How to polish inside a 1.2mm brass tube ? - Page 1
Forum section: electronics / general

author: mr ed
author_rank: Regular Contributor
timestamp: March 31, 2025, 09:56:56 pm
content: I'm not a machinist. I want to polish smooth the inside of a brass tube. Tube is tiny.1.37mm OD 1.2mm ID, 12mm long. (Radio: microwave use).Also, one 3.2mm OD with a 2mm ID

author: jpanhalt
author_rank: Super Contributor
timestamp: March 31, 2025, 10:12:15 pm
content: Have you considered a tooth cleaning brush (attachment) with toothpaste (or other suitable mild abrasive)? They are available in various formats and and are designed to fit between teeth. They are long enough to meet your 12mm need.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 31, 2025, 10:26:23 pm
content: I would be tempted to use a rope/textile thread. Pass the rope through the tube, fix the rope thread on its ends, then manually move the tube back and forth, along the rope.Or maybe keep the tube fixed, fix the rope in the chuck of an electric drill, and rotate the rope inside the tube. Eventually suspend a weight on the other side of the rope, to make sure the rope is not pulled out of the tube while rotating.Impregnate the rope with any polishing paste, if required.Please beware that I have no idea if any of these is good advice, never tried what I've just described, and I have zero hands on experience in microwave.

author: mr ed
author_rank: Regular Contributor
timestamp: March 31, 2025, 10:31:43 pm
content: It's kinda rough inside. I like the string idea so far.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 31, 2025, 10:47:42 pm
content: Is it easier to rotate a brush at, say 500 rpm, or pull a string back and forth at the same sfm?

author: coppercone2
author_rank: Super Contributor
timestamp: March 31, 2025, 10:50:17 pm
content: never dull wadding rolled up into little ribbons good luck, you can sandwich it between two sticks and reciprocate it, that way its compressed between the two sticks and its like a tight polishing piston moving up and down. you need a boroscope for inspection and frequent wadding changes. I did ID with silver plate (kool amp) doing this tooI think I hate you for making me think about polishing the ID of a small borethe strings and stuff are not that great because it might change the ID of the tube and you can't really control where its working. And spinning this helps too.and start with a chem dip to reduce work load, try on exterior first to make sure it worksi feel sick thinking about it any more, leave me alone

author: fourfathom
author_rank: Super Contributor
timestamp: April 01, 2025, 01:37:56 am
content: Pipe cleaner? (fuzzy, with a wire core). That, and a polishing compound.

author: Mr.B
author_rank: Unknown
timestamp: April 01, 2025, 01:45:40 am
content: Interdental brushes as @jpanhalt suggested.Mounted in a battery drill with a small amount of fine grinding compound on the bristles.

author: bdunham7
author_rank: Super Contributor
timestamp: April 01, 2025, 01:58:48 am
content: Quote from: mr ed on March 31, 2025, 10:31:43 pmIdeally would you want to polish or ream them?

author: mr ed
author_rank: Regular Contributor
timestamp: April 01, 2025, 02:28:05 am
content: Tried string and variations quickly on the 2mm. Not great. It does not put much pressure anywhere except near the ends.I will try using a small brush tomorrow. Reaming sounds good too if I can get the size.Thanks for the ideas.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 01, 2025, 03:27:59 am
content: Hello mr ed,Gathering and mixing some ideas already provided by others...First ream, then polish.For the polishing method I suggest, the tube has to be longer than its' final lenght (12 mm).About 20-25 mm should do it to solder(*) two 1/16" brass or stainless steel fittings (one at each end).Use aheavy duty miniature peristaltic pumpto push superfine polishing paste through the tube.At the other end, connect the smallestneedle valveyou can find.Set the valve to create the highest possible pressure inside the tube, just before the pump stucks.The pressure will, by principle, be applied uniformly on the whole inner surface of the tube, creating an equal level of abrasion.Once finished, cut off the fittings by using a diamond cutting wheel.Hint: using a PVC pipe inside the pump will allow to create a higher pressure than the usual silicon tube.Hint 2: if surface roughness is still above your goal after the polishing process, proceed withelectropolishing.(*) Silver solder with at least 40% Silver, use ajewellery maker H2O2 micro torch. If too expensive,cheapiescan be procured from Ali.Torch.

author: bdunham7
author_rank: Super Contributor
timestamp: April 01, 2025, 03:47:16 am
content: Quote from: mr ed on April 01, 2025, 02:28:05 amhttps://www.kctool.com/stahlwille-11085-jet-reamer-set/

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 01, 2025, 08:01:01 am
content: Quote from: bdunham7 on April 01, 2025, 03:47:16 amHarvey Toolssells the 1.2 mm reamer having a reach of 14 mm, which should be sufficient for a 'ream only' process.Other diametersare of course available off the shelf (metric and imperial).Ask the manufacturer if he can provide custom reamers having longer reaches so that you can also try the above described 'pumped polishing paste' process.Update #1: changed 1 mm reamer to 1.2 mm

author: Uunoctium
author_rank: Frequent Contributor
timestamp: April 01, 2025, 08:02:23 am
content: The often used tool by Dentists is an mandrel (long shaft fits grindig paper) - mostly the conical form. Its also obtainable in cylindrical, but not with an diameter of 1.2 mm or less. And building such a tool without posessing an eroding machine seems impossible. My suggestion as an poor mans trick: use an normal drillbit of eg 0.9 or 1.0 mm in reverse position an glue with superglue (1/1000 drop) a tiny fraction of an sheet optical ferrule polishing paper (thorlabs from 30µ to 0.01µ) on the shaft.If not at hand, you can use normal paper, 2..3 windings in rotation direction and wetpolishing the bore with added toothpaste or other abrasives like "Wiener Kalk" plus an small amount of water.

author: coppercone2
author_rank: Super Contributor
timestamp: April 02, 2025, 03:49:54 am
content: I don't see how you will polish the inside of a thin tube (likely not super strait) with anything but a flexible object. I suspect if you try these operations without like a chuck your just gonna grind a flute in the tube

author: Simmed
author_rank: Frequent Contributor
timestamp: April 02, 2025, 04:06:10 am
content: Quote from: mr ed on March 31, 2025, 09:56:56 pmwhat did you use to inspect the insideto know it is not smooth ?and what is your criteria of smooth ?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 02, 2025, 10:19:52 am
content: Hello mr ed,New contraption for small diameter tube inner surface polishingDevise a fixture that, like a clamp or a vice, can hold the small tube by its both ends.Both custom jaws (PA6, PVC, aluminium, brass) are machined with a small bore and a donut shaped groove that contains ano-ring seal. The bore goes through the jaw.Once clamped in the fixture, the tube has to be hold tightly against the o-rings and everything has to be waterproof.Instead of the donut groove, you may also try a CNC machined conical bore, where the o-ring seal will be become tighter the more you screw the clamp/vice.At the other end of each jaw, increase the size of the bore, thread it and add a small stainless steel fitting.Setup the hydraulic circuit consisting of a small12/24 VDC gear pump(*), athree way valvethat acts as a discharge bypass to limit the fluid flow, and the above mentionned fixture.To ease the filling of the circuit, add avanilla brake fluid reservoir, followed by a small ball valve.Optionally, add some monitoring stuff to better control and optimize the polishing process : pressure transducer ormanometer,rotary flow meter.Use aDC PWM controller to speed down the pump.As grinding fluid, use a mixture of chalkpower (as suggested by Uunoctium) and silicon oil.Remark: it will require many hours to polish a tube.Hint 1: if the grinding fluid becomes too hot, add acooling coilto the circuit(*) nominal pressure is 8 to 15 barg (116 to 217 psig) depending on pump model. This also means that vanilla copper or stainless steel tube and standard off-the-shelf water valves can be used to built up the fluid circuit.

author: mr ed
author_rank: Regular Contributor
timestamp: April 02, 2025, 12:48:30 pm
content: Inspection: I use a stereo microscope and eyeball it down the tube.I tries the dental gum tool, it wont fit 1.2mm at all. It will go only half way down the larger tube but its workable from both ends.Note: the small tube is very thin wall and is much smoother inside than the larger thick wall tube. Maybe substitute a thin wall tube for the 3.2mm OD and make most of the issue go away? The larger thick wall tube is truly ugly inside.Specs: smoother is always better but 50um or better.Sorry, not going to pump grinding fluid etc - way to much for small quantity. Remember, I'm not a machinist nor have a shop.I will design out the 1.2mm ID tube and have a proper shop drill into a larger block of brass. It would be too fragile anyway.The 3.2MM OD tube is necessary as its an output waveguide and moves back and forth a bit to tune. I think a custom turned piece is the best solution. I can then polish that from a good start by hand.

author: Langdon
author_rank: Frequent Contributor
timestamp: April 02, 2025, 01:53:32 pm
content: You know those brushes they use for cleaning the inside of test tubes?Get a brass-bristled one.Problem solved!.

author: Ian.M
author_rank: Super Contributor
timestamp: April 02, 2025, 02:01:59 pm
content: Pumping grinding fluid is a fairly dumb idea. It will cause greatest abrasion where it goes through the smallest orifice i.e. the needle valve and will destroy anything other than peristaltic pumps. If I had to do it, I'd pump fluid through the tube to be polished, between two syringes, big enough they don't have to cycle too fast, but small enough that the grinding media doesn't have time to settle out. Use a single linear actuator to drive both syringes, and replace them whenever the force to drive them increases too much or they start leaking. However even considering this is a last resort.Sewing needles down to 0.6mm dia (0.75mm across the eye) are readily available, and could be used to move abrasive loaded thread wrapped round the needle near its eye within the bore in a conventional honing operation.

author: Gregg
author_rank: Super Contributor
timestamp: April 02, 2025, 06:05:02 pm
content: If I were tasked with polishing the interior of a small diameter brass tube, I would fish as many strands of string as I could fit through the tube. The string could be coated with abrasive compound such as buffing compound that comes in stick form or my favorite, SimiChrome polish. If it is really rough inside I may start with valve lapping compound.The trick is to twist the string as the tube is run back and forth. The tighter it is twisted, the larger the string diameter will be to fill the tube quite evenly. With one end of the strings tied to something stationary and something on the other end to facilitate twisting, the string can be held straight minimizing excessive wear at the ends. If the tube is going to be trimmed, a slight flare on each end will help keep the abrasive on the string.

author: jpanhalt
author_rank: Super Contributor
timestamp: April 02, 2025, 07:34:10 pm
content: Quote from: mr ed on April 02, 2025, 12:48:30 pmDental brushes come in different sizes. Perhaps you got the ones designed for Michael Strahan.From brush supplier:QuoteI have some 17 ga. (0.047" (1.2mm) ID) 304 stainless. Unfortunately, I discarded the brushes my dentist passes out at each visit, so I can't test. The interior of stainless might be shinier.

author: khach
author_rank: Contributor
timestamp: April 02, 2025, 08:43:08 pm
content: Electropolishing? With tooling like this picture?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 03, 2025, 01:47:50 am
content: Quote from: khach on April 02, 2025, 08:43:08 pmHello khach,Fluid circulation seems to be a no-go for@mr ed: he prefers to dodge those kind of suggestions and try to cobble something with his cordless drill.So, not sure if it's really worth providing additional advice.Provided explanation ishe's not machinist nor have a shop.Quote from: mr ed on April 02, 2025, 12:48:30 pmHowever, his goal is to machine and thengold platemicrowave stubs or guides which has likely something to do with impedance matching or waveguide filter design(*).Fortunately, many subcontractors are capable of designing and manufacturing specialized machinery and equipment to meet specific requirements (like a custom fluid polishing machine), or even simply supply the polished tubes.Actually the stakes are not that high : 50 µm is a fairly rough surface. Electropolishing easily goes down to 0.1 µm (mirror finish).The drawing you provided, taken fromthis document, would need just limited upgrade to handle the small diameter of the tube to be polished.(*) Not later than last month, I had a discusion with a few guys who designed and machined a microwave L band feeder. They explained me the utmost care and machining precision that was required to achieve their goals, in order to catch the last hundredth of dB.

author: khach
author_rank: Contributor
timestamp: April 03, 2025, 10:33:51 am
content: Hydraulic mechanical polishing is very sensitive to oil flow speed. In some places of tube it deeply grinding tube surface. Electropolishing produce more homogeneous surface.

author: coppercone2
author_rank: Super Contributor
timestamp: April 03, 2025, 10:38:37 am
content: I suspect chemical and flow polishing aversion is related to general aversion of fluids and natural materials that is prevalent among enigneers. But they won't think twice about running a dremel through fiberglass, plastic or brass inside of a roombe braveI kept my nose clean for a while and after wards I found that I had severe irritation after sharpening knives on a diamond hone manually by hand after I blew my nose. I wish I could do it under fluid in a beaker without handling the part personally.I.e. never dull, ammonia smell and throwing away stuff contained in wadding is better then dealing with fucking polishing dust by several orders of magnitude. Ireallybegan to loathe dry polishing operations and also dry rust or paint removal. Its foolish (and so are the medical consequences, obscene cost of drugs for treating scarred lungs, etc)And since your part is small, and I don't know your capabilities, if you have the right power supply... you can look at electrolytic plasma polishing. the only problem is your talking about ~200V @ 10+amps, regulated (in a dilute ammonium chloride or other weakly conductive acid)... but it does appear to do everything that you want. I never did more then polished the head of a pin before though.Have you looked at bright dip brass treatments? (the problem might be deceptively scary NO2 gas, rather then insidious particulate contamination). For small parts it should be more then manageable, they have a scrubber problem for big batches that actually is a bit scary to deal with (but IMO, 100% contained and also very easy to clean up with basic industrial resources)I would love to hear from someone that knows how to do flow polishing, a example of a equipment setup, and so on. I understand that the way the process is presented its over whelming, most people here don't know anything other then a peristaltic pump might survive the longest.Can you give us insight on the design of the tank (slurry tank), the fluidic couplers (i.e. for attaching work piece), the control system (for determining flow) and the interconnects that could be used to practically put together such a apparatus? I built a few coolant loops for viscous materials (really cold glycol) and it was rather chock full of pitfalls. Sure I learned a thing or two from the school of hard knocks but I would hardly consider it information that I would pass on to someone because I had to improvise everywhere.

author: jpanhalt
author_rank: Super Contributor
timestamp: April 03, 2025, 11:48:35 am
content: Hypodermic needles for medical use may already be polished on the inside. Google AI says they are, but further searching indicates they "may be." Moreover, it seems that reusable needles are more likely to be polished than non-reusable ones. A downside is that some needles may have coatings on the inside to facilitate flow and prevent absorption of whatever is being administered.Here's a link that describes polishing:http://www.copen.ac.in/proceedings/copen10/copen/9.revised_manuscript_591.pdfNeedles and plain stock tubing are available from quite small bore to bores larger than 3.2mm.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 03, 2025, 01:51:53 pm
content: Hello coppercone2,Quote from: coppercone2 on April 03, 2025, 10:38:37 amI would also love to hear someone who could share his knowledge about that topic.The information I shared about flow polishing came from a colleague that saw such kind of techniques years ago. However, I can not disclose more than the following information :1) the process uses a grinding fluid2) the process is very slow.Other insights about the parts to be used are my personal contribution and just answers the following trivial question : If I had to do that, how would I proceed ?I am of course aware that the fluid will equally grind the tube to be polished and also the whole hydraulic circuit. But for a small batch of tubes (as required by OP), who cares about the wear and tear on the entire equipment, as far as the job gets done ?That's why I actually suggested to use off-the-shelf cheapies from Ali and not hydraulic parts made of sintered CBN. Because I'm quite sure those are unobtainums.As an alternative, these manufacturers sell quality stainless steelpartsandfittings.Silicon tubes and also parts made of rubber are known to be rather resistant to abrasion. If a very low pressure is sufficient, maybe 3D print the fittings and perhaps the jaws by using TPU filament would provide an acceptable result.I suspect chemical and flow polishing aversion is related to general aversion of fluids and natural materials that is prevalent among enigneers.You can't be more right.I may naively think that tinkering with a small hydraulic circuit with a maximum pressure of 15 barg is within the reach of any technician or engineer who does not have two left hands but at least a free access to an NPO machine shop (= fablab).In the negative, it should be for sure in the skills of any HVAC tech or plumbler, and there are thousands of those.

author: mr ed
author_rank: Regular Contributor
timestamp: April 08, 2025, 12:11:17 am
content: I drilled out the rough tube to 2mm ID which removed the ugly bits and which fixed an unrelated RF issue. Then I burnished the inside of the tube. I used aircraft safety wire which is stainless, very smooth and hard. I kinked the wire a bit so it has some spring action inside the tube and spun the tube while running the hard wire back and forth. Not a 0.1um polish but very acceptable.

author: johansen
author_rank: Super Contributor
timestamp: April 08, 2025, 01:45:55 am
content: Quote from: mr ed on April 08, 2025, 12:11:17 amcan you drill a tube out to 1.9mm and then push a 2mm ball bearing through it?

author: coppercone2
author_rank: Super Contributor
timestamp: April 08, 2025, 02:53:00 am
content: lol that is a very creative solution but I am disappointed by the loose diameter spec from 1.2 to 2mm that is 31 thousanths, that is above reamer cutting dimensions

author: mr ed
author_rank: Regular Contributor
timestamp: April 08, 2025, 10:37:08 am
content: To be clear, there were 2 tubes at issue, a 1.2mm ID thin wall and one 1,57mm ID thick wall AT 3.2MM OD. The 1.57mm was drilled out to 2mm. The 1.2mm ID brass tube is really too fragile and will be designed out replaced by a hole in the brass block. Thanks again.

author: coppercone2
author_rank: Super Contributor
timestamp: August 06, 2025, 02:47:27 am
content: Bright polishing dip is actually really good. I made some for copper and its like superb.Small perestalic pump with bright dip will solve this issue but you need to be ready to do a flush with waterBe wary you might only need seconds, I put a 1mm copper tube in there for approx 2 min and it looked like it was made out of crinkled foil. I made another one and I got the results I wanted in approx 5 seconds.

author: EC8010
author_rank: Frequent Contributor
timestamp: August 25, 2025, 11:25:52 am
content: Brasso. If you have a lathe with a collet chuck, you can spin the tube. Maybe even a Dremel with collet would do. I'd wrap a bit of tissue paper round a small drill, soak it in Brasso, and go for it. Then polish with clean paper. Fiddly, but should work.

author: mr ed
author_rank: Regular Contributor
timestamp: September 25, 2025, 01:14:14 pm
content: I will try Brasso too.FYI picture of the brass part with a hole that tapers to 1mm, straight off the CNC machine. Just the hole and taper are critical.

author: coppercone2
author_rank: Super Contributor
timestamp: September 27, 2025, 08:14:22 pm
content: I wanna try bright dipping that so bad. It looks like something I can make on my little proxxon mill with an assortment of cutters. Other then the taper (looks like that might need CNC control)The clearance between the hole and the wall looks very small on the lower right quadrant though.FYI the tube I was working on is very thin, that is why it decomposed after 2 min. That was a micro heat exchanger to protect the shaft of a transducer.FYI from the sense of maximum polish, I read that pure chemical is smoother, I think it can go down under 0.5 ra.. i believe its used as a follow up for electropolishing, with the idea that electropolishing reduces the amount of fumes that are made in large scale production.BTW I bought a big pack of diamond pastes, I ran out of buffs to try, but I got a wonderful finish on my titanium tweezers with fine grit diamonds. I don't know if brass is too soft for this, but damn it looks good.Your whole assembly looks similar to a die. How are you gonna get rid of the mill marks on the interior? I have seen people handle this problem painfully, by using diamond loaded wooden sticks, or somewhat less painfully with special vibrating tools (the one that looks like a tiny trowel and you put a tiny pizza slice of sand paper on it). But they always do it on what looks to be hardened steel, it seems like it would be supremely difficult to get brass in the same circumstances.This is a MMWAVE system right?Also I believe the big benefit of pure chemical polishing is that its less agressive on edges, which suffer from fringing during electro processes, and get extra ware, while pure chemical leaves them alone. Electrolytic Plasma polishing solves this problem by making a plasma blanket around the submerged part during polishing, that I believe basically shorts out the gradient from corners and irregularities, but I am guesstimating that part would require something like 40A @ 350VDC to do that process... (done in a weakly conductive solution)

author: mr ed
author_rank: Regular Contributor
timestamp: September 30, 2025, 09:48:34 pm
content: The parts needing polish are only the hole (arrow) and the taper. The hole is a circular waveguide and (not visible) horn with a cutoff of 170GHz. The smooth surface is a mating surface to the other half. There is a 15 x 11 mm pcb still being designed that covers the interior mill marks so I dont care about those. The 4 little squares are for alignment and component protection at assembly time. The 4 mounting holes are recessed to provide a brass on brass gasket when tightened down to the bottom brass part. That is to reduce rf leakage.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 30, 2025, 10:40:27 pm
content: Quote from: coppercone2 on September 27, 2025, 08:14:22 pmLapping is what you're talking about. An ancient technique for making precision sliding fits. Ideally, you would want the lap to be softer than work piece such that the abrasive charge becomes embedded in the lap and not the work piece. I'll let Clickspring Chris explain how it might have been done 2000 years ago. Although, a 1-2mm bore is pushing it a bit!https://youtu.be/8U6SCL6Y4ZU

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2025, 11:24:40 pm
content: Dental floss with an appropriate polishing compound?

author: coppercone2
author_rank: Super Contributor
timestamp: September 30, 2025, 11:27:29 pm
content: Quote from: Andy Watson on September 30, 2025, 10:40:27 pmIts not really lapping, what they do is cut a stick, load it with diamond compound and rub it. It gets mirror polished on irregular shapes. I thought lapping means that it needs to fit to some form (i.e. flat). Stick polishing with the end grain of wood. It can do like corners which soft objects have a problem with. Usually done for cosmetic appearance on irregular shapes, but I think that the material removal is close to nothing. Takes hours. Its absolutely the most pain staking way to finish something. Not done on a tool, literarly rubbing it by hand. And you go up in grit and do the same thing again. IIRC the labor figure for some hand held thing is like 20 hours per grit. Like a small dowel, somewhat shaped to the object in question. Naturally the wood splits apart and fits the form of the curve you are polishing. Takes experience to figure out how hard to push. Like used for post finishing to get a total mirror appearance on one of those complicated hollow trophy pieces they make for a company director with a 50 axis CNC machine.Don't know what kind of wood. Don't think its a good gift anyway, its usually a pseudo advertisement and people like throwing it in the trash because it remind them of work lol, and the resale value is scrap. If you see one of those on someones desk, ask about it and it will trigger a 20 min rant.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 01, 2025, 01:28:25 am
content: I can't add to the polishing approaches, but thought of approaching it from the other direction. Start with a glass or plastic rod of the appropriate dimension and then electroplate. Dimensions would need to be adjusted for the dielectric constant, or if a plastic core was used you could possibly add mechanical support on the outside and the use solvent to remove the plastic.

author: antenna
author_rank: Frequent Contributor
timestamp: October 01, 2025, 02:46:53 am
content: Electric discharge machining (EDM)?What about electropolishing? I recall a thread somewhere (not here, obviously) where a guy made a gun barrel with rifling using electrolysis and 3D printed inserts.

author: Psi
author_rank: Super Contributor
timestamp: October 01, 2025, 03:18:40 am
content: The best I can think of to get an even polish down the length is if you could get two 1/2 circle rods. (eg D shape, so if you place them together you get ~1.0mm round rod).You then drill a few tiny holes, like 0.3mm, perpendicular to the length on the flat side of each at a few locations and put a small pin through each to lock the two halfs together rotationally but so they are still free to move outwards on the pins.You then push it through the brass tube and spin it quite fast. Centripetal force will force the two halves apart and rub the sides of the tube until smooth.The key would be finding something you can buy that is a 1/2 circle at around 1mm size, and which is a hard enough substance to polish brass.And then trying to drill 0.3mm holes in the center of a 1.1mm surface will be a bit hard unless you have a drill press designed for that.