Looking into a ballscrew, not sure if I should buy. Help

[oldmam4m80s]

There is a ball screw that is very close to what I need for my dream machine. However the ones I have found in the past that have been closer were about five times more expensive. This is a Precision Rolled Screw with a pitch of 40mm and a backlash of .0002. it sure sounds like a winner for my budget. I can always upgrade later after I use this ball screw for the machine. What I would like to ask is: Are there any downsides to using a ballscrew with high pitch such as this one? Will I need a high torque motor to be able to machine at slow speeds? How will this affect my machined parts?

Just anything that you can think of mentioning I would like to hear out. I have read about ballscrews but I know there are experiences that the information has left out. Real world experience... So please chime in, I would appreciate it. Thanks

[irving2008]

What diameter and length of screw are you considering?

[oldmam4m80s]

Sorry for the delay. Cell signal is not what I wish it would be and thus goes my efforts in a quick reply... :( Well I am unsure about exact diameter but I believe it to be about 1 Inch. Well here is the specs from the seller:
Travel: 96"

OAL: 105.63

Thread Length: 99.41

Thread Dia.: 40 mm

Thread Pitch: 40 mm RH

Nut Length: 3 3/4"

Nut Dia. (body): 2.870

Nut Dia. (flange): 4.580

Bearing Dia.: 1.181

Coupling Dia.: .9845

The motor kit I was looking into has 1 hp servos. Unsure at this point if I need to go larger. I know inertia, torque and other details have a big impact on the screw size. I am just unsure if my choice of electronic equipment is lacking. Some guidance is what I am looking for. Thank you all for your time.

[irving2008]

Sorry for the delay. Cell signal is not what I wish it would be and thus goes my efforts in a quick reply... :( Well I am unsure about exact diameter but I believe it to be about 1 Inch. Well here is the specs from the seller:
Travel: 96"

OAL: 105.63

Thread Length: 99.41

Thread Dia.: 40 mm

Thread Pitch: 40 mm RH

Nut Length: 3 3/4"

Nut Dia. (body): 2.870

Nut Dia. (flange): 4.580

Bearing Dia.: 1.181

Coupling Dia.: .9845

The motor kit I was looking into has 1 hp servos. Unsure at this point if I need to go larger. I know inertia, torque and other details have a big impact on the screw size. I am just unsure if my choice of electronic equipment is lacking. Some guidance is what I am looking for. Thank you all for your time.

What weird specs, some imperial some metric. Anyway that screw is 40mm dia nominal and weighs around 25kg and has massive inertia, you're going to need around 4Nm of torque just to spin the screw up to speed with any reasonable acceleration.

Why do you need such a big screw? That one will push some 4.3 tons along!

[irving2008]

Just found your other posts. That is a monster of a machine. What are you planning to machine in steel that size?

With that amount of z travel you're going to have real problems with rigidity. All the effort in worrying about screw tolerances is pointless, the gantry will deflect one or two orders of magnitude more.

I think you need to take a step back and explain why the machine needs to be this big.

[oldmam4m80s]

Thank you for chiming on this irving. Well I am building the machine to be able to handle a wide variety of things. I had one gentleman who approached me to machine some 500-800 lb steel pressure housings. I am unsure what machine they belonged to but they were pretty massive pieces of metal. They had been machined wrong at the factory and they were looking for someone to fix the error. They may have been something like 4'x3'x1.5' in size. just a rough estimate. That is not my main goal though. I basically just wanted to build a machine where I could turn down a minimal amount of jobs as possible. I also would like to make quiet a few custom parts (about 40%) for a personal project that will be large. perhaps not as large as the work area. But it will definitely come in handy. Some parts (about 15%) I want to make will need the majority of the work area. I would not mind making smaller CNC machine parts such as kits that I can put on the market locally to help people get into CNC building. The kit is more of a hopeful venture but the parts are a definite MUST have. I called many machine shops and in order to make the parts I would want I could pay for a CNC machine twice over so I decided to just build one and be done with it.

The only reason I am looking into this specific ballscrew is that it fits the budget better than most others I have found. It would be nice to find one that is a tad bit longer and smaller diameter as well as price tag but this has been the find that fits the build closest for now. I plan to use what I can get to increase income so I can buy new from Manufacturer and sell the used one to recoup some costs. Until that happens though I am left scrounging around for surplus and cheaper stuff that will get the job done.

I think for the pitch size though it may be too high. Would it be better to have a lower pitch such as 10mm or there about, when using weaker servo's? I guess the real question is what type of servos work best with high/low pitch ballscrews? Will it be best to go with a higher speed/torque servo with higher pitch ballscrews? or what is the relationship there, if any?

[irving2008]

There are so many factors here it's hard to know where to start. Building such a large industrial strength machine is a huge undertaking. Have you looked at purchasing an older second or third-hand machine?

The key issue is you have to look at the whole system, not just individual parts. The servo you have suggested has 4096bit resolution so directly driven this screw gives you 40/4096mm resolution, that's .01mm or about 4thou. Is that good enough? The rated rpm for the servo is 3000 so nominally your max feed rate is 40/1000*3000=120m/min which is huge, you'll never be able to use that and keep the machine from tearing itself to bits. But the most telling bit is that servo has only 3Nm of torque (7 instantaneous). Your gantry is going to weigh something like 120kg to get the rigidity over the width, based on 2 lengths of 8" x 4" x .25" bolted together in an L-shape + the y and z axis servos and spindle (they weigh12kg on their own!) and it's going to struggle to give you any reasonable acceleration. You need to run the numbers.

edit: just run the numbers to a first approximation and its just about doable with moderate acceleration.

however, I assume you plan to use this screw for the X-axis (though its only 8ft not the 10ft you'd originally mentioned)? If so, for a Y-axis of 4 to 5ft you'll need 2 screws to prevent racking if you plan on machining steel. That will need 2 servos, one won't cope without gearing down 3:1 and with these loads I'd not recommend belt drive.

[bikepete]

> that's .01mm or about 4thou

Pretty sure 0.01 mm is more like 0.4 thou FWIW.

The other thread where the OP describes the project is at

http://www.mycncuk.com/forums/mill-b...html#post47999

Says we're looking at 5' x 10' working area, Z travel 14".

As Irving says this sounds like a major project and which screws/rails/servos to choose is, I would think, secondary alongside making the structure sufficiently accurate and rigid to machine steel at an economically sensible speed and finish. And resolution is the least of the problems I would think - more important will be making sure everything is flat and orthogonal (even with e.g. 500 lb of steel plonked on it), that steel can be machined at an economic rate without excessive vibration and with a reasonable surface finish... etc.

Just had a quick nose around to see if there's commercial equivalents out there... best I could find:

Milltronics: BR60IL (100" x 60" x 28")

Obviously you don't need to emulate the same speed, horsepower or accuracy necessarily to have a useful home built machine but it's notable that it weighs SIX TONS (14000 lb). You could get away with much less of course but it will compromise the accuracy and finish attainable over those large travels. Rather than guessing on this, I'd probably want to get some calcs done on (as a minimum) how much your proposed structure will deflect under the machining forces you intend to put on it. This'll let you know whether it can achieve realistic tolerances for commercial work. Damping/resonance could also be an issue worth tackling at the design stage rather than later...

You'll need to be confident of its accuracy before taking on paid work, I would think - worth thinking now about how that would be proved - measuring things accurately over those sizes is not trivial...

Some good stuff here on design BTW in case you've not come across it before:

http://www.mycncuk.com/forums/faqs-p...mn-design.html

Apologies if you already know all of this - just some random thoughts after reading your plans so far. Good luck with what sounds like an ambitious build.

[irving2008]

@bikepete. You're right, was a typo on my part... Doh :(

[oldmam4m80s]

Thank you irving and bikepete. Very good viewpoints and insights. This is exactly the information I need to make sure what I have envisioned will actually work. I have only done a few calculations on the project and I know doing the math is the blueprint for getting it done right. So I will be calculating more of what is needed very soon.

I had done some calculations in regard to steel flex of the table and gantry to make sure it would be within acceptable tolerances. I came up with some safe numbers but I do not have them handy at the moment. It was a green light for me to continue searching for the next item I needed though. I lost the deal I had setup with the 4"x8"x.25" steel tubing I had setup and I believe my design came in at about 3800lbs. Definitely not light! As far as speeds go I am in no hurry to manufacture parts fast. I would baby this machine to increase longevity of components as well as tools. Perhaps once it produced enough income to build a heavier duty one, not within such a small budget, I would consider building it to maximize speeds and getting better resolution and accuracy. For this build I will be satisfied to get in the .00X's inch. maybe a very low .0X's but I would rather it be more precise and accurate than not.

In response to the second/thirdhand machine option I have looked into it but I did not feel confident I would be able to find one within my budget. If by chance I did happened to come across one I would then have to learn that machine and troubleshoot any issues with it. I felt as though I would be much more prepared to troubleshoot and deal with a machine I designed and put together so that I would know every nook and cranny. This build to me is much more than just getting a machine to do a job. It is more of a Build it to learn it, troubleshoot along the way to fine tune it and watch it go! :) if that makes any sense.

I did think about the two screw option on an axis and if I had to the shorter of the axes would be ideal. An Engineer at a manufacturer I spoke to advised I go with only one screw per axis. I called them to ask about pricing on parts and WOW! they were very proud of their parts. I am considering a design with single screws at every axis but if I see the design benefits from two screws at any point I will go with that option.

As far as drives I have looked into them just briefly because I figure that there would need to be a lot of time involved in choosing the correct one once the other parts have been selected. I will give them another once over now though.

It is an ambitious project and I would like to become more aware of how this all comes to be. Every detail that has to be accounted for to achieve this build I would like to learn about. It is interesting to me to be involved in a from every aspect inception to reality.

Assembly of the table to maintain projected machinability was one thing I wrestled with. I have come up with a few ideas. I asked a couple of machine shops to Face mill the table top where all rail and screw mounting would take place but they wanted ALOT of money for just this simple operation. I know its not the easiest thing to do but not difficult enough to warrant the amount they wanted to charge. So for this issue I am currently designing an easy to make setup that will allow me to achieve the operation at home. It really is not as bad as I anticipated. So that issue has a cost effective solution to implement.

I just recently purchased a welder that I have yet to try out. It is a Lincoln Invertec 275s. I will TIG weld the assembly together to reduce warping. I will take the utmost care in making and designing the tools I will need to help me accomplish this build and they will be used for the next more precise build I spoke about earlier.

Thank you for the Link bikepete. I did not know it was one till I read over it a second ago to make sure I was responding to everything. I have read a few things on the subject, my eyes are still good so I should read some more :) I am looking into the link as I type. As far as resonance goes, I don't believe I will attain the speeds required in the current design but they are still something I should apply calculations on before buying a part just to be sure. I had not gotten that far since I am shopping around for economically available parts. Resonance changes per available ballscrew. :( This one I am considering is probably a good buy but I do not know it yet. I have contacted the person selling it and they advised they sell the large ones fast so I guess I should make up my mind on it. I just have not decided for sure since I am in hopes of finding one closer to my design.

The deciding factor on this particular ballscrew is strength of the Servo I am going to put it on. Let me find the link of the servo kit I am looking to buy to post that here. Well the link is a strict PDF. The website is not allowing me to view since I do not have a PDF reader on this PC. Here is the link from my other post: http://dmm-tech.com/Files/dmmmaxs14a_p.pdf I believe Irving already read this one and the torque of the motor seems to be just enough. When I posted my choice I had not anticipated the thickness of the ball screw to be as large so I would likely chose differenlt once I acquire the screw if necessary.