How to inspect the inner surface quality of a 1.8-2mm stainless steel tube

I could use a pipe cleaner and some polishing wax from Dremel to improve the quality unseen maybe some different polishing compounds used in jewelry manufacturing to really polish the surface but really I would be working blind. while you can look through it that really only tells you if there are obstructions and gross burrs the human eye cannot focus on the surface inside a tube that small. Does anyone have some hints on how to really look at the surface quality on the inside of a tube like this.
Because really if I could ensure a mirror finish in those tubes I could advance 3d printing quite a bit. one of the problems of all metal hotends is friction, and friction it these short steel tubes is mostly caused by machining. If inspections and after work could be performed less blindly it could ensure that the parts are extremely smooth on the inside where it counts. Any ideas? That don't cost a huge fortune?

terramir

A tube polished on the inside will trap and guide light in the same manner as a step-index optical fiber. If you launch a laser pointer beam into the tube at a small angle, and look at the pattern on a white card illuminated by the other end of the tube, the pattern will tell you how close to a mirror you are. You will need a simple fixture to hold tube under test and laser pointer - the human hand is too unsteady for this to work.

Fiber optics-based borescopes may be worth a look. Scopes are available that go down to less than half a mm diameter. Example:
Borescopes | Pipe Cameras | Fiberscopes | Videoscopes
No affiliation.

-Marty-

Look into extrude honing. This process might just give you the finish you desire.

No help on the inspection technique, but it might be worth searching for 'ballizing' as a process to improve the surface finish of the tubes. It involves forcing slightly oversized tungsten carbide balls through the tube to improve the surface finish and produce an exact diameter.

A few years ago we tested the inner surface of SS tubing for a medical device using a stylus and an LVDT (Linear Variable Differential Transformer). It worked quite well.

To test the surface finish, you need to know what you need to test for. The specifications for a mirror finish are rather generic.

Not sure exactly what you are trying to accomplish but there is a lot more to fluid friction in a tube than just the surface finish. I suspect that the tube finish is not where your issues are. Depending on the fluid flow velocities, viscosity, surface tension, etc., the things you will likely need to address will be in small details at interfaces and transitions.

Having a micro-finish with an Ra of 9 will not guarantee a better flow than an Ra of 50.

There's company in Anaheim, CA that does exactly this, in exactly materials of those sizes all the time. It would probably be worth talking to them. I've dealt with them before and they are good people. You could just buy the material done and to spec way more consistently that would would be doing with pipe cleaners. What you're describing is their only business in stainless tubing.

Stainless Micro-Polish, Inc.

Erich said:

Look into extrude honing. This process might just give you the finish you desire.

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+1 on extrusion honing, have done hundreds of extrusion dies with the gray wax, comes in different grades but a press will be required as well as a cylinder to hold the wax, we used 50 tons but the dies were fairly big, puts a fairly good polish on, we used to do cylinder bores, tubes and all sorts, even seen a manifold on there, it is a plastic material, flows nicely and residual wax can be melted out, it’s conceivable to make the stuff out of wax or even plasticine mixed with abrasive, alternatively wire edm internal finish is fairly good,
Mark

Just how long are these tubes? I would think that the only place where a 3D print head needs to have a truly small diameter is at the point where the plastic comes out and perhaps for half a mm behind that. Anywhere else the tube could be 10, 20, or more mm in diameter and that would reduce the friction a lot more than any change in the surface finish.