Full crosbraces for Prusa Mendel

You've used dibond before, Nophead, why not for the frame? too thin?

granted, I haven't had much luck finding it, myself. I found one printer (as in a purveyor of printed signs) that sells small custom signs for around $10, but most suppliers are selling 4x8 ft sheets (or large sheets at least) for $150-250.

What's nice with getting it form a printer is you can send them an image file with all the holes marked out and cut lines highlighted.

I have used Dibond for the Y carriage as it is stiff and light, but I may use the piece cut from the middle of the frame for that.

I don't think it is stiff enough for the frame though and the sheets need to be thick enough to screw into or tap. The upright sheets could have nuts on the back but its tricky to place them without clashing with the buttresses.

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[www.hydraraptor.blogspot.com]

Ah ok. Yeah the acrylic will look slick, like it's floating :-)

BTW, if anyone is interested in the place i found for dibond, it's here:
[www.858graphics.com]

But i just noticed they have a $100 minimum :-(

On the aluminium extrusion supplier side, I just received an order from KJN (discovered on the wiki they sells to individuals, prices seems correct), and I'm starting to love that over the threaded rods...

At £2.90/m, most of the cost was the t-nut (but their basic t-nut can be inserted in the slot at anytime).
I also ordered an handle to carry the reprap ^^ [reprap.org]

I have been reading with interest the debate as to how effective the cross braces can be.

I have just completed a set for my Prusa and while there is still movement in the frame, the cross brace makes a valiant effort in removing lateral movement.

I have not used the upgraded frame in anger yet, It will be interesting to see if the additional stiffness improves the print at all.

@Justblair: THX for the feedback and comments!

Same findings here: it helps, but it is not the end of the road.

Should I keep that frame, I'll crossbrace the lower side - or a couple of parallell bars supporting Z motors where they belong, at the base of the machine.

I'd like to help getting the guesswork, hear-say and promotional snake oil out of the evaluation of machine frames (and other components). This might foster real improvements...

So I started some preliminary work on acoustic analysis of different frames Have a look at this thread, your comments would be much appreciated: [forums.reprap.org]
Next one to be auditioned (with same firmeware & gocde) is a cupcake. Has always been some sort of musical instrument, since it is built much like a resonance box(and I suspect this is a weak point of boxed frames)

Also: who could provide us with a calculation of the fundamental frequencies of the rods (stainless steel) with the lengths involved in a Prusa Mendell? I started to look this up, I think I found a couple of sweet articles, but things advance faster with several people investigating some points.

I have the printer running tonight printing a wades extruder in 100% fill. Lots of backwards and forwards acceleration and deceleration.

With the braces attached the Prusa looks and sounds a lot more composed. There is still a little movement in the x-plane. But we are talking a couple of mm at the top of the vertex now. This is a fraction of what I was getting before.
in
More impressive is the damping effect. Minus the bracket I reckon that the Prusa may be prone to oscillation, especially when filling into tight shapes. The bracket really brings this right down. The mechanism I think is that the rods on the lower half of the printer are bending and absorbing the shock. Pre-bracket the printer is rocking about due to the slight play in the plastic brackets

So what I am seeing is that there is a double benefit. In the first instance the movement in the X plane is reduced, Secondly oscillation is reduced considerably. I agree that getting a completely solid printer may be impossible on the mendel format, but for the legions out there who have now committed to the design, the brackets are going to be a worthwhile addition. I still am unsure if the quality is better, but with less noise and vibration I will wager that the printer is going to stay in alignment for longer and wear should be reduced.

Indeed. For the majority committed to the Prusa Mendel frame (although coming upgrade options will permit a very hign rate of parts reuse) a couple of meters of threaded rod and three to six printed parts will allow stiffening the base and one of the sides.

What I am seeing in the spectra (more coming soon) is that high freqency resonances in the rods themselves aren't that much of a problem as I initially thought, most of the disturbances and energy are concentrated in the very low to low frequency range, up to 500 Hz, among them the large vertex rocking one can see and feel. I suppose too, play between the thin plastic parts and the rods.

Benchmarking should resort to the accelerometer rather than to a mike. I am having a look at the ADXL335 which features a very decent refresh rate.
ADXL335
Though, I wonder if the Mendel frame vibrations warrants plus or minus 3g ?

Edited 1 time(s). Last edit at 01/18/2012 06:37AM by Lanthan.

Resonance? Try attaching a large mass somewhere to change the natural frequency. Or better still a large mass suspended in rubber attached to the frame. As an added bonus it apparently works quite well if your printing during an earthquake.

How about putting 10mm od/ 8mm id tube around the main frame members with a washer and bolt either end. It would be a relatively easy upgrade to see if it improves things. You could pre tension the frame struts.

martinprice2004 Wrote:
-------------------------------------------------------
> Resonance? Try attaching a large mass somewhere
> to change the natural frequency. Or better still a
> large mass suspended in rubber attached to the
> frame. As an added bonus it apparently works quite
> well if your printing during an earthquake.

Yeah. Tuning. But first we need to determine what those natural frequencies are (hence the other thread and first attempts)

>
> How about putting 10mm od/ 8mm id tube around the
> main frame members with a washer and bolt either
> end. It would be a relatively easy upgrade to see
> if it improves things. You could pre tension the
> frame struts.

Very interesting idea!
However, if the main problem is at the plastic/metal interface rather than with the metal rods, as we are suspecting it, that might not help that much.

Just found this paper. Those guys get experimental data by building a tetrahedron and submitting it to hammer blows. It is possible that this practice would satisfy many a Mendel builder Besides, good starter for anyone whishing to do some FEA on the frame.
http://sem-proceedings.com/22i/sem.org-IMAC-XXII-Conf-s20p04-Modal-Analysis-Tetrahedral-Machining-Structure.pdf

In the automotive field, a good test of a structure is tortional stiffness, where you fasten one end of the structure then apply a twisting load to the other to measure angular displacement (its often quoted as a measure of how good a chassis is). Perhaps this might also be a simpler way of getting a feel for how various designs perform against one another.

The basic principle is shown here and should be familiar to a lot of people.

A simple bench setup could be made to test this, perhaps clamping the base and pulling the top of the machine with a known load in X.

Edited 1 time(s). Last edit at 01/18/2012 04:33PM by martinprice2004.

martinprice2004 Wrote:
>
> The basic principle is shown here and should be
> familiar to a lot of people.
>
> A simple bench setup could be made to test this,
> perhaps clamping the base and pulling the top of
> the machine with a known load in X.

This is a very sound proposal.
By just firmly pressing vertically with one finger on the end of a rod on the top, I can induce a noticeable displacement of the lower Z bar end on the same side, maybe half a millimeter to one millimeter...

If you can in someway secure it on it's side, you can hang a know weight like a kg on the top vertex and then measure the deflection. I think that'd give us a more quantitative measurement we can compare other machines against.

Nothing is ever as simple as in the text book.

Theres a slight confusion here that corner to corner rods are the best solution, they MAY be better than the one in the original post, but they may be worse. There are several reasons for this.

1) It assumes steel is the same in tension and compression. This is approximately true for steel beams, but for thin rods. Euler Crippling Loads come into play, making the effective strength/stiffness better in tension than compression, this makes the simple corner to corner analysis not quite so clear as the mechanism then works in 3 dimensions imparting side loads on all the members. Fastening the cross point at the centre would probably be an improvement.

2) The corner to corner assumption is more valid when analysing simple pin jointed structures with thick struts. What we have here is not a pin jointed structure, so that assumption is not true. The bars are not subject to simple tension or compression, but also side loading (not present in pin jointed structures). The only real analysis which truly reveals which is best would be FEA.

3) The Thingiverse structure shown in the first post has substantial width centre blocks, which stiffen the assembly considerably to resist bending in the rods, A simple cross structure may not have this.

4) The forces applied to the mechanism are complex and vary considerably in service. We are not looking at a texbook strut assembly with point loading.

We need to keep an open mind as we may be surprised what works best on a reprap.

Edited 2 time(s). Last edit at 01/20/2012 04:32PM by martinprice2004.

I was thinking about this today and I have an idea for measuring improvements in stiffness using a line laser, ruler and a luggage scale.

Mount the line laser securely on the top of one of the z-axis steppers, pointing at a parallel wall.

Secure the ruler horizontally so that the line laser intersects the 0 marker.

Attach the luggage scale (or more accurate if you have it) to the z-axis smooth rod at the top of the printer.

Pull till you get a decent amount of shift in the frame

Read the scales.

Apply the upgrade (in this case the cross brace)

Now apply the same amount of force on the luggage scale (ie get the reading to be the same as before.

Read the amount of deflection as indicated by the line laser and the ruler.

I am sure that someone far more knowledgeable will point out the flaws in this method, but it is quick, uses household items and if an upgrade is worthwhile it should at least tell us if the x-axis (which is the weakest) has been stiffened.

If scales are out the question we could also use a weight, string and pulley to apply the lateral force.

Yes sounds a workable solution. Perhaps theres a way to set it up so the laser movement is amplified across the scale. The improvements will likely be in the fractions of a millimeter in X so we it needs to be something quite accurate.

Accurate measuring devices are getting quite cheap nowadays, so don't be put off getting a bit professional, for example dial test indicators (dti's) start at around £10 and digital micrometers around about £8. Theres many to choose from on ebay. A dti is probably best for this sort of measurement. Either buy one with an adjustable stand or just bolt it to a simple wooden frame and zero it on the end of the z axis motor mount.

Edited 1 time(s). Last edit at 01/20/2012 06:16PM by martinprice2004.

martinprice2004 Wrote:
-------------------------------------------------------
> Yes sounds a workable solution. Perhaps theres a
> way to set it up so the laser movement is
> amplified across the scale.

I am not sure we will need to amplify it. Observation says that the difference is considerable. It should be easy to see it using a mm scale.

After reading this thread and thinking about the issue, I've had a go at producing a new frame vertex which aims to stabilise the square section of the Prusa frame. It does this by strengthening the joint from the triangular part to the square part, at the expense of the (as I see it) overly-strong constraint of the triangle, which, as someone posted earlier, is inherently strong. There is also provision in the vertex for full cross-bracing, or using a sheet material to constrain the squares of the frame. The scad file is parametric, so feel free to play around with width and depth of the vertices if you feel that the size I have come up with is compromised in another way.
[www.thingiverse.com]