FAQ's (Motion Transmission,Acme, Ball, Allthread Screws, Cogs, Belts, bearing, etc )

[chuckknigh]

Let's start this section off by making a simple FAQ. The basics of motion transmission seem elusive to the new user, and it's so important to building a CNC machine!

So what were our questions, when we started out? These are the ones that I remember...

1) How do I make this thing move?
2) Screw drive
3) Rack and pinion
4) There are different kinds of screw threads? Allthread, ACME, and ballscrews.
5) Motor couplers
6) Backlash
7) How to deal with backlash
8) Bearings, radial, thrust, etc.
9) Resonance and whip

Any other suggestions, guys?

-- Chuck Knight

[ynneb]

That is a good idea Chuck.
Why dont you submit 1 or 2 FAQs now and let others do the same. In time there should be quite a collection.
I guess its a matter of starting the ball rolling by setting an example.

EDIT: I have changed the title from "Lets make a FAQ" to "FAQ's (Motion Transmission,Acme, Ball, Allthread Screws, Cogs, Belts, bearing, etc ) This will change the idea of starting the FAQ's to actually been an FAQ section.

[chuckknigh]

I guess I'll "lead by example," by answering a few of my own questions. Think I'll start with the easy ones, first! ;-)

1) How do I make this thing move?

The answer to this question could fill a book. But a simple answer is also possible.

Take, as an example, a single axis. There is a framework, a table/platform that moves, and a way to make it move. For this discussion, we'll use a screw drive.

The screw is run along the bottom of the table, and through a mating nut. The nut is firmly attached to the table.

By turning the screw, the nut is made to move in a linear manner, and consequently moves the table.

Here is a picture from my own table, showing the nut and its connection to the table.



The screw can be turned by hand, if you wish, or it can be coupled to a motor. In a CNC machine, the motor is controlled by a computer program.

2) Screw drive

This could be combined with the question above, as it is fairly well answered.

3) Rack and pinion

Rack and pinion is another way to make something move in a line. There is a motor connected to a gear, which rides along a flat rod. The flat rod has gear teeth cut into it, which mates with the rotating gear.

When the gear is made to turn, the whole carriage moves along the flat rod. The gear teeth keep it from slipping.

4) There are different kinds of screw threads? Allthread, ACME, and ballscrews.

There are 3 different kinds of threads that you will encounter. Each has its uses, its benefits, and its tradeoffs.

ALLTHREAD is the threaded rod that you will buy at your local hardware store. It has teeth shaped like a "triangle" which are known as triangular threads, or 60 degree threads. They're a standard screw, and are just like the ones you will use to assemble your machine.

Its benefits are numerous, including easy availability and extremely low cost.

Its down sides are equally numerous. To begin with, its consistency is less than desirable, and the positional accuracy you will achieve using this type of screw will be the lowest of the types.

It also has a fairly low transfer efficiency, which means that you'll need a bigger motor to move your machine.

ACME THREAD is the kind of thread you'll usually see on a good quality vise, or some of your woodworking tools. It's a squarish thread, designed specifically for the transfer of power.

Its benefits are also numerous, including relatively low price (not as cheap as allthread, though) and relatively easy availability. You may have to find a specialty store, or use mail order for this type of rod.

Regardless, it is usually made to higher standards than allthread, and its positional accuracy is better. It also is a more efficient way to transfer power, which means that a smaller motor is necessary, to move the same load.

BALLSCREWS are the best and most efficient type of screw drive available.

Its benefits include reduced friction (so little, the nut will "fall" down the screw from gravity, alone) and the highest accuracy. It is a helical ball bearing, and works amazingly well.

However, it has a major tradeoff for many homebuilders. Its cost is the highest of the three, and its availability is comparatively limited. They must be ordered from a specialty company, like www.bsa.com or salvaged from relatively high end equipment.

5) Motor couplers

Motor couplers are important in the construction of a CNC machine. They are what actually make the physical connection between the screw drive, and the motor itself.

There are many options, of varying costs. The most obvious is a rigid coupling, but this requires extreme precision in the alignment and mounting of the motors, and is not easily achieved by the homebuilder.

So, a flexible coupling is desirable. There are off the shelf couplers available, which use a rubber "spider" between two rigid parts, to allow a minimal amount of flex. They do not introduce backlash into the system, and work quite well...they're designed for this purpose.

Another option, in keeping with the homebrew nature of many of these tables, is to simply use a piece of reasonably rigid hose to couple the motor's drive shaft to the screw. The air hose with the spiral cords inside makes quite a good coupler, and is available cheaply at any local hardware store.

6) Backlash

Backlash is a very important consideration in a CNC machine.

Backlash, simply defined, is the sloppiness in the system.

If you go out to your garage, get a nut and a bolt, and screw them loosely together, you can feel the backlash. "Wiggle" the nut, relative to the bolt -- there will be a significant amount of play between them.

In a screw driven machine, this looseness produces sloppiness in the machine.

Practically, when the machine reverses direction, you want it to immediately move in the other direction...measuring backlash is a way to quantify the amount of movement that should happen, vs the amount that actually happens.

7) How to deal with backlash

Though it can be dealt with through software, there is a more effective way to handle it, which physically removes the looseness in the system.

A two part nut, pressing against both sides of the screw thread simultaneously, effectively removes the backlash from a system. The picture above shows one such system, which works quite well, and which is quite simple to build.

8) Bearings, radial, thrust, etc. 9) Resonance and whip

Etc, etc, etc... This is the type of thing I'd like to see take shape -- questions and answers, as a basic primer.

-- Chuck Knight

[High Seas]

Regarding Item
5) Motor couplers
The attacged image shows a bit of the range,
from left to right,
1. mini hoseclamps (jublie-clips) and hose bits;
2. Flexible 3 piece "spider" coupling;
3. semi-rigid yet flexible coupling.

They range from cheapest to most espensive - and in my experience from least useful to most likely to work - all left to right.

The mini hose clamps and hose may work for larger diameter shafting - but not at 1/4 inch! The "spider" won't work well under a load (like a heavy zaxis), but the semi-rigid yet flexible give good preformance on staying connected - and tollerating some minor misalignment!
You get what you pay for - or you buy it 3 times over!!

[chuckknigh]

An excellent point, though performance vs cost is not necessary a linear function. Especially when you factor in eBay.

Now, who wants to post/answer some more questions?

-- Chuck Knight

[Zagroseckt]

Just a queston here.
I herd that some of the more powerfull cnc systems use a fixed laser on each axes to determin it's posision.

What is this called and do you know of any examples i can take a peek at.

[Bent]

Regarding Item 5 Motor couplers

You could use a gearwheel and a belt whith teeth on it. It would take some amount of misalignment. One other advantage is gearing of the motors

Bent

[chuckknigh]

A LASER is a good way to determine position, since it's a simple matter of reflecting the beam, and measuring the time it takes to travel the distance. They have off the shelf LASER rangefinders and tape measures that do just that, and the price isn't too horrible.

I haven't seen one with this feature, but it makes a lot of sense...

As for the gear/belt method for coupling, you still need a way to connect to the screw and the motor. You're right about it handling some misalignment, and with a timing belt it would be guaranteed to have zero backlash. A 1:1 gearing ratio might make a viable motor coupler, and let you "tuck" the motor out of the way, somewhat.

-- Chuck Knight

P.S. I'm delighted to see so many new posts in this thread! Now, who has a "What makes this thing move" kind of question, and maybe an answer? :-)

-- Chuck Knight

[m1911bldr]

I'm building a 3.5 x 4.5 router table using steppers, acme lead screws, Igus rails/carriages, Turbo CNC and the HOBBY CNC board. It seems the best I can do is about 20 IPM with the lead screw arrangement. You guys now how to calculate what gear Diametral Pitch to use if I upgrade to a rack and pinion arrangement? I would like to use something that gives a VERY close inch-type travel per step. I just don't understand Diametral Pitch enough to know where to begin the calculations.

[chuckknigh]

Good question...and one which I can't answer. I'll open it up to the others...any ideas?

I believe DP is essentially nothing more than the "size" of the teeth. (A relationship between the diameter and the number of teeth, I think) Do you have a "Machinist's Handbook?" I know there's a whole chapter in there, relating to gears and DP.

-- Chuck Knight

P.S. What about just changing your lead screws for something a little coarser? I know you can get 5 start ACME thread that has like 2 turns per inch. Eric (Balsaman) mentioned it in one of his numerous posts.

[trubleshtr]

Hi
DP= Number of teeth on the gear divided by the pitch diameter in inches.

PD (pitch diameter) is the diameter of the pitch circle (imagine the half way point of the height of the tooth on the gear when looking at it from the side) it is an imaginary line (circle) all the way around the gear. The Pitch circle (PD) is important to get the correct meshing of drive and driven gears with respect to clearance between the mating teeth, binding will occur otherwise. (the diametral pitch is usually a whole number)
Another thing to consider is pressure angle, but if you buy a rack and pinion together (set) you should have no problems with clearance or pressure angles......
hope this helps and doesn't confuse....

[trubleshtr]

Hi again,
I forgot to add, The DP is sometimes called "circular pitch" which is essentially the distance measured from the center of one tooth to the center of the next tooth (when looking at the gear from the side) The same way you would look at the pitch on a belt or screw, the distance travelled is a function of the "pitch" or distance between the centers of the meshing components. Man, I wish I knew how to quickley draw things on my computer. A picture is worth a thousand words......