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3D parts printing undersize

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lee webster17/10/2019 21:08:20
8 forum posts

I watched a video on youtube this evening where the poster showed that 3D parts print undersize because of filament shrinkage, about half a percent. I don't know if this applies to resin printed parts. I hope I can post a link to the video here.

(5) How to 3D Print Dimensionally Accurate Parts - YouTube

What really interested me was that I am designing parts to print out and I would have to enlarge the design by the shrinkage of the metal being used in the casting. I thought I would have to do this in my cad programme, it seems it can also be done in the printer software. Does this mean I could design full size parts, a scale of 1:1, and then tell the printer to change the scale to suit? factoring in shrinkage for the filament and the metal of course!


Andrew Johnston17/10/2019 22:29:06
5400 forum posts
621 photos

You can design in any scale you want and then scale to final size in CAD. Having said that I normally design to the finished size of the part. An exception is gears that are going to be machined on the CNC mill. In that case I design as 1DP and then scale accordingly. Personally I do any such scaling in the CAD system. When scaling a 3D print for shrinkage I use the slicing software for the 3D printer. One problem is that the shrinkage of the printed part is anisotropic and also varies with part size, shape and layer thickness. I've recently 3D printed a trial cylinder flange for my traction engine to check fit on the boiler:

cylinder flange.jpg

I scaled this by 0.5%, which turned out to be generous, as the length of the flange is about 0.2% too long. I think that the shrinkage of parts created from liquid resin is smaller than parts printed from filament, if only because heat isn't involved.

I think you'll find that shrinkage in a casting is more complex than a scalar value. It will depend upon how the casting cools. Once part of the casting has cooled it isn't going to shrink any more, even if other parts of the casting around it are still shrinking - look up hot tears. The shrinkage will also depend upon any cores and how they fit within the casting.

Of course it all depends what sort of accuracy you hope to achieve, but it isn't always simply a case of applying an overall scale factor. For reference the flange above was about 14 thou oversize in 7 inches.


Bandersnatch17/10/2019 22:29:37
1586 forum posts
53 photos

Slicers (Cura etc) generally let you rescale before they prepare the gcode for the printer.

For my money, you are better off making this kind of correction at that point (the slicer) and keeping some sanity in the CAD model by leaving it at the theoretically correct size .... particularly if you get into later revisions.

Vidar18/10/2019 00:09:48
56 forum posts

I can just agree with the above replies.

A singular scale value can only get you some generalish correction. That might be good enough, or it might not. If possible design up ahead to expect variations while still keeping the important parts as needed. Or plan ahead for later clean up machining?

Personally if I need an accurate part, in either just 3D print or 3D print + casting, I try to be religious about following a repeatable procedure. I then make one part, see where it is off, and aim to correct the original model for that. Second time is usually quite good, but if you have a truly repeatable process then you can repeat and iterate closer and closer.

old mart18/10/2019 17:08:44
1516 forum posts
136 photos

Could you make a simple part and actually measure the finished product to verify the expected change?

Brian Oldford18/10/2019 18:28:51
619 forum posts
12 photos

This is something pattern-makers have been doing for eons.

jimmy b19/10/2019 07:16:23
620 forum posts
38 photos

I know my printer well enough to know that I need to scale to 99% and get prints bang on size.

I just do it my slicer.


lee webster19/10/2019 09:26:42
8 forum posts

I will take on board all suggestions. I am very much 'playing it by ear'. My printer should be here on Monday. I will do a trial print of the head or block, less plastic to print a head, and then measure. I am not adding any of the stud holes to head or block, I think it will be easier to drill them in the castings.


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