Z Axis Design Help Needed (by tonight!)

[amishx64]

Hi all.

I would like to show off...errrrr...criticize my Z axis design and get some opinions on it. This is my first SW drawing.
Background info:

I want to cut mostly plastics and aluminum with the possibility of doing a bit of steel if I have to.
I am planning a steel-construction moving gantry 3'x4'x1' machine. I have not yet fully designed the machine.
I have bought all of the linear rails I intend to use as well as the Z axis ballscrew.
I want to order the aluminum by tonight from Enco while they have their Cyber Monday 20% off + free shipping. This is kinda important due to budgeting.

Questions:
1) What thickness aluminum plate should I be shooting for?
I figure the plate that meets the gantry could be 3/8" or 1/2" and the router-mating plate should be ~1" thick. I see overkill as a good thing.

2) Should I consider elevating the linear rails in order to get around having to remove the middle section of the router-mount plate or is it better to just remove a portion of the plate (as shown)?

3)The amount of Z travel is based on the ball screw I bought. Should I leave the travel the way it is or attempt to reduce it to minimize flex?

Approx plate dimensions:
270mm wide x 500mm tall or ~10.5" x 20"



P.S. I somehow misaligned the back bearings slightly. Ignore that.

[awerby]

[QUOTE=amishx64;1027811]Hi all.

I would like to show off...errrrr...criticize my Z axis design and get some opinions on it. This is my first SW drawing.
Background info:

I want to cut mostly plastics and aluminum with the possibility of doing a bit of steel if I have to.

[This looks like a heavy-duty machine, and it will probably be able to cut aluminum, if you keep it all as sturdy as it seems. But I doubt it will be a good machine for cutting steel.]

I am planning a steel-construction moving gantry 3'x4'x1' machine. I have not yet fully designed the machine.

[Why don't you keep designing, before buying materials piecemeal? You'll doubtless need more aluminum for the rest of the machine, and you might as well get it all at once. It's amazing how things change, as you consider how the different parts interact. If maximum rigidity is a consideration, then a moving-table design beats the moving gantry approach.]

I have bought all of the linear rails I intend to use as well as the Z axis ballscrew.
I want to order the aluminum by tonight from Enco while they have their Cyber Monday 20% off + free shipping. This is kinda important due to budgeting.

[While Enco's a good place for import tooling, I don't believe that they really put out that great a deal on metal stock. Shop around a bit for your aluminum; you might be surprised...]

Questions:
1) What thickness aluminum plate should I be shooting for?
I figure the plate that meets the gantry could be 3/8" or 1/2" and the router-mating plate should be ~1" thick. I see overkill as a good thing.

[I'm not sure about that - at a certain point, it's just dead weight that your motors have to push around. 1/2" is probably plenty thick enough.]

2) Should I consider elevating the linear rails in order to get around having to remove the middle section of the router-mount plate or is it better to just remove a portion of the plate (as shown)?

[Yes, you should deeply consider how this all fits together. But it might make more sense to mount the ball-screw on the back side of the plate you're mounting the router on, and to make clearance for it without sacrificing the integrity of the structure. Of course, it's also good to keep the router as close to its supports as possible, to minimize adverse leverage.]

3)The amount of Z travel is based on the ball screw I bought. Should I leave the travel the way it is or attempt to reduce it to minimize flex?

[It's hard to say - what size parts were you thinking of making with this? You need your longest tool to clear your highest part.]

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[amishx64]

awerby,

Thank you for your response.

Let me start by saying that you make very good points. I have not designed the entire machine, but I do know how it is going to go together for the most part. The reason I wish to buy Aluminum from Enco is that other online distributes charge the same plus shipping and I have a once-a year deal here (I keep tabs on them). Unfortunately, there is absolutely nowhere local that can supply me with Aluminum. That is why I only intend on using it for a small part of my machine - the Z axis. I do have a local steel distributor which I will be buying from.

My first machine was a moving-table design. I do appreciate the rigidity that it provides but in order to maximize working area, I will have to go with a moving gantry design. I don't want to spend $1000 on rails so that I can maintain the same working space with a moving table design.

As for the Z axis:

I agree 1/2" should suffice. I think I will go with that for the back. I read somewhere that the router plate should be thicker however, so I am still considering a larger size there. I do think that 1" is a little thick though.

I don't think mounting the ballscrew on the back of the plate is ideal. I would introduce a lot of moment forces (1.5+ inches distance) that way. It will have to go on the front but I will have to add in something to give me room to eliminate the ugly gap.

As for the parts I am trying to make... I am not sure. I want a machine that can take care of most of my needs. I'd guess that in the future most things will be 4" maybe 5" or less thick. I'm not really much of a machinist yet and my needs will grow. I am trying to accommodate that future growth to a certain extent. The good thing about not having that much cash to spend is that I don't have tool based constraints yet. I only own a couple of small HSS endmills and one carbide.

I will mock up a spaced out version of the Z axis and see what happens...

[amishx64]

I have made adjustments. All plates are now 1/2". I added two square 1/2" pieces to add to the height of the bearing block, then another 1/2" 'bridge' to jump the two, then the third 1/2" piece which will hold the router.

Any opinions on the rigidity?

[ger21]

If that plate is 20" long, I'd say it will flex quite a bit hanging down that far. I'm not a fan of that type of design. I'd prefer the bearings to be much lower, and let the spindle hang down in it's mount, if it's long enough.

[amishx64]

If that plate is 20" long, I'd say it will flex quite a bit hanging down that far. I'm not a fan of that type of design. I'd prefer the bearings to be much lower, and let the spindle hang down in it's mount, if it's long enough.

I am going to be using a M12VC for a while, so I'm not going to be able to let it hang down much at all.
M12VC pics in first post

I've decided to follow Andrew's advice and not purchase anything else material-wise until I have modeled the entire machine. So in the mean time, do you have any suggestions on how to rearrange the components? If I understand what you are getting at, you would extend the base plate and move the rails down with it but leave the ball screw in place. I would also need a longer 'router plate' to connect the ballnut and bearing blocks. That brings me back to the short router/spindle issue. I could add an adjustable collet and shank to fix the issue, which I do have. Runout would be my next smallish concern. What do you think?

[vegipete]

Like Gerry says, I'd be worried about flex too. If you left room, you could add vertical bolt on ribs to stiffen the plate, as I've sketched in green on your top view image. Or the two ribs could be mounted farther out, as sketched in red. But I would be still be worried about flex down at the cutter tip. There are only the two z-axis bearings on the rails to provide all required stiffness...

[amishx64]

Hmm yes very good idea. I think angle aluminum supports would work. I'd likely put them on the outside that way there is more room for them to branch over past the bottom plate.

Now I'm really stuck between doing this and making the plate thicker, say 3/4". Maybe I'll do both. Thoughts? I'm almost more worried about the bearing flexing than the aluminum. I do have very high quality, new bearings however.