machine rigidity

[frankv]

As i am sure all of you know a fundamental design aspect of milling machines is structural rigidity.Having three (at least) moving axis and ensuring that the
tool follows the designated path accurately is essential. Indeed all other aspects such as control software , and drives depend on the machine design.
I am inviting those interested to share what information we have on small
(x300mm , y200mm , z50mm as guide to size) machines capable of working steels up to say 316l stainless.

[murphy625]

I wouldnt think it matters much on size.. If your trying to mill 316L stainless, you better not skip on quality.
Why 316L ??? (From what I know, 316L is used mostly for welding because the "L" stands for "Low Carbon" and prevents the weld heat from making the carbon migrate to the weld area which would make it rust)
I think normal 316 / 308 / 304 etc are much easier to machine than thier L counterparts.
One thing for sure, your not going to be able to take cheap shortcuts when you build this thing.

Just my .02 hope this helps.

Murphy

[frankv]

The machine is up and runing it will eat soft materials but have now remachined the head to closer tollerances as i was getting some judder on mild steel. As to why 316L it has a slightly better heat resistance than the 316 and i use this material for turbine nozzle guide vane and turbine disc.
They do not last as nemonic steels but at least these i can machine.
The guides of the machine are BO1 tool steel and the head moves on phosphor bronze bushes machined to as close tollerance as possible.There is no need for any high positional speed so these bushes work fine.All moving parts are machined to a slightly tight fit and honed to size. For the stainless steels i use tungsten carbide cutters.Do you know of anyone who is in to
building such metal eaters?

Thanks for your reply

[Mcgyver]

frankv, sounds like an interesting machine - how about some pictures? I am interested in homebrew ferrous cutting cnc mills, but have not commenced contstruction.

[frankv]

Will be posting a cou[le of pics soon . I had to dismantle the head assembly as i had some vibration. I remachined some parts and only today have got it back together.
Tests on mid steel with an 8mm endmil cuting to a depth of 6mm have produced good results.I have a couple of jobs in mind which should show the true capbility of the machine and will be keeping you posted with developments.
From what i read on this site i think there is too much emphasis on drivers and software . To me having a good robust machine is fundamental to achieving accuracy
and repeatability. Most times the control system is capable of much greater accuracy than required. It is the machine that due to vibration and deflection gets you out of tollerance and this is more so on milling machines.
I would apreciate some of your views on the subject

[Mcgyver]

I don't know that I’d put it as being too much emphasis on control etc, I'm a relative newcomer and think people contribute where & how they can. For me, just starting on the electronics learning curve I'm glad there's that expertise. I do agree that there could be more superstructure stuff, and I don't think you'll any objections to you or others contributing it.

I agree that vibration/deflection will affect accuracy, but it’s relative. For example, a very well made machine used to produce wax or foam pieces obviously wouldn't perform cutting stainless. Robustness, vibration, deflection, accuracy etc is equally important to that owner/design, however the design parameters are not going to what we'd want for ferrous.

imo homebrew cnc interest is more weighted towards machines for wood or light alloy work. I think the reason is that the 2800lb+ cast iron machine, that 100 years of machine tool evolution tells us is required for serious ferrous work, is awfully bloody difficult to make at home!

My superstructure idea was a welded fabrication of quite heavy structural steel, normalized and then fill with epoxy/crushed crushed granite - total weight 1000-1500lbs. There's a thread here on it and lots of members helped refine the idea.

I look forward to learning about your ideas

[frankv]

Exactly why we are all here,to share and learn.I have built and am continually refining a small milling machine x/300,y/200,z/50 capable of machining stainless steel.Mass is one way of obtaining rigidity and reduce deflection but
i am more interested in overcoming these problems thru structural design.If there is anyone who is interested i would be glad to share the little i have learnt.My machine is a moving head type. X axis rails are 32mm tool steel
bars and y axis rails are 20mm same material.Solid steel blocks 40mm thick support the head.I will be posting recent pics soon.In the mean time i have some pics of the first design. The machine was pneumatically assisted on all axis. All pneumatics have been removed and the head is now driven by a 300W 3 phase motor under control of an inverter with infinate speed control and slip compensation.

[georgebarr]

Has anyone considered building the CNC structure on a CAD program and then using Finite Element Analysis software to measure the structural strength and perform vibration analysis to find faults? Finding faults and then redesigning to solve problems can all be done on CAD and other software. I will try to post a picture of my design that I intend to use to cut mild steel.

[coolman]

please erase this message

[frankv]

Originally Posted by georgebarr Has anyone considered building the CNC structure on a CAD program and then using Finite Element Analysis software to measure the structural strength and perform vibration analysis to find faults? Finding faults and then redesigning to solve problems can all be done on CAD and other software. I will try to post a picture of my design that I intend to use to cut mild steel.

Would be interested in any results

[Mcgyver]

George, I think that is absolutely what is needed and what I aspire to: an engineered approach based on calculated forces and performance objectives vs. anecdotal. Not being an engineer, my default has been the anecdotal. With some of the incredible software available maybe it’s within a non-engineer’s grasp.

I‘d be interested in your progress and any pointers you can contribute