[peteeng]

Hi All - Cranking the columns generally speaking has nothing to do with reducing the load on the bearings. It is done to recover the dead space created by the stickout or overhang of the Z axis/spindle assembly. My latest machine is a high rail design which does not have columns. I strongly recommend this arrangement, I don't see myself building columned machines again. Its clearly stiffer then using columns as the "walls" are very stiff. But TTT has gone down a particular path and Tom has to balance many issues in his design. Tom probably time to freeze the design cost it out, digest where you are then make some more decisions.... Peter

[ihavenofish]

Hi All - Cranking the columns generally speaking has nothing to do with reducing the load on the bearings. It is done to recover the dead space created by the stickout or overhang of the Z axis/spindle assembly.

People do think that, but it's absolutely not true. As a side effect you do get a more compact machine, but what you are actually trying to do is get the spindle nose inside the bearing footprint.

As with all of this stuff, if you dont know where to start (as in, you aren't a seasoned machine designer which is almost everyone here) look at machines on the market that perform the tasks you want to do. Also look at their evolutions. Look at biesse, scm, weike etc over the last 40 years. Look at datron. It isn't an accident they have all evolved to the same basic double upright design. The only machines using "walls" instead of uprights are whole room machines for milling boat moulds.

[peteeng]

Hi IHNF - Getting the tool within the "kern" of the bearings is a an ideal situation but this usually results in a bigger machine. This means there are only direct loads and moments on the bearings vs loads/moments and secondary moments. That's the compromise, the bearings are sized to take the direct loads, direct moments and secondary moments if you do the math. There are many machines that use the high rail design and they are not limited to boat moulds. There's no right or wrong answer here its up to the designer/maker to make those calls. Commercial machines need side loading and tool changers etc and this can make the walls the wrong way to go... there are many many issues to balance in a machine design Peter

[routalot]

Triangulation saves materials, but lets not confuse things. Wider = stiffer. If 20mm wide = 1 for stiffness, 40mm wide = 4. a 30mm rib might equal 2 (would need to do the fea). a 120mm rib might = 5, and therefore be stiffer and less material than just 40mm plate. But you introduce more surfaces that need to be machined very precisely, more holes, threads, more work. 5" in triangulation might be a 4.8, saving lots of material over the rectangle plate with basically the same stiffness but with all the same complexities.

The trick becomes, what is the min stiffness you need (deflection and resonance come in to play) and what is easier, paying for thicker plate or making a complex precision assembly.

When you look at mine, the 'stiffness" for the cutting load is actually fine - deflects a few tenths with 20kg force sideways, but resonance creeps in in the front to back direction. You do not necessarily have to deal with the resonance with thicker material, but often this is the easiest way. In my case though the right way is to make the upright bases wider, no the plate thicker. In his case being a much bigger taller machine than mine, thicker plate will help.

But as mentioned before, there may be other weaknesses more important on his machine to address first.

By way of an analogy;if we see a metal framed building erected,it is easy to see how the designers build in resistance to deflection under lateral loads.They don't add a second upright girder,bolted to the corner post,they use diagonal braces to triangulate the assembly.We don't usually seek to lose any space under a gantry by placing braces there and so extending the beam and bracing it to a point close to the gantry bearings is a decent solution.

[TeeTomTerrific]

How are you making these parts, uprights etc?
I have some suggestions, but it will vary depending on how you have to make them.

I'm having the uprights machined by the same Chinese company that I'm buying my extrusions from. the last quote I got for them using 20mm 6061 was $100 USD each for material, machining and anodizing. I'm sure the price is going to go up changing the thickness to 30mm.

[ihavenofish]

Ah, ok. Hopefully communication goes well
My ideas probably won't work in easily, would change the way you set up the parts.

[ihavenofish]

By way of an analogy;if we see a metal framed building erected,it is easy to see how the designers build in resistance to deflection under lateral loads.They don't add a second upright girder,bolted to the corner post,they use diagonal braces to triangulate the assembly.

Only because they want to save materials, or have windows. They could just make the wall a solid concrete rectangle

When you make uprights from solid plate, you have no reason to trim out any material unless you need to lower the weight or need clearance.

The most hilarious thing I saw in bad machine building recently was an upright from solid plate that had triangulated weight reduction machining. I don't know if they thought it looked cool or what, but all they did was make a weak upright weaker.

[dixdance]

FEA is easy and free these days. If you are building a machine without FEA then building machines is above your pay grade :P

Wrong... and rude.

You have no idea what my background is. Wonderful machines have been designed and built for thousands of years without the benefit of FEA, computers, or even electricity. Mechanical aptitude, intelligence, and experience in the shop and field have been more than adequate, and still are. I can see how FEA is useful for value engineering, but that's not my goal.

[ihavenofish]

Wrong... and rude.

You have no idea what my background is. Wonderful machines have been designed and built for thousands of years without the benefit of FEA, computers, or even electricity. Mechanical aptitude, intelligence, and experience in the shop and field have been more than adequate, and still are. I can see how FEA is useful for value engineering, but that's not my goal.

It was a joke, silly goose.
But it is true, FEA is free and easy and I wouldn't design a CNC machine frame without it.

[dixdance]

It was a joke, silly goose.
But it is true, FEA is free and easy and I wouldn't design a CNC machine frame without it.

Oh, sorry, I misread your emoticon, looked like you were sticking your tongue out. Humor doesn't always come thru well on the internet.

I don't doubt FEA is invaluable for a serious designer of CNC machines, especially routers made from extrusions. I just want a larger, accurate machine - I'm ok with overbuilding to ensure stability and rigidity.

[ihavenofish]

Oh, sorry, I misread your emoticon, looked like you were sticking your tongue out. Humor doesn't always come thru well on the internet.

I don't doubt FEA is invaluable for a serious designer of CNC machines, especially routers made from extrusions. I just want a larger, accurate machine - I'm ok with overbuilding to ensure stability and rigidity.

I find the opposite. Actually haas made a video on the topic. They say they used to just overbuild everything. Now the machines are lighter and more rigid and accurate.

Definitely you can look at existing machines and say "well this one is 10mm thick iron and flexes this much, so ill make mine 12mm thick" but FEA will let you say 'I want this much flex, my motors can drive this much weight, where should the material go"

Its just a tool, like anything else, and is not above anyone's pay grade anymore (well unless you draw your machines out on napkins).

[TeeTomTerrific]

Well I just can't seem to leave well enough alone. New version with solid milled X axis like the new Z axis, but backed up with a heavy 40160 for extra rigidity. I gained 30mm on moving the spindle backwards more towards the center of the trucks and then I moved the front truck forward 50mm for a total of 80mm pushing the z axis backward. Not completely centered on the trucks but I can live with that. Now I have to get a bunch of parts requoted.

[peteeng]

Hi TTT - One way to gain a little stiffness is not to crank the column. Good luck with the quoting. $100USD sounds exy for the columns. If you like send me a step file and I'll get it quoted here as well with my usual machinists to see the delta. Anodising is the usual $$$ and can cost as much as the part here. So flicking anodizing which is only aesthetic can reduce cost a lot... The last phase of design is called polishing, so give the design a good polish and your done. Peter

[ihavenofish]

You probably cant get the plate for $100 even at chinse prices.

[TeeTomTerrific]

Hi TTT - One way to gain a little stiffness is not to crank the column. Good luck with the quoting. $100USD sounds exy for the columns. If you like send me a step file and I'll get it quoted here as well with my usual machinists to see the delta. Anodising is the usual $$$ and can cost as much as the part here. So flicking anodizing which is only aesthetic can reduce cost a lot... The last phase of design is called polishing, so give the design a good polish and your done. Peter

Since this the manufacturer of the extrusions they are all setup for anodizing. I really think they are including it at little to no charge because I asked if the custom parts would be anodized.

[ihavenofish]

Since this the manufacturer of the extrusions they are all setup for anodizing. I really think they are including it at little to no charge because I asked if the custom parts would be anodized.

Anodising is cheaper and a scotch brite pad. Some places will have min charges, but if the company making the parts also does the anodising is should be basically free, or close to it.

[TeeTomTerrific]

I do believe I have reached the state of final design. I backed away from a custom machined X axis because the cost for that was prohibitive. However, I have strengthened both the X and Y axis by replacing the small 10mm thick mounting plates on either end for the Motor and BF15 bearing with a full length piece of 10mm aluminum attached to the axis beam extrusion. I also straightened out the funky crank on the support columns. All the table 8080 extrusions are the extra rigid type about double the thickness/weight of standard extrusions. I'm waiting on the final quote so I can place my order. The cost of the support columns went up to $180 each by changing the thickness to 30mm and widening the area that attaches to the trucks so they were spread further apart.. My next task after placing my parts order will be deciding on a controller and software. This CNC adventure certainly is never a dull moment.

[ihavenofish]

One suggestion based on experience. Since you have screws either side, brace the frame along its travel length. This will prevent if from bouncing around and sagging mid travel.

For the control... yeah good luck. ha. I'm still looking for something thats affordable but has high performance.

Mach: no.
linuxcnc: flexible, but no jerk control
Delta; expensive and needs their special DNCnet drives
LNC: cheaper than delta, but apparently a bit buggy

What ones have you looked at so far?

[TeeTomTerrific]

One suggestion based on experience. Since you have screws either side, brace the frame along its travel length. This will prevent if from bouncing around and sagging mid travel.

For the control... yeah good luck. ha. I'm still looking for something thats affordable but has high performance.

Mach: no.
linuxcnc: flexible, but no jerk control
Delta; expensive and needs their special DNCnet drives
LNC: cheaper than delta, but apparently a bit buggy

What ones have you looked at so far?

.
I believe the frame is fairly well braced. If I get any bouncing I can always add a center post on the long side midway down.

I haven't dug deeply into it, but I was looking at Centroid Acorn or Mach 4 with an Ethernet Smooth Stepper but I will have some time between arrival of parts, (at least 6 weeks), and assembly time, (probably a couple weeks), so I can take my time and explore other options as well.

[ihavenofish]

Bracing I mean vertical. post in the middle will go a long way.

Mach I would avoid. Acorn I only know a little about. I've heard some bad and some good, but not enough to formulate an opinion.

I have a DDCSV3.1 on the little machine in the video. Can't recommend. I does work obviously, but it is extremely light on features and performance. Good for cheap as chips machines, but not something like you are making.