What material for PM25V column lift?

[Bill Kahn]

I would love to have a smidge more vertical clearance on my PM25V. So, I am planning to put a 4”x6.4” block of steel, like 2.5” thick, under the column. And attach with longer bolts (still 12x1.75).

Has anyone here done that? How much rigidity do you lose? Can I go to 3” thickness?

And, does the material make much of a difference? I was thinking of just squaring up any old a chunk of hot rolled I can find. But are some steels better for this purpose? Ductile iron? Grey iron? And there are grades there too. This is the first time I have ever thought about steel choice—I normally just pick up whatever scrap might be in the drops bin. But for this I will order what I want.

Thanks.

-Bill

 

[matthewsx]

I doubt material choice will make a difference here if you're going with a solid piece of steel or iron, aluminum may even be an option.

Of course you will sacrifice rigidity but how much of a difference that makes will depend on what you're doing with the extra clearance.

 

[JCMunder]

Respectfully, I'd have to disagree on the materials.
Stiffness depends on the modulus of elasticity, also known as Young’s Modulus, which is constant for a given metal. Because Young’s Modulus for steel is three times that of aluminum, an aluminum part under load will deflect three times as much as a similarly loaded steel part. The thickness and shape of the formed part also contributes to its stiffness.
All steel has approximately the same stiffness, but comes in many different strengths depending on the alloying metals used.

For stiffness, I'd recommend a solid piece of steel. I'd also go with Grade 8 bolts and up the preload (torque) just a little.

Joe.

 

[Bob Korves]

Cast iron will dampen vibration better than steel, but probably the rest of the column and base of the mill are cast iron, so maybe it is not an issue.

 

[mikey]

Respectfully, I'd have to disagree on the materials.
Stiffness depends on the modulus of elasticity, also known as Young’s Modulus, which is constant for a given metal. Because Young’s Modulus for steel is three times that of aluminum, an aluminum part under load will deflect three times as much as a similarly loaded steel part. The thickness and shape of the formed part also contributes to its stiffness.
All steel has approximately the same stiffness, but comes in many different strengths depending on the alloying metals used.

For stiffness, I'd recommend a solid piece of steel. I'd also go with Grade 8 bolts and up the preload (torque) just a little.

Joe.

Joe, I can't disagree with your analysis but Bill is asking about a spacer and I wonder if an aluminum part would suffice. If he were subjecting the part to an end load that had to sustain cutting force then yes, the MOE would be really critical. However, I wonder if the compressive forces would matter in a column spacer, especially if it was bolstered by torqued down grade 8 bolts. If that spacer did move, I would bet that you would need an Angstrom-meter to see it.

 

[benmychree]

Respectfully, I'd have to disagree on the materials.
Stiffness depends on the modulus of elasticity, also known as Young’s Modulus, which is constant for a given metal. Because Young’s Modulus for steel is three times that of aluminum, an aluminum part under load will deflect three times as much as a similarly loaded steel part. The thickness and shape of the formed part also contributes to its stiffness.
All steel has approximately the same stiffness, but comes in many different strengths depending on the alloying metals used.

For stiffness, I'd recommend a solid piece of steel. I'd also go with Grade 8 bolts and up the preload (torque) just a little.

Joe.

Steel alloys or heat treating make virtually no difference in modulus of elasticity, that is, deflection under load.

 

[JCMunder]

I agree that the difference would be small, but I approached this from the OP's position of taking something from the drop bin. If I had the choice between scrap Al and scrap steel, I'd take the steel, since all will have the same stiffness.
Cast Iron could dampen more, but given the cast iron base and column on either side, the steel spacer won't make a measurable difference, since dampening is a function of length and the spacer is short relative to the column and base.

In a bolted joint, once preloaded the bolt loads will not change due to external forces unless the loads exceed the bolt preload. In this case that would be doubtful. This means that the spacer, not the bolts are the load path. Since the base of the column (the spacer) will react the machining loads, the loads are a torque on the base, being amplified by the moment arm (distance from cuter to spacer). Placing a soft material like AL in a stiff column can greatly effect the entire system stiffness.
I'd worry not only about deflection, but the first-mode frequency of the spacer (both are driven by stiffness), meaning that the AL spacer is more likely to react to machining vibrations than the steel. This could be more dramatic than the cast iron dampening comment above.

Though your reply did make me look at this further. I'm surprised to say that I forgot how low a modulus Cast Iron can have. Cast Iron can have a modulus of 10(x10^6) to 24. AL has a modulus of 10 and steels range from 29 to 31. So, if this is for a Grade 1800, 3000 or 4000 cast iron structure with low modulus (13-20), then the AL spacer may not change the stiffness enough to worry about. If this is a Ductile Irons (60, 80, 120) cast iron with high modulus (24), then the AL spacer could have my above concerns.

After I posted, I realized that I also didn't consider galvanic corrosion. In this case, plain carbon and low alloy steels are a match for cast iron, whereas AL can corrode in contact with them.

 

[JCMunder]

Steel alloys or heat treating make virtually no difference in modulus of elasticity, that is, deflection under load.

I concur, all steels have approximately the same stiffness.

 

[pontiac428]

Is aluminum made of sponge cake? Given the difference in the moduli of elasticity between aluminum and steel, how much actual displacement at the cutting tool are you considering to be an issue compared to the amount of movement that will actually occur due to the difference in material? I think you're talking about tenths of tenths or smaller. I don't care about the world beyond .0005", and I certainly don't care about 1x10^(-10) kinds of numbers. Isn't rejecting an aluminum riser as too flexible akin to throwing the baby out with the bath water when put into real-world context?

 

[JCMunder]

Is aluminum made of sponge cake? Given the difference in the moduli of elasticity between aluminum and steel, how much actual displacement at the cutting tool are you considering to be an issue compared to the amount of movement that will actually occur due to the difference in material? I think you're talking about tenths of tenths or smaller. I don't care about the world beyond .0005", and I certainly don't care about 1x10^(-10) kinds of numbers. Isn't rejecting an aluminum riser as too flexible akin to throwing the baby out with the bath water when put into real-world context?

If I have some time this week I'll take some measurements of my PM932 to determine the geometry. The harder part is determining the machining forces. I can just apply a random 100 lbs radial load for sensitivity, but I don't have a feel for the actual loads. From there, calculating the deflection is easy. Until then, the actual displacement would be a guess on my part. Though whatever the answer is, the aluminum displacement will still be 3x larger than for steel.

The OP asked "And, does the material make much of a difference? ", the basic answer is yes. The detailed answer is yes, but the amount might not matter to you. I'll see if I can quantify it with some realistic numbers. And we still have the galvanic corrosion issue of Al against steel.
If the OP had said "I only have AL to use, will that be ok?" I may have had a different answer.

The best way to think of this is to take a steel beam. Cut it in half and then weld in a coil spring between the two halves. The beam halves are not any less stiff now, but the entire system stiffness is much lower, and it is driven by the stiffness of the spring. Aluminum has 1/3 the stiffness of steel and it will drive the system stiffness.

Qualitatively, I thought the trend in Mills and Lathes is bigger, heavier, more massive, more metal, and thus stiffer. Given how much easier AL is to machine, couldn't we make them from cast and machined AL for much less $$ if it didn't matter?