[ma1]

Hello folks,

I want to build a small desktop CNC mainly for PCB prototyping. Now, the biggest thing is "not" to spend a lot of money, since you can get 3 piece two layer 2"x3" PCBs for $50 nowadays. I am hoping to spend less than $75 (not including motors and software). The overall size of the CNC can be ~18"x12"x15". I am not looking for a lot of power, but rather accuracy on the order of 0.01" for small circuit components.

Now, I am brewing a design for a ">90% home depot" based solution for the mechanicals. The design involves drawer slides and allthread screws. I have tested the ball bearing full-extension drawer slides in the store, and they don't have any play in the vertical direction, but have a lot of play out of the plane (which can easily be constrained):

Liberty 22-1/100 in. Full Extension Drawer Slides (2-Pack) - D80622C-ZP-W at The Home Depot


Home depot also sells pure steel ball-bearings under the disguise of door rollers:

Prime-Line Products 1-1/4 in. Steel Ball Bearing Rollers (2-Pack) - D 1502 at The Home Depot


On top of it, melamine coated particle board shelves seemed very flat and straight in the store, and you don't need to paint it:

3/4 In. x 11-3/4 In. x 97 In. White Melamine Shelf - 1605509 at The Home Depot


I hope you are following where I am going with this.

My initial trial with a 1/4 rod and a coupling nut and hardwood ended in the realization that i need a thicker diameter rod, and a different base material to make sure warping is minimum. The first stage whipped slightly as it was moving due to the 10"-cut threaded rod being bent. However, the screw/nut combination was very smooth without any lubrication. Next, I will try 3/8-16 screws.


Now, my question is , what is a realistic precision expectation for home-depot non-acme screws with coupling nuts? Can I achieve 0.01"? If necessary i can put two cascaded plastic nuts to compensate for some backlash. For 3/8-16, and 200 turn motors, the minimum step calculates as 0.00032" movement.


Thanks

[packrat]

The particle board shelving is not very strong and the melamine coating is basically just a sprayed on coating- like a paint. The shelves if stored in a unheated area or a damp area will warp. Is the a kitchen remodeling company in your area? See if they have any Corian sink cut-outs that they would sell you. That would be much stronger/stable.

[atwooddon]

I certainly wish you the best of luck in your quest. If your stated budget is $75 not including motors and software, you may be a bit optimistic. You will need at least 3 sets of drawer glides and that will eat up a significant portion of your budget. You will still need frame material, screws, nuts/bolts and an endless number of trips to Home Depot which will all add up rapidly. I am not trying to discourage you but I think your expectation of a $75 machine and being able to obtain .01" accuracy is a little optimistic.

I would recommend you draw up a set of plans (there are many examples here on CNCZone) for your small machine, roughly inventory the supplies, add up the total and I think it will give you a better idea of the minimum expenditure. Scrounging materials and visiting surplus hardware and building materials stores can help get your total down but there will always be those items you will need to pay retail.

Good luck
Don

[RomanLini]

...

Now, my question is , what is a realistic precision expectation for home-depot non-acme screws with coupling nuts? Can I achieve 0.01"? If necessary i can put two cascaded plastic nuts to compensate for some backlash. For 3/8-16, and 200 turn motors, the minimum step calculates as 0.00032" movement.
...

I think you can get pretty good precision from 3/8 16 threaded rod and plastic (tight) leadnuts.

Most threaded rod these days is made on decent equipment, and the thread pitch remains pretty constant for that whole rod. So you just need to make sure there is no slop in your plastic leadnut and calibrate for the pitch of your threaded rod once you can do some cutting tests.

Also you are best off with solid couplings to the stepper motors on a small slow leadscrew setup like this. That will help with precision too.

[ger21]

I'd use a cheap acme vs all thread.
Enco - Guaranteed Lowest Prices on Machinery, Tools and Shop Supplies

[ma1]

Thanks for the replies guys.

I looked at ACME screws, they are not that expensive (unless it is stainless steel), but the problem is that the nuts are either expensive ($20-30 for precision 3/8-12), or plastic nuts are not readily available. I can't make one myself either because the acme taps are $75. The cheapest option in the acme world looks like:

GP acme 3/8-12, 36" standard steel screw+ brass nut = $10+tax+ship
at Mcmaster

Compared to Home Depot:
Allthread 3/8-16, 36" stainless steel screw + nylon nut = $9.20+tax+pick up


Now, how do these compare in terms of nut durability, backlash/repeatability, screw strength, long term screw durability, etc.?

Remember that my machine is going to be small, intended to cut max 3"x6" circuit boards.


Also, I found this site, which features <$100 cost Mantis-9 , and that is very close to what I want:
Wiki Home - Make Your Bot!

One question is that they are using MDO instead of MDF. I looked it up, and it looks superior to MDF, but supply might be limited. The only MDO that home depot carries is 3/8 thick:
3/8 4x8 4-Ply MDO P1s Plywood - 208553 at The Home Depot

Can i substitute this for MDO/MDF , what do you think:
Trubord 19/32 in. x 4 ft. x 8 ft. OSB Square Edge Sheathing Board - 339696 at The Home Depot

or:
3/4 in. x 4 ft. x 8 ft. C-3 Red Oak Domestic Plywood - 165956 at The Home Depot



One last thing, what is your suggestion for a cheap stepper controller board? I was going to build it myself with Allegro chips, but might be cheaper to buy it.

Thanks

ma1

[Drools]

If I had to do it all over again I would get one of the Microcarve machines I think he is up to the A-4. It is a few hundred bucks sans electronics but they look very well made.

[atwooddon]

I would recommend the HobbyCNC kits for a stepper controller as well as stepper motors. The kits are very easy to build, I have built 3 of the EZ 3 axis kits now and they worked right off the bat, no problems. Support is very good via the Yahoo group. Lots of users.

HobbyCNC | CNC for the desktop machinist and hobbyist.

Don

[ger21]

It's very easy to make a tap for plastic nuts from a scrap piece of acme.

Here's a way that may be even easier.
Making Acetal leadscrew nuts the easy way - The Home Shop Machinist & Machinist's Workshop Magazine's BBS

[mcphill]

No tap needed (from the MicroCarve thread):

Making Acetal leadscrew nuts the easy way - The Home Shop Machinist & Machinist's Workshop Magazine's BBS

[adprinter]

If you are planning on driving a Pen, the all thread will probably be OK. However, if you plan on driving a cutting tool of any kind, you really NEED the deep valley grip an ACME thread provides. Don't get me wrong, you might succeed in building something with the allthread rods. But, after very little use, you will begin to see those threads wearing down, to a point they will eventually strip off completely when they are put under the load of driving a cutting tool. You should research a cheap supplier for ACME lead screws to use in your project.

[Spiv]

Surplus printers- these little monsters are full of motion rails, motors, slides, and belts (if you could get away with belts. That would be a pen only thing I think). I built my first CNC for cutting foam and wax molds for ~$50, $28 of which was a crappy dremel.