Gantry mill

[jsheerin]

I spent some time finishing up an initial model for a moveable x-y table yesterday. It has slightly more static deflection but with smaller beams than I was using before. I ran a few more iterations with some additional stiffening which improved things slightly. A screen shot showing deflection under a 50lbf load plus gravity is shown below. Once again this is a symmetric model so there would be another spindle next to the one shown and actually loads applied to this spindle would also be applied to the other spindle. I think I'm at a pretty good place right now. A lot of my deflection is in the vertical direction, but it's just deflection of the structure under gravity (about 0.0005"). But under a vertical load, the compliance looks to be about 0.045" per 10^6 lbf of applied load once symmetry is accounted for which is about 20x better than my target of 1"/10^6 lbf. Parallel to the cross bar, compliance is about 0.4"/10^6 lbf, and perpendicular to the cross bar it's about 0.63"/10^6 lbf but those two numbers would probably be a bit different due to symmetry.

Next I'll add the rough design of the x and y axes to the model. I might also change my model a bit to try to more accurately model how I'd weld these tubes together. Then I need to design the support structure for the z axis screw.

[jsheerin]

I found a long ball screw last week which gives me all the screws I need for this machine. However in thinking about it some more, I realized it might work better to change how the dual spindles are oriented and place the long ball screw differently. See the sketch below. The spindles are now placed next to each other in the x direction. This will let me make the gantry narrower which should improve stiffness and make it easier to package the ball screws into the mill.

Now my y axis screw will be 10" longer than it needs to be though. It already has the end blocks, so I don't really want to shorten it. I'm pretty sure I can package it as is to get my desired travel in the minimum size package, but I'm contemplating just giving myself an extra 10" of y travel. This would give me a working envelope of ~31"x34"x22" instead of 31"x24"x22".

[Khalid]

I really want to see this in finished form.. The efforts you put in 3D modeling and stress analysis is awesome and plausible..
Regards

[jsheerin]

Thanks Khalid. I'm sure it will be quite a while before it's finished, but hopefully I'll get there eventually.

I received my last ball screw yesterday. Below is a picture of the x axis ballscrew (the long one) and the second z axis screw. The z axis screw is from a Mori Seiki MV45/40 mill and includes one of the bearing blocks and triple bearings at the other end. The x axis ball screw will give me 49" of travel and the z axis 22". I must be getting pretty good at estimating screw travel from pictures with rulers in them, because this is almost exactly what I estimated these screws would do (ebay sellers didn't give the actual travel measurement).

The x screw appears brand new and unused. I'll have to find bearings for this one and fabricate bearing blocks, but that shouldn't be too bad.

I've also been spending some time Machine Tool Reconditioning. It's incredibly dry, but it's got all kinds of suggestions for how to check alignment of machinery which is giving me lots of good ideas of how to build this thing. I've also been reading Michael Morgan's scraping book and watching his dvd trying to get a handle on scraping. It's good stuff - http://www.machinerepair.com/. I picked up a scraper and some Dykem hi spot and will be giving that a try soon. I've also been thinking about how I'm going to stress relieve the finished machine frame after welding. I've started doing the calculations for how much power a stress relief furnace would take and it seems reasonable to maintain the temperature in a large enough space. Now I have to figure out how much power it will take to get up to that temperature (so I can figure out what kind of heating elements I'd need). I've built a small propane fired forge before out of stacked fire bricks (see pic below) and was thinking of a similar but much larger setup for this - something that I'd build around the machine frame, use once and then disassemble. I was thinking of electric heating elements instead of gas though. I've also been reading about vibration stress relief, for example: http://www.stressreliefengr.com/ and http://www.meta-lax.com/. I could easily build one of these units - a 3 phase 1/2HP motor driving an off center mass run by a vfd (as I have plenty of vfd's sitting around). But opinions seem mixed on whether it's a real effect or snake oil.

[jsheerin]

I've been doing some thinking in my free time about evolving my design, and I realized I've been modeling my frame somewhat incorrectly. I have the z axis fixed to the gantry through the linear bearings in x, y and z directions. When I first started out, this was probably somewhat realistic. While the bearings obviously would not be supporting the z axis in the z direction, the ball screw would be mounted fairly closely to the bearings so the vertical load would still be applied to the gantry in roughly the same spot. However on my newer designs such as the sketch in post #14, the ball screw will be attached to another frame on top of the gantry which will distribute the vertical load to the outer portions of the gantry instead of the middle (assuming this works well when I model it). This means the vertical forces that I'm currently modeling as being transferred through the z axis bearings are not accurate at all and are possibly causing me to over design my gantry. So now I need to figure out how to model and constrain the z axis so it's close enough to reality to be a useful design tool. Looks like I'll be learning some new ANSYS techniques...

[ebrewste]

That's some monster you are designing there Exciting stuff, especially since you are actually doing some analysis and not just guessing.

In addition to remembering that the bearings are free to move in one direction, don't forget that they also have compliance. They HSRs are mercifully the same stiffness in four directions, but they also have another stiffness in torque. You may want to model this if you think you are getting any significant cantilevering effect, though I can't see much in your drawings.

Also, don't forget ball screw deflection. They can be modeled as a spring on a spring (ball screw column axial deflection + ball nut axial deflection).

Not sure how much of the above you have already considered. Ignore if you've already gone down that road.

Awesome stuff -- I can't wait to see your progress.

In Moore's "Foundations of Mechanical Accuracy", he seems to really rail against vibration as stress relief, but I can only state this as a believing reader, not someone with any real experience in a controlled experiment. Fantastic read, if you can find a copy somewhere.

Erik

[ebrewste]

Regarding the annealing / stress relief: You may want to see if there is a race car industry nearby. I couldn't tell where you live. Race car roll cages should be annealed / heat treated, so there is likely someone with a big furnace that is used to heat treating tube assemblies, likely for cheap.

If you just want heat treat yourself, please take pictures, I and others will be fascinated!

[jsheerin]

Thanks for the comments and compliments.

Because I'm doing solid modeling for my fea, the deflection of the screw itself under load would be accounted for, but the compliance of the ball nut would not. Similarly, I know the HSR's have compliance but I haven't modeled that yet. I have a spreadsheet where I've done some initial calculations on what loads would be and the life expectancy of the bearings based on that, so I could probably add that in there. I guess my position on this is the deflection of the ball screws and bearings will be whatever they are, and I've tried to pick ones that will be appropriate for the load ranges I'll use them in. In fea I'm looking more at just what the frame does and I'm trying to make as many simplifying assumptions as possible so I can get a reasonable design. I usually tend to over complicate things and I'm trying to avoid that as much as possible here (although I'm sure some would argue I'm already doing that), but I do want to have a reasonable idea that what I build will have good performance without a lot of unnecessary expense and mass in the frame.

I have heard of the Moore book somewhere - I think I have it on my Amazon wishlist at the moment. I'm currently slogging through Machine Tool Reconditioning (it's great for falling asleep with). Maybe Moore's book will be my next one.

I think I might heat treat myself. My interest in doing it myself is that I don't really have the equipment to haul my completed frame somewhere to get it heat treated. So my options are to buy / build / borrow a trailer and appropriate crane or come up with a way to do heat treating in my garage. The heat treating would seem to be more interesting and give me more capabilities in the future, so that's why I'm leaning that way. Whatever I do, I'll definitely take pictures although I bet it'll be a while before I get there.

[jsheerin]

I believe I figured out how to simulate something more like the reality of linear bearings in my design. For anyone interested, I used contact elements in ANSYS. I setup my model so that there was a gap between my linear bearing and the rail and then setup contact pairs between the bearing surfaces and the rail using separation only always and selected key options to close the gap before starting the analysis. A refinement of this which I might look into would be specifying the compliance of the coupling. I could use this to simulate the compliance of the THK bearings from catalog data.

Attached is a screen shot of the displacement of my test model. It's a vertical rail constrained in x,y,z at the bottom of the rail. It has a bearing on it, and there's a vertical piece that holds the bearing from the top of the rail (sort of like the screw in my real design). There's a big block attached to the bearing with a cylinder (cutting tool) coming out the bottom, similar to my milling head. There's gravity applied as well as a force in the y direction on the cylinder. You can see the bearing has slid down the rail and that the block has gone down with it and twisted to the side. Harder to see is that the top of the rail has deflected in the y direction in the opposite way. The bearing is transferring the moment applied by the force on the cylinder into the rail and deflecting it in the correct direction.

So now that I know how to model this I can go back to working on my frame design.

[jsheerin]

To be more specific (and just for my own notes), to do contact this way, I'm setting keyopt 5 and 9 to a value of 1 and keyopt 12 to 4.

[ebrewste]

That's some great work there. Really cool to see the displacement showing that it's working. Top notch.

[rowbare]

Have you read this? It is an interesting read for anyone wanting to design a machine.

http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf

bob