Thread: DIY mill...
Your points are valid, but I think I'll go without the centre support for now, not least because it severely limits the useful length of the table in the X direction as I will have to make the table longer to allow the maximum travel and the length of the pillow blocks means the overall travel is now reduced, see the pic below and compare to the previous one
This arrived today... 500W, 6mm collet, no measurable runout. 70mm square by 250mm long and a standard 43mm clamp. And it only weighs 1.7Kg... not bad for £40
Last edited by irving2008; 28-07-2008 at 09:31 PM.
Been doing some structural analysis of my proposed gantry frame, planned to be constructed from 50 x 25 x 3mm aluminium rectangular tube.
I found this structural analysis program called 'Framework' downloadable for free from: http://home.wanadoo.nl/wolsink/
It produces load, stress and importantly, displacement analysis for any arbitrary 2D and 3D frameworks.
Here is a screenshot of the calculated displacement for a gantry frame as described with a 10mm thick aluminium spindle support on 16mm round rails - the max displacement under 100N cutting load at the tip of the cutter is 1.01mm in this example (which is clearly too much - and they are exaggerated in the diagram) and most of the displacement is due to bending of the spindle support. I have yet to experiment with making it a U channel instead.
I suspect in reality 100N is far too big a load for a mill doing light cuts on PCBs and Aluminium. Calculations using the torque/HP of the spindle motor and a 4mm 2 flute cutter at 300mm/min feed suggest 20N is more likely giving a 0.2mm displacement which should be easily resolved with a thinner U channel for the spindle support. More work to come tho...
Last edited by irving2008; 08-08-2008 at 08:26 AM. Reason: spelling!
A 500W motor (approx 0.75HP output) at 32000rpm gives .17Nm torque, so the max cutting force before the motor stalls on a 4mm cutter is .17/.002 = 85N, however it probably wouldnt get close to that in reality.
Another way to look at it is the cutting force = chip size x chip removal energy = h * w * E
where E = power * tool efficiency / volume removal rate
Aluminium requires about 0.6W per mm3 so a 300W output motor will only acheive a removal rate of 180mm3/sec
E = 300 * .75/1.8e-7= 16.7e6 and therefore force for a .5mm deep cut on a 4mm endmill is around 16.7e6 *.0005 *.002 = 16.7N
Some more calculations using the 'Framework' program.
Option 1 - 50mmx25mmx2mm aluminium rectangular tube for uprights 300mm high (50mm width in X plane), 1 length across the back at the top (50mm width in Z plane). 2 x 16mm horizontal precision rails centrally mounted at top on uprights vertically spaced 100mm (i.e one at 200mm above table and other 300mm above table).
Option 2 - 100mm x 19mm MDF for uprights, 1 length across back (100mm width in Z plane obviously). Rest as above.
in both cases the spindle mount is modelled by a 300mm long x 90mm x 10mm aluminium plate located centrally in Y plane, and at bottom of Z movement. There is 1.7kg dead load (spindle motor) and 20N force in both X and Y planes on the bottom of spindle mount.
The lattice and load diagrams are shown in the attached picture.
The interesting thing is that the X/Y displacement of the spindle tip under 20N load is, for the Aluminium version, 0.22mm/.003mm and for the MDF version is 0.43mm/.004mm which shows that an MDF version is definitely fine for PCB milling (where the load is much smaller) and can probably do lightweight aluminium work but the aluminium frame is prefered for anything more strenuous. These calculations assume all joints are perfect and non-moving. Another interesting thing is that the MDF version weighs 5.4Kg while the Aluminium one is 3.6kg (both including the 2 x 16mm rails and the 'spindle mount'). Aluminium is definitely the way to go, its considerably lighter and more rigid (but not significantly so - doubling the force doubles the offset but thats mainly in the distortion of the spindle mount and rails rather than the frame).
I am trying to compile GCam for Linux to get everything working on one system.
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