If you search for "2525 ball screw" on ebay you find sellers who sell 25mm dia, 25 mm pitch screws. They can probably make them to your specs. Example: 2525 L1100MM Ball Screw Dia 25mm Lead 25mm High Lead ballscrew with Ballnut | eBay
Linearmotionbearings2008 does not have an option coarser than 10mm pitch. I did learn that he can provide 32mm diameter screws. I did not ask the price. The nook calculator and adjusting the length closer to my need produces <175 inch per minute with the 32mm diameter screw. I am not sure if the Chinese ground balls crews would qualify for higher rotational speed than rolled screws? I don't remember who said it, but someone early on mentioned that wood likes speed. This does not seem fast compared to other peoples machines. Are there any other suppliers out there that can provide a complete package with blocks and end machining like Linearmotionbearings2008? The only other option I have found is Roton 1 X 1.000 Right Hand Recirculating Lead Screws & Nuts for Power Transmission - Roton Products, Inc.
Thanks for the link to Atlanta Drives. That integrated rail/rack is very elegant!
If you search for "2525 ball screw" on ebay you find sellers who sell 25mm dia, 25 mm pitch screws. They can probably make them to your specs. Example: 2525 L1100MM Ball Screw Dia 25mm Lead 25mm High Lead ballscrew with Ballnut | eBay
I am looking at them now. Thanks so much for this!
That is true but varies with the wood and other parameters. Frankly you can see this in most wood machining processes that are done manually. For example running Cherry through a table saw or other machine to slow will lead to burning. You can even see what happens with a handheld router if you go to slow in the material you are working on.
What you need to determine is the common feed rates for the materials you expect to work on. Note I said feed rates not rapid movement rates. Just be careful that the suggestions aren't for high performance industrial machines.This does not seem fast compared to other peoples machines.
I was under the impression that most ball screw suppliers can build to print - for a price.Are there any other suppliers out there that can provide a complete package with blocks and end machining like Linearmotionbearings2008? The only other option I have found is Roton 1 X 1.000 Right Hand Recirculating Lead Screws & Nuts for Power Transmission - Roton Products, Inc.
By the way, once you start talking about large diameter screws you really should be looking at the inertia to see if that will be a problem. Many of the solutions you see in the forums here are in a sense canned, that is the mechanical performance is well understood, once you start looking at solutions outside the norm you really need to consider doing the engineering to see if you have a workable solution.
Interesting, I have to wonder about the cost though. You will also be looking at a servo for this axis but that might be the case anyway.
Thanks for the link to Atlanta Drives. That integrated rail/rack is very elegant!
Thanks to all for posting in this thread, as I know it has helped me a lot in my design phase, and Im sure many other CNC newbe's have found it invaluable.
The screws are reasonable. The prices for the ballnuts are breathtaking!
I'm still not convinced 25mm is large enough for a ballscrew spanning over 8". Unless, however, you conceive a spinning nut design, which would eliminate problems due to whipping. I think Techno-Isel uses spinning nuts for their long axis. Phife has a build thread where he uses a fixed-end bearing block for the ballnut. I believe the Grumblau plans utilize a spinning nut. A surplus hollow-shaft direct drive motor is another option, albeit expensive.
Wizard, you really got me thinking with this comment.
Louieatienza, great point. I think this concern goes to the point that Wizard was making.By the way, once you start talking about large diameter screws you really should be looking at the inertia to see if that will be a problem. Many of the solutions you see in the forums here are in a sense canned, that is the mechanical performance is well understood, once you start looking at solutions outside the norm you really need to consider doing the engineering to see if you have a workable solution.
I wish that I had the ability to figure out the answers to these questions. My strength is as an artist and woodworker. That said, I am going to continue to work on the rotating screw design because it makes a significant difference with my need for a narrower machine. A preliminary drawing has revealed that the actual distance between the bearings is 2650mm. 25mm or 50mm pitch will allow the screw to rotate quite slowly but I am concerned about such a long unsupported screw. Once I am a little farther along I will reach out the ebay sellers and see if they can provide larger diameter screws. I am very aware that I am beyond my limits, but I can't let the spinning screw solution go just yet.I'm still not convinced 25mm is large enough for a ballscrew spanning over 8'.
I have realized that my other design needs to focus on the "Canned" solutions. I take that to mean 20° pressure rack with CNC Router parts PRO Nema 34 reducers with the spring tensioner. The machine will have to become narrower but there should be fewer unknowns as this is a well used solution.
I feel that I need to take this part of the design slowly so that I make the right decision for my particular situation.
Thanks for all of the help
Before I designed anything for a large long ball screw, I would first buy a small one to get the design concept and functionality down. No sense buying a big truck to see if you can rotate the transmission to turn the wheels when a junk yard Volkswagon would do.
Lee
Nook's metric critical speed calculator says a 21.7mm root dia (25mm ballscrew), 2650mm length, end fixity B, gives critical rpm of 374rpm. If the lead is 25mm, that's about 370 ipm -- not bad. And that calculator assumes no stability contribution of the ballnut, which is normally not very near one of the ends, so most of the time the unsupported screw length is much less than 2650mm.
The other issue is droop due to gravity, which might exacerbate whipping. The max deflection (for simply supported at each end) is (5*w*L^4) / (384*E*I).
- w is the weight per unit length. The screw's cross-sectional area is probably about (Pi/4)*(0.92")^2 = 0.66 sqin. Density of steel is about 0.29 lb/in3. So w = 0.66 sqin * (1 in unit length) * 0.29 lb/in3 = 0.19 lb per unit inch length.
- L is (2650mm - 70mm ball nut length?) = 2580/25.4 = 102" worst case.
- E is 29E6 psi
- I is (Pi/4)*(root radius)^4 = 0.026 in4. [EDIT, error the first time, propagating below but now corrected]
So max deflection is (5*0.19*102^4) / (384*29E6*0.026) = 0.37". That's a fair amount, might be a problem. I'd use a bigger screw.
Alternately, IIRC there was a nice large format machine (maybe 6'x12' or so?) from the UK on cnczone last year (?) that had the racks near the rails at or just above the table, and the motors mounted vertical above the gantry feet, shaft facing down, behind the gantry tube. I think he used compact gearboxes. I don't think anything stuck out to the side. But this is a vague memory. I do remember it was a nice machine in a old large room--an interesting contrast. Anyone know the link?
Last edited by dmalicky; 01-31-2014 at 02:24 AM. Reason: calc error
David Malicky
Thanks David,
I was concerned about the screw sagging! Thanks for the heads up on the 6' x 12' build. I am searching for it
My 2500mm long 25x10mm rolled ball screw can spin up to 550 rpm before it starts whipping, when the gantry is at the ends. I kind of wish I had designed a rotating nut when I had the chance, to reduce the inertia and increase the speed.
You guys are aware that they make precision rack and pinion, right? How much precision do you really need over 12' and doesn't speed and budget matter?
Lee
Found it! http://www.cnczone.com/forums/diy_cn...g_machine.html
Suitable gearboxes can be found on ebay for reasonable prices, but it takes a lot of searching and research on part numbers and brands. Louie gave some great advice on these back in post #77. We looked at them for our 4x8 machine but we already had Nema34 steppers and at the time we couldn't find 2 input-compatible boxes with the right ratio, size, backlash, price, etc. Most of the reasonably priced ones are for Nema23 or smaller. I would suggest around a 5:1 ratio. 4:1 is about the steepest I'd want for good acceleration and it would be plenty fast.
Or a vertical motor can certain be done with a belt drive or a DIY gearbox. We went DIY gears for ours, pretty economical. Pics and part numbers available if of interest.
tahustvedt, thanks, that's a good point of reference. The calculators have a small safety factor built in, and often the end fixity 'B' is better in reality, especially when the ballnut is near the 'free' bearing mount end, as it effectively makes that end 'fixed' (double-bearing), turning the system into end fixity 'C'.
David Malicky
I would definitely go for rack and pinion if I was making a 12' long machine.
Always interested!
Thanks for the link to the post. I am looking forward to reading it tonight.Or a vertical motor can certain be done with a belt drive or a DIY gearbox. We went DIY gears for ours, pretty economical. Pics and part numbers available if of interest.
LeeWay, I started out with rack and pinion and have come full circle. There seem to be too many concerns with a long ballscrew for my particular application. A spinning nut design would solve those issues but I might also loose the advantage of a narrower stance. And then there is the reality of the budget!
Yes, rack is a good solution for a long axis. If only cutting wood, I'd probably use a CNCRP's Pro drive, since it's easy and reasonable $: PRO Rack and Pinion Parts | CNCRouterParts
If cutting aluminum, a gear drive would be far stiffer, and offers more reduction for more acceleration and resolution. Ours uses 84T:20T for the reduction (4.2:1) and another 20T for the pinion. All gears are 20 pitch, 20 deg pressure angle. The big gear and pinion are on a 5/8" hardened dowel shaft, located by precision needle bearings. Bronze thrust bearings locate the big gear between the alum plates. The motor mount holes are a bit oversize to adjust gear-gear backlash. As you can see, some nut and bolt holes were counterbored to clear the big gear and rack. The build is a little rough, but they work great. Parts cost per drive is about $130, plus aluminum and the stepper.
Parts
Motor gear: Boston 20T, 1/2" bore for our NEMA34s.
Boston Gear YA20 Spur Gear, Steel, Inch, 20 Pitch, 0.500" Bore, 0.500" Face Width, 20 Teeth
Boston Gear YA20-1/2 Spur Gear, Steel, Inch, 20 Pitch, 0.500" Bore, 0.500" Face Width, 20 Teeth
The less $ one is plain bore, so needs 2 setscrews tapped.
Big gear: Boston 84T, 5/8" bore.
Boston Gear YA84 Spur Gear, Cast Iron, Inch, 20 Pitch, 0.625" Bore, 4.300" OD, 0.500" Face Width, 84 Teeth
Cast iron helps reduce noise.
Axle bearings: precision needle roller, 5/8" bore, 1 1/8" OD, 3/4" long.
New TORRINGTON HJ 101812 Bearing | eBay
Needles are compact and extremely stiff. If placed above the table, probably use a sealed bearing: McM 8258K21.
Axle thrust bearings and washers: McM 5906K515 & 97022A261
Axle: 5/8" hardened dowel pin, McM 98381A812
Needle bearings required a hardened axle. We ground flats for the 84T set screws, and chamfered the case away for welding to the pinion.
Rack pinion: Martin 20T, 1/2" Bore.
Martin TS2020 Spur Gear, 20° Pressure Angle, High Carbon Steel, Inch, 20 Pitch, 1/2" Bore, 0.500" Face Width, 20 Teeth
We reamed it on a lathe to 0.625" to fit the axle; the gear must be plain bore to do this concentrically. Martin gears are a little harder than Boston, so a good choice against the rack.
Rack: Moore Gear R2020x6
Deciding between 1/2" and 5/8" axle size is a little complex. At 1/2", FEA models showed that bending of the pinion shaft was a big source of flex (we were aiming for a ~50k lb/in machine). So we upsized to 5/8", which also fits the 84T. But a 5/8" axle requires reaming the pinion, which didn't leave much steel for the setscrews, so we light press-fit the axle in the pinion and welded them at the end (welding elsewhere would cause runout).
With a less stiff machine, a 1/2" axle is likely fine. Then the only question is how to get a big gear with a 1/2" bore.
a) Adapt the Boston YA84 to the 1/2" axle. McM 2867T57 may work if concentric enough.
b) Use Martin's 72T steel gear with 1/2" bore: Martin TS2072 Spur Gear, 20° Pressure Angle, High Carbon Steel, Inch, 20 Pitch, 1/2" Bore, 3.7" OD, 0.500" Face Width, 72 Teeth
Steel is noisier than cast iron. Ratio with a 20T motor gear is 3.6:1. Easy.
c) Switch to 14.5 deg pressure angle gears, which have both 80-90T and 20T in 1/2" bore.
Martin Spur Gear, 14.5° Pressure Angle, High Carbon Steel, Inch, 20 Pitch
These have more backlash than 20 deg gears, but probably not significant compared to the rack backlash. Steel. Better ratio than the 72T. Easy.
d) Switch to 24 pitch brass gears: Brass Spur Gear, Brass, Inch, 24 Pitch
I'd need to analyze to see if strong enough, but probably fine. Probably a better solution if using a Nema23 motor with 1/4" shaft.
David Malicky
Thanks for posting this Dmalicky. The detailed explanation is so useful! If I remember correctly you used a balls crew for the Y axis. If this is true I would be curious to know why? Price is my guess. I would also be curious to know if you used reduction with that axis? Did you try to match the inches per minute of the x-axis.
thanks again for the great post!
I find nothing wrong with the affordability or usefulness of ball screws on a shorter axis. 4' or less. The differences become dramatic when you start getting longer.
My router uses a rolled ball screw that is 48" long. I was able to use it initially at lower speeds. Say 150 IPM. Cutting speed. When I installed a double nut, I was able to increase it by double the speed. 300 IPM. The entire machine became more stable with a double nut on X and Y. That tells me more math needs doing in order to get an accurate idea of what is actually needed for a long ball screw. You can do what others have done, but question them on satisfaction of rapid speed.
Lee
Maybe my machine would have been better off with R&P, but it definitely wouldn't have been more affordable, or easier to design/build, for me as I can get by with a single ball screw. A single 2510 ball screw and motor costs $350 including shipping. I am not that satisfied with my rapids now at 520 rpm (reduced slightly for headroom). Maybe I'll figure something out eventually, but I'll manage somehow.