You can add solid modifiers that either become part of the model (like for casting sprews for example), or infill modifiers to make areas solid, or even voids in the model etc. Depends on your slicer program. I use Prusa’s fork of open-source project Slic3r. It will work for most printers and should work for your resin or FDM. Even has text capabilities now. Lots of tutorials available. Some pretty cool tricks can be done.
It’s unnecessary with resin printers since the infill is solid. I use Cura and Zortrax slicers for my FDM prints. I’ll take a look and see what options are there.
Does the infill have to be solid on resin printers?
I only have a Form3 from FormLabs. There is no setting for infill. It’s all homogeneous.
I recently got into resin printing and use a 3rd party slicer (Lychee).
Slicer features include the ability to hollow out a solid body with a wall thickness of your choosing- like a chocolate Easter bunny 
. You can also call out a drain hole for internal liquid resin to escape for post processing. Otherwise, all resin printed parts are solid provided your 3D model is solid to start with.
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I have tried this website… Printed two types of bolts and nuts… none actually work… to tight, any suggestions?
I am picking the right pitch… 1.25 or 0.75… for the bolt and nut… but after printing… its impossible to get them in the nut…
Love you to teach me how, i like this.
What type of printer are you using? I print in resin using a FormLabs which is highly accurate. I also have success on a Bambu X1C. If your printer isn’t accurate enough, you could try uniform scaling of a very small amount. Or perhaps non-uniform scaling in either the diameter, or pitch. But I’d recommend scaling the diameter a small amount first. Such as 1% perhaps.
Im using FFF creality ender 3 v3 plus.
You can do non-uniform scaling in Shapr. Assuming the threads spiral along then Z axis, just scale up the object on the X and Y axes to add tolerance.
If you want functional 3d printed with matching nut, lot will depend on your printer tolerance level and material strength. No use printing a screw that breaks easily under normal stress, I have a 3d printed thread that broke off so it embedded itself in my camera.
Also instead of McMaster samples get 3d printable versions: https://makerworld.com/en/models/70851?from=search#profileId-86384 there are lot of pre-mades out there that done the hard work with guidance on printer settings.
Print few of these and see how they work, check the strength, if you need more tolerance for the nut or less, like others suggested scale in the X & Y with increased wall loops to make the print solid with no infil.
Once you have a good working nut and threads you can use that as a reference to make custom nuts and bolts in Shapr.
