Well... I'm at a bit of a crossroad in my design, I have altered the Z axis to the conventional method which should please Jazz! but the machine is now 1004mm wide by 1400mm long with overall cutting area of 620mm x 1100mm and I'm now thinking do I need it that big?
The original idea was to have a machine capable of machining soft metals or even steel if it could cope but I also wanted it to be big enough to machine 600mm wide sheets of MDF so I could machine my arcade cabinets, the problem is I would need a machining area of 1800mm lengthways to produce them in one cut but I am now thinking should I make a smaller machine with say a cutting area of 400mm x 600mm for aluminium and possibly steel (Is steel doable or a definate no go?) and when that is built, use it to make parts for a dedicated mdf/wood router that would take the full length sheets at a later date, This machine could hang of the wall with a slight incline to get over the space issue.
I am also thinking that 1610 ballscrews with 1.8 deg steps will only give a resolution of 0.05mm, If I built it for metals and plastics then 1605 ballscrews could be used bringing the resolution down to 0.025mm.
What are your thoughts on this?
One other thought, If I only have a cutting width of 400mm would I get away with a single ballscrew, if not, what is the maximum width I could get away with or is it just better to go with 2 when machining metals?
Last edited by CharlieRam; 18-08-2014 at 01:53 PM.
For machining metals, esp steel, you need rigidity, so a mill arrangement, table that moves in X & Y and a rigid fixed Z column is the best solution. a rigid router style machine can mill ali if strong enough but its a compromise at best.
To improve resolution for machining metal you need to gear the motors down. a 4:1 reduction improves resolution by 4 and increases torque, necessary to overcome the inertia of a heavy moving table and increased cutting forces.
Single ball screw is out of question if you machine metals on the cnc, meaning aluminum, not steel. many though machine iron from time to time. Steel is out of question if not annealed.
1610 ball screw makes one turn and moves 10mm. so for 1mm it makes 1/10th of turn. To make one turn a 1.8 degree motor- 360/1.8=200steps per turn. When you apply 1/8 micro stepping which is the usual 200x8=1600 steps per turn.Any decent driver will do that. So to move 1mm then 1600/10=160steps per turn. 1mm/160steps=0.00625mm resolution. 1/4 microstepping will give you 0.0125 mm resolution
I would worry more about rails and ball screws parallelism and perpendicularity in all directions, as this is the culprit for the real life imprecision of a machine. Do yourself a favor and equip with precision straight edge long enough that can be laid on both the long axis rails perpendicular, and 2 precision squares, cause without this i dont see how you can even be in the <0.5mm ballpark when you reach the moment to mount everything together.
Your design is a very long constant length lever that never changes no matter what depth we cut.? Your lever length is distance the spindle extends from the spindle mount plus the tool extension. Lets say total extension of 150mm.
Now if you swap the rails around your lever is variable length lever and only ever reachs 150mm when cutting really thin material and at full extension.
Every where in between and the leverage drops and the stiffness increases so there's less deflection at the tool. At full height our extension is pretty much just the spindle nose and the tool length so order of magnitude better than your design.
May seem like a small thing to you but it makes all the difference when things get hard.!! . . . . Your design is ONLY good for wood anything harder will show it's weakness quickly.
Also with your design then using thick steel for the back plate will add very little to the design other than making the Z axis motor work harder lifting more weight. Your gaining very little strength over aluminium because it's supported at the 4 corners and not extending, if it was extending like a lever then yes it would give more strength.
Regards the size and use of machine then I'll repeat what I've said many many times it's always best to build a machine designed to do the job intended and do it the best it can. JACK of ALL TRADES is always a compromise in some department and this compromise increases with size.
Small machines do it best because they are easier to OVER build.!
Good to read that you've switched to the conventional Z axis, although I hope you've done it due to understanding the problem rather than peer pressure.
To to lay it out again, using the calculations for the simplified example previously shown:
Conventional Z axis deflection of between 0.19um - 10um depending upon Z axis extension
Unconventional Z axis deflection of 10um - FIXED (i.e. the worse case conventional at all times)
Your unconventional sketch in post #39 looked like it might get around the 'tuning fork' problem but it would not be very practical as it was drawn at full extension and this meant cutting would be up near the gantry! Your only option for practical work would be to drop the spindle down in the mount to hang down out of the clamp, but I think this would not be especially stiff. Your only other option in the design would be to raise the lowest bearing block to allow the Z axis to drop further but then you are back around the loop of an unconventional Z axis as per the above.
I understand what they are saying, although I don't fully agree because although with the conventional method at miniumum travel it is stronger because the tool tip is about 40mm from the bottom bearing block, at max travel it would be far more flimsy than my design, there must be a 'magic' point on Z where aluminium can be machined ok and if I knew that distance then I could try and work it out in my design.
None of that matters now ( I think!) because I have gone back to the drawing board with a completely different design using a fixed gantry based off a Video Jazz posted in another thread where the Y axis raises and lowers with Z. It may come to nothing and I may change my mind again!
Jazz, I know what you say about jack of all trade machines, it would help if I knew what I intended to use the machine for but I don't! I like working with wood as much as metal, I think now I am leaning more towards metal machining because more practical things can be made but I would also like to have a play with my artistic side with woods and plastics, This was why I was going for a router type build initially but the size required for some of my wood type projects have put a bit of a dampener on that :-( unless I go for a vertical machine like yours.
There is a magic number.? On my machine.!! It's 0-50mm and past this finish deteriates to point that's not acceptable to me, but that doesn't mean my number will match your machines number.!! . . . . This is essentially the problem in that every machine is different and the finish you get will be a direct results of how strong and how good the design along with many other variables. This also holds true for feeds n speeds to some degree.? Each machine will perform different for same material.!
Only few weeks ago there was a Cincinnati CNC VMC on ebay for £1400 24 tool changer and full bag of tricks. At that money even if screws and rails where worn out you could convert control and replace components for same money or not much more than building a DIY machine for cutting steel.!
To give an example I picked this CNC Mill up at scrap value. 1100 x 500 x 500 cutting area. DC servo's, 7.5Kw Cat40 spindle, Was fully working apart from needing PSU for the out dated Hindeman control which at some point around 2060 when get time I'll be replacing with Mach3 and CSlabs Analog controller. DIY isn't always the best option esp when you get into milling steels.!
I see three problems with buying a mill.
1. I couldn't say I made it myself ( big one)
2 It's bloody huge! I would never get it in my back garden and even if I did, I wouldn't have anywhere to put it.
3 Divorce is expensive :-)
#1 It would be an achievement in it's self for someone new to CNC to Retrofit something like this and they could be proud if they did it.!
#2 This should tell you everything you need to know about what's required to cut steel.!! . . . . It's nearly 3000KG and mostly cast iron.
#3 Turned out to be the Best thing that ever happened and worth every penny(And there where many 1000's. . Lol).
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