[lgalla]

Bruno good post my agree ratio is 100%.
Here is some unqualifyed blurbs.
Epoxy does not shrink,esp with 90% filler.
A large batch of raw mixed epoxy can reach 170 C.This should never happen to us as the high filler loading absorbs the exotherm heat.If it did,would this temp be high enough for rebar to expand?If so I forsee problems as the steel would expand and the epoxy would not.TCE of aluinium is similar to epoxy I beleive.
I could not see reaching these temps with the low epoxy ratios we demand.
Beleive me,a pot of one gal of epoxy will"smoke if not spread in thinner layers.
Here is a copy of previous observations:
Temperature:
Heat lowers viscosity and accelerates cure dramatically: from 25~ to 50~ C, viscosity drops over 90% (e.g.: 10000 to 500 cps). 12 hours hardening @ 25/C becomes 10 minutes @ 80/C or 5 min. @ 120/C. Therefore, winter & summer, outdoor & indoor applications may require vastly different initial viscosities and pot lives. Below 10/C (50/F), cure and viscosity can be so affected that special compounds are needed.

Larry

[lgalla]

Guys some of the engineering experts have PM'd me to state it is not clear what posters expect of the material or their designs or ideas.Many posts ago like 7 or 8 hundred members posted ideas for machines.If you could give some specific spans,loads,expectations,etc,the engineers can help.
Larry

[lgalla]

Since I made the suggestion I'll be first.
The router table I want is 6'X12'check post # 203.Jerry was kind enough to draw for me as my drawing and computer skills are low.One correction in the drawing.Concrete should read polymer concrete.
The frame will be bolted or possibly brazed to avoid stress.Possibly I will use re bar if deemed necessary
Do you think W4X13's are up to the task?The gantry will be 2 W4X13's 7'long and weight with payload 600+lbs.I will try to attach PDF'S from member LOCSTER13 which was an inspiration for my design.The main goal of the design is to produce an accurate surface plate to mount the rails and reference plate to align the gantry.Also the wide I beams provide an easy mounting surface for the rails and rack& pinion not requireng drilling the E/Q.
The Zone and members are a polite bunch,but replys such as U R an Idiot are welcomed as failure in a huge costly project is....Costly.Am I inviting ridicule?Maybe but I can handle it.
Thankyou
Larry

[romihs]

Sandi,
There a many ways of reinforcing E/Q, and I believe that large manufacturers will not use rebar specifically but the pipes for lubricant and/or vacuum, conduit for wiring, air lines... all of them can and are used as reinforcement purpose in addition to their main purpose or vice-versa.:idea:

I you look back at the casting done on the german site, I am sure that what looks like 1 x 2 fb was not used only for fastening the linear slides on the X and Y axis, but also as reinforcement because the flat bars are also anchored in the E/G. Not only that but all major fastening points are reinforced with anchored steel plates.

Best regards

Bruno

Hi Bruno,

Yes, you have a good point there. I never looked at those flat bar inserts as reinforcement. I always only saw them as mounting poits for the linear rails...

Thanks.

Regards

Sandi

[romihs]

Hi all,

I plan to build a mini CNC mill with X, Y and Z travels of ~150mm (I know, it's small, but I got some 280mm long rails which I want to use...).
I estimate that the its final dimentions woulf be ~ 300mm x 500mm x 500mm, with a total mass of about 50kg (just a guess).
I began building this out of aluminium, but I would glady switch to E/Q. :cheers:

I began designing a lathe, constructed using weldments, which I would like to build out of E/Q. I was thinking of making the lathe about 1m long with a width of about 400mm. This is still a long way away.

Regards

Sandi

[LeeWay]

My first intended use is simply to fill extrusions and use the polymer to dampen resonance and to add weight to the extrusions.
I had been contemplating a bridge type mill in the future and depending on how my first one works out and how everyone else's tests and machines come along, it may get designed and built using this stuff. Maybe a 36" bridge.

One real failing on the design of the little X2 that I have is the center of gravity over the cut. It is way over extended out front and with one mounting point, is a weak spot. Most of the accuracy issues stem from that deflection there. The dovetails aren't perfect either and require a lot of fiddleing to keep any type of reasonable accuracy with mild steels and very minimal cuts.
My 80/20 machine is using large linear ways and has a better mounting for the column, but the cutting forces will still be out front.
It should work better than the X2, but I think a bridge type mill will equal out some of those forces and put them in line with the structure of the bridge itself. Less flexing under load there.
The drawback is the table size limit, but if the bridge is large enough, it would cut some decent sized parts.
The base would also be polymer of course.

More on rebar.
Reinforcement doesn't have to be rebar or rebar like.
Rebar works so well because the aggregate actually locks the ribs of the bar in the mix and when pressure is applied to it, the rebar is like in tension and isn't as flexible as it normally would be.
A smooth pipe will provide some added umph, but not as much as rebar because of the ribs.
You could put bends on the ends of the smooth pipe and make them less likely to slip through the pour under tension, but it still won't deliver the same ultimate shear strength as the rebar.
Filling extrusions or steel members are a good substitute to rebar on gantry spans. Think of it as an exoskeleton and rebar as an indoskeleton.
Anchor points will be needed for bolting parts on and when designed well, can be incorporated as part of either type skeleton to add strength.

More on heat.
I mentioned pours less than 4" as thin. I think I read the other thread where he said he doesn't pour thicker than 2". I think that is plenty to mix at once. It would give the opportunity to add the structure if indo during the second or subsequent pour. I think this thickness is more controllable and far less likely to produce any kind of threatening heat.

[BobWarfield]

Here's a post I showed a long ways back. Reinforcement as well as a scheme for leveling the insert grid are inherent in the design. Fabrication is straightforward weldment, relying on the grid leveling and the natural "surface plate" settling of the E/G (may pour topmost layer with minimum or very fine aggregate).

The table starts out with 1/4" plate bottom, 3" thick wall DOM tubing frame, and 1" rebar reinforcement grid:



You could use I-beams as in the previous example, but I think the tubing will be stronger while still presenting a profile that "locks" the E/G in place. Of course the DOM tubing could optionally be E/G filled as well if desired.

Inserts are hex stock with "lock-in" grooves mounted on socket head cap screws that are welded to the table base plate. The threads allow the inserts to move up and down so the insert tops can be leveled relative to one another before puring E/G mix:



It takes a boatload of those inserts, but they'll be easy for me with my CNC lathe. I think the insert grid itself will also stiffen and strengthen the table. The grid before pour will look like this:



As you can see, the grid layout clears the 1" rebar very nicely without any interruption. These inserts will be used to mount the gantry rail system as well as for work holding. If one wanted to dedicate as such, you could use piping instead of the inserts and create a vacuum table as well. For that case, you lose the threaded leveling of the piping, so I would use the gantry mill itself to grind protruding pipes level with the table.

Lastly, before pouring, the table is supported on a heavy 8" DOM sub-frame to support it from underneath:



I would plan to fill the 8" DOM sub-frame tubes with E/G and rebar as well, largely for dampening.

You'd want to do the pour after the weldment is complete including subframe and the insert grid has been leveled so that the structure has done whatever bending it will do before the epoxy cures. If all goes well, you wind up with what I would think is an extremely rigid, very well damped, and very heavy table on which to create a gantry mill:



BTW, the idea for this structure is very similar to the structures described in Bamberg's PhD thesis "Principles of Rapid Machine Design." If you haven't read it, you should. It's all about building extremely high performance CNC machines using weldment frames filled with concrete (not E/G!) for dampening. The technology worked extremely well and he "tells all" in the thesis. I'd provide a link, but it moves around. Just Google on Bamberg and Principles of Rapid Machine Design.

Now if one of the MechE's has some FEA software, we can probably make good progress converging on what works and what isn't adding value in these machine designs. For example, there are a lot of obvious questions about this design, especially surrounding the subframe:

- Are dual tubes sufficient?

- Should there be 3 tubes?

- Is an "H" pattern for the subframe better?

- How much will the whole thing flex before and after pour?

Cheers,

BW

[brunog]

Since I made the suggestion I'll be first.
The router table I want is 6'X12'check post # 203.Jerry was kind enough to draw for me as my drawing and computer skills are low.One correction in the drawing.Concrete should read polymer concrete.
The frame will be bolted or possibly brazed to avoid stress.Possibly I will use re bar if deemed necessary
The Zone and members are a polite bunch,but replys such as U R an Idiot are welcomed as failure in a huge costly project is....Costly.Am I inviting ridicule?Maybe but I can handle it.
Thankyou
Larry

Larry,
I saw all 3 pdf's:

Don't worry I WILL NOT (chair)!!

If your moving gantry is set on a square tube structure, what is the use for the rails on the table. I know you have made the table design first and maybe it's an OOPS.
If so may I suggest The folowing:
1- using channels instead of I beams.
2- add 1/2" or 5/8" allthreads at every 36" attached to the sides as reinforcement regardless of suggestion #1
3- Try to incorporate vacuum pipes embedded in the table, regardless if you have the pump to hold that amount of succion. (think of future needs)

Best regards

Bruno

[brunog]

My project is fixed gantry E/Q cnc mill with a 16" x 32" E/Q movable table with 12" travel Z axis. I want to be able to mill whatever I get my hands on from ash to zirconium LOL.

Best regards

Bruno

[brunog]

Bob,
C'est quoi ça FEA???

Excuse my french, what does FEA stand for???

Bruno

[turmite]

Bob,
C'est quoi ça FEA???

Excuse my french, what does FEA stand for???

Bruno

finite element analysis


Mike

[lgalla]

Good to hear what you guys are working on,it has helped me with different ideas for my own design.I guess this was a flood for the engineers.Hope to hear some feed back.Bruno possibly hollow steel tube for the vacuum imbeded in the table can replace re bar?Thanks.
I will ask a simple engineering question.
Take a 4"X4"/1/4" A36 steel tube.What would the deflection be on a 4'length,8'length with a 200lb centre load?Would filling withE/Q make it stiffer?
Thankyou
Larry

[brunog]

BTW, the idea for this structure is very similar to the structures described in Bamberg's PhD thesis "Principles of Rapid Machine Design."

Bob,
Isn't it the same Bamberg that wrote "Principles of summarizing" which comes in a 12 volume set??

Best regards

Bruno

[BobWarfield]

Bob,
Isn't it the same Bamberg that wrote "Principles of summarizing" which comes in a 12 volume set??

Best regards

Bruno

Must be a different Bamberg you've encountered, Bruno.

I just found a home page for him that hadn't been there in prior searches. It has a link to his PhD thesis:

http://www.mech.utah.edu/~bamberg/people/bamberg.html

Best,

BW

[ckelloug]

Igalla, I owe you a calculation on this question and I should have one out in a day or two. I don't have FEA capabilities but I am planning to write a program to compute the deflections of some of the beams of interest. For anybody who truly cares about the details, I'm planning on using Lagrangian methods and Castigliano's deflection theorem.

I unfortunately got called in today despite being on two months of leave to go to a meeting and then had my SO put me to work drilling cord holes in a bunch of new cabinets so I didn't get any actual calculating done.

To answer briefly and qualitatively, filling a 4x4 tube with E/G will significantly increase the tube's stiffness. Without getting the calculation done, I can't say how by much right now.

[fyffe555]

I will ask a simple engineering question.
Take a 4"X4"/1/4" A36 steel tube.What would the deflection be on a 4'length,8'length with a 200lb centre load?Would filling withE/Q make it stiffer?
Thankyou
Larry

Hey Larry - we did this ages ago You can use the spread sheet from post #5 http://www.cnczone.com/forums/showthread.php?t=14227.

Ckelloug - Please write something to calculate this properly. The spreadsheet is basic at best and ignores a lot of stuff although its close enough for home built ..

Anyhow, ignoring the deflection of the beam due to its own weight (as it's constant);

A 4' 4x4x1/4 steel tube with centre 200lb load and *fixed* ends would deflect 0.0008 or less than a thou'.

The same beam at 8' and same load centred would deflect 0.0064 or about 6 1/2 times more than the 4' beam.

Fill the same beam with E/G and it gets a touch more complicated. Do we have a concencus on the modulus for E/G? accurescasting.com web site has a modulus of 4.5x10^5 which seems a touch low, hardwood is usually taken as 4.9x10^5, steel used in the beam is 29x10^6, so E/G is 64.4 times weaker than Steel.

You can simplify the beam and E/G filling problem as its doubly symmetric in cross section and the neutral point will be located at the centre of the beam with or without E/G. You can modify the E/G section to replace the E/G with an equivalent steel section at the centre to use the same modulus for the calculation and then simply add the two Area moments of inertia from the tube beam and E/G equivalent and calculate.

Before the engineers amongst us scream, yes this does ignore any and all sheer caused by the E/G glued to the inside of the tube. I've no idea what the sheer strength for that would be and it's not going to be a major contributor under these loads anyway. The variables of a good verses bad E/G cast, resin ratios, resin elasticity and agregate sizes will will affect it more.

So, if the modulus of E/G at 4.5x10^5 is right(?) then steel will have the same strength at ~ 1/64th the cross sectional area. The 3 3/4 sq section = 14.06sqin. 14.06/64 = ~0.22sqin or ~0.47" square section in steel. In otherwords a 3 3/4 square cross section E/G beam is equivalent to a steel 0.47" square cross section beam.

Adding the E/G equivalent MOI to the tubes MOI you/re adding 0.004 to 4.854. Not much at all and it will have no significant affect on stiffening the beam in the conditions given.

That is, an 8' composite beam with 200lb centre load will deflect 0.00643" . the same beam without E/G to the same five figures will deflect 0.00644'.

If the E/G had any tensile strength the calc's would be different but since it doesn't this method is a decent approximation and a measure of the effect of composite beams in this scale.

It gets more interesting since the E/G will weigh enough to cause a static deflection of the composite beam about 20 times more than the E/G will reduce the deflection under load.

So Why Do It? the E/G and beam will deflect under gravity alone - but that is naturally accounted for during construction and machine alignment. The E/G will not contribute to strength in bending untill you get to very large sections and 64x steel seems to be the measure - until someone shows me I've used the wrong modulus but the ratio is still large.

The biggest benefit is Larry's original ideas - E/G is castable and it's a relatively cheap means to massively dampen vibration by adding easily formed mass where you want it.

Andrew

[Geof]

....The biggest benefit is Larry's original ideas - E/G is castable and it's a relatively cheap means to massively dampen vibration by adding easily formed mass where you want it. Andrew

But if you are building a steel structure which you then want to dampen the easiest and least expensive is the concrete mix with added aluminum powder to prevent shrinkage as described in the Bamberg thesis.

And if you want the ultimate vibration damping you use the rubber bags inside the steel members and then filled with the concrete. This casts in place a constrained layer damping system.

[lgalla]

Thanks Andrew.
The question has nothing to do with my machine,but is interesting info.Will help other designs.If we turn the same beam around and call it a mill colum and have twisting forces,will the E/G add more benefits other than damping?
Thankyou
Larry

[fyffe555]

Thanks Andrew.
The question has nothing to do with my machine,but is interesting info.Will help other designs.If we turn the same beam around and call it a mill colum and have twisting forces,will the E/G add more benefits other than damping?
Thankyou
Larry

Twisting forces are somewhat different from a simple deflection. Twisting starts introducing such nasty things as warping shear stress and warping moment and the assumption the sections might change areas in deflection.

Usually in structural uses the simple straight bending forces are calculated first and torsion or shear forces second and the two added, in most cases the material section required to resist bending is more than that required to resist torsion in most circumstances...

To answer the question; adding E/G doesn't appear to resist torsion or twist by any more than the tube alone would. The calculation is a straight multiplication of the materials modulus, the warping moment, cross section and how far it's twisted. Since we currently seem to think E/G modulus is 1/64 that of steel it suggests the E/G will increase the strength against twist by the same proportion.

However this ignores any shear forces involved between the E/G and steel tube and also that the warping calculation assumes the structure has changed cross section and E/G is good under compression, IF the tube section significantly reduces area and the E/G is compressed then the composite tube & E/G could be stronger.

It's not likely to happen though since we're trying to build a machine to hold to a few thousands of an inch, A column holding straight to thousands isn't going to change its sectional area and if the tube is suffifiently strong to achieve this the E/G isn't likely to have much effect, other than dampening.

EDIT; just read this again and its about as clear as, well, E/G. Apologies but you get the drift..

Andrew

[fyffe555]

And if you want the ultimate vibration damping you use the rubber bags inside the steel members and then filled with the concrete. This casts in place a constrained layer damping system.

Now there's a good idea.... I'll have to go find this paper..