Friday, 14 November 2014

Accurately Printing Holes Parallel to The Build Plate

Broadly speaking my 3D prints fall in two categories: aesthetic or functional. Aesthetic prints just need to look good. They typically do not need to be accurate. If they look correct to the naked eye, then that is enough. Functional prints perform an engineering role. The print finish is not important. Accuracy is. To print objects accurately it is often necessary to adjust the model and print parameters. 
Functional prints are typically combined with hardware, usually nuts and bolts. The majority of designs require such hardware to be installed vertically or horizontally, often both within a single print. I endeavor to create an accurate print that requires little clean-up. 
Note that the example print in this document would ordinarily be printed flat on the build plate. Whilst there are issues printing holes perpendicular to the plate, printing an overhang should be avoided if possible.  The holes in the example print have diameters; 20mm, 10mm and 5mm. 

Printing Without Supports
An overhang can be printed without supports. A cylindrical hole can support itself, however, the resulting print is poor.

Print settings: Maker Ware high-res default settings with no supports.

The following time-lapse images shows how some of the inaccuracy occurs.

As the depth of the overhang reduces, it begins to curl upwards. The top of the print is no longer horizontal; this causes print media to build-up on the overhang. When the printer begins bridging, this build-up distorts the hole. Subsequent layers force the build-up downwards, creating more distortion.
The largest hole in the example print has a diameter of 20mm. This is within the printer’s parameters for bridging. However, as the hole is completed it overhangs more and becomes weaker. This results in sagging. In addition, the bridging elements of the print are untidy. The build-up and bridging can be cleaned-up, but this won’t reverse the distortion of the print.

Printing With Maker Ware Default Supports 

Supports are the solution to collapsing overhangs, but the default Maker Ware supports are not adequate. The hole still collapses. (EXPLAIN)

Print settings: Maker Ware high-res default settings with default supports.

The Maker Ware Supports are untidy. They, and the bridging, are still distorting the print despite cleanup.

Support and bridging parameters can be adjusted in a custom profile. However, the design of the supports and the presence of bridging is the real problem. 

Custom Support 
Modelling supports into the print object give the operator complete control. Supports, and equally as crucial, their spacing, can be accurately defined. I create supports 0.4mm wide and 2mm apart, placed from the centre of the hole outwards. The supports are placed across the hole.
The width of the support is one print line width of the extruder at default settings. It can be easily removed with a finger. Spacing supports 2mm apart creates support across the entire hole, preventing any curling during the print and avoiding any bridging.

Modelling supports into the object creates a tidy print that is accurate (enough) and requires little cleanup.

I do not model the support into my working object, instead adding them to a duplicate when my design is complete. A support template can be imported, adjusted and combined. This is fast and non destructive. Below is a summary of the support design
that can be used for all overhangs.

Support width: 0.4mm 
The default line width of the extruder.
Support spacing: 2mm 
Working from the centre of the hole outwards. If the space between a support and a vertical edge is less than 2mm it is omitted as it typically creates a support that is hard to remove.

Dead Holes 
Dead holes do not pass entirely through an object, they have an, ‘end’.
The same approach to supports should be applied here. A space of 0.4 mm (if possible) from the end face aids removal of the support. This is an effective approach to printing nut recesses perpendicular to the build plate 

Modelling supports into an object is more time consuming as regards preparation and print time,  but the results are better quality and require less cleanup.

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