Small Mill very early design stage

[Robin Hewitt]

If you are worried about thermal effects then you will definitely need the consider dynamics as well, beam/column frequency and moving masses etc.

I see the big problem is that the spindle connects to a slide that connects to a slide that connects to a slide that holds the workpiece. All this with a total slop of < 0.001" so there is something left to allow for flex in the system.

I just don't think it's possible using commercial bearings, there simply isn't enough contact area to take the load.

Robin

[leadinglights]

...
If you are worried about thermal effects then you will definitely need the consider dynamics as well, beam/column frequency and moving masses etc. just adding more mass can lead to resonance not to mention problems with accel/decel....

Hi Ross,
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.
Mike

[irving2008]

.... The same mass of aluminium should be more rigid than steel...

about 3 times more rigid... but also 1.7x linear length, i.e a beam 5cm x 5cm steel will need to be 8.5cm x 8.5cm in aluminium for the same mass, or 6.5cm x 6.5cm for the same rigidity, but because it is less dense it will more easily transmit vibrations from motor to workpiece partly destroying the benefits...

[leadinglights]

about 3 times more rigid... but also 1.7x linear length, i.e a beam 5cm x 5cm steel will need to be 8.5cm x 8.5cm in aluminium for the same mass, or 6.5cm x 6.5cm for the same rigidity, but because it is less dense it will more easily transmit vibrations from motor to workpiece partly destroying the benefits...

Hopefully easier to address than many of the other potential problems. Modern motors, whether stepper, servo or BLDC tend to be smooth and powerful and compliance for damping in the mountings of less consequence than in ballscrews, guides etc.

Mike

[Ross77]

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.

[Robin Hewitt]

0.025mm doesn't sound that unachievable.

Set your micrometer to .001", look at it and decide how you're going to divide this tolerance up between the slides, the screws, the spindle runout, the surface finish and flex in the system.

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:

Robin

[John S]

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:

Robin

Robin has nicely quoted an example of what happens in the real world regardless of endless calculations.

[ptjw7uk]

Robin,
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.

Peter

[Robin Hewitt]

Robin, 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.

Hi Peter

The twist is too small, like .005 degrees. Adding extra tension wouldn't help, neither would concrete for that matter but it might make me feel better :heehee:

Robin

[leadinglights]

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.

Mike