One more DIY first born CNC

[get_f8]

Boxes are many times stiffer than flats so use boxes. Cheers Peter S

Sounds good and goes well with the Keep It Stupid Simple way

Thanks for all the explanations, even we can not integrate all ideas in one project its always good to have the knowledge. I might use the sandwich idea for the spindle holder on the Z axis though. I found two plates of 6"x12" yesterday (measured thickness is 6.14mm). I would have liked the X chariot to be wider but 6 inches wide could be enough.

I've redesigned the whole thing to go with a fix gantry and moving table. The gantry and base will end up being one assembly. I put the gantry in the back so the cutter would be off center. The Y axis rails will be fixed to the base and the bearings to the table. The table will also be a torsion box.

I have a question about positioning the bearings under the table. I understand I cannot put them too close to each other as this will cause the table to act as a lever and cause flex. Are there basic rules to follow when designing the moving table? Can I extend the table pass the bearings, and in what proportion is it reasonable to do so? Or should I make a table as deep (the front to back length) as the bearing span?

Cheers

[peteeng]

Hi F8- There are two main schools of thought:
1) Overhangs are fine you need to make the overhang stiff for plunging loads. There is a name for the proportion of the overhang and it slips my mind at the moment. But it's for heavy beds that have to take the weight of the bed into account so it evens out.
2) Put the bearings at the edges. Then the middle has to be stiff enough for the plunging loads of the cutter.

Putting the bearings at the ends uses the most space, putting the bearings inboard uses less axis space. So pick your geometry to suit. A good method if you are using cheap bearings is to use 3 bearings per side (end, middle, end) this solves many flex issues and improves the movement of the bed. The cost of a bearing is usually small in proportion to the rest of the $$$.

Cheers Peter

[get_f8]

Here's the latest update.

I switched to a fixed gantry design and a bigger spindle.

There's a couple things left to do.
- Wiring shouldn't be a problem but I have to make sure all the little components like the switches will have it place.
- I'll probably add a fan to make sure that the Y-axis motor does not overheat.
- I think of using an aluminium plate for the table and attach a waste board to it instead of what we see on the pictures.
- I thought of covering the tracks and screws with accordion dust cover, is there a common way to protect the motion parts against dust?

Overall size of the main frame is 50"W x 30"D x 22.5"H

Axis travels are X:15.75" Y:33.00" Z:4.50"

Please comment before I start assembling stuff.

Cheers

[wizard]

This has been an interesting series of posts. A few suggestions and then some comments.

1. Pick a construction material and stick with it as much as possible to control construction costs. This is cost is a huge priority which it is in most home builds.

2. Really get a handle on what your expectations are. This can make a massive difference in build costs and satisfaction after the machine is built.

3. Seriously consider steel for a potential cost savings over Aluminum.

4. Very few people ever complain about building a machine that is too stiff. On the flip side many have complained about a machine that is too flimsy.

Here's the latest update.

I switched to a fixed gantry design and a bigger spindle.

There's a couple things left to do.
- Wiring shouldn't be a problem but I have to make sure all the little components like the switches will have it place.
- I'll probably add a fan to make sure that the Y-axis motor does not overheat.

Why do you think that the Y-Axis will be a specific problem? By the way the only way to tell if a stepper is overheating is to stick a thermocouple on it. Many steppers are rated with a 60° to 80° C temperature RISE that is way to hot to touch.

- I think of using an aluminium plate for the table and attach a waste board to it instead of what we see on the pictures.

The table arrangement you have is not the norm. The short distance between linear bearing blocks means that there will be a very high potential for twisting or even binding with a single lead screw. You will likely need two lead screws to address that potential problem. Generally with moving tables you need the bearings on the long side of the table so that the spacing between bearings on a rail is greater than the spacing between rails. A single screw moving table can work, you just need to make sure the table will not twist under load.

The second thing that comes to mind is what happens if you want to machine very long pieces on that table. With the back blocked off you can’t slide anything through the gantry frame. This might not be a problem on a purpose built machine, say a PCB router, however on a general purpose machine it might be a big problem. This is another one of those things that depend upon your expectations.

- I thought of covering the tracks and screws with accordion dust cover, is there a common way to protect the motion parts against dust?

This is one approach. Dust, true dust not chips, can be a huge problem if that dust gets into motion components. I’ve literally have seen bearing blocks stop moving due to dust creating a putty after mixing with the grease in the bearings. There are other choices for dust protection including sliding metal covers. The problem with covers is that it often prevents proper maintenance. For example sample before greasing the linear bearings and lead screws should be wiped down. By the way the other reason for these covers is to keep crapoff your product.

.
Overall size of the main frame is 50"W x 30"D x 22.5"H

Axis travels are X:15.75" Y:33.00" Z:4.50"

Make sure your usable work space is what you need. Remember your gantry saddle (Y-axis) takes up considerable space on those gantry rails. Depending my upon what you are doing you may need space for fixturing

Please comment before I start assembling stuff.

Cheers

Well I commented.

If the machine meets your needs I think you have an interesting solution. It isn’t a cookie cutter machine but rather a unique solution. Frankly I’d take a day or two off and just keep it out of mind as a project. Then sit down and think about how you will use the machine and see if the design fits your needs. No machine is perfect but it should be able to handle most of the tasks you imagine for it with ease. Otherwise back to the drawing board.

[get_f8]

This has been an interesting series of posts.

* blushing *

A few suggestions and then some comments.
1. Pick a construction material and stick with it as much as possible to control construction costs. This is cost is a huge priority which it is in most home builds.
2. Really get a handle on what your expectations are. This can make a massive difference in build costs and satisfaction after the machine is built.
3. Seriously consider steel for a potential cost savings over Aluminum.
4. Very few people ever complain about building a machine that is too stiff. On the flip side many have complained about a machine that is too flimsy.

I know I said I wanted to use 8020 alu channels but got convinced and changed my mind to torsion boxes.

An important thing I hadn't done was to write down my objectives and refer to the list during the conception but fortunately Peter brought me back on track.
My objectives ended up being:
1 & 1 (equally important)
- Precision which mean sturdiness and pushed me toward a fixed gantry.
- Easy to build, hence the reason I chose torsion boxes. I know it's debatable because many people will say that 8020 channels are easy to use and I won't disagree, it's just that I'm a wood guy. Steel would be great yes but it goes against this objective, at least for me as I have very little experience with welding. Ease of built is way up high because, knowing myself, I need it to be simple in order to get to the end of it. Many people suggested to keep it simple... best advice for me.
3
- Cost: partially to go easy on my lovely other half and also because it's my first router. Gotta start somewhere, make money with it, then it will be easy to justify any further purchase. Right?
4
- Powerful enough to cut aluminium It's okay if it can't do it but I'd be so happy to be able to cut aluminium and make parts for my son's R/C (Traxxas happen to be a true deception in this house!)
5, 6 & 7 (sub-objectives, things to keep in mind)
- dust management to keep it running trouble free
- clamping table you know, to clamp things down
- work area? My real (and specific) need for this machine is to cut wood that fits within 8x3 so I intended to spend time on thinking about work area but all I'm thinking of is getting this done.

Why do you think that the Y-Axis will be a specific problem? By the way the only way to tell if a stepper is overheating is to stick a thermocouple on it. Many steppers are rated with a 60° to 80° C temperature RISE that is way to hot to touch.

Because it's IN the torsion box plus I intended to solidify the structure around the motor and I figured it would cause the motor to be enclose in a tight space which would prevent to dissipate the heat.

The table arrangement you have is not the norm. The short distance between linear bearing blocks means that there will be a very high potential for twisting or even binding with a single lead screw. You will likely need two lead screws to address that potential problem. Generally with moving tables you need the bearings on the long side of the table so that the spacing between bearings on a rail is greater than the spacing between rails. A single screw moving table can work, you just need to make sure the table will not twist under load.

I'm adding right side views (without the sides so we can see something). I modified the spacing between the bearing blocks from 7.5" to 9.5". The 9.5" option is also shown with the table at minimum Y position.(front)
On the picture from my previous post, the table is a short torsion box but I thought of replacing it with an aluminium plate with some tapped holes in it so I can screw on a waste board to it. This was to save time and it would be easier to replace the waste boards. It would also reduce the table height, either to gain Z travel or to gain sturdiness by lowering the height of the machine. I don't see an aluminium plate flex enough to prevent the bearing to move. Am I underestimating the sheer strength here?

If I wish to stay to a one leadscrew setup, would it be better to put the leadscrew closer to one of the two tracks or to leave it in the middle.

Worst come to worst, I would either:
- make a short table (on the width) and put the rails closer to each other
- or invert the length of X and Y so the Y axis would use the 1000mm tracks. I did a quick switch in the drawing to see that this move would easily give a work area of about 23.75" x 23" (550in2) compared to the 13.75" x 33" (450in2) with the previous configuration. Oh boy! Here we go away! Decision, decision, decision! Haha! I love this! Some would close the books right here but ... don't we love this? (Picture added)

The second thing that comes to mind is what happens if you want to machine very long pieces on that table. With the back blocked off you can’t slide anything through the gantry frame. This might not be a problem on a purpose built machine, say a PCB router, however on a general purpose machine it might be a big problem. This is another one of those things that depend upon your expectations.

Thank you! What's more to say?... very much? Thank you very much! It wouldn't be dramatic as I plan to only cut small things but let's be smart, I'll cut the opening.

(on dust management/protection using accordions protectors)
This is one approach. Dust, true dust not chips, can be a huge problem if that dust gets into motion components. I’ve literally have seen bearing blocks stop moving due to dust creating a putty after mixing with the grease in the bearings. There are other choices for dust protection including sliding metal covers. The problem with covers is that it often prevents proper maintenance. For example sample before greasing the linear bearings and lead screws should be wiped down. By the way the other reason for these covers is to keep crapoff your product.

I work with CMMs, the accordions are attached using Velcro at one end so are easily removable for maintenance. Sliding metal covers sounds great too, and maybe cheaper.

Make sure your usable work space is what you need. Remember your gantry saddle (Y-axis) takes up considerable space on those gantry rails. Depending upon what you are doing you may need space for fixturing

That's good advice. My current needs will be covered no matter what but there's no reason to waste valuable work area just because I don't expressly need it now right?

Well I commented.

And looked good doing it. Well done, but seriously thank you very much for your inputs.

If the machine meets your needs I think you have an interesting solution. It isn’t a cookie cutter machine but rather a unique solution. Frankly I’d take a day or two off and just keep it out of mind as a project. Then sit down and think about how you will use the machine and see if the design fits your needs. No machine is perfect but it should be able to handle most of the tasks you imagine for it with ease. Otherwise back to the drawing board.

Before switching the X and Y rails I thought everything was fine... but that was before. Now I need to figure out if I want a square or a thinner&wider table. I bought the rails and ball screws kit because it was a quick and easy solution. It gives a bigger machine than I currently need and any configuration would be okay but it would be foolish to put aside design efficiency simply because I don't need it now. It will open up many possibilities to have a well designed machine and once again I sincerely thank you for your help.

[routalot]

Its good to see a thorough assessment of the objectives and this has to help with arriving at a good solution for your needs.I do have one concern after looking at the proposal in post #25 and it is the possible flexing of the vertical panel on which all the upper hardware is mounted.I would regard some triangulation as absolutely essential and the Z axis is amply braced.I can see that avoiding any obstruction to the table moving is essential and consequently I would suggest adding a pair of vertical webs at the right hand side of the machine as it wouldn't prevent locating the machine any closer to a wall than the essential stepper already does and it would add stiffness.

[davida1234]

It might be possible to install the Y stepper in front of the back brace and save some depth:

[davida1234]

You might also consider to use profile rails for the X axis as the load condition of shown shafts is at their least optimal orientation.

[routalot]

That would certainly add stiffness,not sure how much impact it would have on the working envelope of the machine though.

[ger21]

It might be possible to install the Y stepper in front of the back brace and save some depth:

Or turn the motor around and use a belt to drive the screw.

[davida1234]

Or turn the motor around and use a belt to drive the screw.

His rendering shows a stepper with dual shafts. So, I did actually turn the motor around and used the rear shaft so it could still be mounted on the cross brace.