Concrete machine bench?

[John Hill]

I am slowly getting the new workshop sorted but the machines are still on blocks or stuck in temporary places.

Here is the idea for them, my 12x36 lathe, tiny shaper, drill press and maybe slow cut saw will all go on one 1200 x1600mm table that will be made of concrete!

I have been talking to someone who can cast such a thing, the 100mm thick top and the support will be in one piece weighing quite a bit, close to a ton.  The base part will be in a shape of a letter 'H' (laying flat) with two solid ends and a connecting part through the middle.  The top will be as flat and level as possible with a gutter cast around the edge.

Everything except the lathe can be just bolted to the table top but I am still thinking of how to mount the lathe so that it can be levelled,  it has four bolts at the headstock end and two at the tailstock end.  My idea at the moment is to set bolts in the table with nuts between the table and the lathe casting so that I can wind them up to level the lathe, once it is level I will fill the space with grout, any future levelling will need shims or I will be able to drive wedges under the feet of the table.  I am not really settled on these details as yet.

If the machines are carefully placed and I do not fit the chip sheild on the lathe there will be, albeit somewhat restricted, access to the backside of all the machines plus of course one surface will be easier to clean.


Anyway, concrete bench, daft or what?

[Bernd]

John,

Here is a thread from HMEM that might interest you. Kermit built a "sort" of concrete table for his lathe. Concrete Lathe Bench

Bernd

[loply]

What are the advantages of making the bench from concrete?

If it's mass for anti-vibration you're after I would suggest topping the concrete with a layer of nicely finished plywood, and hard varnish it.

Repairing the concrete as and when it chips, pits and stains will be more difficult then resanding/revarnishing/replacing a sacrifical wooden top. Heaven forbid it should eventually crack...

If you don't hugely long for the antivibration properties of such a heavy bench then I would just make a thick wooden one?

[John Hill]

Thanks Bernd, that bench Kermit made is a work of art!  He showed some good ideas there.

Loply, the attractions of concrete include mass, stability and hopefully vibration damping.  It should also be cost effective compared to steel or wood construction.   I am not overly concerned with dings and chips as most of the surface will be covered with machines and I do have a wooden top workbench for all that hammering and bashing activity,  good point though and having plywood sections to cover areas between machines would be a good idea. One of the ideas I have been discussing with the concrete casting people is to have a wooden batten around the outside to make a more 'people friendly' edge.
I will be pretty upset if it cracks!

[PekkaNF]

I don't know. It is an interesting design challenge.

Few random thought comes into my mind:
* Might be a good idea to make the base separately. You could level it after curing and place it over a thin rubber mat to pervent it from walkking and reduce further vibration
* You need a way to seal and paint it. You don't want oil and chemicals into concrete.
* You need to consider hardpoints in advance. Concrete works well in compression. Plastic or conduit pipes into mould to make holes for screws?
* You need to design the machine mounts such way that it will allow shrinkage of the concrete. Concrete shrinks rather long time.
* There are books and stories about concrete table tops.

PekkaNF

[srm_92000]

Hi,

My dad made a bench for his wood lathe as basically just a wooden box (fairly thick top piece) with legs, then, when all in place filled it with about 5or more bags of sand - doesn't move anywhere fast! but you can always get it out if needed.

Cheers
Steve.

[75Plus]

I will be pretty upset if it cracks!

I recently had 35 cu. yards of concrete poured for a driveway and parking area. The concrete guy assured me that there were 3 positives about concrete.

1.  It will get hard.

2.  No one will steal it.

3.  IT WILL CRACK

Joe

[owenh]

I made a concrete benchtop for my little atlas a few years ago, just built a form on the shop floor upside down with the holes for attaching the bits cast in place. I used bits of plastic water pipe. I finished it with automotive plastic body filler and painted it. i'm quite happy with it.

[John Hill]

3.  IT WILL CRACK

Joe

I think you used the wrong contractor to pour your concrete.  We use a lot of concrete around here, driveways, houses, boats, furniture and we dont expect it to crack.  Driveways etc we put saw cuts in to prevent cracks.

[PekkaNF]

3.  IT WILL CRACK
Joe

NO it will not!.
* You need correct amount of reinforcement, probably two meshes or what ever is that net made out of about 5-6 mm steel. You need spacers to place them to proper depth. Here you can find a plastic standoffs with varying height.
* Proper mix of concrete (too much water and it WILL shrink and crack), cast right and then don't waddle with the surface until you get the water out.
*Depending the air temperature, you may need to spray with water for a week and cover it with plastics to keep it from drying too fast.
* You also want to design it to avoid wild cross section variation, different thicknesses will dry/cure different speed AND shrink different speed AND that generates the cracks. Therefore, you probably want to make base and top separate.

Just my personal experience, I did need to find out basics of concrete when I did some home renovation.

Pekka

[John Hill]

You also want to design it to avoid wild cross section variation, different thicknesses will dry/cure different speed AND shrink different speed AND that generates the cracks. Therefore, you probably want to make base and top separate.

Thanks Pekka, a very good point.

[PekkaNF]

You also want to design it to avoid wild cross section variation, different thicknesses will dry/cure different speed AND shrink different speed AND that generates the cracks. Therefore, you probably want to make base and top separate.

Thanks Pekka, a very good point.

It's an great honor for a junior member to help a senior practioner of applied science! 

For a well seasoned metal worker another difficult concept of composite materials is to understand that they are COMPLETELY different from homogenous materials. E.G. Concrete is really good in compression, but sucs in loading of any other direction. You need to load it in compression. Therefore you need rebar or other type of reinforcement. Like table top. You place a load on top of it. Compression on top = good for top. Bottom not so good....therefore you need a mesh or rebar. On bridges and such they go even further and tension the tiebars to keep the whole contraption on compression.

Pekka

[John Hill]

Yes Pekka the intention is to have steel reinforcement in the slab but I had not thought of tensioning it but nothing is impossible.

[David Jupp]

Post tensioning might be a bit simpler to arrange than pre-tensioning.  Either would be significantly more complex than simple reinforcing.

[PekkaNF]

Yes Pekka the intention is to have steel reinforcement in the slab but I had not thought of tensioning it but nothing is impossible.

Sorry I did't mean that table top needs to be prestressed, just trying to explain the need for rebar. Prestressed concrete would not be my first attempt with concrete. See this article:
http://en.wikipedia.org/wiki/Prestressed_concrete

I was thinking more like this on basics:
http://en.wikipedia.org/wiki/Reinforced_cement_concrete

There must be better than this instruction somewhere, just could not find it fast:
http://www.diynetwork.com/how-to/how-to-build-a-concrete-countertop/index.html

That has all the basic stuff.

Here it would be cheaper and easier to buy concrete on the bags, just add water, you can choose right size agregate for the required thickness. I also would use somewhat thicker mesh than hog fence. You want the reinforcement close to the surface, but not too close. This will have something to do thickness. Do you have an access to local building codes? Most countries have typical cross section drawings of floors and such, they probably have lot of useful instructions and I'm pretty sure you'll find those materials locally easier.

Just tell me to shut up if I get too longwinded

Pekka

[Davo J]

Just a thought if this is going to be an island type bench, a hole somewhere for your electrical plug/s to go down through instead of running it over the side.

Dave

[75Plus]

 The concrete guy assured me that there were 3 positives about concrete.

1.  It will get hard.

2.  No one will steal it.

3.  IT WILL CRACK

Joe

Last INSIDE JOKE I'll post here. 

Joe

[dickda1]

John,

Cnczone has an insanely long lived thread on concrete bases and concrete-epoxy.

http://www.cnczone.com/forums/epoxy_granite/30155-epoxy-granite_machine_bases_polymer_concrete.html

-Dick

[DaveH]

Hi,

All concrete will crack, especially with reinforcement bars.

Guess why the stones are put in - crack stoppers.

Vibrations will also cause cracking.

Well thats my 2p's worth 

 
DaveH

[John Hill]

Pekka,  I wont be doing this job at home. It is not the mixing of the concrete or the lifting up in buckets to fill the form, nor is it the building of the forms and the cleaning up of the mess afterwards, it is all of those and considering we have a local concrete casting company interesting in doing the job I intend to let him do all the hard work!

There is a type of reinforcing now where they mix short steel 'staples' and it will be interesting to see what the experts have to say about that, so far I have only heard of it being used in building foundation pads etc nonetheless I expect we will be using .6mm mesh and if we decide to cast in one piece there will be reinforcement between the parts.

[John Hill]

Post tensioning might be a bit simpler to arrange than pre-tensioning.  Either would be significantly more complex than simple reinforcing.

I think internal reinforcing and design of the base to keep unsupported spans to a minimum.

[John Hill]

Just a thought if this is going to be an island type bench, a hole somewhere for your electrical plug/s to go down through instead of running it over the side.

Dave

Good point Dave however I have the electrical coming down from the ceiling.  I have gone to a bit of effort, not much, to get power cables out of the way in this new shop and the grinders, drill press etc are all wired from under the bench.  I have outlets for portable tools on the front of the bench which saves cables trailing over/through the work in progress on the bench.

[John Hill]

John,

Cnczone has an insanely long lived thread on concrete bases and concrete-epoxy.

-Dick

Dick, that stuff sure would be the way to go for a really, really serious job but somehow I suspect the retirement funds might not stretch to that!   

[John Hill]

Last INSIDE JOKE I'll post here. 

Joe

 

[dickda1]

John,  

I am well intro retirement myself.

Some of these guys are looking into the damping properties of concrete/epoxy in the iron casting cavities of existing parts.  You (and I) are, I hope, beyond this obsession (Quorn anyone?).

Concrete is absolutely an option for a lathe/mill/shaper.

Dick