Newbie- Desktop Mill - Questions about Axes

[Reed92]

I have been doing tons of research, but am still confused about what parts go into building an axis, and how they fit together.

At the very least, I would like to know what would be the best setup for a small - Approx. 20x20x6 - mill that will be used mostly for milling PCB's, some wood, and possibly some urethane.
I would prefer to keep my costs to under $100-$150 per axis. That is neglecting the cost of motors, of course (which will probably be Pacific Scientific 144oz. in., NEMA 23, Dual Shaft)

[awerby]

Well, for each axis you need some kind of sliding parts, and something rigid to mount them on. Then you need some driving parts, and a way to hold onto them so they can interface with the motor (there are linear motors that combine these two functions, but not on your budget.)

So you can get round rails, preferably the supported type, with ball bearing sleeves that attach to the table or whatever, or the more deluxe square-section linear rails with ball-bearing trucks. You can mount them to aluminum extrusions, or flat-ground pieces of steel. You can use screws to transmit motion, either ballscrews or acme type, with appropriate nuts. Or you can mount a gear rack and put a pinion on the motor.

PC boards take a lot of precision, and they tend to be pretty small. You might consider making a small mill for doing them, and consider it a learning experience that will help you make a larger router for bigger projects. Or reverse the order, and make a large sloppy mill first, to see where the problems are in attaining the precision you want for the PCB mill, and make that one next.

We'd all prefer to spend less money, rather than more, but unless you get pretty lucky in your scrounging you won't find all these high-precision linear-motion, power transmission and structural components for the amount you want to pay. Figure out what your top priority is - Precision? Size? Cost? - and plan accordingly.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[Reed92]

Thanks,

I got a phone call from the igus guys today, and I'm seriously considering the DryLin W series rails. They come in my size(with a carriage) for about $90/axis. I'll have to purchase a lead screw and have end blocks made.

My real question, I guess, is what happens at the end of the lead screws? I know there is an end block with bearings in it, and a coupler on one end for the motor, but I'm not sure about what else goes there. The best visual I've found is ACME Axis Diagram, but I still find it somewhat confusing; especially since I don't really plan on having to use drill rod at the end.

[vger]

Reed,

On my first 2 machines I used 1/2" 10 TPI Acme rod for lead screws. Precision was good enough on the first machine (7" X 5" X 2" moving table) to do circuit boards and leave traces as narrow as 0.010". You need to ensure that the lead screw is secured so that it will not move in the axial direction. The way I secured the drive end of my screws was done like this...

1. Turn down one end of the acme to 5/16" dia 2 1/2" long.
2. Turn down the same end to 1/4" dia 3/4" long (for coupling to motor)
3. Thread about 3/4" of the 5/16" section for 5/16" 28 TPI nuts
4. Make a bearing plate with insets on both sides for 5/16" ID bearings
5. Assemble onto prepared end, bearing, bearing plate, bearing, 2 nuts (one to lock), and coupler to motor.

It's kind of the "poor mans angular contact bearing" assembly, but it works very well.

Steve

[Reed92]

Originally Posted by vger Reed, On my first 2 machines I used 1/2" 10 TPI Acme rod for lead screws. Precision was good enough on the first machine (7" X 5" X 2" moving table) to do circuit boards and leave traces as narrow as 0.010". You need to ensure that the lead screw is secured so that it will not move in the axial direction. The way I secured the drive end of my screws was done like this... 1. Turn down one end of the acme to 5/16" dia 2 1/2" long. 2. Turn down the same end to 1/4" dia 3/4" long (for coupling to motor) 3. Thread about 3/4" of the 5/16" section for 5/16" 28 TPI nuts 4. Make a bearing plate with insets on both sides for 5/16" ID bearings 5. Assemble onto prepared end, bearing, bearing plate, bearing, 2 nuts (one to lock), and coupler to motor. It's kind of the "poor mans angular contact bearing" assembly, but it works very well. Steve

Thanks Steve!
That simple list clears some things up for me.

[RomanLini]

You can check out my build thread, my machine uses Drylin rails and some simple solid couplings and simple leadscrew endblocks. It works very well and is quite precise;
Diy_hobby_small_plastics_mill_router

As for your first build, especially if making PCBs is important I would recommend just making a smaller machine say 10" x 10" by 1.5" which will be great for PCBs and many small flat sheet jobs (acrylic, urethane etc). But with those dimenstions it will be easy to make rigid and accurate. On a small machine like that you can also use threaded rod for your leadscrews which is cheao and easy and will give you a lot of accuracy from the fine pitch.

When you make a larger machine later as funds allow the small PCB machine should be easy to sell although you will probably want to keep it for PCBs and use your larger machine for wood.

[Reed92]

Originally Posted by RomanLini You can check out my build thread, my machine uses Drylin rails and some simple solid couplings and simple leadscrew endblocks. It works very well and is quite precise; Diy_hobby_small_plastics_mill_router As for your first build, especially if making PCBs is important I would recommend just making a smaller machine say 10" x 10" by 1.5" which will be great for PCBs and many small flat sheet jobs (acrylic, urethane etc). But with those dimenstions it will be easy to make rigid and accurate. On a small machine like that you can also use threaded rod for your leadscrews which is cheao and easy and will give you a lot of accuracy from the fine pitch. When you make a larger machine later as funds allow the small PCB machine should be easy to sell although you will probably want to keep it for PCBs and use your larger machine for wood.

Oh wow, I just realized you don't have a fixed support on the motor end of the lead screw! Is there a reason for this? Do I not need one for a small machine?

I've been getting quotes on parts. I still want to stick close to my original dimensions. I believe I misrepresented the size of my machine, though. The outside of my frame is being restricted to 19"x20" so it fits on my rollie. The actual travel of my XY axes will be around 10"x14"
I'm still going back and forth about the z axis...probably 2"-4".

[RomanLini]

My machine has no axial support for the leadscrew. This is quite ok on a small machine with solid couplings. The stepper motors are rated for about 6kg continuous axial force and higher peak forces, which is plenty for a small machine making light cuts etc.

Using solid couplings makes it simpler and easier to build, and is also more accurate as there is no slop or flex in the couplings. The far end of the leadscrews are just held radially so they don't whip, there is no axial support at the far end either.

You goal of a 20" square machine with 10"x14" work area sounds good. It's a good size that you can make rigid, cheaply and without too many troubles.

The more you reduce Z travel the closer you can put the gantry down over the work, this pays back in rigidity and also in less leverage (and slop) in the Z axis.

Personally I found that 99% of stuff I cut is less than 20mm high with most being flat sheet up to 12mm. People have different needs etc, but I think you are better going for less Z travel and MUCH improved all round performance in rigidity and accuracy.