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View Full Version : A daft question about vibration damping

D.C.
07-10-2012, 06:10 PM
Sorry for the all the silly questions, I usually prefer to think a lot and buy a little. In the ballscrew designs for shifting your gantry around, would it possible to dampen vibrations in the thread by attaching a free moving block with bearings in either side of the gantry?

Lets say you have a 3m long screw:

Frame -weak spring - dampening block - gantry - dampening block - weak spring - frame

When moving the gantry would come into contact with dampening block and just shove it along in front of it compressing the spring, as the gantry moves back the spring moves the block back into it's 1m position.

As the dampening block is connecting your thread to your linear rail would this set up allow you to dampen the movement of the thread or would it just go horribly wrong, would too much friction be added?

JAZZCNC
07-10-2012, 07:00 PM
This principle is often used in very large and long Lathe's. They use sliding blocks either side of the moving gantry that cups the screw. You can see one here at 1.40min
Heavy Duty Lathe/Heavy Duty Precision Lathe/Lathe Machine/CNC Lathe/Lathe - HD Video by S&J Corp. - YouTube (http://www.youtube.com/watch?v=QrvOm5IEnxI&feature=related)

blackburn mark
07-10-2012, 07:07 PM
my first instinct tells me that adding mass to the screw would lower the frequncy of the screws sympathetic resonance or just devide the screw into a higher frequency harmonic... it may be possible to tune the resonace out of the reach of the vibration causing it?
im guessing it would take a lot of work in either trail and error or some funky maths...you would lose some of your axis travel and getting a good solid contact between your mass and the screw would take a bit of doing

are you talking about resonance or whip?
the way i see it, resonance is a flex mass vibration type of thing (guitar string style)
whipping is the screw acting like a skipping rope

if your talking about whipping i would take a look at jonathan's revolving ball nut design

D.C.
08-10-2012, 03:05 AM
I was talking about whip without even knowing that was the correct term for it, ooops.

It's nice to know that some of the design solutions I'm thinking of have also been used in high end machines but I'm getting the nasty feeling that I'm about to attempt to design the best machine ever created only to find I've shot financial wad all over a machine that won't work because I'm a clueless muppet.

Plan d would be to go for a ghetto build as cheap possible where I can learn from mistakes without excessive financial harm and progress onto bigger and better when I have a clue.

My ultimate goal is for a medium duty machine that has a five axis spindle, a moveable lathe, a gantry overrun, enough precision to do PCB work and enough 'ooophm' to do occasional metal work.

This means that on one machine I can make accurate dovetails by mounting wood vertically at the portion of the rails that overshoot the table, carve detailed balusters using the lathe portion, carve out 8x4 sheets in wood or PU foam for concrete casting when the lathe is lowered, create small and smooth running brass gears and do PCB's.

Do you think it would be better to aim for the machine to end all machines first time round or build up to it over 3 or 4 builds, bearing in mind this is more a hobby project and not a roof over the head depends on it situation? I'm happy to be patient and learn but not happy to piss money up the wall on something useless.

Secondly I'll settle for a first build capable of knocking out good quality wooden kitchen cupboard doors (Do you detect a woman's influence in these design requirements? Why yes sir you do!) and PCB's (No female satisfaction required in this part, she will watching strictly come dancing) while I spend a few months learning the ropes, would an mdf build do this and bearing in mind I plan to upgrade the size of the cutting area 12-18 months down the line and also start tackling more demanding jobs, should I spend some cash now on decent motors/electronics and scrimp on the rails etc so I have the cash for future improvements or will this prove to be a nightmare?

PS I have an IT monkey background and much prefer linux over windows so I'll almost certainly be using linuxcnc/emc2 over Mach3 if it makes a difference.

irving2008
08-10-2012, 07:26 AM
Starting with the last Q first.. While most seems to prefer Mach3/Windows either will suffice... I use both.

Making a machine to end all machines is an 'interesting' goal. It will be difficult (but not impossible if you throw enough money at it) to meet the conflicting demands of machining PCBs and small metal parts (high acceleration, short throws, high accuracy to .01mm or better) with that of large wooden items (large size, higher speeds, but relatively low accuracy 0.1mm). You'd be better off with 2 machines, a CNC router for the wood and a small CNC mill for the other parts. The cost of two machines may be less than the cost of building one 'super' machine. Drive electronics could be shared if you only run one at a time, at least to start with.

As to a lathe... if you need a metalworking lathe buy one. A 4th and 5th axis are less useful unless you have specific projects in mind and adding a 4th axis to a router isnt so difficult, but its not a lathe in the conventional sense. A 5th axis needs a whole different way of looking at the machine as achieving sufficient rigidity, even on a small scale, is very hard to do.

Personally I've always though the idea of building a small 'sacrificial' starter machine to learn on isn't a bad idea of a waste of money, since it can be used to make parts for the larger one.

D.C.
08-10-2012, 07:07 PM
Thanks Irving, bearing in mind I plan to do a cheap and cheerful MDF build first and then improve on it, would these cause me any problems?

Makita RT0700C 700W Router - For £90 it seems to be a steal and more suited to cnc use then the Bosch Colt which gets used a lot.
Makita RT0700C 700w Router/Trimmer £90.98 (http://www.lawson-his.co.uk/makita-rt0700c-700w-router-trimmer-rt070-p143148)

And these 4nm Nema23 steppers - seem like overkill but I can reuse them in a bigger machine later on.
Nema23 Stepper Motor 4Nm x 3 (http://www.cnc4you.co.uk/index.php?route=product/product&path=20_27&product_id=71)

irving2008
08-10-2012, 08:56 PM
you won't need 4Nm steppers any time soon, and bigger isnt always better. stepper motors are a small part of the costs and even large routers rarely use more than 3Nm ones. Why spend £40 more than you need?

The Makita looks a good deal, it has a bigger lower bearing (17 x 35 x10) than the Colt (15x32x9).

D.C.
08-10-2012, 10:31 PM
So the 3Nm steppers would be fine for a 4x8 cutting area with one of those 2.2kw chinese spindles?

irving2008
09-10-2012, 12:31 AM
As an actress once said... its not how big it is, its what you do with it....

The vast majority of machines built by the members here are using 3Nm steppers or smaller; very few have needed to go bigger. When considering a 2.5m axis you have to think long and hard about how you're going to drive it. Spinning two long heavy screws is one option and that needs powerful motors but for machining wood 3Nm is usually OK. But a better option is spinning ballnuts and that needs a much less force and is far more efficient. An alternative is a static belt drive or rack and pinion... all need relatively small motors though there are tradeoffs to be made. If you were talking about a large steel mill, a Bridgeport or similar with a heavy table on dovetail ways then 4Nm might be the only thing that'll work...

D.C.
09-10-2012, 12:36 AM
3Nm motors it is then.

Btw the reason I was hoping to one day build a mutlifunction machine is that I have a small garage, if I fill it with machines I will never be able to fit in and actually use the things.

I was inspired the doughty drive adaptions:

5 axis cnc routed Pineapple file - YouTube