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

[dabear3428]

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....

in that case you are aligning two divorce mounted pieces of equipment with a common shaft between, in that case a perfectly flat starting point is VERY important. in this case we are talking about an entirely different animal and the effort to get a perfectly flat mounting surface and he is talking about much flatter that .002", like i said having it that flat is nice but not worth the effort IMO, effort that can be used elsewhere to have more effect on the final outcome.
the base of the mill conforming to the concrete base so that it is not warped when bolting to the concrete base, and if the base of the mill is not flat then attaching it to a perfectly flat base will warp the mill and make it worse if you really want to have a positive impact shimm it level and apply a metalized epoxy between the concrete base and the mill base before tightening the base to the concrete

 

[dabear3428]

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.

again you are talking about level, level is not flat. there are 2 components to an interface if both are perfectly flat then great but if one is not flat then making the other perfectly flat is a waste of time. other wise i agree with you

 

[dabear3428]

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.

BTW how do you like the pm-940V I'm deciding between that mill and pm knee mills like the pm-949TV, how do you think they compare?

 

[MikeInOr]

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.

From previous post my point was a piece of granite counter offcut with a concrete slap poured on the bottom of it should be plenty flat enough and granite surface plate flatness is a waste. I disagree that a granite counter off cut will lead to any problems down the road. From what I have seen online it is flatter than what most people use for a base for their bench top mill. I stick with "my" belief that stiffness and stability is much more important than surface plate flatness.

But the OP poster as well as everyone else on the forum is welcome to build their base any way they think will work best for them.

 

[MikeInOr]

again you are talking about level, level is not flat. there are 2 components to an interface if both are perfectly flat then great but if one is not flat then making the other perfectly flat is a waste of time. other wise i agree with you

This makes perfect sense to me!

 

[verbotenwhisky]

BTW how do you like the pm-940V I'm deciding between that mill and pm knee mills like the pm-949TV, how do you think they compare?

I like my mill, it has the standard square mill traits which I can live with. It has plenty of power, it'll run a 3" face mill .125" cut through steel without bogging down. The knee mills are great but to get a knee mill with the work envelope as large as the PM940V-PDF has it would cost twice the price. The Z axis has 20" vertical movement with 22" table clearance 26" X-axis, 28" if you pull the power feed and 12" Y-axis movement. I wanted the PM940M-PDF but I am glad now that I have the PM940V-PDF because many of the guys with the gear heads have issues with seals and the weight of the head. Mine has a 2HP 3Ph motor capable of plenty of torque and up to 5000 RPM.

 

[dabear3428]

I like my mill, it has the standard square mill traits which I can live with. It has plenty of power, it'll run a 3" face mill .125" cut through steel without bogging down. The knee mills are great but to get a knee mill with the work envelope as large as the PM940V-PDF has it would cost twice the price. The Z axis has 20" vertical movement with 22" table clearance 26" X-axis, 28" if you pull the power feed and 12" Y-axis movement. I wanted the PM940M-PDF but I am glad now that I have the PM940V-PDF because many of the guys with the gear heads have issues with seals and the weight of the head. Mine has a 2HP 3Ph motor capable of plenty of torque and up to 5000 RPM.

yea the V makes a lot of sense to me, i know that knee mills are inherently more rigid but not sure that i need it and in the 940V PDF you get a lot of extras that would drive up the price of the 940TV. If i decided to CNC it the bed mill is much easier to convert. what are the standard square mill traits that you are referring to? other than rigidity i'm not aware of any down sides to the large bed mills like the pm-940

 

[verbotenwhisky]

From previous post my point was a piece of granite counter offcut with a concrete slap poured on the bottom of it should be plenty flat enough and granite surface plate flatness is a waste. I disagree that a granite counter off cut will lead to any problems down the road. From what I have seen online it is flatter than what most people use for a base for their bench top mill. I stick with "my" belief that stiffness and stability is much more important than surface plate flatness.

But the OP poster as well as everyone else on the forum is welcome to build their base any way they think will work best for them.

I might be incorrect in what I thought you were saying as you and DaBear3428 seem to be talking flatness and I am looking at the leveling and concerned about the pads where the mill will be bolted being level in reference to each other to simplify mounting the mill. Personally I would not actually set the mill on the concrete because it will absorb moisture and mold it which isn't good for the metal, I would build a shelf a bit lower, half way down to add mass and lower the center of gravity like I did on my wife's wood lathe. I do not disagree with your contention that stiffness and stability is important, actually adding mass and lowering the CG would play into that 100%. My contention is that what ever you set the mill on would be best if flat and level so there is a known reference surface there to start with.

Some times I note that we tend to disagree to agree from misunderstanding, most everyone here is sharp/smart and trying to get to the same destination, we are all headed to the same place...

 

[verbotenwhisky]

yea the V makes a lot of sense to me, i know that knee mills are inherently more rigid but not sure that i need it and in the 940V PDF you get a lot of extras that would drive up the price of the 940TV. If i decided to CNC it the bed mill is much easier to convert. what are the standard square mill traits that you are referring to? other than rigidity i'm not aware of any down sides to the large bed mills like the pm-940

Really the only one I contend with is I have to remember to lock the Z-Axis to negate the forward nod of .0015, lock it once it is set and that goes to less than .0001. The 940V is heavy so it doesn't seem to have the rigidity issues that the smaller mills suffer with. I have owned a few Bridgeports over the years and do not see the rigidity problem with this mill, it is solid at 1200 pounds. I added an extra joint to the arm holding the DRO to get it out to the front so I do not have to lean over the table to get to it and I replaced the mud flap that hung off the front of the table over the Y-Axis crank with an accordion cover and added lights.

 

[dabear3428]

Really the only one I contend with is I have to remember to lock the Z-Axis to negate the forward nod of .0015, lock it once it is set and that goes to less than .0001. The 940V is heavy so it doesn't seem to have the rigidity issues that the smaller mills suffer with. I have owned a few Bridgeports over the years and do not see the rigidity problem with this mill, it is solid at 1200 pounds. I added an extra joint to the arm holding the DRO to get it out to the front so I do not have to lean over the table to get to it and I replaced the mud flap that hung off the front of the table over the Y-Axis crank with an accordion cover and added lights.
View attachment 485446

sorry for drooling on the photo

like i said i'm on the fence for what to get after i get my shop air conditioned but i sure like the pm-940V
i did not look at your location, i work in Corpus and live north of there.
i have the same issue with locking my z on the grizzly and i'm mulling over the idea of adding a counterweight to keep tension on the head at all times. how good is the motor/vfd that ships with it? good low speed tq?