Molding a concrete slab to use as a base for a bench mill.

[verbotenwhisky]

Inexpensive bench mills do not come with a bottom that is machined perfectly flat to begin with. Just the act of placing the mill on a perfectly flat base, or any base, is going to cause it to to flex and contort. Further bolting it down to said base, flat or not, is going to cause it to distort more.

No matter what you do the bottom of the mill will distort. I have never seen a procedure for placing mill and removing all distortion from the mill/base interface. Maybe I am just inexperienced???? I bet distortion is just a fact of life and bolting a bench top mill to a heavy ridged stabile surface gives you more distortion BUT also gives you a consistent repeatable starting point that will also add rigidity to the mill while also dampening vibration.

IMHO "0.00013" over the whole 18"x24" surface" for a base will buy you absolutely nothing in the ability, functioning and repeatability of the bench mill. Rigid, stabile and vibration dampening, in my opinion, is what you should be chasing... not flatness. If you are going to chase flatness you had better disassemble the mill and take the base to someone that has a Blanchard grinder to flatten the bottom of the mill if you want your flat base to have any positive affect.

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If you take the time to build the best, level-est and stiff/heavy base then setting the mill, shimming and anchoring become far less traumatic. Taking the attitude that its already distorted then not working to build the best base possible will only lead to headaches later.

 

[MrWhoopee]

If you take the time to build the best, level-est and stiff/heavy base then setting the mill, shimming and anchoring become far less traumatic. Taking the attitude that its already distorted then not working to build the best base possible will only lead to headaches later.

Much like building a house. You can still get it right if the foundation is not square and level, but it's a whole lot easier if it is.

 

[dabear3428]

Much like building a house. You can still get it right if the foundation is not square and level, but it's a whole lot easier if it is.

yes but if the house is out 1/16th over 10ft is that really that much better that 1/4" over 10ft?
and how much more work was put in to getting there that could be used someplace else to have a better return on investment?

 

[nobog]

The rigidity of the mill come from the interface of the base, column and head and the size/mass of such. Cutting Delrin? who cares. Making titanium NASA parts? Something heavier duty than a bench mill might be in order.

 

[verbotenwhisky]

yes but if the house is out 1/16th over 10ft is that really that much better that 1/4" over 10ft?
and how much more work was put in to getting there that could be used someplace else to have a better return on investment?

The point we were making is, the effort you put into the foundation will improve your final result and make the work on top of it easier..... Aim small miss small. More to the point, he is setting a machine with a much smaller tolerance, the proverbial 1/4" becomes a big pain in the ars when one starts chasing .001's with shims....

 

[dabear3428]

If you take the time to build the best, level-est and stiff/heavy base then setting the mill, shimming and anchoring become far less traumatic. Taking the attitude that its already distorted then not working to build the best base possible will only lead to headaches later.

level stiff and heavy does not equal perfectly flat, don't get me wrong perfectly flat is great but the amount of work involved to get there can be better spent elsewhere IMO. IMO make it as rigid as possible, connect the mill with as much rigidity as possible and adding mass/vibration absorption where you can, are the biggest gains for the work involved in getting there.

 

[dabear3428]

The point we were making is, the effort you put into the foundation will improve your final result and make the work on top of it easier..... Aim small miss small. More to the point, he is setting a machine with a much smaller tolerance, the proverbial 1/4" becomes a big pain in the ars when one starts chasing .001's with shims....

if the base of the the mill is out 3 thou how is that any different than the concrete base being out by that much?
if you really what to go for broke take the base of the mill off, wrap it in a release agent and cast the concrete against that or use metalized epoxy to provide the perfect fit between the mill base and the concrete base

 

[dabear3428]

The rigidity of the mill come from the interface of the base, column and head and the size/mass of such. Cutting Delrin? who cares. Making titanium NASA parts? Something heavier duty than a bench mill might be in order.

kind of my point if the accuracy is that important a better use of resources is spent on a more rigid mill or reconditioning/moving a used mill with that level of rigidity/mass already built in.

in my case i have a grizzly g0758 6X20 mill and the rigidity on the tram of the head is of more concern than the flatness of what it is mounted to.

 

[verbotenwhisky]

if the base of the mill is out 3 thou how is that any different than the base being out by that much?

I work in a world where they machine skids, 10' long and 8' wide, on a huge mill to flat for a 1000 HP motor and pump to +/-.002" so they can shim out the discrepancies. Why do that if you know the motor and pump are out +/- .060"? The answer is simple, if you have at least one reference surface as close as possible setting the equipment and aligning the equipment becomes easier and simpler with less shim material required.

I understand your point; however, when I do something I work to the most accurate tolerance possible knowing that I will not hit it but I'll be close, it simply matters to me....

 

[verbotenwhisky]

kind of my point if the accuracy is that important a better use of resources is spent on a more rigid mill or reconditioning/moving a used mill with that level of rigidity/mass already built in.

in my case i have a grizzly g0758 6X20 mill and the rigidity on the tram of the head is of more concern than the flatness of what it is mounted to.

I have a PM940V and when I set it I leveled the base as close as possible before I set the mill on it and then shimmed the mill on one corner to get it leveled up with the machinist level, it was a pain but has made my life so much easier.