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View Full Version : BUILD LOG: Made of aluminum, Made for aluminum: Would love design critique!



gamingan
22-06-2017, 09:27 PM
So, I squeezed in a few hours every night for the last 5-6 days to design this one, using experience from my tiny first machine (moving-bed style for wood) and exploring other people's builds online.

Pretty proud of the effort as a beginner, but not so confident (read: nervous) about how this machine will come out in reality.

I do have some unresolved questions (see further below), so I would definitely appreciate some feedback before I get going!

Below are pictures of the best design that I could manage to create based on:

my dimensional requirements/constraints
the goal of cutting aluminum
taking advantage of at least some components like rails and bearings that I already had lying around


21980

21981

Here are the questions/concerns that I have (Q1, Q2, etc. are marked in red on the first picture above):


(Q1) Do I absolutely need a support-bearing block for the Z-axis ballscrew? I did this for X and Y, but the Z-axis construction is really tight on space, so I'd prefer to leave it out if that's fine.

(Q2) I presently have two Hiwin HGH-15 bearing blocks for Z-axis; ordering more would add cost and more importantly time due to re-shipment, not to mention the reduced Z travel. Is two bearing blocks good enough?

(Q3) Any improvements I can make on these plates? I used fairly thick plates (15mm), but I'm more concerned about the shape and positioning.

(Q4) Steppers: 350 oz-in. Are these going to struggle with moving the amount of weight all the parts add up to? I am mostly concerned about Y, even though I have dual-drive (i.e., two steppers) on that axis. I don't need too much speed even on rapids, as this is mostly for occasional personal use. I do want the router to run without skipped steps obviously.

(Q5) Any other general feedback on overall structure/geometry? I'm excited to begin assembling this, and see where it goes.

Nr1madman
24-06-2017, 08:30 AM
Hello!

Im no expert so take my advice with a huge pinch of salt :)

The recommendations for the gantry side plates is usually 20mm thick instead of 15 for aluminium routing.

I do think that you need the extra bearings on Z axis.

The motor strength is fine if the motors have low inductance?

Also I don't know about the strenght about mounting the gantry atop of the ballnut like that.. The side plates should be connected to the plate that has the hiwin bearings..

Im sure you will get more feedback!

Otherwise very nice cadwork :)

Nr1madman
24-06-2017, 08:31 AM
Wait.. don't you have any bearings for Z ballscrew? Only direct connection with motor?

routercnc
24-06-2017, 08:57 AM
Q1: For Z you need a ballscrew bearing block with a pair of angular contact bearings to take the axial thrust and support the weight of the spindle. The bearings in the motor are not designed for that amount of load. This is usually at the top of the ballscrew.
At the lower end of the ballscrew it is better to fit a bearing but I think you can run without one.

Q2: I would not use 2 bearings on the Z axis, it needs 4. Also I would swap them around so the bearing blocks are on the Y axis and the rails are on the Z axis. Then add a Z axis plate to mount the rails to on one side and the spindle bracket on the other.

Q3: 15mm side plates are OK, 20mm is preferable.

Q4: For what you have drawn a pair of Nema 23s with the right power supply (~70V) will move that no problem.

Q5: Can't quite see the detail around the interface of the gantry side plates, but make sure there is a good connection down to the bearing blocks.
Do you already have the 15mm rails for X and Y? If not, go to 20mm.

Zeeflyboy
24-06-2017, 02:32 PM
As above, while you can get away without a bearing on the floating end (you just end up with a lower critical speed which isn't generally a big deal on a Z-axis) you absolutely do not want to go with the way it looks like you've designed it there with the screw attached to the motor. Ideally you want a set of angular contact bearings on the fixed end.

Given the design you already have, it wouldn't be too much work to raise up the sides and then sit the gantry flat on the rails - that will give you much more rigidity than using the longer gantry sides, then just move the ball nuts to the side. Unfortunately I'm very limited in width and I couldn't get away with that design on my own machine given the work area I wanted, but it's definitely the way to go with a machine like this one.

what is the span on the gantry? how thick is the extrusion? looks to me like you'll get some fairly significant deflection fore-aft which may limit your surface finish and speeds/feeds in alu. I'd consider beefing that part up personally given the fairly decent looking span.

Neale
24-06-2017, 08:15 PM
.
Do you already have the 15mm rails for X and Y? If not, go to 20mm.

Consider 20mm rails for Z as well. Not needed for strength, but if you go with the two-plate Z structure as recommended, 20mm rails make it much easier to fit the ballscrew and nut between the plates.

gamingan
26-06-2017, 07:01 PM
Thank you all for the incredibly useful suggestions; learned a lot by trying to think about why each of those suggestions made sense.

I've refined the design now (along with some significant changes in overall geometry) -- let's see if you guys like it.

Here is the 2nd version of the design...

Front view: http://i.imgur.com/Ik9s7uV.jpg
22007

Back view: http://i.imgur.com/iKuChuA.jpg
22008

Below are the changes I made:


Most important of all, altered the geometry so that the rails and ballscrews are protected. This also simplified the Gantry side plate design a lot!
Gantry side plates now are 20 mm thick (instead of 15 mm).
Strengthened the X-axis (i.e., the horizontal "gantry axis") by using 80 x 160 mm aluminum profile instead of 40 x 160.
Replaced ballscrews with 20 size (instead of 16), and linear bearings + rails with HGH20 size (instead of HGH15).
Added a fixed bearing block (with AC bearings) for Z-axis ballscrew (previously it was fully floating beneath the shaft coupler).
Added two more linear bearings (HGH15) for Z-axis.

Feedback?

I hope this one is OK to start assembling!

Zeeflyboy
26-06-2017, 08:47 PM
Significantly better in most regards, nice job.

Consider going one step further and putting the rails on the top of the side extrusions, you can quite literally get rid of all unsupported gantry side arms and it is certainly easier for alignment purposes than side mount.

larger extrusion on X is a good decision, I would still go for heavy profile though if that's an option in that size.

Also consider flipping the Z-axis around so the router mounts to a plate with the rails and the carriages are fixed to the gantry side... 'tis what all the cool kids are doing these days. Gives better rigidity. I'd also say just upgrade those to 20mm rail too for simplicity's sake.

routercnc
26-06-2017, 10:41 PM
... you can quite literally get rid of all unsupported gantry side arms and it is certainly easier for alignment purposes than side mount.



In case it is not obvious you have trapped the long axis rails between the gantry sides, which means you cannot easily control the pre-load on them when it is all assembled.
As drawn the side extrusions which sit on the bed - one of them needs to be adjustable laterally so that the pre-load on the rails can be set. Otherwise if you bolt this together rails may wear prematurely or bind.

gamingan
27-06-2017, 08:13 AM
In case it is not obvious you have trapped the long axis rails between the gantry sides, which means you cannot easily control the pre-load on them when it is all assembled.
As drawn the side extrusions which sit on the bed - one of them needs to be adjustable laterally so that the pre-load on the rails can be set. Otherwise if you bolt this together rails may wear prematurely or bind.


Consider going one step further and putting the rails on the top of the side extrusions, you can quite literally get rid of all unsupported gantry side arms and it is certainly easier for alignment purposes than side mount.

Hmm... I will try to go this route but I really prefer the rails mounted on the sides because it also does a bit of dust shielding. Is there any reliable strategy you can think of (screw-based adjustment perhaps?) to adjust/align things if I *HAD* to go with rails on the side?

And to be honest, I don't see how top-mounting the rails allows adjustment either... routercnc mentioned the need to do lateral adjustment, which I agree with, but how exactly does this become possible in practice for the case of top-mounted rails?

Zeeflybloy: Regarding flipping things (rails <-> bearings) on the Z-axis: Will check to make sure it doesn't conflict with any dimensional constraints I have, and give that a try.

Neale
27-06-2017, 08:42 AM
These Hiwin rails, for all practical purposes, have pretty well no alignment tolerance. They must be accurately aligned to, say, 0.01mm, in all dimensions. Can you control the width of the bed assembly, side extrusions, etc, gantry beam length, gantry spacer thickness, so that the total error in dimension is less than 0.01mm? The one big difference between home-built and commercial machines is that we don't have a giant machining centre that can give manufacturing accuracy. We have to build in adjustment at all key points. That's why the rails on top of the sides will let you build a good machine and your design will be a nightmare to set up. I understand your point about dust protection but keep asking yourself the question, "how can I adjust this on assembly?" For example, you could machine/shim the spacers to allow for gantry length errors, but how will you guarantee that the rails are parallel? Epoxy beds, bolts in oversize holes, etc, are all viable techniques in our workshops, however much they might be frowned on commercially!

You can spend as much time setting up as building, but that ability to tweak for accuracy is key. It has the potential to be a good machine if you can just sort out a few details like this.

Lee Roberts
27-06-2017, 10:38 AM
Neale, great explaining!

If the machine cant move, there will be no dust :disturbed:

Zeeflyboy
27-06-2017, 12:23 PM
Neale explained it there!

It is worth pointing out that I am in the process of building a machine with side mounted rails so I've spent some time thinking about how to do it correctly, so I definitely can't say don't do it - just that you need to know HOW you are going to do it.

You have a few significant issues with mounting them on the side. First, how do you make sure they are perfectly aligned so that neither rail goes up/down in height relative to the other along the axis travel... for me my solution to this is a 20mm thick precision ground eco-cast bed plate which will serve as my flatness reference, and the design allows me to insert a spacing shim between carriage and bed to get vertical alignment.

The second issue is how do you ensure the rails are perfectly parallel (e.g. that they aren't further apart at the back than the front, and they don't have any bend that takes them closer/further apart along the run. This needs to be done to a very high accuracy - something in the region of a few microns. Your extrusions will not have sufficient flatness, straightness or dimensional tolerance to ensure this level of accuracy so how do you achieve it?

For me, I switched to using more expensive extrusions from misumi which are milled on two sides give a proper precision milled surface on which to mount the rails. That only solves part of the problems however, as you still need to ensure that the rails are the same distance apart at any given point in the travel. My machine is quite a lot smaller so I managed to fit my cross bars (a job being served here by your extrusion bed) in the current CNC and measure them with a probe to 0.01mm resolution. I came out with a variation of around 0.06mm and have used a variety of 0.01-0.05mm shim material to hopefully take them to approximately the same length. My plan is to then make a bar that mounts across the span which can float if the rails diverge or converge along the travel, with a dial indicator mounted. I will then move along the axis and where the dial indicator diverges by more than a few microns I will insert some shim material.

So, now that you've sorted all that out - next problem, how accurately do you think the gantry length is cut? If you are lucky it is probably within 0.3-0.5mm of what you have asked for. Problem is your rails are a fixed distance apart, and now you will either not be able to fit them between the gantry side plates (if the gantry was slightly shorter) or you will have a gap that then applies a load to the bearing carriages when tightened up. You will need to diligently shim either the gantry beam or the space between the side arms and carriages to make sure that there is no gap to cause problems.

If you manage to figure out a solution to all of those issues, then there is little to worry about with side mounting - but I hope you can see that it is not as straight forward as you might think to avoid issues of misalignment.

Personally, given your overall design there I would mount them on top (I would also personally go with some extrusions from misumi milled top and bottom for the side pieces there if the budget allows but it's not necessary) and figure out a little dust/swarf shield arrangement for them.

Food for thought anyway.

edit - to answer your question, lateral alignment is extremely easy on top mounted rails. You simply fasten one rail down as square as you can and then that serves as the reference for the second - you would just move the gantry along the travel fastening the second rail as you go. The issue with using extrusion is that vertically you are relying on the flatness and straightness of the extrusion which is not ideal but certainly not as problematic in that orientation (the gantry would likely twist to accommodate any small variation before causing damage to rails) but you will see plenty of people using self levelling epoxy pouring with a bridge to provide a flat mounting surface between sides. Easy option to get a respectable level of precision in that plane would be the top/bottom milled extrusions I mentioned but it depends upon your budget and precision desires.

A_Camera
27-06-2017, 12:33 PM
I really prefer the rails mounted on the sides because it also does a bit of dust shielding.

Dust is going to be an issue, regardless how you install the rails. A word of advice is that you already at this stage think about dust extraction. Not only the linear rails which needs to be protected from dust but also the room where you are planning the use of this machine. So personally, I'd mount those rails on the top, as said by others, but I'd also design a dust extractor, a simple dust shoe, as well as some side walls, which not only provides some protection for the rails, but also personal safety.