1 thou , 0.001 inch, 25 microns for the youngsters, is quite easy over a small distance with a well adjusted machine, but gets progressively harder over long distances. I have aimed at this over the 200mm by 200mm by 100 mm range, quite tight enough - and even if I get close it should be good.
I have rejected converting an old milling machine on a simple basis, space available is of the order of 1000mm by 600mm including access so say 600mm by 600mm.
It was the mass/rigidity that convinced me that thermal probles should be addressed. The same mass of aluminium should be more rigid than steel, but has about twice the thermal expansion and also has much better thermal conductivity - hence easier to control the temp of the whole device.
Hidden agendas are, aluminium is also much easier to work with with the equipment that I have and my workshop temperature varies from freezing (4C) to bloody'ot (33C)
on the C1 ballscrews, I keep looking on ebay.
So has there been some confusion over the level of accuracy then??? :whistling:
0.025mm doesn't sound that unachievable. when I was looking for the xy table for my mill I'm sure I saw some precision tables in that travel range for around the £300-£400. bolt it to tee slot table you've all ready got, add a solid fixed gantry for the z and go from there on a trial and error basis.
I'm sure most of the accuracy you require will be achievable by finding the machines best running conditions for each material type and maybe establish a warm up routine so its not used when really cold......then it wouldnt matter if everthing was tight and binding when cold.
On a good day with an obliging material I can cut better than .001" in the Y, but the X cuts around +0.0015" or worse. All bearings and ballnuts are preloaded, the Gibbs have fine adjustments and pukka slide oil, I even have a pneumatic quill lock to de-slop the Z, BUT, it's a round column mill and even wound right down that 6" diameter, cast iron tube at the back is more willing to twist than it is to bend. It's a kind of torsion bar suspension.
You might think that I could just go around again and whisk off an errant .0015" but it doesn't work like that. Once you get inside the flex parameter it prefers to rub rather than cut. Wind it in until you pass the flex point to get a cut and whammo, instant undersize.
You might think that I could cut the X undersize on the finishing path, but it's erratic. I think I've reached the limit of the machines' capability.
As soon as you skimp on the iron you also get to worry about sympathetic vibration. There's a reason why the troops break step when they cross an iron bridge even if it can carry a much heavier load than them.
Perhaps milling to fine tolerance with less than 2 tons of cast iron to back up the tool is a black art. I could try hanging around midnight crossroads in case Old Nick turns up so I can do a deal for some fern seed but I may just try filling the column with ferro concrete first :whistling:
Is your round column hollow, if it is you could put a screwed rod down the middle and put the column under tension as I'm sure that would help without adding extra weight.
Status of the project.
Having been disabused of my idea that I could get the kind of accuracy that I want in a desktop sized milling machine, I have settled back to trying to get the best I can with my limited resources. Previously I had the new milling machine all planned out down to the level of suppliers and part numbers, but all of my plans now lying in ruins. So far I have come up with three options which are in varying stages of taking shape in the immagination of my Turbocad program.
The first option has reached the stage where I can show you a graphic of the X and Y axes. Construction is intended to be in one inch thick aluminium tooling plate for frame as well as the table. Frame parts held together with 10mm socket screws with 6mm dowels fitted after it has been trued up. Initial trials with a wooden mock up shows that the geometry is stiffer than you would expect. Cost of tooling plate is quite expensive, but not breathtakingly so - the bed at 520mm by 240mm of 1" plate is $108 from an American company (Onlinemetalstore) - I assume not absurdly different in Blighty. I am awaiting with trepidation some quotes for milling the edges - including four accurate datum faces.
Second option is begining to take shape and is more like a normal milling machine with an XY table. The basic idea is to have two large "L" shaped pieces of 7075 aluminium bolted to separators to form a stout "I" beam structure.
Final option is the same as option 2, but is based on a rusty hulk of a milling machine(?) of unknown make. The vertical beds would be milled and hand-scraped flat to fit the guide rails - simlarly the horizontal beds. A new cross table would be built from scratch. So far this is only a picture in my mind.
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