# What material for PM25V column lift?



## Bill Kahn (Oct 7, 2019)

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


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## matthewsx (Oct 7, 2019)

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.


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## JCMunder (Oct 7, 2019)

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.


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## Bob Korves (Oct 7, 2019)

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.


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## mikey (Oct 7, 2019)

JCMunder said:


> 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.
> 
> ...



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.


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## benmychree (Oct 7, 2019)

JCMunder said:


> 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.
> 
> ...


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


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## JCMunder (Oct 7, 2019)

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.


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## JCMunder (Oct 7, 2019)

benmychree said:


> 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.


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## pontiac428 (Oct 7, 2019)

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?


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## JCMunder (Oct 7, 2019)

pontiac428 said:


> 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?


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## pontiac428 (Oct 7, 2019)

Believe me, I get it. I just don't know if the juice is worth the squeeze. My issue isn't with the consideration of stress on the system. It's a question of proportion (if the earth moves one inch closer to the sun, how much shorter would the year be? Picoseconds?). Obviously, relatively small resonances in a lathe can affect the finish- but is it the same for a mill, that rotates (vibrates) a fluted cutter against the work and has many more coupled elements in between?

Galvanic corrosion is hard to chemically balance without an aqueous electrolyte to pull electrons through. It exists, but it will be glacially slow and the metal salts have nowhere to go. Besides, the contact surfaces can be passivated in the home shop.

You can get very robust equations for calculating cutting forces on any axis from machinery's handbook. It should be easy to plug some useful numbers in to find the loads on the machine.


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## mikey (Oct 7, 2019)

Hey Bill, bet you didn't think putting a 2.5" spacer under your column would result in this, eh?


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## matthewsx (Oct 7, 2019)

My statement that aluminum may be an option wasn't based on any serious engineering calculations. Rather that the PM25 is a small machine which already isn't capable of taking big cuts so any additional deflection from a adding 2.5" spacer is probably a trade-off worth taking if you need the additional capacity on the Z axis.

I don't know if the OP intends to leave it in place permanently, but I suspect there is a specific project in mind and it will be removed after that is done. 

From a practical standpoint the cheapest option will be structural steel, if the project involves only light cuts it might even be possible to use a section of box beam. 

John


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## JCMunder (Oct 7, 2019)

I tell my young engineers the following:
Believable:  WAG (Wild A$$ Guess) is less than INFORMED OPINION is less than ANALYSIS is less than TEST.  Test is best.
Laziness:  WAG (Wild A$$ Guess) is easier than INFORMED OPINION is easier than TEST which can be easier than ANALYSIS.  

Thinking about this on the way home, I realized that this is an single variable problem is easier to test than analyze.  So, I did a simple test.
The column and base stiffness is unchanged, so this is really a question of how stiff is a thick aluminum shim.  And a simple cantilevered beam stiffness test, set at the appropriate moment arm and load should be a reasonable approximation to at least determine the magnitude of the problem (are we talking mils, tenths of mils or angstroms).

I took a 2" steel box beam and determined that the top of my vice is 11" above my column base.  It's not unreasonable to machine above the vise, so, lets say 12" to make things easy.  My test moment arm is 12"

What's a good machining load?  Hell, I don't know, but I have a 55lb Chineseium Anvil that actually weighs 48lbs (too munch Bondo?).  So, 48lbs sounds like a great side load due to machining and feeds.

I clamped the beam to my table with 12" cantilevered forward. Using my 15-0-15 gauge (.0005/division), I set an indicated deflection (.015" for the first test).
I then carefully suspended my anvil from the wire and recorded .003" deflection (a reduction in the indicated value).
I next clamped the beam in place on a ~1.25" steel plate (all I had on hand) and repeated the test.  Same clamping and torque.  I recorded .003-.0035".  So, as expected no real change.
Lastly, I clamped the beam in place on a ~2.625" aluminum block (6061-T6) and repeated the test.  Again, same clamping and torque.  I recorded .0105" deflection.
I got an additional .0075" deflection by just adding the AL block!  That's a 150% increase, and real numbers not angstroms.  

Now, if I used an infinitely stiff bar and recorded zero deflection with the all steel set-up, I'd still expect to record the same .0075" from the additional AL shim since the shim was the only variable and it's the weak spring in the system.

The results are scalable, if my 48lbs is too high, 1/2 the load (24lbs) would produce 1/2 the deflection (.0038").  Too low and, well, you get the picture.  Now, this is just a static load test.  A dynamic loading due to the cutters would probably produce smaller numbers (maybe 40-60% of static), but still significant. 
Please, can a more experience machinist comment on the loads, it's out of my wheel-house and I didn't have the time to research it.

In the end, my opinion is that using an Aluminum shim could produce real and noticeable errors due to deflections.  I'd personally buy some steel for this purpose before using free aluminum.   Again, just my opinion.

PS. Please feel free to mock my hokey, "stop it, you're scaring me" setup.  But I only had a free hour before soccer drop off, and still managed to take 3 reading for each test configuration.         This has been a fun exercise. 
Quote from my son, when I asked him to get my anvil "You want WHAT!"


Regards,
Joe.


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## matthewsx (Oct 7, 2019)

You are certainly willing to invest quite a bit of time and energy into this question.

I'm not an engineer, or an experienced machinist but I do write for a living. 



matthewsx said:


> 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.




*may*
auxiliary verb

*Definition of may*
 (Entry 1 of 4)


           1a                                                            —used to indicate possibility or probability          



Cheers,

John


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## JCMunder (Oct 7, 2019)

Oh, I'm just excited to be here.  
I'm a Newbie and a hack on the forum and had to pick a first topic.  
I came hoping to be a sponge, helping me to better learn machining.  I guess I didn't figure that my engineering experience could be useful here as well.  Stiffness issues is something I've dealt with for 30+ years.
I cannot offer this forum examples of beautifully machined items, my skill just isn't there yet.  But maybe I can bring design engineering skills and information to offer.

I made sure to say things like "Respectfully, I'd have to disagree on the materials." and "in my opinion".  I surely don't mean to offend.  Sorry if I did.

Regards.


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## Bill Kahn (Oct 7, 2019)

mikey said:


> Hey Bill, bet you didn't think putting a 2.5" spacer under your column would result in this, eh?


You are so right—I had no idea! This is fascinating the depth of the considerations.  I really had no thought to use aluminum, but I love the topics being discussed.  (I happen to have a very nice large piece of Magnesium available.  If the analysis says in practice it doesn’t matter what I use maybe I’ll save myself the trouble of ordering something and just cut off a chunk of that (yeah, the corrosion thing and all I am sure indicates I shouldn’t.  Would sort of be fun to do, and post back three years from now saying that for my hobby work (anything accurate to .002” for me is perfect) the Mg worked great). But given the trouble of actually squaring up a chunk of anything, and the trouble of lifting the mill up, and the trouble of rejiggering tilt and nod, I do sort of want to get it right in one try.  If it doesn’t work, I will, of course, go back.).

I almost wonder how to empirically test anything I might put in.  I am constantly messing up my milling.  All sorts of vibrations—like I am facing and am doing a final finishing pass, and some tiny resonance starts and the visuals of the surface finish are lousy (can’t feel it, but can see it). Sometimes going still slower or shallower barely helps.  What tests could I think about to compare base case to 2” steel riser block, to 3” steel riser, to 3” Aluminum.

Sounds though like I have my key question answered—I can use any steel.  Buy the good bolts. Torque them well.

I am sure others have done this before—any reports from the field are still of interest.

Thanks so much—what a wonderful group here.

-Bill


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## Firstgear (Oct 7, 2019)

Bill Kahn said:


> 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?
> 
> ...


Given the size block of metal you need if it was me I would have no problem using aluminum as I have a bunch of it.  You will not be able to measure the size deflection you might get given the cutting forces.  It’s not like you are going to see 10,000 to 20,000 lbf.

Use grade 8 bolts and nuts and if you must thread into the aluminum block stick some coil inserts in there. Use 2X bolt diameter for thread engage length, bit over kill but you have the length. Torque the bolts to grade 8 specifications with lock washers.

sometimes in pursuit of the perfect solution we run right past good enough.


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## matthewsx (Oct 7, 2019)

This is a great place to ask and get answers, I am constantly humbled by the knowledge here and the willingness of people to share

My late father was an engineer and worked on stuff that had to work right, no room for error. I'm pretty much just a hack who is willing to keep refining my designs until I get the results I want. 

Cheers,

John


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## Bob Korves (Oct 7, 2019)

Interesting thread...


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## francist (Oct 8, 2019)

Agreed, and thought provoking on a number of different levels. Not just about materials and how we design for particular characteristics, but also how and when we decide to be satisfied with “good enough”. 

Most interesting indeed.

-frank


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## BGHansen (Oct 8, 2019)

Nice thread here.  I think the saying goes, "Experience is the best teacher", but it's tough to argue with the math.  I tend to be on the conservative side.  If I was going through the work to make a riser, I'd use steel.  Then you don't have that nagging question in the back of your head gnawing at you every day you use the mill, "should I have used steel or is the aluminum good enough?".  Take a little more time to hog it out of steel and you won't question yourself.  Reminds me of another old saying, "no one ever got fired for buying IBM".  

Bruce


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## Nogoingback (Oct 8, 2019)

BGHansen said:


> Nice thread here.  I think the saying goes, "Experience is the best teacher", but it's tough to argue with the math.  I tend to be on the conservative side.  If I was going through the work to make a riser, I'd use steel.  Then you don't have that nagging question in the back of your head gnawing at you every day you use the mill, "should I have used steel or is the aluminum good enough?".  Take a little more time to hog it out of steel and you won't question yourself.  Reminds me of another old saying, "no one ever got fired for buying IBM".
> 
> Bruce



+1
There are a number of threads on this forum where members have expended a fair amount
of effort trying to improve stiffness in their machines.  Folks with round column mills fill the columns or create bracing, lathes
get solid plinths to replace the compounds and so forth.  The light weight machines that hobbyists use often need all the help
they can get, and folks that make these improvements report noticeable results.  Small improvements
frequently matter.


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## Latinrascalrg1 (Oct 8, 2019)

Other the the dielectric/galvanic corrosion issue (which i consider to be a Major Sticking point that would Absolutely need to be addressed before proceeding down the aluminum path) what would you be loosing besides some of your time and some scrap material to try it for yourself?  You will be able to actually verify if the Aluminum would be strong enough to handle the job YOU need to complete.  If it works for Your Specific needs leaving you satisfied by its performance then what does it matter if its not as stiff as steel?  If theres a problem then either adjust  to accommodate, change paths or revert to the previous setup without the riser/spacer.


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## pontiac428 (Oct 8, 2019)

I really wouldn't worry about galvanic potential.  Aluminum needs hydroxide ion to make an electric cell.  In a dry system, it's not much of a concern.  If it was something to be concerned with, the quarters and dimes would be cooking off in our pockets, because copper and nickel have a difference in standard electrical potential of 0.5 volt.  If you put a clad coin in water, it will corrode, but on your workbench there ain't much going on.  Now when you bolt your aluminum heads on an iron block, that's different because the coolant is working as the electrolyte.  Even so, it wasn't a big deal for the Big 3 auto makers, who mixed metals with impunity for decades.  The 200,000 mile motors end up with some corrosion, but by the time they get there they're already spent and rotting in a junk yard, past their service life before galvanic corrosion became a problem.


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## JCMunder (Oct 8, 2019)

pontiac428 said:


> I really wouldn't worry about galvanic potential.  Aluminum needs hydroxide ion to make an electric cell.  In a dry system, it's not much of a concern.  If it was something to be concerned with, the quarters and dimes would be cooking off in our pockets, because copper and nickel have a difference in standard electrical potential of 0.5 volt.  If you put a clad coin in water, it will corrode, but on your workbench there ain't much going on.  Now when you bolt your aluminum heads on an iron block, that's different because the coolant is working as the electrolyte.  Even so, it wasn't a big deal for the Big 3 auto makers, who mixed metals with impunity for decades.  The 200,000 mile motors end up with some corrosion, but by the time they get there they're already spent and rotting in a junk yard, past their service life before galvanic corrosion became a problem.



Being new to this, I am rather unfamiliar with the common cutting fluids used.   
Do you think that water based cutting fluids used on the mill would provide such a hydroxide ion? And if it splashed onto and wicked into the joint could that raise the risk?
Still probably a low risk, but I'm curious about the forums experience with corrosion due to cutting fluids. 
Also, my personal shop is also the family garage, so my humidity goes way up when wet and snowy cars are parked in the garage/shop.


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## pontiac428 (Oct 8, 2019)

Yes, water based fluids will provide a hydroxide ion, and the wetting agents added to them will help with wetting, but the reduction in surface tension will inhibit capillary flow.  But sure, water based cutting fluids can help corrosion along (pretty sure Kool Mist has some corrosion inhibitors, the stuff is similar to antifreeze in every other way).  When I drip water-based fluids on my stuff, it leaves a mark on cast iron if I don't wipe it up right away.  There's so much oil on all of my stuff that it's not likely for real rust to form (flash rust that wipes off with an oily rag does not count).  With known sources of humidity in your garage, it would be wise to keep everything oiled or coated with lanolin.  Small increases in humidity compound rust's insidious desire to get on tools.  If your machine's all buttered up with petroleum, water-based cutting fluids just roll off.


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