Author Topic: Another homemade milling machine  (Read 2840 times)

Offline AdeV

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Re: Another homemade milling machine
« Reply #50 on: April 18, 2017, 12:05:28 AM »
Since it's only the weight you're after, and not the constructional qualities, why not just use a bag of sand? Seal the bottom as best you can, but leave a removable panel; pour sand in the top until desired weight is achieved - it'll find its own way past the internal structures obviously. Then, when you need to move the machine at a later date you can simply open the removable panel and let the sand out...

The only thing you need to be sure of is that the sand is bone dry before you pour it in, wet sand will cause untold rustiness...
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Online awemawson

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Re: Another homemade milling machine
« Reply #51 on: April 18, 2017, 02:18:34 AM »
If it's a closed box structure i.e. No holes for stuff to flow out, dry sand or pea shingle will work just as effectively as concrete, and will settle into a dense packing over time. Also in extremiss it can be removed if you make a suitable plugged hole in the base.
Andrew Mawson
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Offline PekkaNF

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Re: Another homemade milling machine
« Reply #52 on: April 18, 2017, 03:26:57 AM »


I like the idea of filling with concrete. Adding mass is one of the simplest things you can do to a small machine to improve it.......

PK
I do too but Pekka disagrees and I see his point about shrinkage. However upon doing some extra reading it seems that most of the shrinkage comes from adding more water than is needed to make the concrete move better it seems that if the added water is calculated correctly then little shrinkage occurs but the mix is extremely thick. Will have to look into it a bit more.

The thing is that concrete shrinks for 400 years. First faster and then slower. Many factors affect on shrink rate and some you have influence and some not.

Worst shape for concrete "filled machine" is a hollow thin steel section that you fill with concrete...some machine manufacturers tried this -70/80:s and failed. Concrete filling will shrink in time and delaminate from the skin...if you weld rebar to avoid that it will pull the skin out of alingnment. There are ways to reduce the shrink rate or compensate it, but it will not work in machine building scale.

You can make a servisable machine if you know what you are doing. Basically you cast more or less square section, no voids in it, you may cast some features to it. You don't need rebar and other "iron" on it, unless you have hanging beam or such bad idea. And then let it cure and age some time - longer the better. Then use this cast as a sandwitch between steel members and have it on compression. You must use epoxy concrete, injection epoxy or grind the concrete surface flat prior mechanically mounting the steel sructure to it.

Epoxy concrete works.

And as Andrew et.al. said sand or such will dampen "ringing" pretty well too, it's cheap, reversible and has all good qualities to home build.

Pekka

Offline shipto

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Re: Another homemade milling machine
« Reply #53 on: April 18, 2017, 05:13:57 AM »
What about a dry mix? www.precast.org

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Online awemawson

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Re: Another homemade milling machine
« Reply #54 on: April 18, 2017, 05:38:13 AM »
It will eventually harden from moisture in the air and you'll lose the advantage of being able to remove it.
Andrew Mawson
East Sussex

Offline shipto

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Re: Another homemade milling machine
« Reply #55 on: April 18, 2017, 06:07:15 AM »
Won't want to removed it there's nothing I need to do from inside. The only advantage of being able to remove it would be weight but once it's in place it would be my kids job to get rid of it 😃

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Offline mattinker

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Re: Another homemade milling machine
« Reply #56 on: April 18, 2017, 06:32:14 AM »
If you want weight only, lead!

Regards, Matthew

Offline shipto

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Re: Another homemade milling machine
« Reply #57 on: April 18, 2017, 06:42:16 AM »
It's not just weight it's helping rigidity too. However as I have said I won't make a decision until it's running and I have tested to see if it's needed, fingers crossed it won't need anything extra.

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Offline PK

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Re: Another homemade milling machine
« Reply #58 on: April 18, 2017, 06:49:35 AM »
If you want weight only, lead!
In my experience....

Mass is the main advantage. Particularly on a small machine as it reduces the resonant frequency of the structure below the excitation frequency of the spindle+tooth count combination. 
This resonance (and some terrible engineering in the case of the X3 mills) is what causes the most grief, and it's why the 20KRPM Chinese spindles for chump change have revolutionised the CNC router business. They increase the excitation frequency above the resonant frequency of even the tiniest mill

But there are others.  When the structure is made up of parts of different densities, a portion of energy that travels through it as waves (rather than displacement/vibration) is reflected at each interface. This forces a portion of these waves to travel long paths and get absorbed by the substrate.

Online awemawson

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Re: Another homemade milling machine
« Reply #59 on: April 18, 2017, 07:53:09 AM »
Blimey PK that's a bit impressively technical  :bow:
Andrew Mawson
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Offline PK

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Re: Another homemade milling machine
« Reply #60 on: April 18, 2017, 09:26:04 AM »
Blimey PK that's a bit impressively technical  :bow:
Yeah, sorry 'bout that. It was a big chunk of my PhD.... Buy me two beers and I can bore you to death with the numbers... Oh, and I've had two beers......

Offline PekkaNF

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Re: Another homemade milling machine
« Reply #61 on: April 18, 2017, 10:15:38 AM »
Blimey PK that's a bit impressively technical  :bow:
Yeah, sorry 'bout that. It was a big chunk of my PhD.... Buy me two beers and I can bore you to death with the numbers... Oh, and I've had two beers......

By all means. I'll get you drunk. Please tell more! I used go on trips with one professor, we hooked acceleration sensors and other transduces to measure vibration and find out excitation sources/sympathic frequences. Laymen had no idea how involved in real life it is.

And granules are different animals. We measured some of them too in one big machine...Big beam filled with sand or glassballs...So, which one is "better"? it all depends what is your question.

So...all this is pretty basic (from post #44):
http://www.mech.utah.edu/~bamberg/research/ConcreteCastDamper/concreteCastDamper.html

Everyone seriously considering should least read this:
https://pdfs.semanticscholar.org/5487/82f4c63376150991dd323d0ef15a7da2fee8.pdf

At this phase of this project it's all pretty academic and best to forge forward.

Pekka

Offline shipto

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Re: Another homemade milling machine
« Reply #62 on: April 18, 2017, 01:28:29 PM »

At this phase of this project it's all pretty academic and best to forge forward.

Pekka
I agree I didnt intend it to get this much of a discussion its more a "what if" situtation at the moment I could get it set up and it runs like a dream or it could be so bad that I scrap all but the expensive bits and not even want to waste a bag of cement. I hope it the first option  :lol:
« Last Edit: April 18, 2017, 03:18:21 PM by shipto »
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Offline shipto

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Re: Another homemade milling machine
« Reply #63 on: April 23, 2017, 04:28:47 PM »
So I have now turned my attention to the knee slides and in yet another operation that would have been done better with a mill I made 4 elongated slots in each and made some blocks to help me get them lined up straight and keep them there. I have decided that I will get one side lined up as best as I can to the column and pin it, then everything will be lined up to that.
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