Some times I can create a nice image and other times I cannot figure out why not.
I believe my setting are the same or at least i start with my notes and work off from there. But you can see big differences between the images. I also find my self taking a long time to get the scale right (not prfect). Not sure what to do or where to find my answers.
One can have to do with the other I think. Scale is important since the amount of calculations/rays depend also on the size of the subject as well as the resolution you want.
If I look at for example those ventilation holes I’ve the feeling the entire object rendered was a lot smaller than the actual size of the object so basically the renderer shoots rays of light at the object which is so small you can’t get a good result.
In 3D modelling actual units don’t really matter since what ever unit you use, the vertices are just located somewhere at a 3D grid on certain locations. A lot of 3D modelling tools even use their own units.
Of course when you want to use a model for a render, in a game or for a 3D print units start to matter so you export the model for a certain application with a certain unit connected. So the model will be interpreted in inches/metres whatever you want. It won’t change the model.
I try to use metres as units throughout my entire workflow. That’s because I’m from the EU but the metric system itself is smarter than working with inches, 1/8 inches, feet and other stuff. It makes scaling also easier since it’s based on factor 10.
So as long as you have the model exported in the same units as you will import it in Keyshot it should be ok. But… some file formats don’t even include a ‘unit’ so you have to pick them by import. So it’s important you pick the one which was also used by the people who created the model itself. Again, if you always use the same units it helps to prevent issues.
Since the quality of your second model is so bad I really think it has the wrong scale. Maybe some other people have some comments on my explanation, I’m no guru, but I think it can be caused by the scale.
I’m by the way no fan of how you render in Keyshot and decide the quality of the result by either giving a max time or max samples. Both numbers say nothing about the result you will get since change with complexity of the scene. An approach as V-Ray would make much more sense, telling the program a certain noise level is acceptable and let it render until the noise gets below the trashold. Finalizing it with a denoise pass if wanted.
Good luck! And feel free to share a scene if you want someone to take a look.
Just want to give a small tip in regards to rendering at max time or max samples.
If you use region rendering you can relatively quickly check different areas that you assume are prone to noise and high render times, and see in the info display(I know it’s called something else, but I can’t remember it right now) how many samples it takes before all the noise is gone. Then then I usually add just a bit to that sample number to be safe and use the resulting value as max samples in the rendering settings.
That way you make sure you get renderings without noise without having to guess the time or number of samples and end up with a bad result or render times that are unnecessarily long.
Heads-Up Display (H) it’s called.
I’ve the same approach as you by using the region render and get an idea. But it’s time consuming and you have to guess what will be hardest to render. A noise threshold would saves you a lot of (render) time since the renderer itself checks with every frame if a certain level of noise is reached and moves on to the next frame.
You’ll only need to tell it how much noise you think is acceptable either with or without denoising.
But will drop this in the feedback/idea area 
I agree with what Oscar and Joakim are both saying, and just want to add that inconsistency in scaling will also play havoc with your material textures. That could be contributing to what looks like noise.
thanks for the replys. We use inch as a scale but on import to keyshot it always opens is metric. Is there a way to default keyshot to open in inch?
