[jerseyguy1996]

I noticed that you changed the Z axis design to have the rails on the back of the spindle mounting plate.......it has drawbacks in that mode.

Thank you for all of this guidance! Regarding the statement that I have quoted, can you elaborate on the drawbacks? I am looking to understand the tradeoffs that I will be making by doing it this way versus having the blocks on the spindle mounting plate and the rails on the Z-Axis plate. My concern is that if I put the blocks on the spindle mounting plate, the greatest depth that I can get will be the length of the barrel of the spindle minus the length needed to clamp it. I will draw it up in sketch up this evening so that it will be easy to understand what I am talking about.

[handlewanker]

Hi, a lot depends on the actual size of the machine and the travels etc, but if you have the rails on the spindle mounting plate they will hang down and the further down you go the more unsupported stick out length you have.

Having the rails on the fixed saddle portion means you can go higher or longer with the rails and place the bearing blocks further apart without impinging on the space beneath the gantry as the increased spacing of the blocks allows you to go upwards more.

Spacing the blocks further apart also gives you a better aspect ratio or longer is better than wider in slide configuration.

I went to a model build specifically to inch my way into getting the most travel without compromising the stiffness of slides and I used some of the stiffness of the spindle body as a means to get the 130mm travel without having a long slide hanging down......it's was six of one and a half dozen of the other.

In the photo I posted a couple of posts back, the Z slide is at the bottom of it's travel and hardly sticks out beyond the rails and having the spindle body to go the rest of the way to get the 130mm clearance I want.

I'm also having the Z axis slide going up more than the 130mm, probably about 180mm as it's just longer rails up in the air as this will allow me to have long drills in a drill chuck and still be able to have the maximum 130mm height for any job I want to put on the table, such as a box or solid block to be carved etc.

It's paid dividends to make the model as you can step back and evaluate it as if it were the finished job and see where you could have made gains without having to recut metal......having torn the model apart three times to make major modifications etc which in a finished steel build would be practically impossible.

I'm a visual person, and can get more feel from a solid object in front of me than a drawing, although the principal and concept originated with scaled free hand sketches.....drawings for me come after the build is finalised, mainly for archive purposes, and only then to have a record of the dimensions when it's all welded together......in case I wanted to make another or a bigger one.
Ian.

[ger21]

I think in this case, it's advantageous to have the shafts mounted to the spindle plate, as it increases the rigidity of the plate, especially when it's hanging down below the bearings.
Personally, I think you'd be much better off eliminating the side plates, and dropping the gantry right down onto the rails.

I will probably never need that much depth in a machine....as I will probably spend almost all of my time routing sheet materials no more than 3/4 inch thick. I'm building it with all of that Z travel as a "just-in-case I need it" kind of thing.

What this does, is give you a weaker machine. realistically, unless you're cutting foam, you can't really route more than about 2" deep. So all lifting the gantry gives you is the ability to mount tall work pieces. Give yourself maybe 5" under the gantry, and it should be able to accomodate just about anything you might want to do, and will make the machine more rigid.

My biggest concern would be the "Stacking" of the bearings on the gantry, as well as their orientation. Your moving the Z axis very far away, which is increasing the forces placed on the bearings. Also, mounting open bearings in a horizontal position is probably their weakest position, as they'll want to pull off the shafts from the forces placed on the Z axis. Be aware that these bearings tend to have some play, and are not the most rigid option.
Have you considered placing one rail on top and one on the bottom of the gantry? This brings the Z axis closer, and puts the bearings in a more favorable orientation.

[jerseyguy1996]

Currently I do have the gantry right down on the rails. Is this what you are referring to?

Gantry by jg1996business, on Flickr

Regarding the Y-Axis, I would love to have it snug up against the gantry horizontal beam, but I always run into the problem of what to do with the ball screw. I need at least the height of the SBR Rails to fit the ball screw nut in there. Can I put some low profile linear slides on there and then just push and pull from the bottom rather than the middle?

Ball Screw Clearance by jg1996business, on Flickr

[LeeWay]

There is really no rule that says you have to have the ball screw in the middle. In fact, putting it at the top or out back behind the gantry will help keep it cleaner. Most guys put them in the center for ease of design and installation when they have machines to help with the build. Those that don't, seem to find other ways.

You could split the gantry into two tubes or extrusions and back it up with a large plate. That would give you room to sink the ball screw into the gantry. It would help protect it better as well. Then you could use some low profile rails.

The main issue I see with your setup as is will be the cussing going on when you try to install all those bolts in your bearings. You will have to consider the best way to handle that.

[handlewanker]

Hi, there is a price to pay with having the rails like you have in the last photo.........the bearing blocks are bolted from their top sides and placing two back to back with the plate between them makes the placement of the bolts difficult as the bearing blocks need to be off set to one another.

In the bottom photo of your last post you have the Z axis bearing blocks mounted up from the bottom of the plate.......with the rails down at the bottom of their travel you have more unsupported rail stick out.....that is why I suggested you mount the rails to the Z saddle.........the rails also screw down from their top side.
Ian.

[jerseyguy1996]

@Leeway - Great idea splitting up the gantry. This is what it looks like using the CRP rails and splitting the gantry. Obviously there will eventually be a steel plate that bolts across the back. I just don't have it in there yet to make it easier to see the ball screw positioning.

Using CRP Rails for Y Axis by jg1996business, on Flickr

@handlewanker - I'm going to experiment with different positions for the blocks and rails on the Z-Axis next and I will post some updated images.

[handlewanker]

Ah Ha.....I see you've drawn the flat plate and ballrace slide way system for the X axis.........this would work and could be cheap to make as well as solving a lot of the bearing problems, just be aware that the ball races will/could indent the soft mild steel surfaces of the slideway, but I expect that won't happen unless you get impact abrasion to the X axis carriage way.

I envy you your ability to produce those drawings that give so clear indications of where each part fits.......almost like the real thing.

I've just loaded Turbocad V16 to my computer, but the damm thing is designed for Win XP only and it hardly runs on Win 7.....grrrrr......maybe I can get a Win 7 upgrade or driver without breaking the bank.

The design is looking good........bolting the parts together is going to be a challenge to find material thick enough to tap in to without having a plethora of angle brackets at each change of direction.........that is one reason I'm going for the fully welded frame work on the build I am on, but that is also because I have welding equipment and can use it.

There's no reason to stay with a bolted design.......you could draw a parallel version using welding to do the construction........alignment due to distortion is also a problem with welding if you're not careful.

BTW, splitting the gantry cross beam is a very practical way to get the ball nut housing clearance and I'm using that method too with two 80 x 40 tubes and 10mm thick plates welded to the front for the bridge, but using 20mm HIWIN linear rails on the face of the plate.

I see in the last drawing you have the X axis tube bolted to the upright with through bolts........you'll get crushing of the tube without being able to secure it with that method.......if you drill the tube bigger and fit inserts/bushes between the back and front face of the tube you can pull the bolts up tight without the crushing effect......or you can just weld plugs in on one side only and drill and tap directly into them, but if you don't have welding facilities then a straight through bush will work also.
Ian.

[jerseyguy1996]

I use Trimble Sketchup for all of this. I am using the freeware version of it although they do have a Pro version for about $600. So far I have not had any issues with the limitations of the freeware version. There is a slight learning curve as with all software applications but within a day you will find yourself using it fairly effectively. The nice thing about it is that I can group components that will move together, such as the Z assembly, and then slide it back and forth to make sure that parts aren't going to come in contact with each other.

[handlewanker]

Wow, if you can get those results that you posted with a freeware version, who needs $600 outlay.....probably if you were in business to make money etc.

I paid $45 on EBAY for the Turbocad V16........maybe I'll get my old slow as a wet weekend computer with Win XP up and running and see what Turbocad can do........after learning how to use it....sigh.........all my designing is done with MS paint and a Wacom graphics pad....OK for 2D rendition but a bit of 3D work would be cool.
Ian.

[jerseyguy1996]

I see in the last drawing you have the X axis tube bolted to the upright with through bolts........you'll get crushing of the tube without being able to secure it with that method.......

That's kind of a leftover from the first iteration of this design. My plan is to cut big holes in the front face so that I can get a socket wrench through it to tighten up the bolt that goes through the back face and into the upright. The upright are 1/4 inch walls so I will just tap them directly. Actually I am hoping that I will be able to construct most of this with socket head screws (the black screws that are all over the place). McMaster-Carr That way I only need a small hole in the front face of the tube to get the end of the allen wrench into.

[jerseyguy1996]

Well here is the Y and Z axis with the CRP rails. I had to bring the ball screw out to the front to make it fit. It is really snugged in tight to the gantry now. What do you think? A few questions that I have: Is it reasonable to have the bottom of the ball screw unsupported? Will a thrust bearing above and below the bearing block at the top of the Z axis ball screw be enough to allow free rotation? In other words given the orientation and the direction of forces, would I need a roller bearing?

Using CRP Rails for Y and Z by jg1996business, on Flickr

Using CRP Rails for Y and Z Front View by jg1996business, on Flickr

Using CRP Rails for Y and Z Close Up by jg1996business, on Flickr

[pippin88]

Fine to have one end of a Z axis unsupported, it will never reach the RPM to whip.

[handlewanker]

Hi, as you have spent hours doing the drawing and thinking to get to the latest design, forgive me if I voice some criticism.........it is meant purely as a constructive tool not as a destructive yet more comment trail that some seem to delight in.

Firstly, having spent many years in the trade of engineering, it would say that if you can get those two X axis rails dead blind parallel I'll eat my hat........the span of the two separate slides makes it a difficult job to have a long slide dead parallel over that width and length.

I suggest you make the top slide as the deciding factor and have the ball races fitted to this one only for guidance on all 4 faces, that is top and bottom edges and front and rear faces......this configuration will give you the long narrow slide that sliding ways require to work without having tight or loose spots.

The plate that the ball races run on will also have to be above and below the top tube that they are attached to so you'll need more clearance between the two gantry tubes to clear the nut housing and the bottom bearing etc.....easy peasy, just make the top tube go a bit higher

The bottom slide then only needs to have 4 ball races, 2 on the front and back at each end of the slide to prevent rotational forces of the carriage.

In other words, have the top slide as the major guide way, and the bottom slide as the secondary slide.

Attempting to span a wide slide as you have drawn will not give you a true track guidance to run on as even a couple of thou difference in the plate spacing will make the slide loose or tight.

It's only a small mod in actuality, the top bearing plate just needs to be wider to protrude above and below the top tube to allow the ball races to run on it's front and back and both edges, (making that plate dead parallel is not too difficult) while the bottom plate can stay as it is only protruding below the tube to allow the bearings to run on the front and back faces only.....NOT THE EDGES.

I hope this is clearer than mud, but I could do a sketch if needed......MS paint in Jpeg is simple to illustrate a point.
Ian.

[jerseyguy1996]

@pippin88 - That is the answer that I was hoping for! Thanks!

@handlewanker - I greatly appreciate all of the advice that you have given me! One of the nice things about 3D modeling in the computer is that I can work out all of these details without having to do any trial and error with actual materials. It's interesting that you brought up the issue with the parallel slides in that one of my concerns with splitting the gantry was that it adds a few more degrees of freedom that I will have to contend with when getting everything lined up. I completely agree with your idea of using the top slide as the guide and the bottom slide just to handle the rotational forces. I'll draw it up and post it and you can tell me if it is what you were thinking.

[jerseyguy1996]

I think this is what you are describing handlewanker:

3 CRC Y Axis Bearings by jg1996business, on Flickr

Although now I am thinking about going back to a 2X6 inch tube for the gantry and just have a single wide cold rolled steel plate for the linear rails and I'll move the ball screw up to the top. At this point the gantry is 10 inches wide which seems excessive for a 40 inch span.

[handlewanker]

Hi, yes, that is how I envisage it.

The gantry tube width on the face can be as wide as you want, better wide than narrow....it doesn't obtrude into any part of the structure by going high and it will stabilise the Z axis by being broader than it is long........a long narrow single cross beam will not be too stable and due to steel having inbuilt spring tendencies having two tubes one above the other as you have drawn it is in my opinion good design, as long as you also have a plate spanning the two tubes and bolted to it at the back and for the full length between the side supports.

The big problem with gantry routers is the need to not carry too much weight in the moving parts due to inertia forces, which tends to encourage the lighter gantry design and that induces some degree of twisting to the crossbeam, so if a lighter tube design like two tubes and a back plate can work it will be one step conquered.

On a 40 inch span I would use two pieces of 100 X 50mm tube (100mm side upwards, and a 50mm space between the tubes) and that will allow the ball nut to stay in the middle and still clear the plate at the back.

That gives you a potential 250mm wide crossbeam and I think that will be beneficial to keep the crossbeam from twisting and the Z axis from swaying forward and back with cutter forces.

You already have the side supports right down on the Y axis rails so with the wide crossbeam that is win win in my opinion......better than having the Z axis sticking up without any backing support.

The next problem is to overcome the effects of gantry slewing when the cutter is to the left or right side of the table and exerts more force to one side of the gantry than the other.
Ian.

[jerseyguy1996]

With regards to gantry slewing, the plan was to have a ball screw pushing on both sides. Will that be sufficient?

[LeeWay]

With regards to gantry slewing, the plan was to have a ball screw pushing on both sides. Will that be sufficient?

I think that will work great. That is also what they mean by racking.

[handlewanker]

Hi, one solution creates a further problem.

Yes, it will work, and in the ideal World is a perfect solution..........but.......you need to have both stepper motors deriving their signal from the same point......easy peasy, just plug them into the same point.........if one stepper misses a step or fails to respond you get one side of the gantry under huge forces and at worst it will just stall the motor after it has pushed the gantry off to the one side.

If you miss a couple of steps on one side it won't be noticed and this will give you a gantry out of square all the time unless you have something that detects the out of squareness and can realign it.

CNC machining is hugely dependent on the accuracy of the stepper motor/ballscrew increments, and at the same time the squareness of the dimensions that create the job are dependent on the alignment of the XY and Z axes.

You could drive both screws from one stepper motor in the middle with twin toothed drive belts.........very difficult with this design as you have the gantry on top of the rails and the table below it so where do you place the screws?

If you have the screws next to the Y axis rails you can't couple them together....the table will get in the way.

In many, if not all, of the 3020, 3040 and 6040 gantry routers currently sold on EBAY the gantry has the side plates connected beneath the base frame with a wide plate and the ballnut housing is usually mounted in the middle, that is for single screws..........if you have a double screw with that design it is simple to connect them together at the ends with a toothed belt to both screws and either single or twin stepper motors......the toothed belt with twin stepper motors ensures that if one motor fails the other will maintain the squareness of the gantry or stall if it can't drive both, but no harm will come to the gantry.

You could of course cross your fingers or pray to whom you please that it won't happen,

In a twin rack and pinion drive this cannot happen, but R&P is not everyone's cup of tea.

There is a way with a passive rack and pinion for synchronisation, (takes more than a one line explanation to describe how it fits and works) but no doubt you will have your own preference in the drive method.
Ian.