Concrete-filled machine-tool bed casting - ideas please

[Troup]

I've purchased a Willson slant-bed lathe which has the advantage of a very large swing, considering the compact footprint and silhouette.

The bed is exceptionally deep, and with a good width between the guideways, which are hardly worn. (see photo below, with grateful acknowledgement to Tony G's great website at lathes.co.uk)

However, to my eye and gut feel, the bed casting is rather thin-walled, particularly given the swing, and the depth of the bed.

In a few months time I need to turn a number of large slabs of stainless, right up to the 16.5" swing, and I'm exploring possibilities for adding some mass, damping and stiffness to the bed.

I've had excellent results from adding 'ballast' to a smaller lightly built lathe, in the form of a slab of thick steel plate between lathe and benchtop.

In this case I'm considering something a bit more radical and integrated, like tipping the bed casting so that the apertures are level, and filling it full of concrete.

Due to the unusual configuration, there is no need for any cavity in the bed whatsoever. There is a stainless cover down which chips slide - they don't pass through the bed as in the usual layout.

I know big CNC machines are fairly routinely fabricated and then filled with concrete or similar, and I know this method of improving castings dates back to WW1.

What I need is some practical guidance on procedural issues, such as how to cope with shrinkage, selection and proportions of ingredients, and any other lessons learned from actually trying something along these lines.

 

[lincoln]

i would use sand first, i would hat to have to clean out concrete if something when wrong. it is common to grout the base of cnc's large pumps and other vibration making things. sand in the casting would have a better damping effect. the vibration would be absorbed and turned into heat rather than transmitted thought the hard block.

my .02$

link

 

[pilgrimtt]

On some of the huge horizontal mills that I worked on the floor plates (table) were lined up and bolted down. Then they were filled with some cement like slop. I think they called it grout. If I remember right it would flow in and fill the voids. Makes a huge difference for rigidity.

 

[johnoder]

Maybe someone whose has actually been involved with manufacturing such things will appear here and provide the gospel

In the mean time this is the bottom line:

Anything that has even the slightest possibilty of disturbing geometry, dimensions and other important things is never done after the bed is finished.

This would include but not be limited to flame hardening of ways. This would no more be done after the bed was finshed than the man in the moon.

Unless you are prepared to send the bed out for precision grinding (and all the subsequent issues that process raises) after you pour in the concrete, the simple fact is that you might just scrap the bed.

 

[bsg]

Don't do it!
When we use to prep stroker motors out of stock blocks for racing we would do just what you are thinking of doing to strengthen and stabilize the bottom of the blocks and cylinders.
We would fill the water jacket up to the freeze plug and that was all you needed.
The problem even with the mixes that were recommended for the application is all the machine work would have to be done after the grout had cured; it would expand after it cures.
Don't ask me how we found out.
Kevin.

 

[USMCPOP]

Here's a thread discussing epoxy-granite stuff, with nearly 3,000 posts. There's some pretty good stuff in there if you don't mind wading through it.
http://www.cnczone.com/forums/showthread.php?t=30155

 

[Forrest Addy]

If it aint broke...

 

[Willy]

I haven't tried it on a lathe, but a good engine block stiffener as used on drag race engines might take some of the guess work out of the equation if you are determined to try. I've used Hard Blok in the past with excellent results...not cheap...about $70-$80 for 28lb.

Hard Blok link....http://www.hardblok.com/info.html

 

[g-coder05]

when weve installed large lathes we tend to use grout instead of concrete. much easyer to beat out when you need to.

 

[Alpacca Fortyfive]

I'd take a good long think about Forrest's advice.

However, if you must do it, there are some cements used for high strength flooring which are water soluble.

http://en.wikipedia.org/wiki/Sorel_cement

Plaster of paris mixes can be disolved out using hot water with a high concentration of chloride ions in it.

I can't remember the details as its about 15 years since I was up on such things.

Be very careful that you don't use a mix which will expand during or after curing!

and don't forget to let us know how you get on

Best regards
Keith

 

[Engine Room Artificer]

Bob Warfield is filling the base of his IH mill with epoxy and details it at his site:

http://www.cnccookbook.com/CCMillEpoxyFill.htm

I think possibly he has distilled the ideas from the CNCZone thread shown above. Also I think it should be noted that this is to improve the rigidity of the base not any precision surface.

Once you get the Wilson up and running, if not already, wouldn't it be worth running the sort of job you are considering beefing up for first, to see if there are any rigidity issues before commiting and potentially causing damage or headache.

Al

 

[HelicalCut]

Leave it as is, I recently turned some 18" material on an 18" swing lathe, even with a custom overhung tool holder I had no problem removing metal in the same manner that I would have done with a smaller part. It looked impressive loading a 350Lb slab onto the face plate but once it was on there it turned like any other part Avoid interrupted cuts they seem a bit worse with large parts. If you have doubts, look at some of the flimsy old lathes and what was turned on them.
Get used to shoveling chips if you have a lot of metal to remove.

 

[Norman Atkinson]

Concrete filling

During the last war or shortly afterwards, Model Engineer carried the story that someone- I believe on a ship- made a complete concrete headstock for a lathe.
This was in days of severe shortages!

Clearly, you have a precedent! After all, why not? Cast iron is miserable stuff.

Cheers

Norman

 

[Joe D Grinder]

I can't say do or don't, but there are a couple of points to consider;
1. concrete has a nice ring to it. Harmonics? Critical vibration of a lower order?
2. How many pounds in the middle of the bed to cause the bed to sag?
Hmmm.....Joe

 

[smallshop]

There used to be a machine called a gt express. Used a polymer and crushed granite "casting". I would check into something like that. Was supposed to have real good dampening. Fadal used to make a weldment machine that was concrete filled.(vmc 15) I think they quit making it. Don't know if it was poorly received or if it didn't work well, or what.

 

[BobWarfield]

A couple of thoughts:

I would prefer epoxy granite to concrete. The concrete can cause corrosion unless you bag it (some machine tool mfrs put concrete in rubber bags for this purpose), and it changes size as it sets. Epoxy granite (E/G) is stable in size and does not have corrosion issues. It also bonds much more firmly to you castings.

Your application is very similar to the way Hardinge uses epoxy granite (also called polymer concrete). You can see more about their "Harcrete" in this link:

http://hardingeus.com/usr/pdf/hardturn/ASME.pdf

I'd be surprised if the E/G warped your bed given that there is little to no expansion/contraction, but I can certainly see wanting to be cautious.

I'm not sure the weight involved is all that much in this application. I've poured quite a lot into my machine base and it changed the weight 80-90 lbs heavier. What would you guess the effect on your lathe's ways to be? 150lbs max?

It is very easy to mold the E/G material and conduct a trial. I would try to measure the volume you're working with, make up a similar volume in a crude mold, and just set it in place atop the ways after it has cured. Run a test bar and see what happens. If it's warping the ways, give it up. It wouldn't be very time consuming to run such an experiment. Just don't pour the E/G into the machine until you're sure because once the epoxy has cured it ain't coming out!

Something I would be kind of worried about is pouring while the machine is laying on its side. I'd personally prefer to pour in an attitude where I was sure things were straight and level because the pour will tend to lock the machine into whatever shape you pour to. If you you have things lying so the bed is twisted, the pour will tend to lock in that twist.

It's also easy to try some work and see whether augmentation is needed.

Lastly, for your lathe, I'd be tempted to fill the cavities in the castings under the tailstock and saddle more so than the ways. They're smaller, they're closer to where the chatter is happening, and it just seems like they'd be easier and less potentially harmful to the lathe to start with. If you didn't have a ton of gears in there I'd be tempted to fill the headstock too, but that's a non-starter here.

I will say the E/G is quite interesting. You can pick up a precision surface with it. The stuff molds very accurately.

Cheers,

BW

 

[Charles Dolan]

What do you hope to achieve? If it is just more mass why not just bolt a couple of 3" hot rolled plates to the bottom of the pillars at least you know that that will not strain anything but the floor!

Remember that all concrete expands to some extent when it cures, so if you are going to fill cavities use a polymer mix that is certified not to expand at all.

But I think Forrest has it right, Leave well alone, if you need to do a big job, access a big machine.


CD.

 

[chuckey]

concrete expands ?

You guys who say concrete expands, can you put some figures on it. I have rendered the walls in 3 rooms in my house with 1:4 cement sharp sand with plasticiser. Over the top of every door where the render was only about 9" high, there is a vertical crack, like the whole "sheet" of render was in tension and it cracked at the narrow (weak) points. Or have I got settlement problems?
Frank

 

[IdahoJim]

You guys who say concrete expands, can you put some figures on it. I have rendered the walls in 3 rooms in my house with 1:4 cement sharp sand with plasticiser. Over the top of every door where the render was only about 9" high, there is a vertical crack, like the whole "sheet" of render was in tension and it cracked at the narrow (weak) points. Or have I got settlement problems?
Frank

The concrete shrank, which is typical. The break occurs at the narrowest point, which is usually above doors and windows. That point also gets the most movement when the house moves with the wind or other loadings, so that adds to the possibility of breakage at that point. On concrete slabs, when it breaks at an inside corner that type of break is called a re-entrant break, or re-entrant crack. Normal concrete doesn't expand, and it never exceeds the dimensions it was at when placed. There are specialized cements that can be used to make concrete that will expand at about the same rate as the drying shrinkage so the concrete shrinks very little. THat material is called shrinkage-compensating concrete. It is very spendy, and is used only in special circumstances, like in nuclear plants, etc., where cracking is very undesirable. In this thread somebody mentioned grouts that expand. Those grouts are called non-shrink grouts and work about the same....the idea is for the grout to stay at the same volume after it's cured. It's possible that at some point the grout may actually be very slightly larger in volume than it was when placed, don't know if that increase could damage a lathe bed, or not.

 

[Troup]

Thanks to all for the excellent advice, information and links.

I will certainly proceed, if at all, with caution.

Including evaluating what I already have, to see if it is adequate as it stands.

I do however believe there can be shades between 'broken' and 'fixed', and have never yet regretted a modification to a good machine tool which was motivated by intention to make it better. Bad machines, OTOH, I have found are generally bad to the bone, and don't repay the effort.

I think if I do proceed I'll be trying out the selected course of action on something non-critical, like the bed casting of a drill press (there's a friendly CMM at the end of my alley - I could do a 'before and after' contour map, then check again in a few months)

Sand is an interesting idea - rammed, as in a foundry situation - but I imagine it would need to be treated in some way so as not to dry out and lose cohesiveness, and not to encourage corrosion.

Seems to me one interesting possibility would be to use something along the lines of shell moulding, but where a gas rather than heat is used to set the sand. Strikes me the gas could possibly be infused through coiled pneumatic tubing with pinholes, rammed in with the sand.

Would certainly be less 'irreversible' than polymer concrete or such.

Thanks also for the 'rubber bag' tip- I'd not heard of that. I can imagine lining the cavities with something like thick butynol roofing - if the objective was purely damping and mass, without any attempt to add stiffening, and if there were concerns about expansion on setting (as well as corrosion). Could conceivably improve the damping situation.

Thanks again to any and all, this really is a great forum....