Weighing in...

[JasonC]

I have a simple general question. Why are so many parts built so heavy duty? I under stand the need for rigidity but from what I have seen theres parts that could be refined for weight reduction and still maintain it strength?


Jason

[gooberdog]

Hi Jason,
Without reference to specific parts it's hard to discuss. Weight (mass) is not always bad. Another factor is that the plan accommodates a simply equipped shop and uses standard readily available structural steel shapes.
Chuck

[JasonC]

Got ya. Didnt think of that.

Jas

[Gerald D]

parts that could be refined for weight reduction and still maintain it strength?

We want stiffness in addition to strength. (Stiffness is very different to strength). More stiffness gives more rigidity and gives less vibrations. We also need some mass to dampen vibrations.

None of these factors were ever calculated or analysed. All shapes and thicknesses were selected on experience, gut feel and comparison to similar machines, to get a compromise between cost, availability and performance. It seems to have turned out fairly okay, but I would be the last to say it is perfect.

In retrospect, I can't think of any significant part that can be reduced in mass without expecting some decrease in performance.

[JasonC]

Cool. I guess looking at it a little more and a little "my" logic the moving parts are the only parts that I see that could be decreased mass by just useing aluminum vs steel, gear reductions plates, Y car ,Z slide..etc.

Has anyone tried to reduce vibration with apropriate dampers. I use Dynomat on my audio builds and it works great.

Jas

[Gerald D]

Why would you be happy to reduce the stiffness of the moving parts?

[IN-WondeR]

You cant use something like dynomat to reduce vibrations on a CNC. Vibrations are not caused by sound, it's caused by the fact that you have a cutting head which moves through a piece of material. Only weight can reduce vibration there.

The Mechmate is already a light CNC, so making it lighter will only increase things like vibrations, and make it less rigid..

[JasonC]

I dont believe I said anything about reducing stiffness, just replacing heavy steel parts with equally and structurely strong alum parts. Looks like this might be a venture ill experiment on my own..lol.

Jas


Why would you be happy to reduce the stiffness of the moving parts?

[Alan_c]

For aluminium to have the same strength as steel, it has to be 3x thicker and is more costly!

[Axel1966]

Hi Jason,
I asked the same questions fiew times ago.
Trying to make the MechMate Lighter is not a good idea.

The fact is, weight and heavy duty construction is important to get a nice cut quality.
Especialy with the MechMate engineering, with V-Rails and V-rollers system.
Now I've maked up my mind, and I think about oversize some components
to be sure to get the best cut quality possible.

Don't waste your time on this subject, any possible angle
will lead to the necessity to build strong and heavy. ;)

Some people would like to build a lighter cnc table, because they'll manage light
forces : because of the use of a laser cutter instead a spindle for example.
But they'll need other systems too, like suported round rails and linear bearings,
much more expensive, but which allows lighter structures.

I asked this question, because I was trying to save money on structure.
It was realy a bad idea, because the table might be the cheaper chapter,
in the construction of your MechMate.
Stepper motors and drivers may be more expensive than the table itself. ;)

[JasonC]

Thanks Axel

You have made the point perfectly and the other guys too. This is probly a case of "lighter isnt always better". Im slowly building mine and am seeing the general overall mass of this sucker(10 foot x 5 foot). I guess I should just finish the build first and see the end result.

Thanks guys
Jason

[WTI]

Sure, if you were only going to cut foam for modeling, then you could make it lighter.

But for the rest of us who cut wood and the occasional aluminum, we need all the weight and stiffness we can get.

Look back at the Shopbot archives and see how many people would have to make the machine stiffer, stronger, better....the MM already incorporates all of these improvements - no need to reinvent the wheel.

If you want to add mass to the MM, you can fill the hollow base/leg tubes with sand.

[KenC]

Any lighter, MM will hop around one's shop when a cat bump into it... If one consider that as an advantage.

[Kobus_Joubert]

What about a concrete bed...or a brick and mortar base. I am going to build my new workshop in the near future and was thinking of putting my MM on a solid base as my existing table is a bit on the light side.
Any comments.

[TechGladiator]

@Kobus; It should be fine as long as you use Steel Rebar..:D

[Axel1966]

What about a concrete bed...or a brick and mortar base. I am going to build my new workshop in the near future and was thinking of putting my MM on a solid base as my existing table is a bit on the light side.
Any comments.
Well, for any reason you may want to move out someday, or sell your MM (for a better one).
It can be wise to rather reinforce the existing structure or to upgrade lightest parts.

[KenC]

Remove the table structure & bolt the U beams on concrete floor...
Less work & now you have your whole building to damp your cutting vibration :D

[Gerald D]

It is interesting that guys who have built and operated MMs, who then consider building other machines or making big modifications, all want to go with more weight.

[Kobus_Joubert]

I don't care about 'IF' I move....the working part of the MM is still there, only needs a table.
Gerald you have seen my table made from the round pipes of an old radio mast.
That thing sometimes shake so bad that I have to stabilise it between the existing 2 walls.
Shaking like that affects the cut quality as we all know.
When I move to my new shop I would like to make it BETTER. ;)

[KenC]

Kobus, you need lots of cross braces. :)
How fast are you running your MM? on small work piece, about 100 x100mm work piece very high density carving, my machine will jerk around like crazy at 1200mm/min feed rate.

[Kobus_Joubert]

You telling me stuff that we all know. I was just thinking that I could make a real SOLID base while I have the opportunity.

[Axel1966]

Some people uses polymer-concrete structure.
I think it's better than bricks or concrete (even fiber-concrete).
Concrete can erode to dust with vibrations. No probem with resin-base.
Through, it's possible to make dismantable parts.
Some info here : Polymer-granite on CNC Zone (http://www.cnczone.com/forums/epoxy_granite/38800-index_epoxy-granite_machine_bases_thread-2.html)

[TechGladiator]

It is interesting that guys who have built and operated MMs, who then consider building other machines or making big modifications, all want to go with more weight.

Although I have only been using mine for a short period of time, I can see why people would like to add more weight. I am guessing it has to do with acceleration and feed rate BUT there are some nice jolts every once in a while.

[Red_boards]

My (very heavy) table is linked to its feet by 12mm bolts. I'm sure these and the rubber feet bend in response to gantry direction changes, so it sometimes looks as though the table is hopping, even though it isn't. I'm guessing some "give" is not bad, and might even prevent failure of a rigid brace?

Kobus,
One of the machines I contracted to in the past had its rails bolted to the floor with a 3m tower riding on them.

[Gerald D]

Some guys only use their CNC routers for cutting rectangular cabinet parts - they can build lighter / more flexible.

Shopfitters who cut gentle curves on counter tops can build lighter / more flexible.

But, if you have a lot of rapid stop/start motion (for example, cutting letters into signboards) then you will see a lot of shaking and consider building heavier / stiffer.

[Axel1966]

Good to know. Thanks

[normand blais]

Hi Kobus
I would go for concrete at near 2 ton a cubic yard .you could even cut nice arch form on MM. Get the mixer to backup to your shop
Gerald ,do you remember the guy who ''swisscheese '' his shopbot ,it was cromed if I remember right .

[KenC]

Just cast whole table leg right up to the beams in situ.
That should do it nicely.

[Gerald D]

or, bolt the main beams direct to your floor and work on your knees :)

Normand, I remember those holy shiny pictures!

[JasonC]

I was doing some drilling last weekend on the MM. i climbed up on the table and now i see where the mass is needed. I was able to move the table by shifting my weight back and forth fast. wow! the table is solid but I was suprize on how easy it was to wiggle. I may evan consisted floor anchors if it is a problem when Im finished. Im also creating a space at the bottom to store sheets of MDF or ply for added mass.

Jason

[KenC]

Jason,
LOL! if you wiggle hard enough, the whole table will hop!
Now you have 1st hand experience in know how bad an idea it is to lighten the table structure any further :)
I agree with Red that some give is good in this case. Anchoring the table may incur more problem than it cures.
Don't worry about the wiggling, IMHO it is actually an advantage, Excessive jerking force need to compensate some where, & wiggling allow certain degree of dampening, so that there will not be any adverse effect on cutting quality.

Some Tai Chi philosophy, move with the force, be the force. ;)

[Axel1966]

To help on vibrations problems, we use in audio some cones :
http://www.jefferson-hifi-video.com/produit/image/ATOHM-Maxi_cone_de_decouplage_avec_accessoires-4902.jpg
http://img73.imageshack.us/img73/7489/pointededecouplageno0.jpg
The vibrations diffuses through the cone and cannot climb back through it.
It prevents the "resonance" effect.
I think about manufacturing some for the MechMate.