[Cheeseduck]

I've finished designing my 38'' by 18" budget CNC in CAD and I'm just about ready to buy the parts and start building it. But before I start, I want to make sure that there aren't any problems with my design.
The majority of the CNC router will be made out of 11/16" plywood (the support beams are scrap) that I have lying around. The leadscrews will be 5/16 allthread, and the whole thing will use the angle-aluminum-bearing technique for linear motion. Cross dowels will hold the whole thing together. A cheap Harbor Freight router ( 2 Horsepower Fixed Base Router ) will act as the spindle.
Am I ready to start building, or did I screw something up?

[OCNC]

It seems like you have enough stiffness where you need it. I would not use the allthread. 5/16-18 will require 1800 rpm to get 100 inches of travel per minute. This is not going to be possible and I think you'll be disappointed if you can't cut at least 100ipm.

Chris

[harryn]

I am a little concerned about the table stiffness. Is it just one layer of the 11/16th ply ?

Consider taking some in your hand and seeing if you are happy with it's stiffness.

[Cheeseduck]

I forgot to mention: Although the pics don't show it, the table is made out of two sheets. The whole table will be 48" long in total.

As for the allthread problem...The only other alternative that I can afford is 3/8 12-ACME. Will that give it a significant speed boost?

[jckstrthmghty]

I'd just save a bit longer for 1/2-10 5 start or something similar. Anything all tread just doesn't cut it on a 48" table.

[OCNC]

As for the allthread problem...The only other alternative that I can afford is 3/8 12-ACME. Will that give it a significant speed boost?

So with 3/8-12 1 start you now would need a stepper rpm of 1200 to get 100 ipm of travel. This is still too high for a stepper motor. The 'wheel house' for steppers is usually below 600 rpm. The 1/2-10 5 start acme (1/2" travel per rev.) brings 100 ipm down to 200 rpm while 1/2-8 2 start acme (1/4" travel per rev.) brings 100 ipm down to 400 rpm . These are much better choices.

[vtx1029]

From the pictures it looks like that front piece of wood across the gantry would interfere with the router going up.

[Phife]

Id be concerned about your table sagging in the middle. especially when the gantrys weight is over the center, even with two sheets bonded together it will still sag. I have 2 sheets of 1" MDF bonded and bolted together and it sagged atleast 1/8 in the center over a period of about 1 month, and was continuing to move.

Try using some bracing like you did on your gantry sides to help support the table bottom.

You will want to put on a spoil board on your table as well, you dont want to accidentally cut the table as it will be a pain to replace as its also part of the linear motion.

Also, I'll have to agree with the others, ditch the all thread and get some proper leadscrews or ballscrew. Really with the price of the ballscrews available from Chai and the sorts, its gotten pretty cheap to use ballscrews.

[Cheeseduck]

From the pictures it looks like that front piece of wood across the gantry would interfere with the router going up.

Sorry if the renderings are a bit fuzzy; the supports actually don't get in the way of anything. I'll also try to add more supports to the table (maybe even another sheet of plywood!?!). The spoil board will be a scrap piece of MDF. As for the allthread issue...at the moment, even cheap chinese leadscrews are out of my range. I'll definitely try to replace them when I get the money, but they'll have to do for now.

[ger21]

If you can swing $60, 7 ft of this should be enough.
1/2 X .250 Right Hand Hi-Lead Lead Screws & Nuts for Power Transmission - Roton Products, Inc.

Get some delrin on Ebay, and make your own nuts with a homemade tap from a scrap piece of screw.
You'll get 5-10x the performance of 5/16 allthread.

[TechGuy5002]

FWIW: It appears you going for barebones materials. Whats worse than spending money on better quality materials is spending money using cheap materials that fail to perform. I hate to see you throw the money you have into a machine that does not work, or fails to perform the tasks you need it to do. If it fails, will be back to zero cash and still end up with out a machine that works.

The only practical advice I can suggest is go for better materials and buy just some of the materials you need to complete a portion of the machine. Build and pay as you go! For instance start with the Z-axis. Its probably going to take you several weeks to build it. which hopefully affords you additional cash to contribute to the budget. In additional if the materials you use fail to measure up to your expectations, you are only out the small cost of materials used to build the Z-axis and not the entire machine.

You make also wish to consider using thinner plywood and laminating peices together to make an equivent or thicker plywood. Typically thick Plywood uses filler between sections of veneered wood to make it thicker. this filler is usually saw dust or other wood material that is held together with a soft binder. I think it will reduce the rigidness of the plywood. I think if you laminated thinner plywood together it would be more rigid. Perhaps if you have some small scraps of plywood you can test this.

I would also consider constructing the machine so that you can upgrade the frame materials at later date, if you think you have more money to put into it over a long period, but not enough to build a new machine from scratch. Perhaps start out will better lead screws and rails which than could be reused with better frame materials at a later time.

If you are unable to afford better materials, What about getting a part time job to bring in additional cash to close the gap? I use to know a guy who was a director at a major financial company that was pulling in big money. He took on a part time job working at UPS during the morning shift loading packages onto local delivery trucks for exercise instead of going to a gym. "Why pay to go to the gym when you can get paid to work out!" was his way. Another friend of mine worked at restaurants during nights and weekends to save extra cash so he could start his own business.

[Cheeseduck]

This whole thread seems to be telling me to switch out the allthread, so I guess I'll be doing that. Will McMaster-Carr work?
Techguy- I can understand why Unfortunately (as a student), a low quality CNC router really is is all I can afford right now. If I build another router when (if?) I do have more spare cash, I'll definitely follow your advice. For now, luck will have to fill in the gaps in quality.

[OCNC]

Is building the cnc related to your endeavors as a student? Do you have a specific use in mind or is it more about the machine build?

Chris

[Cheeseduck]

The machine is just for my own projects. Who knows, though, it might come in handy one day.

[DonFrambach]

This whole thread seems to be telling me to switch out the allthread, so I guess I'll be doing that. Will McMaster-Carr work?

I bought "precision" acme lead screws from McMaster-Carr. Here's a link: McMaster-Carr The single start screws are only a few dollars more than the general purpose screws and "precision" grade screws are available in multiple start versions (for more money, of course). You can buy nuts from CNCRouterParts and dumpsterCNC - anti-backlash solutions for home and industrial linear motion.

[Mauser]

Laminating two pieces of plywood together will not produce nearly as much rigidity as taking that second sheet and cutting it into strips and attaching them at right angles to the bottom of the first. Just like a 2x4" is stronger in the 4" dimension than the 2" dimension.

If you made the bottom of that table look like a set of cubicles, it would be much, much stiffer, and stronger yet if you skinned the bottom with one more sheet.

That's called a Torsion Box. And it's the same principle as the honeycomb in a hollow core door (or the wing of a fighter jet).

[ger21]

Laminating two pieces of plywood together will not produce nearly as much rigidity as taking that second sheet and cutting it into strips and attaching them at right angles to the bottom of the first.

Yes it will. Laminating two sheets is actually in effect a thin torsion box. It will be more rigid then just adding ribs to the bottom. Without adding the bottom skin, the ribs will only add a small amount of stiffness. Of course, using 8" ribs will be stiffer than 1" ribs, but without the bottom skin, it's not worth the effort.

Also, I believe that in the context of the post your referring to, he's talking about making parts for the machine, and not just a table. Adding ribs would probably not be an option.

[Cheeseduck]

A torsion box sounds great. I think that I'll build the CNC router with one sheet of plywood for the table. As soon as it's finished, I'll use it to cut out all the ribs for the torsion box and upgrade the table.

[louieatienza]

You could even just get a few more of the aluminum "L" you're using for the rails, and use them to stiffen up the table.

The other thing is, you might want to beef up the gantry as well, as you are planning to push a 2HP router with it. It may be better to double the legs rather than brace them. Wouldn't be a lot of material to do so.

[Cheeseduck]

I just realized that my previous idea made no sense, so I think I'll just use wood bars to support the table. I do have quite a bit of extra plywood, so doubling wouldn't be a problem.