Opinions wanted: Granite milling machine configuration.

[Swiss]

Hi all,

I would be interested to hear any opinions about a 4-axis milling machine I am designing.

These are just some preliminary sketches. The dark grey parts are solid granite, screwed together. The light blue bits are the rails. The base is 600mm x 600mm (2' x 2').

I guess one thing I'm worried about is stiffness. Ideally, I'd like the spindle to be nicely positioned directly between the rails, rather than overhanging as it is on the images. But I can't think of a sensible way to do that.

The linear motors and the 4th axis are not shown.

Hugo

[WilliamD]

Looks like an interesting design. I've never seen something like the z axis you have setup. Only thing I could point out was how to you plan on lifting/moving the z axis? Some sort of mechnism on each side?

[Swiss]

Well, unfortunately I haven't CADed up all of the parts, but the idea is to have a pair of linear motors on each axis, separated as widely as possible. So the Z-axis would have one motor at each side.

Ideally, each axis would have a pair of rails, and a pair of motors, with the axis inbetween. I think this would make the machine quite stable. I have managed to do this with the Y and Z axes, but I can't work out how to do it with all three axes.

There would also be a pair of long springs, extending some way above the machine, to support the Z-axis when the power goes down.

Hugo

[Swiss]

OK, I've added the linear motors.

Hugo

[NEATman]

Hugo-
I think you're worrying a little too much about the stiffness - most THK style rails are very stiff and have very high load ratings. It would probbably be best if you have the spindle out front, so you can access it easily. Designing in two linear motors per axis will work, but where do you plan on mounting the linear encoders? The best place to locate them is closest to the work piece. This way, any abbe offset or thermal movement of the machine will have the least impact on the accuracy of the part, as the linear motors will hold position based on the encoder position. Another trick is if you mount your encoders on something that is thermally stable (like low expansion glass) then as the machine "heats up" the accuracy won't change, as the encoders will stay the same dimension. Also, with two motors, you will want to make sure that the amplifier is capable of driving two coils if you wire them together. You will need to make sure that the magnet tracks and coils are fairly closely aligned so that the magnetic phasing is close, or one coil will be fighting the other.

On the air bearing gantries I've designed, we took great pains to position the linear motor coil at the CG of the moving mass for each axis. This was done to minimize the torque moment created when accelerating or decelerating the different axes. Also of concern is the magnet tracks - these tracks typically have at least grade 42 NeFeB (rare earth) magnets and can be somewhat dangerous if you are working around the machine with steel tools. I would reccomend "way covers" of some type, especially if you intend on cutting ferrous materials!

You will also need some sort of solid hardstops for each axis, as if the motors run-away, they are an impressive slide-hammer! Keep an inch or two between the encoders and the magnet tracks, as if you are planning on renishaw encoders, the magnet tracks can sometimes trip the limit switches or the reference marks.

Keith
NEATman

[Swiss]

I think you're worrying a little too much about the stiffness

In what way? Do I have too many rails, too many carriages, rails too far apart? I am more concerned about the stiffness of the granite structure. The reason I was thinking about having the spindle facing away was to let me have bigger granite side pieces. I do worry about those parallelograming because of high accelerations of the X axis. Although, come to think of it, the X axis is quite low mass.

where do you plan on mounting the linear encoders?

That I am still not sure about. The X and Y axes are no problem I think, but for the Z axis, the easiest thing would be to mount it next to one of the motors. Assuming the X axis can't rotate about the Y axis much (because the rails are stiff enough), this shouldn't be a problem.

we took great pains to position the linear motor coil at the CG of the moving mass for each axis.

That's exactly what I'm trying to do by using pairs of motors. So that the centre of the force is at the centre of the gantry. I think that the Y and Z axes are well balanced, but I can't work out how to get the X axis balanced in the same way, without using a crazy number of rails.

I would reccomend "way covers" of some type, especially if you intend on cutting ferrous materials!

Yes, definitely. I had a really nice idea for the Y axis covers, to completely prevent any swarf getting to the motors, rails and encoders. I'll see if I can work out something similar for the other axes.

You will also need some sort of solid hardstops for each axis, as if the motors run-away, they are an impressive slide-hammer!

That's something that really scares me. I can just imagine the machine smashing itself to pieces the first time I turn it on. I'm going to have to carefully calculate the minimum stopping distances it can cope with, and put in some buffers and cut-out switches.

Keep an inch or two between the encoders and the magnet tracks, as if you are planning on renishaw encoders, the magnet tracks can sometimes trip the limit switches or the reference marks.

That's something I was wondering about. Would it be better to use magnetic or optical encoders? The magnetic ones are less prone to dust, but might be thrown by the powerful magnets.

Hugo

[NEATman]

Hugo-
Regarding the stiffness, two rails with two bearings per rails should be sufficient. The side granite pieces are reinforced against parallelogramming due to the horiziontal piece tying them together. It acts like a gusset plate.

About mounting the encoders, the closer to the work piece the better. For a reference of what Abbe offset error is, this is a great resource.
http://www.danaherprecision.net/MotionHandbookPage.aspx
Also read the slow down to speed up section.

I like the dual motor idea - just be careful, as the majority of the cost of linear motors is the magnet tracks.

As far as crashing the machine, I duggest when you first start it up put some shipping foam at each end of the axis that you are working on - once you get it tuned and are used to running the machine, standard rubber bumpers or shock absorbers would be fine.

Optical encoders have a much finer grating distance (4 microns or 20 microns) as compared to magnetic (about 250 microns) so you will get better repeatability and finer resolution from an optical encoder. Most magnetic encoders are very resistant to stray magnetic fields, so you should be OK with using one of those as well.

Keith

[Swiss]

this is a great resource.
http://www.danaherprecision.net/MotionHandbookPage.aspx

That is a great resourse. Very interesting reading and very thorough too. That should be a major resource for cnczone.

the majority of the cost of linear motors is the magnet tracks

Yes, it's a big cost. But, amazingly, even if you add up the cost of all the nicest parts, the total still comes in at half what you'd pay for a full machine. I really like it too because it means the axes are nicely balanced, and also kept out of the way of the spindle.

hugo

[Zumba]

Hugo-
I think you're worrying a little too much about the stiffness - most THK style rails are very stiff and have very high load ratings.

THK rails are wet noodles by themselves, even the big 55mm ones. The math speaks for itself.

[NEATman]

Zumba-
I meant to convey that the bearing blocks are rated for very high loads - and when properly mounted, have excellent stiffness. THK Rails are also not very straight by themselves - they must be mounted on a qualified surface, and it is best to use a laser, autocollimator or qualified straight edge and indicator to align them straight.

NEATman