Newbie- Some simple questions that I know nothing about.

[ThePerceptor]

Hey folks! My name is Sam and this is my first post. I decided to join here to ask some questions that I knew nothing about and was wondering if you could help me understand a few things to do with making a CNC machine.

Okay, here it goes.

With the threaded rod that's used to move the axis things, is special threaded rod needed with the thread being at a certain angle and a certain distance apart? Or can I use just ordinary galvanised threaded rod for a hardware store and then input it's specs into the CNC software?

Also, when machining whole 3D object by making it in 2 halves, what stops the object that is being machined from sliding and moving around the table? I thought it might have been some sort of vacuum underneath the table that holds it in place. I honestly have no idea.

I appreciate your time and efforts in answering these questions. I know i'm a noob and appreciate your patience.

Also, if this is in the wrong place I apologise, I am used to very different forums like on the 405th.

Thanks a ton guys, Sam

[randyf1965]

Normal threaded rod can be used won't be the fastest moving but will work as a proof of concept. You have to calculate the steps per inch based on the rod you use

Securing the material being cut I have used double stick tape, screws and homemade clamps. A vacuum table would be nice but for now I am CHEAP.....

[microcarve]

Hi Sam,

I shouldn't do it, but I'm gonna stick my neck out here....

I'm probably the only guy here who'll say something like this.
But. I'm beginning to think there are some considerable benefits to
plain old threaded rod as leadcsrews. Here's a few reasons why...

The threaded rod you get from the hardware store that's galvanized
may be dipped or electroplated. In either case, it will need to have
a die run down it's length to take off burrs from too thick of a zinc
coating or simply to correct dinged up threads from rough handling...
...as they're not considered any sort of precision component and they
get handled with basically no care at all.

The die IS much closer to a precision tool. It's also cheap, easy to get,
and will make the threads very uniform on a fairly good piece of threaded
rod with a few passes.

Acme threads are rolled on acme that's affordable enough. I have ordered,
machined, filed, sanded, straightened, and generally fixed literally thousands
of them in the last several years...as a hobbyist....(I'm not a pro with $$$
machines and tools).

I have often wondered how I can pay premium prices for something that can vary
quite a bit from one shipment to the next. Yes they're all acme in the size
I order, but threads will be too thick in one place, too narrow, the O.D. too
much or too little. A very few thousandths of an inch matters whether
they fit in the bearings that hold them in place.

It normally goes all well and fine, but sometimes nuts have to have extra
time to make them as backlash free as possible. Many get wasted sometimes.
The rolling process that creates the threads is apparently a little different
on whichever machine produces them......and that's actually to be expected
with anything mass produced when you think about it.

IMO...the threads would have to be ground on acme to make them as "precision"
as it's often thought they are. Then they get too expensive.

Acme (rolled) is generally good at .006- .009" error per foot. Again, the less
expensive kind many people do use. Leadscrew error.

So, It's not going to work any better on a small to medium sized, home built
machine that a good clean slightly polished piece of common threaded rod.

From several dozen practice nuts made for threaded rod here....by a common
and inexpensive tap, so far every one of them are more backlash free than
what may come off my $80 acme tap that may be a little different from
the Acme threads on my leadscrews. Sometimes it take 2-3 passes with the
acme tap. Sometimes the 1 extra pass makes the nut too loose.


Then....if you need to make several acme nuts, you need to make your own
tap if it's not one of the very few readily available sizes. If it is....a tap can
cost well over $400. They often are specially ordered from Bosnia.

It's not hard to make your own tap, but it'll take some time unless you have some
good tools.....like a lathe at least.

Or, you can buy nuts. They can be $$$ or cheap depending on where they
come from and the size.

With a common tap and a $12 stick of 3/4" delrin, you can make all the nuts
you want whenever you want. And on a reasonably well built machine the
main thing that may need replacing most often may the the plastic nuts.

All the screw does is drive the axis. The software will account for the screw
pitch and make it work correctly. It doesn't matter if it's acme or threaded
rod. You have to take a little time with the threaded rod to clean it and
give it a light polishing, but it will work as well....if not better...than acme.

Of course there's no fancy multi start screws in threaded rod readily available,
but those have even more variations in them from one maker to the next.
There are no standards for multi start screws. Each maker makes them to their
own specs. They also have specially made taps for those threads.
So your "single start" screw won't give blazing rapids. It will have
a better mechanical advantage.

Making leadscrews on this hobby level mostly goes well. But sometimes it's
not at all obvious there's the slightest variation in a shipment of screws
until they're cut and being fitted. Then it takes a lot of extra time to get them
working as they need to be.

A nicely cleaned piece of readily available...(but decent quality)..threaded rod
will be as relaible and problem free as any rolled acme.

That all assumes a small-medium home built design. I'd consider the acme on
an axis that's more than 3 feet or so. But I wouldn't hesitate to try the threaded
rod.

BTW...I would use Delrin for nuts. I have had HDPE fail a few times. Friction
from heat can change the properties of lower cost, cheaply made HDPE....though it
should work fine for a fairly long time.


John

[foobillious]

As an alternative to buying a tap, perhaps heat forming the nut would be workable, especially for a beginner with minimal tooling?

Making Acetal leadscrew nuts the easy way

I'd imagine the process would work fine for threaded rod too.

*warning: i'm a newb and don't know what i'm talking about*

[microcarve]

Originally Posted by foobillious As an alternative to buying a tap, perhaps heat forming the nut would be workable, especially for a beginner with minimal tooling? Making Acetal leadscrew nuts the easy way I'd imagine the process would work fine for threaded rod too. *warning: i'm a newb and don't know what i'm talking about*

It probably would, but a common tap is cheap. And I'm just experimenting
with taking threaded rod serious myself, but so far, I can't find a thing
wrong with the idea. A dozen or so attempts work like a charm every
time.

These are brass. I tried that because --mainly for looks....but because
of the ease it would/should/could provide getting the threads all
uniform and polished.

Turns out, I could've just as easily used much cheaper steel for the
same results. And I have....

My purposes are getting as precise of work as I can on my small
machines and threaded rod seems to be great for it.

So Far....


John

[ThePerceptor]

Wow, thanks heaps for all your advice guys.

The main reason I wanted to use just ordinary threaded rod is because I work at a hardware store and my Grandfather owns a hardware store specialising in nuts and bolts. So I thought it would be the most convenient (and cheapest) solution.

And as for accuracy, I don't think that it matters too much if it is 0.01 of an inch out. The whole project is pretty much to help me make props for my drama class at college. So if you guys think that ordinary threaded rod will do the trick then threaded rod it is. And if it all goes to plan I will probably end up using the machine to help build a machine for personal use.

I really appreciate your help guys, and I hope that after I learn more and start to plan I can help noobs like me out in the future.

Sam

[microcarve]

Just inspect your threads carefully. Some are so crappy they have
slag & actual bubbles in them.

Use the die. Then a light sanding to verrrrrry slightly round off the
...now sharp from using the die....top of the threads with 600 grit
sandpaper. In a lathe if you can....being careful not to let the thing
whip and kill someone.

Some polishing compound will make the thing look like it cost $$$ and
it'll glide smooth as satin.

A little work, but it's not at all unusual to have to do the same with
more expensive fancy screws if you have to make a lot of them....


John

[ThePerceptor]

Oh, I hate to double post, but I need to ask a few more questions.

I was thinking of using this machine to carve both wood and foam.

1st Question: How fast will the spindle need to spin in order to carve wood.

2nd Question: For the foam, do most people buy foam blocks or 2 part mixed foam? I live in Australia in Canberra and I have no idea where to buy high density foam blocks from. But I found this stuff on the internet:

RFOAM100 Rigid Foam 2kg [RFOAM100] - $52.50 : Adelaide Moulding & Casting Supplies

Will it be alright to make this stuff in a box, carve it with the CNC, then make a mould of it? I already know how to make and reproduce parts from moulds (art in high school) i'm just not sure if there will be open cavities in the foam making it hard to carve.


Thans again guys, you've been a lot of help

Sam

[ThePerceptor]

Thanks for that man, I'll post some pictures here when I am done. Sounds like it will work well

Sam

[microcarve]

Most anything from a cheap dremel to a 3Hp router would do wood.
The Lighter the tool, the more passes you could need.

I don't know that particular foam, but it should be fine to cast a block
and machine it.

I'd probably go with something a little better than a dremel for the wood,
though. May as well for what it costs.


John

[Fastest1]

Sherline has been using regular threaded rod for years and getting great results. Sure a ballscrew might be better but for most it works great. I too think that a heat formed nut would be great.

[allrounder]

I don't see where anyone has suggested anti-backlash except by good luck or heating plastic nuts. There are many sounder methods of removing slop in the threads. One is to use one nut behind the other and devising some means of forcing them apart or together until between them there is no play. Imagine if you cut your delrin internally threaded cylinders in the photo in half so that there were TWO complete nuts, each being half the length. Then if you screw them onto the rod until they meet, then screw them apart about one turn, and then mount them or fasten them together in some manner, so that they are forced together slightly by the mounting. There will be NO backlash. And it can be adjusted to maintain the no slack situation as things wear. There are DOZENS of ways of doing this. You can have two threaded strips for the nut and have a pair of nuts and bolts keeping the two strips together. Brass is good if you must use threaded rod. If the thread on the rod is rough it will wear the nuts too quickly. But you can clean it up first. I worry a little, though just how constant the pitch would be as you go along the rod. That could be important. You can achieve real el cheapo antibacklash just by putting a coil spring between the slack bits. (Between table and Frame) Not recommended though.