I think the flex caused by the weight of the Z axis and spindle due to gravity would greatly exceed any flex caused by a moving Y axis.

 

Really?

There is potentially quite a bit of force on the cutter when it's working. It's the deflection caused while work is being done, not merely the movement. Perhaps some calcs are in order... This is why I was hunting for an analytical model in that other thread.

Allistar, do you have a basis for your conjecture, or is it more of an informed hunch? Just trying to learn more about the way you are thinking about this.

Much thanks!

 

Perhaps I'm underestimating the amount of force while cutting. The reason for my comment is that I've just spent a couple of hours ensuring the double X gantry on my Lead 1515 build is perfectly parallel after mounting a 4.5kg spindle to it. If the weight of the Y axis and spindle do deflect the X axis it'll always be deflecting it (while the Y gantry is away from the ends) whether you're cutting or not (and no matter how hard it's cutting).

If you're concerned about deflection you could go with a double X axis with a slave gantry link the Lead 1515 high Z mod build. You could run the two X axis's rotated 90 degrees as per your original suggestion. Or have the driven gantry rotated and the slave gantry not. This all depends on how much Z height you have to play with.

 

Thanks for your thoughts Allistar, a couple points/notes to your comments....

We're working on a tweak of the Z-Axis, and rotating the X-Axis in CAD right now to evaluate the alternatives....

The forces are impressive, I'll do some calcs/simulations once I get done with the CAD work.

Yes, a lot of work since these machines have to be trued quite manually! I've been contemplating ways to help this process to make it more systematic. I agree this is a LOT of work.

Actually, in fact, per that tutorial you linked to above, of course the deflection will CHANGE based on where the spindle is on the X-Axis. This is worse than it "always" sagging because that sag will be a function of X-position! Ouch!

Further, in fact, the "sag" will change as you engage your workpiece because the workpiece will support the bit (at least for a while). So, depending on the force, and how long you stay in a position, your cuts will also vary in depth.

I'm sure we're agreed this is neither the intended behavior, nor what we want!

Actually what we're going for is "stiffness" in all three axes. We're primarily cutting aluminum in our application of this unit, and want to get the repeatability to an acceptable level (still TBD). When doing wood working, often the tols are not as high as when making metal parts that have to mate. Hence this is a GREAT unit for signmaking, and even for your application (from what I've gathered, a wood lathe). But if you are off by 0.5mm, probably not a showstopper (correct me if I'm wrong). For interconnected metal parts, that's kind of a lot!


The suggestion to rotate the X-Axis beam actually comes from reviewing the beam deflection tutorial you linked. The stiffness in the longer dimension is SIGNIFICANTLY higher in the larger (80mm) dimension. Hence it makes sense to orient that in the direction of the largest forces. For your application, as a lathe, I sense you are most concerned with Z accuracy, so perhaps it makes sense to use the stiffness in that direction for your application. My questions come more from the perspective of the generic performance of the design.

I would point out, however, you may experience flex in the Y direction if you cut fast... with lathes though, you are usually taking small cuts and not "hogging" like for making signs.

The other motivation is that the current design is actually not great because it's side-loading regular ball bearings, which were not designed for this type of loading. There are special ones you can get which tolerate side loading, but we're not using those in this machine. By rotating the X-Axis, we load those bearings in their intended orientation for most operations (except Z). But one might assert (even for a lathe application) that the Z-forces will generally be smaller than X or Y for typical applications. (use the bit to do the work, not excessive force).

We also have to consider ROTATION of the Z-Axis around the spindle mount, and/or around the interface between the X-Z axis. (Basically the carriages) This you may care about with a lathe application because you will certainly be side loading your cutter. Your "taller" config helps with X-Axis rotation (different kinda flex than we were talking above) but still important.

So, as you can see... we've been noodling here on a bunch of small details. Really just doing our best to understand the machine before we get to work building it. Constructing the entire thing in CAD has been a great exercise to get a deeper view of the machine and how it works.

Once we have our Z-Axis ideas sketched up with the rotated X-Axis, I'll post some pics so you can get an idea of what we're thinking... There is an upside with this approach that is also provides a bit more Z-Clearance by lifting the X-Axis.

If we end up finding that your intuition is correct, we'll also experiment with adding a second C-Beam, essentially making the X-Axis into a "box". By tying them together you end up with (in effect) a laminate which is highly resistant to flex. So this would be our next step in reducing the "sag" part, while improving stiffness all around, based on above points...

Sorry for the long message, we're right in the thick of the designs, so it's top of mind! Thanks for continuing the conversation!

Cheerio!

 

The gantry flexes A LOT!
I'm considering adding the High Z mod for the same reason - I'm hoping it will strengthen the Gantry. I am cutting HDPE for now also and get a lot of flexing in the Y direction, and a moderate amount in the X direction. It creates a lot of chatter marks in the upper left and upper right quadrants when cutting a circle. In fact, I am getting a 0.003" inch deflection caused by the X motor torque that I will cover in a different post. I will be adding a second mount to the router, I think that will help a lot in the X direction and I'm sure a second rail on the gantry will help in the Y direction.

 

Thanks Bee!

This was our intuition based on the overall design... Let's see if we can make some improvements! That's the whole point of this being in opensourceland!

Stay tuned for our tweaks to the X-Axis, and based on your experience, we'll consider proactively adding a second beam to the X-Gantry. Have some ideas here too!

Appreciate you chiming in to the conversation and helping with your observations!

Cheers!

 

How soon do you think you will have something to look at? I'll hold off ordering if there is something that might work better.

 

We've been working on it steadily the last couple days (since my initial post about rotating the Axis). I'm pretty close now to having an assembly put together.

Be warned, it requires a few "custom" bits'n'bobs, but may be worth it. I'll try to explain the rationale for each of the tweaks. Mostly, it uses the OpenBuilds components.

We're going to build the "Horizontal X" version as well, but it'll be our first build of any of these, so we will have nothing to compare to. I'm curious how you are measuring your beam flex? Also, how long are your gantry/axis beams? Obviously this is a factor as well.

All of that said, I'll post up what we've come up with as soon as it's done, and we can discuss the pros/cons and get to the best option to try.

Also, if we find that there is still too much flex in the X-Beam (now in the Z-direction) Doubling them up is still an option, but perhaps it can be done more compactly than the published "High-Z" mod. This would be done by simply stacking a second C-Beam onto the X-Axis, and tying them together. This will make a significantly stiffer beam. Probably also require some X-Carriage tweaks.

Stay tuned, and we'll have something to discuss soon!

P.S> What CAD environment are you working in? We use Inventor, and can export into STEP and 3D PDF for sharing and 3D viewing. The latter being much more compact, but requiring Adobe Acrobat. We'll probably post the 3D PDF first for knocking around.

 

Bee, (and everyone else!!)

Here is a 3D PDF of the re-imagined X-Z Carriage Assembly. Still working on the rest of the frame structure, but that's the easy part! Below, I'll quickly explain the motivation for the tweaks we've made. Really not too much though it looks pretty different...

- Angle Brackets which connect the X & Z carriages. These provide a rigid and well registered connection between the carriages. Attachment uses the threaded holes in the plates already present. Countersink/Flathead screws help prevent the connection between the carriages from sliding around.

The most important objective here is that the Z-motion is completely orthogonal to the build-plane. This mount pretty much assures an orthogonal connection between the carriages, so we're back to having to assure orthogonality via the other adjustments (wheel adjustments, spindle mounting, etc...). Orienting the Eccentric axles on Z-Axis as shown allows for adjustment of this in the Z-direction, but also requires that this be checked.

- Replaced the Z-Axis single Anti-Backlash Nut with the same assembly as found on the other axes. Rotation of the Spindle due to forces exerted in the X-direction is possible. This plus the next tweak is meant to reduce this possibility. It may be overkill, but it's probably more stiff than the "stock" setup.

- Added a second set of wheels, and tied them both together and rigidly to the carriage plate. This uses a short piece of extrusion, and a spacer which can be purchased from Misumi Industrial. It's a "custom" but about the same price as the plates from OpenBuilds (about 12 bucks). We've been ordering from Misumi a lot, and they seem like a decent and reliable supplier. This mod provides further rigidity and resistance to rotation about the carriage plate.

Also, this distributes the cutting load here on two bearing-sets instead of one. It's two everywhere else in the system, no reason to short-change the Z-Axis. If nothing else, will bring the service interval of these bearings in line with the others. Also, if these wear faster, then you have to readjust the squareness of Z on a regular interval because the bearings will "wear-in" more quickly, since they are exposed to 2X the forces of the rest. This helps even things out.

It should be noted, that if one wanted to keep using the "Vertical X" version of the system, they can still take advantage of the "enhanced" Z-Axis, as this is independent of the X-Axis mods. We just did this to try to address all the places we saw there could be improvements in rigidity of the components of the system.

Stay tuned for the actual "Horizontal X" frame mods... working on those next.

Please feel free to shoot any Q's, suggestions, or observations over on this if you like!

Attached is a 3D PDF. You should be able to view the content if you use an official Adobe Acrobat viewer. Otherwise it comes across just as the static picture.

Enjoy!

 

Hello Again!

OK, here is the rest.... This file includes both the X-Gantry assembly and the X-Z Carriage assembly in place. A few things to note:

- See this RepRap article for a bit of a conversation on rigidity.

- We have opted for a 4-point end-plate because we use a vibration damper in the motor mount, and we tension the lead screws using the OpenBuilds nuts. This will work equally well for setups which use the standard non-tensioned design approach. (Excuse the long lead screw, we're still determining the final machine width for our setup)

- There has been a gusset plate added to both sides of the X-Beam, and 4x4-point truss connections between the X-Beam and the long-side of the Y-Truck assembly. The Truss brackets we used are again from Misumi. We choose these because they are longer than the OpenBuilds "corner bracket" style. The reasons for these choices are that in order for beams to flex, they have to stretch on one side, and compress on the other. The collection of brackets helps resist this elongation/contraction so aids in stiffness. We have not calculated how much, but the brackets are fairly inexpensive in the grand scheme. Note, none of the brackets are "supporting weight" because the X-Beam is resting atop the Y-Truck upright beam.

- These brackets also help with squareness of the assembly (X-Beam to Y-Truck)

- Cable Tray mount is TBD. Perhaps some others can help us understand the best position for this based on how well the drag chain works and the best working radius for the "U-Turn".

- The Gusset plates also help resist parallelogramming in the X-direction.

- As can be seen, there is not much that has to change on the structural facets. Same length beams are used, etc... A side effect is that the Z Axis is raised a bit by this change. This could be beneficial for folks, or a pain. Depending on which this is, one can shorten the beams used in the Y-Truck uprights to lower the Z back to its original height... Or perhaps this gives you *just enough* extra Z so you don't need the High-Z mod at all!

- Z Travel can be expanded/contracted by simply changing the length of the Z-Beam & associated Lead-Screw.

- Overall, this design is intended to reduce the "axial" or "thrust" loading on the bearings when possible. Here is a nice article that explains the different ways a bearing can be loaded: Nice Ball Bearings – Selection Guide. Also a quote from another part of that web site:

"Deep groove ball bearings by nature of their design can carry axial loads either alone or in combination with radial loads. Axial load capacity of standard bearings is 0.5 times the Basic Static Load Rating (CO) of that bearing. Smaller bearings should not be subject to a load greater than 0.25 times CO. Excessive axial loads can lead to serious reduction of bearing service life."​

- Axial Bearing Loading:

* Cut in X-Direction => Y-Bearings (Load / 32)
* Cut in Y-Direction => Z-Bearings (Load / 16) (in the original design this was (Load / 8) on Z)
* Cut in Z-Direction => X-Bearings (Load / 16)​

- By rotating the X-Beam, we exchange stiffness in the Z-Direction for stiffness in the Y-Direction. If we reference the data given in the Beam Deflection article mentioned by Allistar above and in other places on the site, we see we have swapped the Area Moment for the X-Beam between Z & Y. The result, the X-Beam is 4.8x stiffer when cutting in the Y-Direction now. We've traded this for Z-Stiffness. This means, a "lighter touch" when making plunge cuts or other Z-Axis operations, but once stabilized, Z should be fine. Once you have cut to depth, then there is little axial Z-load on the cutter or machine.

- Undoubtedly, folks will be quick to point out that this could cause some inaccuracy on cuts in the Z-Direction, which for folks making Lathes, may matter. A couple of considerations, still TBD:

* How much does the beam really sag with the Spindle attached?
* How do the Z-Forces compare to X & Y?​

For an answer to the first, we can estimate this using calcs from the article referenced. We also may be able to make some measurements. For an answer to the second, this comes down to how aggressive one drives Z.

It's possible to improve Z-Stiffness (and Y-Stiffness) by adding a second beam. The High-Z Mod does this, but it primarily stiffens Z, disproportionately vs. Y. If we were to make a change to stiffen Z, we would add a second beam directly on top of the current one, in the same orientation. We'd tie them together, and create a "laminated beam" which will exhibit excellent rigidity in Z, more than doubling it, while also doubling the Y stiffness by doubling in height. (this would require a change to the X-Carriage assembly as well to span 80mm vs 40mm)​

We acknowledge this design change may not be suitable for everyone, but for many, it may be ideal. It comes down to what you use your machine for.

Please accept our apologies for those who find there to be a lot of material here... we're just trying to be as thorough as possible in explaining the reasons for the changes we made. We would VERY MUCH WELCOME others add their thoughts here. We may have overlooked something obvious in our changes, and there are many out there with vastly more experience with these machines.

Warm Regards!!

 

I put on the High Z mod onto my 1010 build. I mounted the top rail with all of the angle brackets tightened down. My friend throughout the build has been a set of 1-2-3 blocks. To get the spacing between the two C-beams parallel, used the 2 inch side between the bottom of the top rail and the top of the bottom rail. Held the beams in place with squeeze clamps and put in the mounts. Issue solved.

1-2-3 blocks have been my friend throughout the build ensuring square and flush joints.

 

Thanks for the tip. How fast can you make it travel along the X axis? Mine is set to 3200mm/min max as anything about that and there's a mechanical binding. I can get 4000mm/min out of the Y.

 

Allistar,

I'm curious the significance of determining the "max speed" on the axes? Do you normally run jobs at these kinds of speeds, or is this just an attempt to find the maximum capabilities of the machine in order to program safety limits into the controller?

 

The biggest issue I have with my current machine is it's slow in both speed and acceleration. I know that the time a complex carve takes is more down to the acceleration that the speed. It's been important to me to have a machine that's faster than my first one. I would never cut at 4000mm/min but rapids from one side to the other on large designs are better off being faster.
My first machine has an acceleration on 65mm/s^2, the Lead 1515 is 350mm/s^2 which makes a massive difference to complex 3D carves or V-carves.

 

Thanks Allistar, I didn't see your post at first because Peter was so quick behind you!

Where do you see the mechanical binding on your machine?

Thanks also Peter!!

 

When it binds in the X axis the two gantries start making a resonant vibration sound that gets louder until the steppers bind. There is some kind of friction in the system that is noticeable above 3200mm/min. I was wondering if this is caused by the slave gantry being "pulled" by the driven gantry via the connected Y axis C-BEAM and twisting. I've been playing with the tightness of the wheels to see if that makes a difference but I haven't found a cure yet. I've made sure the two X beams are perfectly parallel.

 

I'm driving the top beam, that's what the plans called for. I wonder if driving the bottom beam would be more balanced given the weight of the spindle is below that.

 

Would racking between the beams be eliminated if it were center-driven like a dual-rail-single-screw Y axis? I know there's probably not a plate for that right now, just wondering if it's something some people might be better off looking at over the longer term. It would probably need more screw protection from dust, but might still be worth the effort.

 

I agree here as this would also minimize tilt in the x-axis and I will make the move of the x-axis screw to the lower gantry. To this point, I would also consider rearranging the machine so that the majority of the movements, if possible, have the leadscrews in tension and not compression, especially when significantly away from the stepper motor. The current position of the X and Y steppers allow for this with the proper writing of the g-code program as tension is in the +X and +Y movement directions.

 

No third party parts, it's all kit bought from makerstore.com.au. I'll take a video of it binding at speed so you can see what I'm experiencing. Maybe the sound it makes will have an obvious cause to you.

Thanks. I'll move it to the lower beam and run some tests. It makes sense to have it on the lower beam. I'll do this by carefully swapping the rear X gantry plate that has the lead nut in it between the top and bottom gantries. I've pulled these off before when I mistakenly put the eccentric screws on the wrong side of the gantry. I must say that doing this is a mission!

 

I moved the X lead screw from the top X gantry to the lower gantry. Now the top gantry is the slave. This has fixed the issue. Previously X was limited to 3200mm/min before it would bind. Now it can go 4500mm/min without any signs of binding. Y can go that fast too. I've dialled it back a bit a set the max limit to 4200. At 4500mm/min there is very little whipping of the lead screw (it barely wobbles off centre).
Driving the lower gantry seems like a better choice for this design. It also provides more room for the drag chain (now the bend in the X drag chain isn't as tight as it was before).

Thanks for your help with this.

 

I moved the x-axis screw on mine from the top to the bottom rail. The idea is better and reduces the length of the moment arms present. I will have to rethread the screw in the blocks as they lined up too tight (both screw sets slammed to the edges).

 

fixed.

 

I am finishing up the install of the High Z mod on my new 1010 build and, I was wondering if anyone could post a picture of the back of a machine that is setup with the Z beam moving up and down not just the router (the configuration shown on page 2).

Specifically I would love to see how and where the drag chain should be mounted. I am trying to figure out if the drag chain should remain on the top rail or move to the bottom rail.
I am also planning to to move the lead screw to the bottom Y rail and drive from there. Would the nut block for the Z axis be attached to the bottom or top gantry plates?

 

Hi John, here's a few pictures of my 1515 with the high Z mod. The lower X gantry is the driven one. This shows how I've attached the drag chain. Excuse my messy garage!