[LazyMan]

I'm thinking about filling some bosch rexroth t-slot extrusions with concrete or cement for added weight and strength. Does anyone have any info on this or any thoughts at all?

Will the concrete or cement cause problems? Will in shrink or expand? When it does will it crack or will in warm the extrusions?

[jsheerin]

Concrete and steel usually work because they have an almost identical coefficient of thermal expansion. Aluminum and concrete won't, so you could have problems. What about something like sand? You'd have to seal it up well so it wasn't getting grit in all your bearings.

[bibster]

LazyMan,

Watch out with sand, use dry sand! We speakerbuilders (other hobby) frequently use... cat litter box filling stuff, for making boxes stable!
Like in the Fonken floorstander, en though it says dry sand on the drawing.
(Even though it's for Aluminium, not wood....)

P.

[Grandad]

I have used a product called "Diamondbrite" to to fill the x axis bars on my second machine. Diamondbrite is a quartz aggregate rich compound with a cement base, it does not shrink, well I can't see any visible shrinkage and it certainly reduces sound levels. The rails are made from 50mm square stainless x 1.5 mm wall thickness x 1.50 metres long.
Incidentally Diamondbrite is used to surface swimming pools

[jsheerin]

If your machine isn't in an environment where the temperature changes a lot, it shouldn't matter. If it's like mine and will sit in the garage and go from 0F to 100F, well, maybe you should be concerned. But coefficients of thermal expansion are:
concrete ~ 6.4
steel ~ 6.7
stainless ~ 9.6
aluminum ~ 13
Values are in microinches / inch*deg F.

So if you have a 65" long aluminum rail filled with concrete and you cool it 70 degrees, the aluminum would shrink about 0.059", the concrete only 0.029". If you had a 6" height rail (like I do), then the same temp change would make the aluminum try to shrink about 0.003" more than the concrete would want to. Going up 30 degrees, the 6" height of the aluminum would grow about 0.001" more than the concrete. I'm not a concrete engineer, but I'm not sure this would be the best thing for your structure.

[Grandad]

I take your point regarding the different thermal coefficients, but I admit to taking a chance in using this medium as to some extent my machine lives in a reasonably controlled environment. I did consider mixing quartz agregate with epoxy but the cost of resin is somewhat prohibitive. If I do have a problem with split rails they are easy to change and fairly cheap

[LazyMan]

Thanks for the info guys!

Ill have to think about this. My machine will generally be in a fairly stable environment but I would worry for the few times that my machine must be left in fluctuating temperatures such as during transport. I'm now considering just using sand as suggested for the purpose of vibration dampening. I was initially hoping that I could use cement and it would also add strength to my extrusions.

I guess its better in this particular aspect that aluminum has a higher coefficient of expansion than concrete. At least you wont have to worry about the concrete expanding and swelling up the aluminum.

I guess my main concern in the very end is whether fluctuations could cause permanent deformation and damage.

Jsheerin, do you by any chance know the coefficient of expansion of epoxy resin?

[rowbare]

An epoxy/granite fill would probably work well. http://www.cnccookbook.com/CCMillEpoxyFill.htm

[jsheerin]

Unfilled epoxy is around 40 microinch / inch*deg F, so much higher. I looked most of these values up on www.matweb.com. The concrete info I was reading said the filler tended to have a substantial impact on what the final CTE was. For example, mineral filled epoxy is listed as around 14microinch/inch*deg F, so a lot different. Looks like this would be a good match for aluminum.

Also, you say the concrete won't expand inside the aluminum - well, it won't, but if you cool it down, effectively it does. The aluminum will shrink faster than the concrete. As long as it doesn't do this too much, you'll just put stress on the aluminum. As long as this is not to high, when it warms back up it will be fine.

[harryn]

Locally there is a tire recycler that sells ground up tire rubber. It comes in a variety of sizes - shaped more or less like small gravel.

I am considering filling up various parts of my router with that, as it would be a good vibration dampener, even if not completely full, and pretty cheap as well. I am not planning on bonding it together.

[dielectric]

I would go with sand. The big boys put sand in the columns of drill presses and milling machines for vibration mitigation. My father worked on a nuclear submarine, and says that the drive shafts for the main screws are filled with a sand-like substance rather than being solid steel or whatever. If it's good enough for a nuclear sub, it's good enough for me!

[LeeWay]

I built both a mill and a lathe using the big 3060 extrusion from 80/20.
I did intend to use some hydraulic cement inside of these extrusions, however, once they were completed, I had no resonance issues. Nothing needed dampening. They just work fine the way they are.
I am still considering using something in the lathe bed simply to add weight. It probably only weights maybe 150 to 170 pounds right now. It doesn't need to be a solid mass inside, but it does need to be restrained from movement.
For this reason, I am considering something that would be totally reversible.
I intend to pack small 3/8" rebars into the largest openings and use some lead birdshot I have been hoarding. Then I will install a grease fitting in each chamber. Since the ends are sealed, this should work out pretty well. Grease has some weight and it will also keep out any condensation. This will be a little farther down the road.

You could also just fill them with flood coolant if the ends are sealed. That would add some weight for you.

On the sand idea, you could use used glass beads from a blaster if you have one. That stuff it pretty weighty too and would surely stay dry.

[LaserImage]

How about lead shot? Plenty of density and easy to pour in.

[jcoats]

or quarter minus gravel, dry it well first. quarter minus is gravel small enough to go through a quarter inch seine or sieve. Normally easy to come by at your local concrete batch plant since you only need a 5 gallon bucket or two. Bring coffee and the operator might give some to you

Personally I was thinking of building a rig using pre-stressed concrete. Almost need to make the forms from steel to get enough strength to make the concrete beams when putting tension on the reinforcing steel (steel wire, not re-bar)

[LazyMan]

Very interesting stuff on cnccookbook.

Unfilled epoxy is around 40 microinch / inch*deg F, so much higher. I looked most of these values up on www.matweb.com. The concrete info I was reading said the filler tended to have a substantial impact on what the final CTE was. For example, mineral filled epoxy is listed as around 14microinch/inch*deg F, so a lot different. Looks like this would be a good match for aluminum.

Mineral filled epoxy sounds very good. What type of minerals do they generally use? Silica sand seems to be one of them. Perhaps some of this stuff mixed with some epoxy would do the trick? What do you guys think?

http://www.mclendons.com/item.asp?sku=10169720