Starting to design out my build, looking for critique

[Chrono]

While I am busy getting all the detail into the CAD and making sure all the dimensions are correct, a question came up, that I want to ask the collective of experience in this forum.

This question is, if I can use the chinese linear rails (like these), or if there are any known problems with their accuracy or surface finish that makes them bad alternatives? I am asking this, because of just how much cheaper they are to get. Same question goes for the ballscrews.

[MartinS]

Hi Chrono

It seems I have messed up my explanation here. I do not plan on using four rails, but instead three. Two linear HIWIN-style rails on one side, mounted atop each other. The other side only has one rail, and a different type, namely the cylindrical Igus-Style rails that you also often find in 3D printers, like the Prusa MK3. That will still take some work aligning, I get that, but it should technically work to help let the machine compensate. I know, technically is a nice word, but practically often takes precedent :D I have added another picture, with the HIWIN rails in their place, to hopefully make that more clear.

When I first saw this design, I couldn't help but think that was far, far too flimsy

Now with your explanation of the gantry driven from one side only (above)......it's getting worse!

Don't forget that to reduce cutter breakage, wear, chipping, improve accuracy and surface finish, the whole machine must be as rigid as possible and subject to as little resonance and vibration as possible. This is why machines traditionally are made in cast iron or synthetic granite (Granitan etc) that are "dead" materials, they don't "ring".

In the latest design, you are driving the gantry from one end only (and I still can't understand what the benefit is of the lower ballscrew). This goes against evrything that you will read on this forum, "ballance your forces", " place the cutter forces in between the bearings" etc. It is akin to digging a hole in the garden whilst standing on a diving springboard!

And the 6mm thick gantry sideplate.......

Compare yours



with this from Boyan.



Yup, Boyan's is built like a brick sh!thouse. Some may say that it's over the top, but I bet it does the job!

Or just taks a look at Andy's build

http://www.mycncuk.com/threads/11552...-2x1-0m-Gantry

Sorry if I seem somewhat hard on you but I don't want you to waste time and money unnecessarily.

Any other of you forum guys have a comment?

Good luck Chrono, you'll get there.

Martin

[Chrono]

Hi Chrono



When I first saw this design, I couldn't help but think that was far, far too flimsy

Now with your explanation of the gantry driven from one side only (above)......it's getting worse!

Don't forget that to reduce cutter breakage, wear, chipping, improve accuracy and surface finish, the whole machine must be as rigid as possible and subject to as little resonance and vibration as possible. This is why machines traditionally are made in cast iron or synthetic granite (Granitan etc) that are "dead" materials, they don't "ring".

In the latest design, you are driving the gantry from one end only (and I still can't understand what the benefit is of the lower ballscrew). This goes against evrything that you will read on this forum, "ballance your forces", " place the cutter forces in between the bearings" etc. It is akin to digging a hole in the garden whilst standing on a diving springboard!

And the 6mm thick gantry sideplate.......
Martin

First up, I am still driving the gantry on both sides. Just one side is resting on two HIWIN rails, while one is resting on a linear bearing on a smooth steel rod-type rail. Here is the model if you want a more detailed look.

Secondly, I appreaciate the feedback, but what exactly are the problem areas here, what needs to change? Most design decisions come from advice I got from the mechanical engineers at my place of work, some of which have been building similar highly-dynamic gantry machines for Zeiss, taking measurements. This is also where the rail division comes in. Looking back at my technical mechanics textbook, this should protect the linear rails from having to take on torque, while still handling static loads. So that is the reason why. I, obviously, cannot tell you if that will work or not, but that is why I am here, after all :D

Edit: Some pictures to illustrate the whole X-Axis setup, excuse the dreadful mouse-written writing.



Another constraint I am having, is that steel box section, for example, is pretty difficult for me to work with. I do have access to people who can weld for me, etc., but I pretty much have no way to assemble the machine where I need it to, in my workshop, or get it out of there, once assembled. It is on the first floor in my garage, with the only access being a small staircase barely wide enough for one person. Hence the aluminium profiles. If that is a no-go, the only other option i have, is a small hatch, not even high enough to kneel in, which will severely limit the machine's size, overall.

Then there is also my budget, which is pretty close to being used up, even with that design.

On a side note: do you have a link for the post from boyan? I would like to further read up on that build.

Thanks for the feedback,
Chrono

[MartinS]

First up, I am still driving the gantry on both sides. Just one side is resting on two HIWIN rails, while one is resting on a linear bearing on a smooth steel rod-type rail. Here is the model if you want a more detailed look.

Chrono, apologies for missing the second ballscrew, that makes a big difference. You at least have a brick under the end of the springbard

Most builders here use rails both ends of the gantry to as best they can, constrain "flexy" elements (don't forget, most materials are looking for any excuse to bend, twist or vibrate).



When your spindle is cutting at the 6mm plate end of the carriage, Hiwin rails will help resist the twist on the gantry better than the Igus ones. And either way, increasing the distance "C" between them will help.

I mentioned Boyan Silyavski's build
http://www.mycncuk.com/threads/6457-...my-first-build
not to push you towards steel, Al is fine, but to demonstrate by design, reducing the chance of flex and vibration.

Re. the 6mm gantry end, I think that you mentioned that it is to "...let the machine compensate". I guess that you mean "to accommodate assembly/build errors" One way that others achieve this with rigid side rails and substantial side plates, is to mount the Hiwins on the top so that with clearance holes in the sideplate mounting, they can find their own best position on the carriages. See AndyUK's approach. Again, I am not pushing you to change, just showing other approaches to your best friend "rigidity". Also, bracing across the end is another approach.



I have a background in manufacturing with machining centres and lathes with capacities up to 1200mm, precision grinding and measuring with CMM's in controlled environments. Whether it be a CMM or a cutting machine, we want to be assured that the probe or cutter is in a specified and known position. The difference betwen the two are the forces in the system. Those in a cutting scenario being far greater....

Regarding the side rails, it wouldn't harm if you added one or two extra pillars and fillets per side. Also, bracing across the end is another approach.



I hope that this post is more positive than the last one. There are soooo many ways of getting "there" and you are nearly there, Y and Z look great.

Crack on lad......

[Chrono]

That cleared things up a lot, thanks! I gotta admit, for a second there I thought the aluminium profile construction was "out of the question" entirely, so to speak, so I am relieved on that end

I also thought about putting in some more support pillars, so I will do that. They are so short anyways, that they don't cost enough to not put in. regarding a crossbar, I could easily put one in in the back, without even impeding X travel, since the gantry's body will sit on top of it, and it will run out of rails before the spindle would even get a chance of touching it.

You also put in angles in the middle pillars. I definitely see their point, but I am looking at them protruding into the machining area, so I might try and make them removable, with accessible screws, so I can put them there for smaller projects, and take them off, when I need the space. That way I get the best of both worlds, so to speak.

Also, regarding the distance between the bearing blocks on the X: That is one of the parameters I can just change on-the-fly, so changing that is easy-as-can-be :D I am mostly leaving it as a kind of place-holder while I am working on getting everything into place. Afterwards I just need to open the variables panel, and go to town dialing everything in.

Finally, on the topic of the thin plate, and the associated double-stack of rails, with one opposite linear rod bearing: It is not that much about building tolerances (I mean I am not perfect, so there will be those), but also things like thermal expansion, and so on. I am going off of the one lecture in mechanical engineering I got in my electrical engineering course, but with the linear rails on both sides, you would enter statically-overconstrained territory, which is exactly the reason for me to go for the steel rod and linear bearings, exactly because they cannot take on torque. however, if this ends up not panning out, changing the lower rail over, would be quite easy, since all i would have to do is cut down the gantry extrusions by 7mm (which the linear bearings are higher by, compared to the rails and bearing blocks), or shim it with another plate, and swap in one of the rails, not a big hassle.

And I found a picture of the Zeiss machine I was talking about:

As you can see, it has a similar design, with rails on existing on the right, while the left either rolls on the table or hovers on it, I cannot remember if the hovering was another machine, though, and a quick search did not throw up any info. Either way, looking at the YZ-Plane, one end is fixed, while the other is allowed to expand/move. Of course, that does not mean that it works the same for me, but I thought I would post that for illustratory purposes :)

I also plan to add some slits in the plates themselves, serving as holes for potential cross-supports, should it turn out that they are, indeed, too bendy. I will attach a picture hereafter, looking at the right plate, with such a support in vibrant pink. I am afraid that picture can explain this much faster than I could in words. Those plates should serve to add further rigidity, in the same sense as the bends all around the Z-Axis do, and if I find I need them, I can put them in, weld them in place, and go from there.



(also seen in the picture, in the front left of the machine, the WIP of new angle plates. The right is not in yet, because I still need to move the right ballscrew vertically in between the two rails)

What do you think of those measures to address these problems? Is there anything else I should be doing to increase the rigidity?

[MartinS]

You're planning to beef up the side rails, add the bed whilst you're at it.

Much advice has already been given to you and there is sooo much more on this site from some VERY talented, knowledgeable and experienced people :

http://www.mycncuk.com/forums/253-DIY-Router-Build-Logs

Don't get too hung up on the design of CMM's.

Time you put a new blade in your hacksaw!

[Chrono]

You're planning to beef up the side rails, add the bed whilst you're at it.

Much advice has already been given to you and there is sooo much more on this site from some VERY talented, knowledgeable and experienced people :

http://www.mycncuk.com/forums/253-DIY-Router-Build-Logs

Don't get too hung up on the design of CMM's.

Time you put a new blade in your hacksaw!

I have to completely agree, without all the advice from the people here I would not even have remotely come this far!

Currently I am reading through some of those build logs to hopefully find a good one about the PSU section of the build. All the electronics are pretty clear to me, but the interface between machine voltage and net voltage. I see that a lot of people build their own transformers, which is definitely an option, but I am still looking into that. Most of what I am still missing on the machine side mostly related to the spindles and which I want to have in hand, before commiting there.

If all goes to plan, I thus want to sort out the electronics over the next two weeks and then move on to buying parts. That is when I want to make a final pass and move on to the next step, if there is not a flaw that needs sorting out. Once I do move on to buying, I will probably make a proper build-log post as well.

I am also scouring the posts for something related to cutting forces and speeds, to be able to dimension the steppers. As far as I could gather from various posts here, I would pretty much always like to use the steppers in bipolar parallel connection, and using that, getting all the info and all the steppers I can find, I have not found a single one, so far, capable of driving two 2510 ballscrews at 7.5mm/s rapids... who would have thought? :D So I am thinking about moving to a one-stepper-per-screw setup. With that, however, I am still looking at one of AndyUK's first pieces of advice to me and hesitating. So I am thinking about putting a timing belt between the two motors to synchronize them as well, but I don't know if that is a good idea or if it even works. Looking at the motor calc sheet, however, there is a motor that would permit me to get 6.1m/s rapids with only a single motor, as well as letting me cut at 2.1m/s with 50N of cutting forces. But then again, I am still searching for posts on speeds to look out for.