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Sliding Mitre Saw Table Insert & 3d Printing Problems.

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philf:
A good friend gave me his old sliding mitre saw a few days ago.

All that was wrong with it was the plastic table insert was broken and he couldn't get a spare. His daughters had given him money for Christmas so he bought a new one.

I've had a 3d printer for some time and have made lots of whistles and Xmas tree decorations but thought that I'd have a go at something really useful.

Although not a newbie to 3d modelling I am a novice with Autodesk Fusion 360. I find it not very intuitive and some things which are very straightforward in some other 3d packages are seemingly impossible in Fusion - so you have to find workarounds.

After some time measuring and a few hours learning how to do things on Fusion I came up with this model.



At 384 mm long the part was far too big for my printer's 200 x 200 capability. Fortunately Fusion lets you split a model into parts so I split the model into 4 parts.



Conveniently, Fusion lets you export an STL file for each part.

I sliced the models with Cura 2.7 and used 100% fill with a 1mm wall and base.

Here's a pic of the original part with all the printed parts.



The parts looked basically OK (they haven't been fully cleaned up in the photos) but I was very disappointed with the holes. Considering I went for 100% fill and a 1mm wall they look decidedly ropey!





Is there a setting in Cura to improve this? There's certainly not a 1mm wall.

On assembling the insert I found the distance between the mounting holes was a few mm out - fortunately it was too long so I could cut 2mm of the ends of the centre sections and slot the holes. If I print any more I'll either compensate for the difference or leave the holes out at one end and drill/csk them to suit.

When I cut the ends off on the mill I was pleased to see that my 100% fill really was 100% and there was no evidence whatsoever of any voids.

Here's the insert in place. It isn't perfect but serviceable.



I did a test cut on a piece of scrap timber and the saw gives an incredibly clean cut. My friend even gave me 2 spare blades - one brand new!

Now I need a stand for it - I don't think that will be a job for the 3d printer!

Phil.

Joules:
Phil, where a hole is critical or you need a threaded insert, I add small top hat bushes.   They are trapped under the part, the larger bush outer diameter spreads the load over more of the print where you can fit them in.  That is quite a common problem with the bore shell coming out and leaving you with a loose fit or ragged hole.

However, nice work on breaking the part up, my preference these days is to weld the parts and scrape them flush.   Welding can be done tig style with a soldering iron, use some filament as tig rod and chamfer the joints for welding.   Very large parts can be fabricated and best use of print grain used.

Try to find the option in Cura for overlap in printing and shell/skin.  I run 25% overlap to guarantee proper weld of the shell.  However I don’t use Cura, so not sure what they call this function.

philf:
Thanks Joules,

I couldn't see overlap to start with but I discovered that in Options I could activate Overlap in the menu. There are hundreds of otherwise hidden parameters which I'll never try to understand.

I'll try a few test prints to see if it solves the problem. Failing that I could just do a pilot hole and drill and countersink afterwards. (Only an option using 100% infil density.)

I'd keep this as a screw together assembly as I can replace individual parts if and when they are damaged.

I can't visualize what you mean about adding top hat bushes!

Cheers.

Phil.

philf:
Hi Joules,

I designed a test piece to try out the effects of printing parameters on holes.

Overlap (25%) made a huge difference and the top of the test piece looks very good. The first layer however wasn't perfect as the filament didn't appear to have stuck properly to the bed. (nozzle to bed distance was 0.1mm with a 0.2mm layer height).

I read up a bit about Cura settings and to get the first layer to stick better it recommends that you use a thicker first layer! This seemed exactly the opposite to what I would have expected and, sure enough, the print was poor. (Hovering over the parameters in Cura gives the same advice!) Someone else recommended using 90% of the layer thickness as a first layer - I've yet to try that.

I ran another print with small chamfers (0.25 & 0.5mm) added to the holes and the results (combined with the overlap) are excellent. Generally if you were machining in metal you'd countersink the edges of holes to remove burrs anyway.



The photo isn't that good - I just took it with my phone with the parts balanced on my laptop.

The top one is with overlap (25%).

The middle one was with 25% overlap and a bottom layer of 0.3.

The bottom one is with 0.2 bottom layer, 25% overlap and a small chamfer added to the holes - I'm happy with this outcome.

Cheers.

Phil.

Joules:
Sadly using the overlap you loose some of the accuracy on components.   Holes can have a tolerance range as the overlap will cause a bulge in the adjacent filament that has been laid down.   One reason I print test holes, well thick walled rings to check I have the correct compensation in my models.  It can also vary filament to filament as they have different viscosity.  Printing with a chamfer at the start is the best way for first layer compensation.  The chamfer only needs be 3-5 layers deep, so quite subtle.

Don’t be afraid to close up that first layer gap, all it means is you end up with more squish.  If you hear the extruder clicking it means the gap is too tight.

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