Pre-stressed makerslide

Extrusions/Tubing/Hinges/Etc.

Pre-stressed makerslide

Postby dzach » Thu May 03, 2012 9:32 pm

This is a thought I'd like to have tried before I posted, but since I have no way of getting the necessary makerslide I'll describe it now and hope that Bart, or someone else who's got a good length of makerslide extrusion, can test and report the results.

Pre-stressed concrete is a well known method to overcome concrete's natural weakness in tension. It's used in the construction of bridges or floors e.t.c.
500px-Prestressed_concrete_en.svg.png
From wikimedia
See the wikipedia article.

The idea is to use the two side holes of the makerslide extrusion, or the center one, to insert a threaded rod along the hole. After inserting the rod, the two ends take a nut (with a washer if needed) and the nuts are tighten to a certain force.

A test for this idea would involve the following:
  • Define the test extrusion length, e.g. 2m
  • Define the test weight, e.g. 10kgr
  • Define acceptable deflection
  • Suspend the extrusion from two wedge shaped supports, wedge edges up
  • Attach dial indicator in the middle of the extrusion
  • Suspend the weight from a string and hang it in the middle of the extrusion
  • Measure deflection
  • Remove weight
  • Install tension threaded rods (no tension)
  • Tighten the end nuts to reach acceptable deflection, curvature up
  • Hang the weight in the middle of the extrusion
  • Measure the deflection
  • Compare and repeat with different values

The test can provide some valuable data about the behaviour of makerslide under load and prove if the pre-stressed makerslide can be used in heavier applications like CNC milling e.t.c.

Bart? Someone else willing and able?
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Re: Pre-stressed makerslide

Postby pyarza » Thu May 03, 2012 11:10 pm

I believe I will not be as good as thought:

Metallic materials as steel or aluminum have "similar" tensile and compressive strengths (completely different from concrete that has a high compressive strength and a very low/null tensile strength).

The use of of pre-stressed steel is usually combined (embedded steel bars) on concrete structures in order to "improve" its combined tensile strength, but I don't believe that it would do any good "pre-stressing" the makerslide extrusion (unless you are going to make it work in a pure traction way, and I'm not sure that is the way you want it to work).

As for the deflection on the center of the beam, it would reduce deflection, but the pre-stressing rod should be installed on the lower middle of he makerslide (from the center of gravity of the makerslide cross-section to the bottom) in order to make an upward pre-deformation, and (if I'm not wrong) the hole in the makerslide is situated in the center of its cross-section.

Maybe a better solution could be creating a new makerslide extrusion with more height: This would improve (lower) its deflection under a vertical load.
A squared section "40x40" would improve deflection issues.

One more thing that should be considered (id doing some tests) is that when you are adding/coupling the rod to the extrusion you are somehow modifying the cross-section (and probably improving the strength) of the makerslide extrusion.

Disclaimers:
I'm not sure about all of the above (and an empirical test could be useful)
Sorry about my English, but it is not my first language
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Re: Pre-stressed makerslide

Postby dzach » Thu May 03, 2012 11:31 pm

Thank you for the response.
The objective is to minimize deflection. The makerslide has a kind of octagonal center hole sized approx. 12x15.85mm. A threaded rod of e.g. M6 size, or a cheaper steel rod with threaded ends, could be placed at the bottom of the hole with a properly shaped end cap, so that the slight initial curvature given to the extrusion is upwards. It would be interesting to see if/how much load can increase for a given deflection with such a method.
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Re: Pre-stressed makerslide

Postby Zat German » Fri May 04, 2012 2:35 pm

This is called "camber" , not pre-stressed. You see this in the beds of those open bed shipping/transport trucks that if you look at will curve up slightly when unloaded.
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Re: Pre-stressed makerslide

Postby TLHarrell » Fri May 04, 2012 3:23 pm

This would be a perfect solution for suspending a static load. Since you are probably thinking of a CNC type machine, the stresses involved are not static, therefore this solution would not solve it. Makerslide is a lightweight structural member, with the intent of lightweight machine design. Anything larger than 18-24" unsupported, and with larger stresses (milling and cutting, carrying more than 10 lbs) is outside of the original design intent.
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Polymer Concrete

Postby bdring » Fri May 04, 2012 3:43 pm

What about filling the voids with a polymer concrete. You could do that with an induced camber. Polymer concrete is quite popular in the CNC world these days.
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Re: Pre-stressed makerslide

Postby dzach » Fri May 04, 2012 5:44 pm

Well, all I'm asking is a test measurement, some engineering data! Why not?
Zat German wrote:This is called "camber" , not pre-stressed. You see this in the beds of those open bed shipping/transport trucks that if you look at will curve up slightly when unloaded.

Camber it might be, it's the tension at the ends that improves the beam's properties. The method of creating the tension is what is known as "pre-stressing" in concrete.
http://en.wikipedia.org/wiki/Camber,
http://en.wikipedia.org/wiki/Prestressed_concrete
TLHarrell wrote:This would be a perfect solution for suspending a static load. Since you are probably thinking of a CNC type machine, the stresses involved are not static, therefore this solution would not solve it. Makerslide is a lightweight structural member, with the intent of lightweight machine design. Anything larger than 18-24" unsupported, and with larger stresses (milling and cutting, carrying more than 10 lbs) is outside of the original design intent.

Is there something wrong with getting the test done? I would have done it before posting but the longest makerslide I have in my possession is 420mm long from a Hadron ORDbot and I find it ridiculous to order a 2m extrusion just to perform a test. Even with that I would have done the test, were it not for my fear that if I break or scratch something, a replacement is not likely to be here within the foreseeable future.

I cannot see how a static or dynamic load would change something to the worse, with regard to pre-stressing. At least not before I or others can perform some calculations and see that come out. Engineering is all about measuring and nothing about believing, so 10lbs and 18-24" might be a good general rule of thumb but not proper data for engineering design.

Filling the voids with (polymer) concrete would probably make the extrusion stiffer and improve other mechanical properties too. It would also make it heavier. This is another "good to have" test, if test data can be obtained and published.

EDIT: I'm not claiming that makerslide will become ideal for CNC milling or anything else but, because it is an open source/hardware design, it would be so useful to have some data published. You never know by whom and how it can be used.

EDIT2: The reason I'm talking about pre-stressing is because it might prove to be a cheap and non destructive method of re-enforcing makerslide in applications that might need it. The same extrusion can be later used in other applications by just releasing the end nuts.
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Re: Pre-stressed makerslide

Postby naPS » Fri May 04, 2012 8:33 pm

The problem with introducing camber on a non-static load is that it will introduce a deflection to the head when the head isn't in the exact middle of the beam. This isn't ideal for CNC applications, and is probably why you don't see it used.
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Re: Pre-stressed makerslide

Postby bdring » Fri May 04, 2012 8:35 pm

It sounds like you are getting a little upset @dzach. We are just bouncing ideas around here and not trying to disrespect you. I can send you a 2 meter section if you want to do some testing.

Concrete has excellent strength in compression but is horrible in every other mode. By pre-stressing it you can get better performance in those other modes and use it as a beam.

Metals are not typically pre-stressed this way. The only pre-stressing of metals I have heard of are more to change the properties of the metals before it is used like in a wire rope cable.

I don't think your idea would change the MakerSlide properties much, but it may yield a stronger beam because of the added material. I also don't think you could predictably deflect the MakerSlide enough in the direction you are suggesting without undesirable affects like bow or twist in other planes.

The added material (if it is metal) would contribute most by being as far off center to the side in tension as possible. Laminating a strip across the bottom or putting a tensioned 8mm threaded rod in the bottom slot would be best.

The goal in a CNC machine is to stay flat. If the load deflects it, that is bad, but isn't starting with a deflection is just as much of a problem?

Realistically if you want to go over 3-4 feet with Makerslide under load you will need to add multiple supports or back it up with more material.
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Re: Pre-stressed makerslide

Postby dzach » Fri May 04, 2012 10:20 pm

Not at all. No reason for me to be upset or feel disrespected, to the contrary, I feel we have a good discussion and I thank all for their responses to this idea. In any case, I'm not preoccupied with this idea which might just prove bad, doing the actual test would require less brain cells than posting about it. I think shipping a 2m piece across the Atlantic just for this test is a little over the top, I can do such a test with a regular 40x20x2000mm alu extrusion and satisfy my curiosity :) . Thanks for the offer anyway.

Back to the subject though, the test I detailed above asks to define an acceptable deflection, be that 1, 0.1 or 0.01mm it doesn't matter. The acceptable for the application deflection (or half that) should be the initial camber. Then the gain in load, if any, can be measured for the pre-stressed case. The idea is to find a cheap way to increase the load that can be put on a makerslide. But as Pyarza said earlier, it might prove cheaper to have a 40x40mm extrusion to do the job.

bdring wrote:I also don't think you could predictably deflect the MakerSlide enough in the direction you are suggesting without undesirable affects like bow or twist in other planes.

You might be right about distortions in other planes; that will make it more difficult to adjuct the right tension and keep other distortions to a minimum.

I think deflection data together with some V-wheel data has been asked before in some other thread. I bet it will keep popping up as more and more people use makerslide in their projects but cannot find such data. Not only for 3D printers or CNC machines but for other applications like the camera slide that I see in another post.
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