Lead Screw Advice

[WhistlePig]

Hello Everyone,

I have a basic machine going, along the lines of a joescnc made mostly of 8020 extrusions and angle iron.

It won't be long before I need to select a lead screw for a CNC router used mostly for wood and plastics of a slightly larger scale, mine is 6X6 foot in size, with a workable size of at least 5x5' in size.

I have read the FAQ and took note of mention of whip in the thread when traveling at higher speeds, so I would like the highest resolution possible but would like to avoid sagging and whip over 6 feet of acme threaded rod. I was shooting for a 10 tpi with a double screw, meaning at 200 steps per rotation .001 of resolution. If I calculated that correctly, that should be more than high enough "resolution" for me for lack of the proper term.

I was thinking that 1/2 acme thread might sag under its own weight so much so that it will cause havoc, and was curious if anyone who has built a similar machine had any advice on how thick a screw would be sufficient to span that distance, I do plan to pen up the one end in two pillow blocks, and the other one, possibly two if I go the belt drive route. I would make the gear captive between two pillow blocks, or mounted bearings.

Thanks, I would certainly provide more information if needed. All I am really shooting for is a basic machine that get me up and rolling at this scale.

Thanks for you help,

[amplexus]

Check out the used market for 1" + ground ball screws. No whip, no slop.
Amplexus

[monte55]

Mine is 60x60 inch and I have 4 steppers and use 3/4" x 6tpi screws from Enco. I did make my own nuts from bronze for a better fit. I use a Xylotex
driver and can get 80 ipm and have no whip. They might whip if turned very fast but I haven't had any problems. See video

YouTube - CNC doing text

YouTube - CNC doing text

Nick

[Metalmog]

I got some 5/8" rollerball screws of the rolled variety, from a guy on E-Bay together with two nuts at very very reasonable prices. These would be ideal for what you require. He also sold a preloaded version of the nuts which only meant you require one nut. They are pretty accurate as well (as in 0.001 per ft) with a 5mm pitch. The roller shafts where something like $15 per foot and the nuts $25 and preloaded nuts like $45. If you really want to pursue this I can go lookup the original invoice to get his deatails. He had a web site you could purchase directly as well - something like HobbyCNC.com

[zimba43]

Hi,
I built a 4ft x 2ft machine and made the mistake of using 1/2" leadscrew which whips and makes quite a noise too. I'm now planning my next project a 6ft x 3ft, i've made a few phone calls to various suppliers who say i would need to go to either 22mm or 25mm ballscrew which in the UK you seem to need a mortgage for. The other thing is what size motor would i need to drive it? So i'm now considering rack and pinion drive, hope this helps.
Simon.

[Matt Adams]

Originally Posted by WhistlePig Hello Everyone, I have a basic machine going, along the lines of a joescnc made mostly of 8020 extrusions and angle iron. It won't be long before I need to select a lead screw for a CNC router used mostly for wood and plastics of a slightly larger scale, mine is 6X6 foot in size, with a workable size of at least 5x5' in size. I have read the FAQ and took note of mention of whip in the thread when traveling at higher speeds, so I would like the highest resolution possible but would like to avoid sagging and whip over 6 feet of acme threaded rod. I was shooting for a 10 tpi with a double screw, meaning at 200 steps per rotation .001 of resolution. If I calculated that correctly, that should be more than high enough "resolution" for me for lack of the proper term. I was thinking that 1/2 acme thread might sag under its own weight so much so that it will cause havoc, and was curious if anyone who has built a similar machine had any advice on how thick a screw would be sufficient to span that distance, I do plan to pen up the one end in two pillow blocks, and the other one, possibly two if I go the belt drive route. I would make the gear captive between two pillow blocks, or mounted bearings. Thanks, I would certainly provide more information if needed. All I am really shooting for is a basic machine that get me up and rolling at this scale. Thanks for you help,

From some quick napkin calculations, it looks like a 1/2" ball screw would sag roughly 1/8 of an inch and have a maximum speed around 150 ipm.

It that what sort of information you're looking for?

Let me know,

Matt Adams
IMAC Motion Control

[zimba43]

Matt,
would you agree that 25mm ballscrew is the way to go and what size steppers would you go for?
Simon.

[Matt Adams]

Originally Posted by zimba43 Matt, would you agree that 25mm ballscrew is the way to go and what size steppers would you go for? Simon.

Simon,

How fast are you looking to move during rapids and during cutting? A 25mm screw would probably only sag .040" over that span, which I personally like a lot more than the 1/8" I was calculating before.

Matt

[zimba43]

Matt,

I'd like to aim for 300 ipm for the rapids, which with a 2 lead ballscrew I hope should be achievable. Most of the time i'll be cutting oak, iroko or softwood so cutting speed will be a case of trial and error to start with plus i havn't decided what size router yet.
Simon.

[Matt Adams]

Originally Posted by zimba43 Matt, I'd like to aim for 300 ipm for the rapids, which with a 2 lead ballscrew I hope should be achievable. Most of the time i'll be cutting oak, iroko or softwood so cutting speed will be a case of trial and error to start with plus i havn't decided what size router yet. Simon.

With what you specified, whipping occurs right around 310 ipm at 25mm diameter. Most of the time a 20% safety factor is put in there, so 250 ipm is what suppliers might quote.

I can get great prices Thomson ball and lead screws, which is a large manufacturer here in the states. If you get me a drawing, I can work up a quote for you. I'm not sure how things would work over seas, but I'd try if you're interested. PM me.

[pminmo]

I have a 42" 1/2" 10 TPI 2 start that whip is noticable starting around 160 IPM as the gantry gets 10" of so from the end of travel. While my drive/motor/ps combo will go faster I had to tune it considering the whip limit. it's not that the 1/2" sags, it's the rotary torque that is the issue.

Ballscrews are great IF, one you can afford it and two you don't have to machine them. Ballscrews are HARD.

This might help you some to understand how stepper torque vs rpm plays out: http://pminmo.com/PMinMOwiki/index.p...chanical_Power

[zimba43]

Matt/pminmo
Thanks for your help, just shows what a great site this is! I'm still working on the drawings but when i get to that stage i might just take you up on that offer matt.
cheers for now,
Simon.