[louieatienza]

Gerry, thanks for the reply. You have to admit that it is kind of funny for you to tell me not to be so worried about resolution, and then yours is even 10x better.

OCNC, you are right, I need to find someone locally with a cncrouter and buy them some pizza and accessories.

I found an interesting, nearly similar concept on cnczone to what I am thinking about for the motion. Also interestingly, it has the exact problem I was wondering about. Please consider to go to the nearly last post in this thread with a picture of his results:

The system uses a nema 34 motor with module 1.5 rack and pinion. The pinion is directly mounted onto the stepper motor shaft, no reduction. It started out that it did not have enough torque, but this was fixed by increasing the power supply voltage.

Now that this was fixed, he made some diagonal cuts and posted a picture. You can see that rather than a smooth diagonal line, it is made up from small line segments. This is exactly what I was worried about.

http://www.cnczone.com/forums/linear..._no_works.html

The gear reduction for nema 34 motors sold by Ahren is only 2x reduction, so I don't think that this is going to solve this problem, nor would I expect 3x to do so either.

What is not clear, is if the OP was using micro stepping or not, but I think so since he talks about setting his controller to 3200.

This is the kind of thing I am spending time calculating and wondering.

Thanks

Harry

I read in another post somewhere about having the spring tension set too high causing "sawtoothing," I believe it was a plasma table. He got higher speeds, at the cost of the "jaggies." Finding a compromise between spring tension and performance was the key.

If you're really worried about this, you can buy planetary gear reducers, even NOS ones, dirt cheap on eBay and run them with servos. Many of the pictures of commercial machines I've seen with rack and pinion have larger pinions and likely higher gear reduction, and almost all of them were helical rack and pinion and servo-powered.

I use 8 start leadscrews and run them direct on my Y axis and through timing belts on the X, and do not have any problems with stairstepping.

[Galane]

Read this whole thread and have some thoughts on making a portable, fixed gantry machine capable of fully accessing a 4x8 sheet of plywood.

First problem, weight and making sure there's not too much of it. A lot of weight could be saved by designing it to feed the sheet lengthwise VS widthwise. You go from needing a machine around 10 feet long to only needing it around 6 feet long.

Gantry weight VS stiffness VS cost. Instead of using a single tube or tubular style structure, how about a composite design? There's lightweight, thin wall, extruded aluminum square tube which is very straight and best of all CHEAP. Sand the surfaces and epoxy several lengths together into a square bundle. Could omit some in the middle to make a hollow space. 10 tubes arranged in a stack like this: (* = no tube placeholder)

0
000
0*0
000
0

The single upper and lower tubes are the guide rails for roller bearings to run on. Better would be triangular extruded tube for those. Triangle tube is stiffer than square and the number of bearings needed would be less.

The entire gantry could be made from this type of tube, if you have someone weld it together. Think space frame, not solid hunks of wood or metal. Build it like a Moulton bicycle.

Instead of a leadscrew along the gantry, use a stationary cogged belt. Fix the belt at both ends then loop the belt up between a cogged pulley and two smooth idlers. There's many versions of that on Youtube. It makes a lightweight, fast and accurate drive system, also used with roller chain.

A variant on that drive could be used to feed the sheets. Attach the ends of the belt to the center of each edge of the moving table. Simple attachment = a couple of pieces of metal sized to fit the grooves in the belt, placed across the belt and screwed/bolted down. Better would be a piece of metal milled to match the belt grooves. Since it will need outriggers to support the table, put idler pulleys on them for the belts. Should have one or two more rollers on each side, mounted to a framework which can be taken apart and packed into a storage box. Definitely need some type of support to hold outward on the outrigger rollers. On the bottom of the gantry, use the same belt looped among cogged pulley and smooth idlers, with a driveshaft across to the other side. A coupler somewhere in there to enable adjusting for a straight drive would be a good thing.

Of course all that will need a way to easily level it during setup. Folding sawhorses, some weights (hang plastic buckets on them, fill with water, rocks, sand, dirt) for stability, some nuts embedded in the structure and bolts as jackscrews. Stick on a couple of bullseye bubble levels and there ya go.

Finally, the moving table. I was initially thinking this is where the honeycomb aluminum sheet might be useful, but coming up with a use/abuse tolerant method of connecting sections together that's also rigid when assembled... To optimize the design for working with sheet wood, I'd go for an aluminum tube grid, made in sections to enable taking it apart for portability. How about some tongue and groove alignment blocks attached to the underside of the grid? Provides X and Y alignment via the blocks and Z alignment by the projecting bits of the blocks against the underside of the grid tubes. Put some gaps in the outrigger rollers for the blocks to move through.

That's where employing a good aluminum welder and machinist would be best so the components will be flat. You'd want the top and bottom of the long edges to not have anything which would cause a bump or wobble as they pass between the support rollers in the gantry. It will need rollers pressing down to keep the table on track and to ensure it can't move up should you be using something like an up-spiral router bit, an end mill or a large drill bit, all of which tend to pull up on what they're cutting.

To protect the table, pieces of 1/4" hardboard on top of the grid tubes. How to attach it firmly, yet removable and non-damaging to router bits should they come in contact with whatever is used to mount the hardboard? Fiddly little details left up to the guy building the machine. Should also come up with a way to attach larger pieces of hardboard for supporting smaller workpieces. Could also design the grid table to be assembled in a shorter length, with shorter drive belts and the outriggers mounted closer in.

I figure it should be possible to pack the outrigger rollers and supporting frames into a box about the size of the gantry section. The table grid sections would survive bare, might want to design a quick way to detach the table drive belts so they could be stored in the box. Other accessories such as folding sawhorses and weight buckets, pack in the van as needed. if a large and sturdy table is available, put the gantry on it, level it, then setup the outriggers and table.

[harryn]

Thank you for all of the ideas. I appreciate your taking the time to read this thread of struggling ideas and a beginner looking for a solution

Where does one purchase cheap, square, flat, straight Al tube ? Most of the tube I have found was either really flimsy stuff or not straight at all.

I have actually considered the belt drive table approach that you mention and it does have some merit. By the time it is implemented with a strong enough belt to do the job though, it turns out that using a long rack attached to the moving table is more cost effective and stronger. My calculations indicate that for this kind of cnc router use with my "goal" tolerances, the belt needs close to a 10 or 15:1 length to width ratio to work. In both cases, a gear reduction is needed. I still like it though, as it is a very extendable concept.

For now, I have switched over to attempting a somewhat smaller machine with less table movement with the hope that the primary barriers will be cost and time rather than the need to invent too much. These two barriers are enough for me to overcome for now.

[LeeWay]

I seem to happen upon straight tube and extrusion every time I need it. Call me fortunate, but there are ways to design a machine that can regulate the amount of bow. Not so much with a plasma machine that needs open space, but certainly for a router that REQUIRE"S a table. The struts on the table hold the rails true. The key to me getting straight stuff so far has been short lengths. Nothing over 6' long. I did have one 80/20 that was bowed a bit, but was getting mounted to a 2" by 3" by 1/4" angle iron that was straight. I made slot in that and the 80/20 conformed. Some guys are math wizzes, some are electrical geniuses, then some can straighten a bowed beam with design. Good luck with it. I was just stalking the new posts and this one came up.

[rebcabarb]

Good luck with it.