[prodigy86]

Hi,

another cnc router question regarding the lead screws for all my axis

my cnc im designing is 1000mm long x 750 wide, using 40 x 40 bosch extrusion and wood/aluminium gantry, but this may change yet.

anyway what i would like to know is what size ACME threaded bar should i be looking to use, would like as least backlash as poss, was looking at inch bar but not sure what TPI would be best and the was something about start up like say 4 or 8, will have to get back on that cant remember, also looked at the ball screws aswell.

any help is much appreciated as im abit unsure right now what to use

thanks

[ger21]

2 turns per inch gives the best balance of speed and precision when using stepper motors.

[prodigy86]

yeah think ill be using stepper motors

will that be pretty quick 2 turns per inch?

[Dylwad]

Depends on what steppers,drivers and voltage,you,will be using, and the weight of your machine. Also depends on how the screws are attached. I can get 700ipm on 1/2-10 5 start without too much trouble, running gecko drives and matched 380 oz steppers at 50 volts with some tension on the screws, angular contact bearings at both ends of each screw.

.00025" per step on my setup btw

Sent from my SGH-T959V using Tapatalk 2

[prodigy86]

you make me realise how much i need to learn, haha

ill find out what stepper motors ill be using and get some pics of what ill use to attach the screws and see if the suitable

while im at it could you please explain whats meant by 5 start / 2 start etc or even a link for me to read up on would be muchly appreciated


am i allowed to ask wheres best to get the acme threaded bar from with all the connections or is advertising other suppliers against any rules?


thanks

[louieatienza]

you make me realise how much i need to learn, haha

ill find out what stepper motors ill be using and get some pics of what ill use to attach the screws and see if the suitable

while im at it could you please explain whats meant by 5 start / 2 start etc or even a link for me to read up on would be muchly appreciated


am i allowed to ask wheres best to get the acme threaded bar from with all the connections or is advertising other suppliers against any rules?


thanks

Usually it's the other wqy around. You need to find out first how fast you need for rapids and max feedrate for the material you need to cut. Then you need to estimate the amount of force needed to overcome the friction of the leadnut and bearings, push the weight of the axis, and cut the material, plus a little overhead. Then you'll know how much torque is needed, and leadscrew/motor combination can be found that will get you close.

[prodigy86]

ah ok, im not really sure what ill be wanting for my feeds/rapid, i really should know all this as im a Metal CNC machinist but in my defence i have only just swapped from using manuals where i did everything by eye and sound :\, these are the times when u wish you listened at college, anyway ill figure it out

ill be mainly cutting wood and hopefully if my machine can handle it, aluminium

think ill finish off modeling the machine and see what i need to do change for what i want

all this reading/learning can only do me good

[louieatienza]

Since you'd generally need higher spindle speeds (i.e. router) and higher feedrates for wood, I would agree with Gerry's suggestion of using a 2TPI (4- or 5-start) leadscrew. You'd probably couple that with a higher tourque NEMA23 stepper; if you use a G540 380in-oz is a popular motor.

[mdonovan]

What resolution do you require? (what's the smallest programmed step that you will want to make?)

How much repeatability do you require? (if you tell the machine to move to the same spot ten times in a row, how much of a spread can you tolerate?)

What's the largest size cutter that you intend to use, and at what RPM and depth of cut?

Answer those questions first, then work backwards from there to determine your best hardware choices.

if you have never done this before, then you are faced with a huge list of challenges, decisions, and potential pitfalls. I've been designing and building motion systems for 30 years and I still don't know much.

go slow and keep asking questions!

good luck!

[rocketflier]

while im at it could you please explain whats meant by 5 start / 2 start etc or even a link for me to read up on would be muchly appreciated

Look at a wood or machine screw. If you look at the end you'll find there's 1 point where the thread starts. Put your nail in that thread and turn the screw. You'll find your nail is advancing by a single thread. That is a full turn of the screw will put your finger down at the second thread grove. That single thread runs all the way to the head of the screw.

A 5 start actually has 5 threads running down the screw. Yeah, looking at it it looks like 1 thread, but there's really 5 grooves there.

Maybe a picture would help?



This is a three start {garage door opener} screw. At the end, there are 3 sharp points. I ran a white string in 1 thread, blue in the next and there's black grease in the third.

Following a single thread a full turn results in you being 3 threads down {white, blue, black, white again}. I just moved farther on a single turn of the screw than I would have with a single start screw. {if the screw above were a single start, a single turn would move you from the white to the very next grove,.. blue}

Pretty good description:
Identifying Screw Threads

What You Ought To Know About Ball Screws | Machine Design


Some descussions:
http://www.cnczone.com/forums/diy-cn...tart_lead.html

[louieatienza]

http://www.cnczone.com/forums/linear..._basics-9.html
http://www.cnczone.com/forums/linear...s_101_a-5.html