I have been thinking of making a cnc wood router for some time now. As I am a college student, I don't have much money therefore I'm looking at cheap ays of building one. I had a thought not too long ago on how to make one really cheap and very accurate. On a standard cnc machine, the axi operate on twin slides and a ball screw to drive it. the problem here is that these polished slides are very expensive along with the pillow blocks. The ball screws are also very expensive. This is what I thought. If one were to make a machine that operated on a rack and pinion. A person could extend the machine if needed simply by adding more racks and the only things to be bought would be the gear and the track, which, arent' that expensive. This also would solve the problem of axis skewing, making the machine more accurate. Tell me what you think...

All you've done is replaced the ballscrew with two sets of rack and pinion. Which still leaves your statement:

The problem here is that these polished slides are very expensive along with the pillow blocks.

You still need to use these parts, or buy/build something cheaper.
To extend the machine you still need to add the rails/carriages or ground rod. Whatever you use to guide the linear motion.

The rack and pinion is just part of the drive train. Same as a ballscrew, timing belt or threaded rod.

I don't see that using rack and pinion will save you a lot of money.

Regards Terry.....

I'm not sure if Terry completely understood your intent, but he's right. A lot of people use rack and pinion instead of leadscrews, but they can't replace the bearings/ guides/rails...

I think you missunderstood me. Instead of the the device operating by sliding on the bars, it will travel on the rails like a train. It would sit on 4 gears, one set will be connected to the one on the other side by a rod. A motor then drives the rod, moving the machine.

Yeah. I guess I'm missing something. Rack and pinion is good for linear motion, but what is keeping it in a straight line as it's moving at 100 IPM with the attached Y axis simultaneously throwing a cutting spindle back and forth. Also, what stops the machine lifting off the rack as you plunge a 1/2" cutter into your material.

I'm not trying to run your idea down. I'm merely pointing out the fact that rack and pinion drive will still need some kind of linear guide.

If I've missed something, then perhaps a picture or drawing of your idea would be of benefit.

I, like many others on here, have been looking for a cheap linear guide system. So far the pipe and skate bearing combo looks about the most reasonable "cheap" system. I'm working on a design, based along the lines of the pipe and skate bearing combo. Still a little way to go with it, but it's looking good so far.

Regards Terry.....

I, like many others on here, have been looking for a cheap linear guide system. So far the pipe and skate bearing combo looks about the most reasonable "cheap" system. I'm working on a design, based along the lines of the pipe and skate bearing combo. Still a little way to go with it, but it's looking good so far.

Regards Terry.....

The skate bearing idea can be quite cheap if you watch E-bay too.. i just bought 100 bearings for about $28 w/ shipping. Not bad, as i will give me plenty to play with for possible future designs as well as robotics usage.

Now I just need to locate a source for the pipe, and check prices on it..

the MDF looks like it will be about $22 for a 4X8 sheet.

Looking for info on drilling & tapping holes in MDF though, if anyone can help. I will also post this question as its own topic.

Eddie

Good point with the lifting up. I would have to find some way to anchor the slides down.

The skate bearing idea can be quite cheap if you watch E-bay too.. i just bought 100 bearings for about $28 w/ shipping.

And, even cheaper if you hit the Goodwill stores in your area. I routinely pay between $.50 - $1 per pair for inline skates. 1 pair yields 16 bearings...2 per wheel. Just look for practically new ones...often someone's kid gets a pair, falls down, and never skates again. Or my personal favorite, someone overweight with a New Year's resolution...almost never used. (I can say this, because I've done it myself)

The bearings are easy to remove from the wheels, and the wheels make good bearing mounts.

Now I just need to locate a source for the pipe, and check prices on it..

Home Depot. Check the "scraps" next-to or underneath the pipe threading machine. They often just throw them out, or will price them so cheap that you can't refuse. Find someone who isn't afraid to get creative (a "closet" engineer) and chat them up...once you find a helpful person, tell them about your project, and they'll often find parts for you at minimal or no cost.

Lowes will behave similarly, but has certain company policies that don't allow quite as good a deal to be brokered, in my experience.

EXAMPLE: I paid $2 for a full sheet of MDF that had been improperly cut into 3 appropriately sized strips. 3 pieces of 16" x 96" x 3/4" MDF was quite useful to me.

Looking for info on drilling & tapping holes in MDF though, if anyone can help. I will also post this question as its own topic.

Tapping MDF isn't a problem.

I know it sounds too simple, but just drill the appropriately sized hole, and tap it with a standard tap. The granular structure will crumble a little bit, but the threads will be just fine. A drop or two of cyanoacrylite glue (super glue) will penetrate into the threads and toughen them up, though it's not strictly necessary.

-- Chuck Knight

Tapping MDF isn't a problem.

I know it sounds too simple, but just drill the appropriately sized hole, and tap it with a standard tap. The granular structure will crumble a little bit, but the threads will be just fine. A drop or two of cyanoacrylite glue (super glue) will penetrate into the threads and toughen them up, though it's not strictly necessary.

-- Chuck Knight

I have seen this remark about the cya (Super Glue) before.. do you just drop it into the hole, or try to let it run down the inside edges of he hole, or do you fill the hole, and re-drill it and tap it afterward?
I know that sounds dumb, but want to make sure what i am in for here :)

Any idea how many sheets of MDF I am looking at.. I am thinking the basics can be done with a single 4X8 sheet, but I may be wrong *Grin*.. I usually am.. (chair)

Eddie

I think you missunderstood me. Instead of the the device operating by sliding on the bars, it will travel on the rails like a train. It would sit on 4 gears, one set will be connected to the one on the other side by a rod. A motor then drives the rod, moving the machine.

What you would get is gone be more like a car driving over the tressels.
Bump, bump, bump kind of motion.
Gears do not make good wheels.

The skate bearing idea can be quite cheap if you watch E-bay too.. i just bought 100 bearings for about $28 w/ shipping. Not bad, as i will give me plenty to play with for possible future designs as well as robotics usage.

Now I just need to locate a source for the pipe, and check prices on it..

the MDF looks like it will be about $22 for a 4X8 sheet.

Looking for info on drilling & tapping holes in MDF though, if anyone can help. I will also post this question as its own topic.

Eddie

Be careful with larger taps when backing them out as they can cause splits in the MDF. I usually mount a clamp accross the mdf to keep it from expanding. Also with the small taps ( 10-32 and smaller), you can drill the holes and then put the tap in the drill, makes a much faster job than hand twisting the tap in.

If it helps, I've come up with what I think is the cheapest way of building CNC router rails without sacrificing robustness, durability, precision. I have gone down the skate bearing path but instead of a circular rod, I am using a square section beam of dimensions 50mm high, 25 mm wide and 2.5mm thick walls.

The beams are my guide rails with the same assembly for x and y except x is anchored to the base. Each assembly (a pair for x and a pair for y) has 2 skate bearings (A) sitting on top of the beam taking the weight; 1 skate bearing under the beam (B) stopping lift caused by using heavy duty cutters and 2 skate bearings on one side only (C) stopping lateral drift and skew. (5 per assembly totalling 20 all together)

The beam is cheap and easy to mount. If I have a problem with precision, I'll get the beams milled.

I am using threaded rod - very, very cheap and fixed at both ends with a drive belt pulley arrangement to achieve movement (rods stationary with drive mechanism on the assembly). I thought long and hard about using a flattened drive belt as a form of rack and pinion but I lost too much on the gearing and precision benefits of threaded rod.

As the drive assembly is to be timing belts and pulleys, the only backlash point will be on the threaded rod which I can overcome with a simple backlash eliminator.

I have attached a drawing of the guide rail assembly. I hope people can understand it. Note that it only works for x and y as it needs an opposing pair to make the inner skate bearing work. My z axis is still waiting further thought.

I reckon guide rails, threaded rod and assembly (less drive) including skate bearings will cost under $200 and only requires a benchpress, an angle grinder and spanners. I have yet to guestimate the drive assembly but I'll be delighted if I can get that under $200.


Hope the attachment works ... first time!!

Cheers

Andy

Terry,

Got any pics?

Rance


<snip>...I'm working on a design, based along the lines of the pipe and skate bearing combo. Still a little way to go with it, but it's looking good so far.

Regards Terry.....

Getting Back to the original question:

BassBuilder:

Actually, I think you have a good idea to develop. However, it may be beyond the ability of the do it yourself comunity. What some of the guys are saying is that the propulsion components (like rack and pinion) don't guide very well; they just propel. And the guide components (like linear rails) don't propel; they just guide. What needs to be done is to find a way for one component to perform both functions.

One idea would be to have gear teeth ground into the tops of the linear guide rails. It always seemed to me that it would be quite logical for the linear guide manufacturors like SKF to add a rack to the tops of their guides and sell matching gears to go with them.

Another idea would be to have a ball screw peform both functions. For example; Imagine a machine that has an axis that rides on ground round bars like thompson shafts, and a ball screw that provides the motion. Imagine merging both functions into one component. If the ball screw were ground on a large diameter pipe and the surface between the grooves were ground smooth, then the ball nuts could have guide bushings incorporated into them. The whole axis could actully ride on the ball screws. Of course there would have to be two of them, and the diameter would have to be large since it would be unsupported (hence the screw ground on the large diameter pipe).

Both of these are solutions that would be so expensive they seem they would be reserved for manufacturors like SKF and Thompson.

Lastly, there is a cheaper and simpler method that has allready been done. Look at the quill on a drill press. It has a cylinder that houses the spindle. This cylinder slides up and down in the housing to provide the guidence, and it has teeth ground right into the back of it to provide the motion. I used to have a PowerMatic table saw that had a chrome pipe running across the front about six feet long. This pipe served as a guide for the fence to move back and forth. The bottom side of this pipe had a flat ground into it with teeth like a gear rack. These teeth meshed with a small pinion in the fence to provide motion. Hence the same component provided the guidence and the motion.

Of course, the manufacturors are working toward having linear rails that have electrical coils built into them with magnets on the carriages - kind of a linear stepper motor.

Maybe these can inspire some DYI ingenuity.

I am using threaded rod - very, very cheap and fixed at both ends with a drive belt pulley arrangement to achieve movement (rods stationary with drive mechanism on the assembly).

Andy


Andy-

What does your drive mechanism look like?

Chris

Hi

I've drawn a couple of very rough and ready pictures and uploaded via my new toy (aren't digital cameras fantastic)

Belt tensioning should not be a problem and the use of a timing belt pulley on a fixed threaded rod should minimise whipping. Assume an assembly that attaches the timing belt pulley to the carriage inclusive of a backlash eliminator.

Feel free to raise questions.

Cheers

Andy