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JohnInOttawa

Don't try this at home?

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So I have a print job that will involve concentric fit.  I need it pretty close and very round.

 

My first thought was, dimension it for lathing afterwards.  Of course, that really begs the question, can a 3D printed part (PLA, nylon, or nylon/CF) withstand the stresses of even a low speed lathe?

 

My hope is yes for at least the PLA and nylon.  I would rather not have to rig the HEPA recovery for the CF dust.

 

Thanks in advance.

J

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Heat would definitely be a factor.  I'm not sure if there would be positive effects or net negative.  But something would be affected.

 

I would use a metal lathe, so a carbide bit.  I also thought that would give me finer control on cutting depth per pass.

 

Small test, good idea.  and a good shield.

 

J

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Depending on the scale of the part, you can start with parts that have a very slight interference, and hand sand them with emery paper to get the exact fit you want.  I have found that when used carefully in controlled manner I can hand sand parts to a quite high precision.  An OD can be sanded evenly by wrapping the entire OD with a band of 220 grit and carefully and evenly turning the part while keeping even tension across the width of the band.  ID's can be done similarly by winding the band just smaller than the bore, inserting and counter rotating the part in such a way that the paper tries to unwind against the inner surface.  You could print a custom sanding tool that is like in ID collet that holds the paper and also expands..  I have done alot  precision square or flat parts with a full sheet of sand paper on top of a marble floor tile.  Here you drag the part across the lapping surface with even and slow strokes, measuring often, moderating pressure to attain squareness...

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A few months ago, I've printed a chain guide for a motorcycle in Nylon for a customer. Looking around online, I found out that this was a replacement part for an old model of motorcycle which isn't in production anymore, and parts are not made nor available. My customer wasn't the first one to print this chain guide in Nylon.

 

I'm not sure what kind of heat is generated by a motorcycle chain when it moves, but I guess it does generate quite a lot, and experience by those people shows that Nylon can handle it long-term. That plus the constant high-speed rubbing as the chain moves.

 

So, I'd say Nylon has a good chance of being able to handle it. Though, of course, as was suggested above, do a small tests scale first just to make sure.

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I found that I cannot drill at all into PLA, nor tap a thread into it, nor use self-tapping screws. It just melts. However, manually filing and sanding is possible. And manually and genlty enlarging holes is also possible, for example to make an M4 bolt pass easily through a too narrow printed 4mm hole, which is actually only 3.5mm. I use a separate drill chuck for this: this gives good grip and allows careful drilling. But even this has to be done very slowly.

 

For higher temp materials, it should be a lot easier, like in commercial plastics.

 

DSCN5622.thumb.JPG.bcd33809236414534d665e6ac120651f.JPG

 

Some materials sand easily into a smooth surface, but some get a sort of fibrous and rough surface.

 

PLA, PET and NGEN tend to break at sharp corners where high stresses are present, like injection moulded plastics. But not along layer lines, in my experience: the fractures cut diagonally through the material, following the stresses. But layer lines may be an issue with some high-temp materials like nylon or ABS, with poorer layer bonding?

 

So I guess lathing should be possible, if done at low speeds and with a small "bite" (I don't know the correct technical term for not taking away too much material at once). But for high accuracy, material creep afterwards due to environmental temperatures, and due to mechanical loads will always be a thing to consider: it may spoil accuracy later on. Just like in most plastics.

 

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I took a look at the technical sheets for UM materials and the max temperatures they can hold.

 

PLA can go up to 50°C, Nylon can go up to 80°C, CPE+ to 100°C, PP to 105°C, PC to 110°C.

 

From personal experience, I wouldn't recommend ABS, as it has a tendency to just break if you go past a certain stress level. It doesn't crack, just breaks in pieces, so Im' not sure it would be able to endure even a low speed lathe.

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This is all great information, thank you everyone.    I'm thinking what I will do next is print off a test part in each of the several materials I have on hand (PLA, Nylon, CF/Nylon), set up a good shield and record the trials.  Just to keep things managable I'll run it all on a Taig micro lathe.

 

If the debris wasn't such a concern for the lense, I'd put a thermal imager on it to see what the heat buildup looks like.  Maybe I'll be able to fit an IR window into the shield or get a shot right after the run to see.

 

I'll see if I can post results here. 

 

Much appreciated!

 

John

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I'd be interested to see the results as well.

 

Perhaps you can protect your IR camera by putting a sheet of transparent plastic in front of it, like what is used for the enclosures of many 3d printers?

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I do have some IR transparent inspection ports that I may set up.  The trouble with most materials that are transparent in the visual is that they are opaque in the LW IR spectrum (8-14 micrometer) used by most uncooled imagers these days, or have big transmissivity loss or variation.  I haven't tested the latest polycarb sheet in my rack, might get lucky.   The idea of setting up a barrier is sound, I may be able to insert the port into my shield over the lathe.

 

I'll give it whirl, and advise.  As with everything, may take some time to get it done.

 

Along the way I appear to have discovered time travel and it is work dependent.  The harder I work, it seems the further I get behind.  Now, if only I can figure out why that is....

 

Thanks !

 

John 

 

 

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I have had success drilling and tapping PLA, but only very carefully and by hand. if you go too fast the thread will melt and strip. (both whilst thread cutting and screwing in a bolt.) it will also obviously not hold as well, and will strip easier than prehaps expected. 

At the moment i usually use a soldering iron on a brass insert, and melt the insert into a hole in the printed part. 

this is very successful.

 

I have also put some on a lathe; using carbide metal cutting tools. Again only with PLA, and was pleased with the results. I did run coolant, and it was similar to machining some other plastics- too slow rubs and melts, needs to be fast enough to actually cut.

 

Hope that helps!

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