Why machine rigidity matter that much if the cutter is the least rigid part?

I'm making a steel frame router and I'm making it heavy and rigid as much as possible and this question popped up today.

Is all the work regarding machine rigidity there to deal with vibration during cutting or is there any other reason to make machine more rigid (after we pass the point of machine lossing acuracy due to flex and lack of rigidity etc.)

For example the machine frame I have attached, is this just wasteful? The cutter is going to flex 100x more than the frame... would this frame be just as good with 4 legs and 4 pipes for bracing...

English is my 2nd language and I'm sorry if I didnt get my questions across. What are your thoughts on this matter?

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Attachment 439970

Your attachment didn't post, but rigidity is important even with small cutters which can flex a bit. If your Z axis, which holds the spindle, is just a little bit loose where it attaches to the gantry or bridge, that will translate to a much larger movement down at the end where your tool hits the material. If that's significant, the cut will waver, even if the tool is perfectly stiff. And if it's big enough, it will snap the tool, especially if it's small in diameter. Similarly, flex between the gantry and the rails it runs on will be magnified at the tool tip due to leverage effects. Your frame cannot be too rigid; any little bit of slop anywhere in the structure will be evident where the cutter meets the material. Tools can deflect a little, but that usually only shows up as slightly undersize parts, not as irregular cuts.

To see how well your machine stacks up, put a dial indicator up against the collet and, with all the motors locked up, push against the spindle. If your indicator only moves .001" (.0254mm) or less, you've got a pretty rigid setup. Much more than that and you need to restrict the types of materials you attempt to cut; the harder they are, the more rigidity you'll need,

When your machine moves, it will shake, vibrate, and deflect. Any of this movement will show up in the cut quality.

router run at high speeds and all your tools are carbide. carbide is very hard and makes for good cutting tools but it is also very brittle so the machine has to be rigid or your tools will break. carbide tools are not cheap so getting the most life out of your tooling is a key factor to making things in a cost effective manor. so the more rigid the machine the further ahead of the game you will be.

This is a frame I was thinking about, is this an over kill or do you think it's worth the money and time to make all this bracing etc.https://uploads.tapatalk-cdn.com/202...175360722e.jpg

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It looks like a lot of extra metal to my eyes.I regard the legs as just the components that keep the working components off the workshop floor.I would leave out the 2 pairs of central legs and rely on the triangulation of the sides and ends to keep everything in place.Then I would focus all of my attention on the massively inadequate spindle mount and it's location as the mounting plate will bottom out on the workpiece long before the tool reaches it.

Post a few more photos and details please.

Not overkill.

Quote:

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Originally Posted by routalot

Then I would focus all of my attention on the massively inadequate spindle mount and it's location as the mounting plate will bottom out on the workpiece long before the tool reaches it.

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this is a very good point but to take it a step further the center line of the spindle is a mile off the gantry and this negates a lot of the rigidity of everything because you have a hug lever now working against you.

Hi Niko - So you build a massively overdone bench yet fill your gantry and Z axis with holes? The Z axis needs to be exceptionally stiff as does the saddle and gantry. Holes do not help you in the rigidity dept. Peter

I'm not sure if it's overkill but to my eye it's over complicated.
Many milling machines weighing up to 1000lbs or more sit on sheet metal stands. Nothing more than sheet metal skin and empty space.
Applying that idea replace the long diagonal brace with a sheet of material. Even if that sheet material is plywood stiffness will be increased.
The same could be done on the ends if storage under the table isn't important. Less to be gained here.

Extra feet can cause more problems than they solve.

This is a design I think I'm going with. Everything is made from steel tubing (legs are 80x80mm tubing, Y axis is made from 120x40mm tubing and X axis gantry is 200x80mm (that's 3x3, 5x2 and 8x3 inches). The X axis is bolted down to the frame for adjustment and the frame is welded. I'll enclose the entire frame with plywood or sheet metal for added rigidity of the frame and to have some storage under the table.

I intend to fill all tubes with sand. I'll fill the gantry too if two Nema34 (8Nm or 1200oz/in) motors can move 40+kg/100lbs without struggling too much.

For Z axis I'll use 20 or 25mm (one inch) aluminum plate and I'll reinforce the motor mounting plate with flat bar bolted on each side for rigidity. Is this setup capable of some light aluminium milling? My spindle will be 2.2kw Chinese one or 3kw if there is any reason to go with more powerful one.

This is my first machine and my budget is tight so I'll love to hear any advice you have to give me. Thanks, you are a wonderful community.

Hi Nikol - Just be careful when you weld in the diagonals. They will pull the top beam down as the welds shrink. This will put a bow in your table top. So just stitch weld them on the sides not the inside faying surfaces. Plus use a smaller tube (its a brace not a main member) so the weld is not on the radius. The radius makes a very large gap that when welded shrinks a lot hence deforms a lot. If you want to cut Al your gantry needs to be square. It's too narrow at the moment and will vibrate in that direction which is the main cutting force direction . Id suggest you weld plates on the ends of your braces and bolt them in vs welding them in this will mitigate the weld distortion....

Make your fits perfect and turn your drive nuts to the correct orientation. Bad form having your nuts twisted...Cheers Peter

Thanks Peter for your reply. I didn't think about about using smaller tubes for bracing, it's a good idea.

I've designed the whole machine so that I don't have to worry too much about distortion from welding. I've designed the Y axis to be bolted on top of the frame and as long as I shim the 2 rails to be in the same plane it doesn't matter if the table bellow is warped because I'll resurface it. Am I correct in thinking like this?

As you can see on the 4th picture I've added plate to brace the gantry and make a stronger connection with the aluminum riser blocks on the Y axis. And I'll weld some rebar inside the gantry tube to stiffen it up even more. Do you think this will be enough or should I make a complete redesign with an L shaped gantry?

I didn't understand the last bit about the drive nuts, can you elaborate?

Thanks.

Gantry should be square - 200x200mm. Gravity is not the major force here, cutting is.

Hello Nikol
1) fatal error No1 either live in the welding world or bolted world. Welding = distortion and always has to be considered when your making a machine that needs to be precise. You have to define "precise" by the way
2) Do not weld "rebar" into the gantry. You just contradicted yourself. You said you "designed" the machine so weld distortion did not have to be considered then you say you are going to weld something into the gantry that will warp it!
3) You have modelled the drive nuts in the wrong orientation.
4) Your long tubes that you are placing the rails on will not be straight from the steel merchant, So a) do you intend to live with the section tolerance or b) are you going to skim the top of these? if so are you going to stress relieve the tubes? or the frame for that matter prior to finish machining?

You realise you have opened the can of worms here? can't put them back in now.... Peter

You mention cutting aluminium and here's the rub, literally. To cut metal you need a very very stiff machine because if the machine moves more then the chip load (or chip width) then the tool just rubs and does not cut. You may be able to do rough rough cuts but to get a surface finish the tool has to be 10x stiffer then the chip load. Say you are doing a finishing cut at 0.003" chip cut per tooth. Then if the machine deflects more than that the cutter just rubs so you don't get a surface finish. There are about 1M words in this forum on cutting aluminium with routers vs mills. Will come down to what depth of cut you need to make parts in minutes or hours, what spindle you have (a mill head is much slower then a router) the list goes on and on and you need to read all the advice in the forum now before you spend 400 hours designing something that will disappoint. I say all of this because you are putting time into CAD which is the correct way to go. Easier to change CAD then metal... Plus you need to write down what the machine specification is that you are aiming at (XYZ dims, feed speeds, rapid speeds, You will need that for the inertia calcs and stiffness calcs that are on their way and the inevitable Q's you are going to ask about motor sizes and screw whip.... all of which are answered in this forum if searches are done... :) have fun

I think you should change the z-axis rails and wagons around: the wagons need to be on the gantry side and the rails on the spindle side.
With your design, the rails will stick out when the spindle z-axis is up, so it will bump into a taller workpiece. Effectively, you will lose a lot of z-height with this design.
You seem to have tried to compensate for this by putting the lower wagons quite high up from the lower side of the spindle plate. This decreases stiffness a lot, which is of course not what you want

One problem with a forum like this is that different people will have different opinions on what a machine ‘can do’. The average home-built steel CNC will be able to cut aluminum (even MDF machines can...), but you will (probably) will not reach the same mirror-shining surface finish that a several 100,000 dollar machine will achieve, or at least not at the same feed. But that doesn’t mean that you cannot cut aluminum with it at all.

I do agree with previous posts that your gantry seems a bit out of proportion with the base and y-axes of the machine, I think it needs some more rigidity there.

Regards,
Reinier

If I change the rails orientation and stick them to the slindle plate, won't I lose clereance if I use longer tools?

In the photo below the Z axis is in it's highest position, if I move the spindle higher than I'll have the spindle plate lower than the spindle...

I understand that this orientation is stiffer but is it worth it? Or am I in a complete misunderstanding?

Thanks

Yes, you lose the ability to move the spindle up above the bottom of the gantry to allow for longer tools.

The solution is to make the Z axis plates stiffer. Add ribs.

Flat plates have no place in a CNC. Box sections are best. C sections are good if box section not possible.

The issue here is that your gantry is too tall, or the Z axis is mounted to low. In your image, with the X axis all the way up, the bottom of the spindle clamp is even with the bottom of the gantry.
But the spindle and tool will hang below the gantry, making this distance unuseable.

Design it so that when the Z axis is all the way up, the bottom of your longest tool clears the bottom of the gantry.
Then allow for your shortest tool to reach the table.

Minimize the amount that the Z axis hangs below the bearings. The machine will perform the best when the spindle clamp is not hanging below the gantry.
If it must hang below, then you need to reinforce the mounting plate with side plates to keep it stiffer.