Concrete counter tops??

[ShedBoy]

Looks good Andrew but I am not a fan of shelves, too hard to get to the back. I made some draws out of lined bread crates after getting sick of trying to get to the back of shelves. Just my 2 cents worth. Are you going to do a build log? I for one would like to see the concrete table build.
Brock

 

[Jasonb]

One thing you need to understand is that as the thickness of teh item goes up you need to use larger aggregate to reduce the risk of shrinkage crack.

The Low heat cement is just that cement no sand or larger aggregate and the use mentioned above is what we would refer to a grouting( not to be confused with tiling grout) and uses pure cement. When used in this way there is a high amount of cement and the "heat of hydration" will be high so the low heat stuff is used. When water is added to cement the chemical reaction creates heat.

The counter top mix is similar but does contain fine aggregate (sand) and other additives but is generally not used over 2" thick so again heat when setting and curing may cause issues.

As said the best way to cure concrete is to put the item under water once it has gone through it initial set, thats what we used to do when testing concrete, after casting the test cube and keeping in a humid enviroment after 24hrs the mould was struck and the cube of concrete placed in a special tank at a constant temp until it was taken out for testing be it 7 or 28day tests.

As for saying concrete cannot be designed well it can if you know how, its been a long time since I did any calculations but I was taught how to do it to obtain the desired final strength and then how to adjust this for things like workability(slump) without affecting the final strength.

One thing that has not been mentioned is the importance of vibrating the concrete once in the mould this drives any air bubbles to the surface and has a considerable affect on the overall strength of the finished item. It also has the added benifit of bringing the fat to the surface which makes getting a fine finish easier, for the same reason tapping or vibrating the sides of the mould will produce a finer surface finish.

J

 

[Jasonb]

Just looked at the drawings again. Are you only casting two little slabs, one to go under each machine?

Cant see that you would gain much advantage with such a small, think bit of concrete.

J

 

[John Hill]

I am very pleased with my concrete bench and I am sure a concrete benchtop would be great too.

For what it may be worth, my bench was cast in one piece, upside down on a steel plate. The top surface is very smooth.

 

[lazylathe]

Okay the more i read this post the more confused i get.... ???

Some people are using a slab of granite to mount their machines on, not a granite table.
Would the concrete slab not have the same effect as the slab of granite?

Of course i am going to vibrate and tap the mould to release air bubbles and achieve a dense casting.
A lot of this stuff i take for granted as i know what to do.
Maybe i should do a write up with very fine detail??
I pour moulds at work every day and use vibrators and tapping to release air bubbles, it is something i do not really think about, just do.
And tenting and keeping the mould wet while drying is very important and will be done.
I cannot submerge it in a bath of water for 28 days, SWMBO would not find that very funny! :big:

I can of course see the advantage of having the whole bench cast out of concrete but then again i do not have the skill
to do that is my basement. And if someone else does it the bench will never make it down into the basement!

Please also take into consideration that the machines i am mounting are a small Myford ML7 and a Sieg SX2L mill.
Not huge pieces of equipment here.

Reading some of the comments i am heading in the right direction while others say i am heading down the wrong road...
Would it be better to just use a slab of granite?
This concrete thing seems to be getting a bit out of hand.

I would most likely want to cast the smaller bit for the mill and see what it comes out like.
Call it a test.
If it is works then the larger slab should also work.

Andrew

 

[f350ca]

Andrew
Sounds like a very reasonable way to add mass to your machines. I think concrete with rebar and steel mesh would be much beter than a granite slab. As stated the rebar adds tensile strength to span the gaps between supports, granite wouldn't have this.
I've cast a few concrete kitchen counters with great success. They have ground and polished tops but the casting process would be the same.
I generally use regular portland cement with granular B gravel if available, in winter I've used the premixed bags with equal results. On the counters I circle the perimeter with 3/8 rebar and tie 6 x 6 mesh to the rebar, you can get fibreglass chop from a redimix plant, comes in bags, this adds tensile strength to stop the micro cracks from forming as the cement ages, water reducer is another must, again available from the redimix plant, by adding this you cut the water required to hydrate the cement and again stop shrinkage.
If you line your mould with plastic vapour barrier you'll get a glassy finish on the sides and bottom, flip the piece and your top is done.

 

[lazylathe]

I think i am going to give it a try anyway and see what happens!

At the moment i am knee deep in house renovations....
SWMBO has ordered a walk in closet!
So after a lot of demolition and cleaning i have reframed and drywalled the new closet.
Now it needs to be mudded etc...
Then paint the whole room and install wood flooring.

Once she is happy then i have time to work on my bench! ;D
In the meantime i will work on the design and look into my options with concrete.

Andrew

 

[ShedBoy]

Yes keeping the Commissioner of War and Finance happy is always a must but usually comes with great reward. Don't let the big words confuse it, concrete can be as basic as you want, most of the big words in the background are happening you don't need to worry about them. You can analyse any subject to death and make it sound more confusing than it actually is. I mentioned the grout idea because it is the most simple way we can do it with miners, who are not all rocket scientists, and still get consistent results at the level we want.

Hi John Hill, do you have any build photos of your bench? Is that a coolant drain around the outside? I really like the look of that table.

Brock

 

[Ken I]

Just an idea - why not use those precast & prestressed concrete lintles side by side and screed over to finish.

2c Ken

 

[lazylathe]

Good idea Ken!

Just the position of the rebar strengtheners may be an issue.
I will have to go and have a look at them and take some measurements.

Drilling doles in that stuff is also usually a blasty blast time!!! ;D

Andrew

 

[robcas631]

Why not go to the junkyard or a steel shop and get a 1/2" piece of steel?

 

[John Hill]

A concrete workbench is the only kind you can carry down into your basement in a bucket!

My early ideas on my concrete bench were to build the support for the concrete slab by setting hollow concrete blocks and filling them with wooden formwork for the top slab. I only had someone else make it when they made me a very good offer.

Did I mention that I really like my concrete bench? ;D ;D ;D

 

[Lykle]

Nice idea, should work.
But be careful with the X2 mill.

I find that the short axis, Y axis?, has a handwheel that is uncomfortably close to the table top.
So I had to mount my X2 on a little pedestal to give my hands enough room around the handwheel.

And I think that adding wooden blocks to increase the height would negate solidness of the wonderfull concrete slab.

Lykle

 

[lazylathe]

Nice idea, should work.
But be careful with the X2 mill.

I find that the short axis, Y axis?, has a handwheel that is uncomfortably close to the table top.
So I had to mount my X2 on a little pedestal to give my hands enough room around the handwheel.

And I think that adding wooden blocks to increase the height would negate solidness of the wonderfull concrete slab.

Lykle

I had thought of that already! ;D
Mine is already mounted on a thicker board on the desk.
The form i am going to build will also be stepped so that this can be incorporated!

Thanks for reminding me though!!! ;D
When i go through this before i start pouring to ensure i have everything ready it will remind me!

Andrew

 

[lazylathe]

Ken also mentioned precast concrete and got me thinking...

What about those precast concrete steps?
They are rated at about 6000psi and are reinforced as well.
Also pretty thick and heavy!

Will go to the garden centers this weekend and have a look!

Andrew

 

[Niels Abildgaard]

Hello Andrew

A lathe needs additional mass and a very stiff support.
Stiff and strong is not the same.
Garden chair steel is not as strong as aircraft maraging steel but stiffness is the same.
If the granite/concrete thing cracks when leveling and aligning lathe You need a new lathe also.

 

[lazylathe]

Most of the precast steps around here are 7 inches thick.
I seriously doubt that it would crack...

I know you have your lathe mounted on a large block of granite.
What would you recommend?
Apart from putting the lathe in the corner and forgetting about it. ;D

This discussion on mounting the lathe always seems to take some serious turns in opinions.

When i go and look at the flimsy steel benches some of the high end lathes come with it makes me wonder
why it is such a big deal.
I know it has to be strong, rigid and be able to handle having the lathe mounted to it and tightened down securely.

And then i look at some of the work done on lathes mounted to wooden benched and wonder if it is really required at all...

Andrew

 

[Ken I]

Most of those lintels etc are pre-stressed as well - so they are always in compression (concrete is always assumed to have no tensile strength) - apart for catacysms - they don't crack.

Ken

 

[Niels Abildgaard]

Hello Andrew

My recommendation is to start with the Myford.
You know it already and can judge if working has been improved being put on rock or concrete.
I have had a Myford myself and the next thing to decide is if You want to keep it original with a lot of belts and a clutch or put a VFD and a single ,short belt on.I am not in doubt if it was mine.
The old WW2 advisory about separating motor and lathe foundation is not valid today.
If I remove the belt and run my three phase,six pole motor at 100 Hertz there is absolutely no vibrations.
This means that the small resonances I sometimes see is from belt or lathe.
I like the feel and smell of granite but if starting over again I would pour concrete and incorporate foundation for motor and lathe as close together as possible in one block.Belt tension can be controlled once a year and if not OK loosen the four foot nuts ,apply some illegal violence and tighten the nuts again.Most years it will be OK.
If You have a dimension sketch of the Myford please show and I will sketch a suitable pedestal.

 

[lazylathe]

Hi Neils,

Attached is a very bad sketch done in paint of the bolt hole dimensions.

The total length of the lathe is 1033mm and the total width is 350mm.
The motor is mounted to the back of the lathe and i will leave it that way.

VFD's would be nice but they are too expensive at the moment for me, maybe in the future.

I was thinking of building a string, heavy workbench from wood and have the concrete pedestal inside the bench top.
I placed a picture of an idea in this thread earlier i think.

Andrew