What material for PM25V column lift?

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.
 
Hey Bill, bet you didn't think putting a 2.5" spacer under your column would result in this, eh? ;)
 
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
 
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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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.
Click to expand...


may
auxiliary verb

Definition of may
(Entry 1 of 4)


1a —used to indicate possibility or probability

;)

Cheers,

John
 
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.
 
mikey said:
Hey Bill, bet you didn't think putting a 2.5" spacer under your column would result in this, eh? ;)
Click to expand...
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
 
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?

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
Click to expand...
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.
 
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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