Aluminum for new carriage plate?

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MeanderBolt
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Aluminum for new carriage plate?

Post by MeanderBolt » Fri Mar 27, 2015 5:12 pm

I would like to cut out a new carriage plate that contains a centered pocket that the Makerslide will fit into. I feel like I have my screws super tight, tighter than probably any other place on the machine, but it still seems like it slips over time. The idea is to provide a way to keep the vertical Makerslide straight and that the only thing the screws are there for is to keep it attached. I figure that a couple of millimeters would be more than enough to keep it locked in. I have a coupe of other little widgets I want to add to the plate as well, but this is the main motivation.

It is strictly a friction held alignment and I remember wondering how it would stay straight while I was putting it together. Now, I will say it has done a far better job than I thought it would do.

My question, is how thick of a piece of 6061 would I need to replace the steel plate. I am not a structural engineer, so aside from the obvious stresses, I have no idea what kind of forces I will be running into.

I guess while I don't have any immediate ideas about cutting other plates out right now, would whatever is recommended for the carriage also be okay for the rest of the machine plates?
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REdington
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Re: Aluminum for new carriage plate?

Post by REdington » Fri Mar 27, 2015 5:30 pm

.250" will work if you keep your pocket depth to about .025" . If in doubt go with .375" but I think that is over kill.

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Re: Aluminum for new carriage plate?

Post by WillAdams » Fri Mar 27, 2015 5:34 pm

Why not just sister the aluminum w/ the new carriage plate?

You'd just need slightly longer bolts, but it should make things even stiffer, right?
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MeanderBolt
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Re: Aluminum for new carriage plate?

Post by MeanderBolt » Fri Mar 27, 2015 5:40 pm

If I am understanding what you are saying about 'sistering' it... Do you mean like face the whole piece and leave tabs / posts so it locks into the first plate, or are you suggesting that I just stack them and the M5 bolts will do the aligning?
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WillAdams
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Re: Aluminum for new carriage plate?

Post by WillAdams » Fri Mar 27, 2015 5:48 pm

I wouldn't go to the trouble of tabs / posts, instead, I'd mill the new piece and include a couple of extra holes --- clamp the new part together w/ the original plates, drill through the new holes and use suitable hardware to fasten the twain together. For extra rigidity you could use an epoxy such as J.B. Weld when fastening.
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cvoinescu
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Re: Aluminum for new carriage plate?

Post by cvoinescu » Sat Mar 28, 2015 2:14 pm

I wonder how much of the behavior of a bimetallic strip you'd get that way.


You can actually calculate how much aluminium is as stiff as steel.
  • Stiffness is proportional to the modulus of elasticity (Young modulus), which measures the rigidity of the material;
  • Stiffness is proportional to the cube of the thickness of the plate*.
So, to replace one material with another, you need to multiply the thickness by a factor that's the cube root of the ratio of the moduli of elasticity.

Given, say, stainless steel E = 180 GPa and 6061 aluminium alloy E = 70 GPa, the thickness ratio is (180 / 70)^(1/3), or about 1.37; mild steel has E = 210 GPa, so to replace that with 6061 aluminium, you need (210 / 70)^(1/3) = 1.44 times the thickness.

If I remember correctly, Shapeoko 2 motor plates are 12 gauge mild steel, so 2.78 mm thick; you'd need 4.00 mm thick 6061 for the exact same stiffness. If you used 0.25", or 6.35 mm, it would be exactly 4.00 times as stiff (a coincidence: both calculations come within 0.1% of exactly 4).

* Why cube? Intuitively, if you add thickness to a plate, you have more material (that's one), the new material is further away from the mid-plane so it has better leverage (two), and the new material needs to deform more for the same angle of bending (and that's the third power).

[Edited to correct formatting and to replace rigidity, a property of the material regardless of shape, with stiffness, a property of the combination of shape and material.]
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WillAdams
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Re: Aluminum for new carriage plate?

Post by WillAdams » Sat Mar 28, 2015 2:52 pm

So long as the machine doesn't get wet, not so much.

If you’re using a lot of coolant, might not be such a good idea, but arguably in that case you’d want a machine w/ stainless steel plates and hardware --- wonder where you could order one, at least w/ such plates?

Here maybe? http://store.amberspyglass.co.uk/config ... lates.html

Anyway, that guy seems to know what he’s about. I suppose if I had a sense of humour, this is where I’d place a smiley.
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cvoinescu
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Re: Aluminum for new carriage plate?

Post by cvoinescu » Sat Mar 28, 2015 2:55 pm

All those people telling you you have no sense of humor are wrong. You do, it's just not the kind that most people appreciate. If you need that emoticon to make me smile, you're doing it wrong. :)
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MeanderBolt
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Re: Aluminum for new carriage plate?

Post by MeanderBolt » Sun Mar 29, 2015 3:57 am

I suppose if I had a sense of humour, this is where I’d place a smiley.
Yeah, seriously Will, I was looking for the 'like' button.
Sometimes, and probably because you say that a lot, I picture you voiced as Marvin from Hitchhikers Guide to the Galaxy. Especially with that last bit. :D

@Cat, Are you saying .25 in. is 4 times as stiff as the steel?
So, I am reading that if I can find 4mm, I would be fine. Of course you would pull out all sorts of math that I would read prior to going to bed (brain is shot for the day).
More, does this piece require the 12 guage steel? Or if I used say 3.175 mm thick 6061, would this be sufficient? Although saying this... I plan on milling out a pocket, so the 6.35, or 4 mm would be a better choice. But would it work for the other plates, or is it just too thin?
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REdington
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Re: Aluminum for new carriage plate?

Post by REdington » Sun Mar 29, 2015 5:32 am

MeanderBolt wrote:

@Cat, Are you saying .25 in. is 4 times as stiff as the steel?
So, I am reading that if I can find 4mm, I would be fine. Of course you would pull out all sorts of math that I would read prior to going to bed (brain is shot for the day).
More, does this piece require the 12 guage steel? Or if I used say 3.175 mm thick 6061, would this be sufficient? Although saying this... I plan on milling out a pocket, so the 6.35, or 4 mm would be a better choice. But would it work for the other plates, or is it just too thin?

That is what I got out of what Cat said. I think the 6.35 would be the best and make all of your plates out of it. I'm going to use 6.35 for all my plates on my next build.

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