Design Study: Rail Inline With Ball Screw

[Bravin Neff]

I'm looking for your comments please. This would be a gantry style CNC router. The Gantry is 150mm x 75mm steel rectangular tube, and the frame is 75mm x 75mm square tube. 1610 ballscrew, 20mm Hiwin HGH20 or similar.

The point of this design is to put the ballscrew directly above the linear guide, with the gantry mount sandwiched between the ball screw and linear guide carriages. I do not believe I have seen this before.

The motivations for this:

1. Ease of machining of the frame. The ballscrew mounts/motor mounts will be machined in the same plane as the linear guide rail surface. Alignment made easy.


2. An attempt to "hide" the ballscrew from inadvertent movement. This doesn't really achieve this, but it gets it away from the outside of the frame.

3. I think it looks cool.

Thanks for your comments.


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[magicniner]

Wobbly thin plate with no triangulation adjacent to the Ball Nut is probably the design's weakest point, to resolve this fully triangulate your vertical and move the ball screws outboard.

[Bravin Neff]

Would you hold to those comments if I told you the wobbly plate was 9mm thick and fully welded with the gantry?

Incidentally I have contemplated adding a triangulating plate to the inside, which would surround d the ballscrew, and so it would be the one item that isnt welded, but bolted.

[magicniner]

Would you hold to those comments if I told you the wobbly plate was 9mm thick and fully welded with the gantry?

You have a right angle welded joint holding a flat plate with no triangulation with your rails and ball nut mounted to it.
You can make it lighter, stronger or both, given these facts would you hold to calling this a "Design Study" ?
:D

[Bravin Neff]

I would. That's what it is. Do you still think it's a thin wobbly plate? Im not being combative, it's an honest Q.

[magicniner]

Then it's perfect and you should build it just as you've drawn it :D

[cropwell]

One of my ideas was to place the ballnut inside the box section I envisaged for the gantry foot. This would give enough depth for the screw covers to telescope within the gantry. The main advantage of placing the screw outside the footprint of the frame is to keep it away from the incredible amount of flying detritus these machines produce.

[magicniner]

Im not being combative, it's an honest Q.

Honest answer, if 9mm is good in that design you can use less, triangulate and make it lighter whilst still being stronger and more rigid.
Or triangulate and make it far stronger and more rigid for little added weight.

[Bravin Neff]

One of my ideas was to place the ballnut inside the box section I envisaged for the gantry foot. This would give enough depth for the screw covers to telescope within the gantry. The main advantage of placing the screw outside the footprint of the frame is to keep it away from the incredible amount of flying detritus these machines produce.

It's a good point. I've been looking at various bellows and related covers that exist on the market to help with debris, and there is a surprising amount that exists.

[magicniner]

Rather than looking for bellows search for "Wire Reinforced Hose" and suddenly there's a huge supply of reasonably priced material suitable for the same job.

[Bravin Neff]

Honest answer, if 9mm is good in that design you can use less, triangulate and make it lighter whilst still being stronger and more rigid.
Or triangulate and make it far stronger and more rigid for little added weight.

I might not be understanding you correctly, but I think you have in mind either gussets between the horizontal and vertical plates, or what I have been imagining, a small vertical detail that connects the horizontal plate with the Gantry, which would fully box in the ballscrew mount and ballscrew. I have been leaning away from that, because the design as is leaves that surface (the horizontal one that the ballscrew mount attaches to) fully exposed on a vertical machining center, making it easy to backside machine. In other words, I am trying to balance the design with ease of Machining these details.

If you can imagine this entire Gantry as one welded fabbed unit, then putting it on a vertical Machining Center, upside down, makes the rail Carriage surfaces easy to machine flat, and then you can back side machine the opposite side, making the ball screw Mount Services flat as well.

[Robin Hewitt]

Is this your first design? What is it supposed to cut? How big is it?

[magicniner]

Basic Engineering Design revolves around Triangulation, without which you will never have optimum rigidity for the weight of your design.

[Bravin Neff]

Is this your first design? What is it supposed to cut? How big is it?

Yes. Wood and aluminum. Roughly 950 x 630 x 135.

[Robin Hewitt]

If you are trying to put the screw close over the linear blocks, why don't you sink the nut in to the plate and drop the screw down? How are you going to keep the cuttings out of the screws? You have made it from heavy section to cut aluminium but that will not help you get the speed to cut wood efficiently. If your gantry weighs 50 lbs then you need 50 lbf plus the cutting force to accelerate it at one G.

OTOH I think you need to build something wrong and then fix it with the Mk2. Understanding CNC design does rather come with experience. Getting it wrong is not actually a bad idea.

[Bravin Neff]

If you are trying to put the screw close over the linear blocks, why don't you sink the nut in to the plate and drop the screw down?

I don't follow you here. Do you mean machine a pocket into the plate in order to get the ball nut lower (i.e, closer to the linear carriages)?


How are you going to keep the cuttings out of the screws?

Guards, covers, bellows. They aren't shown.


You have made it from heavy section to cut aluminium but that will not help you get the speed to cut wood efficiently. If your gantry weighs 50 lbs then you need 50 lbf plus the cutting force to accelerate it at one G.

I don't follow you here. You seem to say it won't be fast enough by saying it won't accelerate enough - is that what you mean? Regardless, I assume there is always a servo strong enough. Some of the machines I sell (for my day job) weigh over 30,000 kg and can accel at 1G. Of course those motors are huge... I'm personally looking at the Clearpath servos. Nema 23 and 34 frame sizes.



OTOH I think you need to build something wrong and then fix it with the Mk2. Understanding CNC design does rather come with experience. Getting it wrong is not actually a bad idea.

I think that's sage advice. What part are you seeing that is wrong?

[cropwell]

Why not put the screw inside the frame :whistle:

[Neale]

I would consider moving the ballscrews just outboard of the ends of the gantry. This allows a more rigid fixing of gantry to carriage feet. If the ballscrew mounts are on outrigger brackets welded to the outside of the frame tubes, mounting pads for the ballscrew bearings could be machined at the same setting as the frame tubes. There is no benefit in having the ballscrews over the slides as a gantry driven at both ends is balanced anyway. It's not like the gantry ballscrew which is more difficult to fit between the linear rails.

The idea of being able to machine the top of the long rails is so far from reality for most of us that I doubt if many have designed for that! I'm in the "if I can't machine it, build in adjustment" camp...

[Bravin Neff]

I would consider moving the ballscrews just outboard of the ends of the gantry. This allows a more rigid fixing of gantry to carriage feet. If the ballscrew mounts are on outrigger brackets welded to the outside of the frame tubes, mounting pads for the ballscrew bearings could be machined at the same setting as the frame tubes.

Great point and idea. I'll try to mock something up. I am definitely motivated by making the machining of the machine as straightforward as possible and only requiring a vertical mill. When I see machines, similar to what I just mocked up except the ballscrews are outside of the frame, mounted to the outside frame vertical wall, this is either (1) asking for a large boring mill or horizontal machining center, with (2) multiple setups, just to machine the ballscrew mounting pads. That starts to add up fast. Or the pads are painstakingly fitted by hand with shims and numerous iterations of racking the gantry back and forth to get alignment, and then I question how flat and parallel the mounting surfaces can be.


The idea of being able to machine the top of the long rails is so far from reality for most of us that I doubt if many have designed for that! I'm in the "if I can't machine it, build in adjustment" camp... I am surprised to read this. Granted, my machine is a little on the small side compared to many I see here, but it will fit on an 80" x 40" machine. The going shop rate is $60/hour near me, so I am counting on all the machining work coming in just a few hundred $. Given how much money people invest into designing and building their own CNC routers, I would thinking have the rail surfaces and mounting pads machined would be easy to justify.

[routercnc]

My current machine uses something similar. I was trying to get the gantry sides as stiff as possible, and make a good connection to the rails. Fitting the ballscrew in there as well was just convenient.

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[Neale]

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I am surprised to read this. Granted, my machine is a little on the small side compared to many I see here, but it will fit on an 80" x 40" machine. The going shop rate is $60/hour near me, so I am counting on all the machining work coming in just a few hundred $. Given how much money people invest into designing and building their own CNC routers, I would thinking have the rail surfaces and mounting pads machined would be easy to justify.
It's an interesting point, and I wonder if there is a difference between UK and US here. I have no idea where I would go to find a jobbing engineering shop anywhere near me, let alone one with that kind of capacity. My own machine top rails are roughly 1.8m long, 1m separation, on a floor-mounted machine. It would be great to have had that machined but like most (UK) folks on this forum, I ended up with an epoxy bed instead. My background is IT so I have very few contacts in the engineering world - I wonder if anyone else knows how likely/easy it would be to find machining capacity like that, available at an acceptable cost, in the UK?

[Bravin Neff]

My current machine uses something similar. I was trying to get the gantry sides as stiff as possible, and make a good connection to the rails. Fitting the ballscrew in there as well was just convenient.

Haven't seen that before. Great job!

[Ger21]

I am surprised to read this. Granted, my machine is a little on the small side compared to many I see here, but it will fit on an 80" x 40" machine. The going shop rate is $60/hour near me, so I am counting on all the machining work coming in just a few hundred $. Given how much money people invest into designing and building their own CNC routers, I would thinking have the rail surfaces and mounting pads machined would be easy to justify.

In nearly 15 years of reading a majority of the DIY forums daily, I can count on one hand the number of people who have had their welded frames machined.
The reasons, as stated above, are probably that most people don't know where they could have it done, at a reasonable cost.

[Bravin Neff]

It's an interesting point, and I wonder if there is a difference between UK and US here. I have no idea where I would go to find a jobbing engineering shop anywhere near me, let alone one with that kind of capacity. My own machine top rails are roughly 1.8m long, 1m separation, on a floor-mounted machine. It would be great to have had that machined but like most (UK) folks on this forum, I ended up with an epoxy bed instead. My background is IT so I have very few contacts in the engineering world - I wonder if anyone else knows how likely/easy it would be to find machining capacity like that, available at an acceptable cost, in the UK?

I am sure regional differences apply, and not just between countries. Where I live and work, there are over 800 CNC machine shops within a 50 mile radius of where I live. I also have colleagues elsewhere in the country that have to travel 500 miles in order to reach fewer shops. I am sure there are hot spots in the UK for manufacturing.

[Bravin Neff]

In nearly 15 years of reading a majority of the DIY forums daily, I can count on one hand the number of people who have had their welded frames machined. The reasons, as stated above, are probably that most people don't know where they could have it done, at a reasonable cost.

I am sure you are right. I have also noticed there seems to be very little overlap between the DIY CNC world and the commercial CNC world. I have worked and operated CNC machines my entire working life, but only have discovered the DIY world a few months ago. And now that I'm excited by it, I talk about it all the time with colleagues and customers, and I have yet to come across a single one of them that knows anything about it or who dabbles in it. I find that remarkable.

[cropwell]

I am sure regional differences apply, and not just between countries. Where I live and work, there are over 800 CNC machine shops within a 50 mile radius of where I live. I also have colleagues elsewhere in the country that have to travel 500 miles in order to reach fewer shops. I am sure there are hot spots in the UK for manufacturing.

Every little job that I have deemed would benefit from machining (generally milling or surface grinding) I have received exorbitant quotes for (I believe they are known as 'F**K OFF' quotes). The main reason given that setting up is the bulk of the work (£200 ish min) and so a one-off job is not economically viable.

If I wanted 1000+ machining, then I would guess the set-up would be amortized quite magically.

Basically, finding a small machine shop in UK that is struggling for work and prepared to talk sensible pricing for small jobs, seems to be difficult.

[zvon]

I'm looking for your comments please. This would be a gantry style CNC router. The Gantry is 150mm x 75mm steel rectangular tube, and the frame is 75mm x 75mm square tube. 1610 ballscrew, 20mm Hiwin HGH20 or similar.

The point of this design is to put the ballscrew directly above the linear guide, with the gantry mount sandwiched between the ball screw and linear guide carriages. I do not believe I have seen this before.



you're on to something there but as the otehr members said the weak point is the upright plate with no bracing for side to side loads. here's the same concept with one form of bracing added to what you initially drew
i like the idea it's very clever & done right could be quite sturdy

[zvon]

I'm looking for your comments please. This would be a gantry style CNC router. The Gantry is 150mm x 75mm steel rectangular tube, and the frame is 75mm x 75mm square tube. 1610 ballscrew, 20mm Hiwin HGH20 or similar.

The point of this design is to put the ballscrew directly above the linear guide, with the gantry mount sandwiched between the ball screw and linear guide carriages. I do not believe I have seen this before.

The motivations for this:

1. Ease of machining of the frame. The ballscrew mounts/motor mounts will be machined in the same plane as the linear guide rail surface. Alignment made easy.


2. An attempt to "hide" the ballscrew from inadvertent movement. This doesn't really achieve this, but it gets it away from the outside of the frame.

3. I think it looks cool.

Thanks for your comments.


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