were very, very expensive. I did not get some of the pricing others mentioned on their blogs. I'm not sure if it was mis-quoted or what. I'm very curious to what nook quotes you for the 5/8 or 3/4" Single ball nuts.

Richard

[ger21]

If in a worst case installation, I use the Roton ball screws and get .009"/ft lead error. Then I could have an error of .018" (nearly 1/32") on a 24" part which is completely unacceptable for metalworking.

You're not going to setup your machine so that it's accurate for a 1" move, but out .018" in 24".

In this instance, you adjust the steps/inch so that your 24" move is really 24". If you indeed had this much error, most likely it would be pretty consistent.

So, if you actually did have screws that were out by .009"/ft, it's still possible to have a machine accurate to within a few thou over 24".

[SCzEngrgGroup]

I checked into the Nook XPR screws and SRT nuts. The screws were very reasonable but the ball nuts were very, very expensive. I did not get some of the pricing others mentioned on their blogs. I'm not sure if it was mis-quoted or what. I'm very curious to what nook quotes you for the 5/8 or 3/4" Single ball nuts.

Richard

Richard,

I paid about $49 each for 1" SRT single round nuts, and about $10 less for 5/8". I used the 1" ones on my machine, with XPR screws, and nthey work beautifully - VERY smooth, and very accurate. I posted the exact prices a month or so back.

Regards,
Ray L.

[SCzEngrgGroup]

OK, Here is exactly what I bought from Nook:


1 18" 0631-0200 XPR RH $40.78 ea. 18" of 5/8" XPR Screw Stock
1 60" 1000-0250 XPR RH $246.70 ea. 60" of 1" XPR Screw Stock
1 24" 1000-0250 XPR RH $98.68 ea. 24" of 1" XPR Screw Stock
2 SBN10325 $33.73 ea. 5/8" SBN Single Round Nut
4 SBN10466 $49.36 ea. 1" SBN Single Round Nut

Regards,
Ray L.

[SCzEngrgGroup]

absolutely untrue statement.

I've produced .0001" repeatability slides with a single rotary encoder attached to the end of a precision ballscrew many times.

Of course the bases were always hardinge CHNC platforms, which may be the the most precise mechanical lathe platform ever built.

at the factory they had to repeat within .00005" and produce a 3 micro finish on a radius before we were allowed to ship.

The servo loop was a single encoder on the end of the ballscrew with a dc servo drive running in torque mode through a timing belt!

yeah I understand if you don't believe it... nevertheless it's true!

And that is relevent to someone doing a CNC conversion on an inexpensive Chinese milling machine how???? For Pete's sake, some people just like to argue! YES, you CAN build special, purpose-built machines for specific applications that are extremely accurate. SO WHAT? We're talking about CNC converting a MILLING MACHINE here. TRY to stay on topic.

Regards,
Ray L.

[ninefinger]

I plan to use single ball nuts, doubled up with spring washers on the X & Y axes to reduce/eliminate backlash. With just a single nut on the Z. My thinking is that the weight of the head should compensate for backlash.

Any opinions on why this won't work for most operations?

Backlash on Z is a problem that needs to be addressed just as much as on X or Y. As mentioned the weight of the head does not necessarily cause it to "fall" down - until you start cutting - then depending how much backlash you have it can start to take a bigger bite than you were expecting (assuming you just touched off to set Z). The smaller the cutter the more this is a problem (ie the increase in cut vrs diameter) and can lead to tool breakage. Even with larger cutters it can cause an uneven facing cut that needs to be passed over again to get that smooth finish and getting a precise thickness becomes a royal pain in the neck.

Downward feeds with drills can be jerky as the head slips and starts due to the backlash.

All of the above happened to me - mind you my backlash is starting to get rather large on my mill - mostly due to the support bearings (must fix - note to self..) and not just the ball nut / screw combination which has relatively little backlash

Mike