New Machine Build- Concept Design- Heavy & Upgradeable

[catmix]

Hello CNCers.

I have been wandering around this site for some time now contemplating what a great and difficult project it would be to design and build my own CNC. I have borrowed many ideas used by others keeping what i believe to be the best for my own design.

I have roughed up a design in Google sketch-up (my new favourite program) of a large CNC mill that i would like to build in the near future. I am seeking comments and advice to improve the design from more experienced mechanical people.

I am a Civil engineer by profession, so i understand mechanics principles but have poor practical knowledge of machine building.

Design
I have designed my model based on the following criteria:
-Machine must be upgradeable (no welding / fixed connections)
-use available sections/materials
-1200 x 900 x ~150mm cutting zone
-High mechanical soundness

The intent is to start the machine with low spec hardware (threaded rods, nuts, skate bearings, drawer slides) and as i go along upgrade componentry gradually to linear rails, acme srews etc.

X Axis
Ive used 'I' Sections for the X-axis purely because they provide good surfaces to restrain a cart using skate bearings. A small angle section is shown which will be used to retain the Energy-chain for the cable management. I am using dual lead screws with dual motors because it seems like a more elegant design. Ive seen a lot of designs with beams or belt systems to distribute the driving force but i dont like the look of them. I know its to reduce costs of an additional motor. I could be convinced either way.

The X-Axis cart is currently channel with skate bearings (no bearings shown).

Y Axis
The Y axis is a rather deep channel section. Ive seen many different designs with RHS, rails, simple narrow beams etc. But i believe the channel is superior because the shear center is closer to the center of the load (spindle) which means the section will distort in a planar manner rather than torsional (which is good i think).

Im using a rail and skate bearing for these (i tried other arrangement but this seemed the best "cheap" option.

Z Axis
The Z assembly is attached to the Y-Axis Cart with drawer slides. I think this is the best for short distance. The screw is offset to keep the vertical projection down. I would prefer if it was in line with the spindle but this seems neater (i havent seen any other machine do it this way).

Please comment on my design if you have useful tips/ encouragment.

Cheers
-Catmix

[ger21]

Start with acme and skip the threaded rod. You'll get far better performance for not much more money.

[pminmo]

Ger21 is right, but I'd go a step further. Skip the cheap acme and spend a few bucks on high pitch acme screws. Here is some info on why http://pminmo.com/PMinMOwiki/index.p...chanical_Power

[cncspear]

Originally Posted by catmix Y Axis The Y axis is a rather deep channel section. Ive seen many different designs with RHS, rails, simple narrow beams etc. But i believe the channel is superior because the shear center is closer to the center of the load (spindle) which means the section will distort in a planar manner rather than torsional (which is good i think).

Yes, but it only applies to vertical inertia and cutting forces. Your cutting forces in the X direction will put a torque into your Y-axis regardless of its cross section. In your case since your channel ends are left open, it won't resist that torque efficiently and the vertical wall of the channel will see bending stresses. At the minimum put a rigid cap on your channel ends. Channels don't do well in torsion, so IMO strictly from a rigidity standpoint, a full rectangular/box section would be better a Y axis.

Also, a possible pitfall with dual screws is synchronization. But I have no solution, just pointing it out.

I don't see a problem with your offsett Z drive. It’s a relatively short offset so racking should not be a problem.

Otherwise, concept looks good.

[catmix]

Good point cncspear.

When i considered the resisting forces i only considered the load of the router itself rather than the cutting forces. I will add the encaps to make that conection more rigid.

I like the channel because i can tuck my cables inside. When i do a few calculations and get an idea of expected deflections under different load conditions I may experiment with an RHS.

[catmix]

pminmo - thanks for the link, thats exactly the kind of info i need to get up to speed.

Are there any books anyone can recommend to get good proctical foundation knowledge in mech engineering?

[ger21]

Lots of good info in these pdf's.
http://pergatory.mit.edu/2.75/2-007_Lectures.htm

[andy_ck87028]

Originally Posted by catmix Hello CNCers. I am a Civil engineer by profession, so i understand mechanics principles but have poor practical knowledge of machine building. Design I have designed my model based on the following criteria: -Machine must be upgradeable (no welding / fixed connections) -use available sections/materials -1200 x 900 x ~150mm cutting zone -High mechanical soundness

I followed similar design principles and leveraged off my background which is IT which has principles leveraged off engineering.

There is a field within IT called object orientation (OO) which formalises modularisation.

During my design / build, I created simple modules that in turn assemble into complex modules. Through discrete sub assemblies, I concentrated on the role that each sub assembly played in the overall schema knowing that I could ultimately replace / upgrade and consequently introduce systematic improvement.

For example, I made the truck with the skate bearings a discrete sub component which is distinctly different from the sub assembly that holds the Y rails and distinctly different from the motion assembly.

When I replace the skate bearings with higher quality componentry, the re-work impact on the machine is minimal.

Each part exists in its own right and can be replaced.

Each part is bolted together into a rigid finished assembly but I did weld the basic components as it adds so much strength.

As each part is removable, I can then utilise the part elsewhere if required.

I found design to be the most fascinating part of the entire project and occupied my mind for many enjoyable days stuck in traffic....lol.

I'm happy to engage in this conversation but it is also your own adventure.

Start with the base principles of engineering and then leverage.

Andy

[catmix]

Hey Guys,

I am trying to get some ideas on wether to build in steel or Aluminium. I have no expirience in machining/using or buying either material.

In Australia i think hot rolled steel section buget cost at approx 3 AUD per kg.
Can anyone confirm this and provide approx. costs for aluminium sections in Australia.

After i get an idea of costs i will look at other pros and cons of each material.

Cheers!

[andy_ck87028]

I went mild steel.

Mass is your friend. Steel is easy to work with.

I used angle grinder, bench press and an old arc welder I picked up for $75.

I've had a ball.

Not familiar with aluminium but discounted it as I wanted a heavy unit and I felt that welding steel was easier than welding aluminium.

Andy