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Lapping Small brass Cylinders and Monel Pistons on a Trumpet.

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Has anyone any advice to get started lapping brass cylinders of about 0.665 dia and matching monel steel pistons for a trumpet ? These are through holes, about 4 inches in length. I have a decent lathe and mill. The suppliers of laps want several hundred dollars in start up tools. Is there a way to make one's own laps ? They have to be finished to a tolerance of about +/- 0.0002
 
Initially the valve casings are bored undersize. In the case of a Bach Stradivarius, where the nominal valve bore diameter is .664" the casings might be bored to .662-.663. Then the casing bores are honed to their final dimension, usually using a Sunnen hone.

The pistons are initially turned oversized, say .665-.666, then are honed to final dimensions as well.

If the casing and piston are theoretically both .664, they still need to be lapped with an abrasive compound. Usually something like 10-15 micron diamond compound is applied to the piston and then it is worked inside the casing until a smooth fit is achieved. The goal is to have about .0005" tolerance between the piston and casing, with a smooth finish. Straightness, roundness, and lack of runout are key.

Once that is done, the action will still be a bit sluggish, so the piston and casing have to burnished against each other too.

I've got two degrees in trumpet performance and am now retraining as a machinist. Trumpet manufacture is an eventual goal for me, so I've looked into this. Hope this info helps.
 
Thanks. All that makes perfect sense for a production shop but not for one-off repairs.I'm just dealing with tiny amounts of material removal. More to my point , I'm curious as to whether anyone had done any precision lapping with home-brew cylindrical laps ? I'm also wondering if there is any reference material out there for making ones own laps ?
 
Maybe this would work for you. They call it a valve burnish tool, but you could also use it with lapping compound. It seems to resemble most “laps” I’ve seen. It also expandable which is useful for pushing dents and high spots out of valve casings if that’s what your needing to do. That’s preferable to lapping since you can retain the overall tolerance. You can order specific diameters. This is a great tool and I have used it to repair multiple horns.

VALVE BURNISHING TOOL (ORDER BARRELS SEPARATELY)
 
I'm curious as to whether anyone had done any precision lapping with home-brew cylindrical laps ?

yes, but in larger sizes, and expanding laps....not exactly what you need. Can you not carefully use an adjustable reamer?


Probably the most common approach to internal lapping is done with loose abrasive. The lap gets abraded as does the work and with a big of care and feel, the lap wears into a perfect cylinder as does the work.

It works, but I'm not sure its for this job. The reason is the brass maybe quite soft. If so it would be all too easy to get lapping compound embedded in the work, and it then becomes a lap for whatever goes the bore. You'd have to make the lap out of something very soft, maybe AL. There are various was to create a expansion lpa, slit the the lap and have a screw going in from end to a blind hole with cone shape bottom is oneway.

The other approach, and this might be better is to a make a proper lap - i.e. a cutting tool that is charged with abrasive and is used without loose abrasive. You can make these down to 1mm. Basically take a piece of drill rod in the lathe, and with emery held between to pieces of wood, put a taper on it with the large end being the finish ID. Charge by pressing and rolling the lap (with abrasive) against a hardened steel plate. The abrasive gets pressed into the surface of the lap and that does the cutting, the lap parent material is never touched. It removes material slowing but does a super job and as I say can be made to work for very small holes.

Finally you could use a garnet based lapping compound like Time Savers loose with an expanding lap. They claim it breaks down and doesn't embed. I don't really believe it and think its more like it doesn't embed as much, more so the risk with soft work, but it is an option.
 
I am surprised by the recommendation to use diamond paste as a honing compound. I would expect the diamonds to embed in the brass and steel and to continue abrading the metal after a good fit is achieved, with the eventual outcome being a sloppy fit. A good analogy is gravel getting embedded into a piece of soap.

An experienced machinist friend pointed me to TimeSaver lapping compounds which have the property that they break down with use and stop cutting. I would think that might be a better choice here.
 
I am surprised by the recommendation to use diamond paste as a honing compound. I would expect the diamonds to embed in the brass and steel and to continue abrading the metal after a good fit is achieved, with the eventual outcome being a sloppy fit. A good analogy is gravel getting embedded into a piece of soap.

An experienced machinist friend pointed me to TimeSaver lapping compounds which have the property that they break down with use and stop cutting. I would think that might be a better choice here.

Thats what the marketing material says. :) Diamond or any other abrasive won't embed if its on a charge and washed lap like i described, i.e. vs using it as a loose slurry
 
I did check in with American Lap and they quoted me several hundred dollars to get set up. Since cast is harder than brass, my concern is that the compound would become embedded in the brass, the opposite of what is supposed to happen. It's my understanding that the lap has to be softer than the material being lapped. I've used Sunnen hones many times and it was a thought but I couldn't find product that went that small and it would be too aggressive.One has to be very careful to remove only a minimal amount or the pistons would have to be plated really thick, which is hard to do. Hand lapping, not power honing seems to be the way to go.
 
Is this a high-dollar instrument, a personal enrichment project, a favor to a friend, or...? I ask because I wouldn't want to attempt this on a high-dollar instrument (Bach Strad, etc.) even if I thought I could maybe pull it off. And if it's a budget instrument, it hardly seems worth the time, effort, and expense unless you're doing it for your own education or as a favor to someone who can't afford either the repair or another instrument, and all that does is transfer the expense to you. The instrument still may not be worth what you're putting into it.
 
Is this a high-dollar instrument, a personal enrichment project, a favor to a friend, or...? I ask because I wouldn't want to attempt this on a high-dollar instrument (Bach Strad, etc.) even if I thought I could maybe pull it off. And if it's a budget instrument, it hardly seems worth the time, effort, and expense unless you're doing it for your own education or as a favor to someone who can't afford either the repair or another instrument, and all that does is transfer the expense to you. The instrument still may not be worth what you're putting into it.

Great points. If you are a brass instrument player, you probably know that there are tons of vintage horns out there but the highest risk you take on buying them is if the valves are shot. I'll practice this rework on an old junker first, to be sure. I get chances at horns like Olds or Conns for less than $100 and they can be awesome horns, even professional grade, if you can fix the valves. The one I have my eyes set on is an Olds Special Tri Tone. It sounds fantastic until the valves don't spring back up because they are too sloppy. My intention is to take the absolute minimum off the cylinders, nickle plate the pistons and lap the two together giving that optimum 0.0003 or thereabouts , clearance.

Now I can buy another used one off of Feebay that looks great and pay say $300 for it. Who knows what I'll get though. Another horn with valve issues ? They want $600 to do it in a shop and I figure they are no smarter than I am. Imagine also if they do a crap job. What would be my recourse. If this works, I might even be able to sell kits to others with the same purpose.
 
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Many of you may know that to make a perfectly flat plate simply requires 3 plates to start and by mating them together in a cycle and scraping then lapping the high spots off, you get perfectly flat surface plates.

One of my biggest issues is whether or not a similar process exists for cylindrical laps ? I have a decent lathe, and a cylindrical grinding attachment, but it certainly isn't up to +/- 0.0001. there's going to be some run out. A fellow did a video that is extremely close to what I want to do and I'll tag it to the end of this post. In the video, I can't recall where (indeed it may have been another reference), he makes passing mention of lapping the external lap against the internal one to get that perfection.

I can get pretty close, say within a thou, but how does one finish a cylindrical lap without having one to begin with ? Is there a process like the 3 flat plates ? Also, is it not true that in order for me to lap brass, I'd need a softer material to make my lap ?

Cheers.

ID and OD lapping simplified. - YouTube
 
Measuring Small Diameters to +/- 0.0001

While we are at it, I only have telescoping gauges and crappy ones at that. I don't even know how I can measure that small a diameter to +/- 0.0001. suggestions are welcome on this issue also. Maybe I'll start another thread for that issue.
 
I certainly wish you all the best in this endeavor. Measurement to +/- .0001" will indeed be difficult in any case, and impossible with your telescoping gauges, I think.
 
While we are at it, I only have telescoping gauges and crappy ones at that. I don't even know how I can measure that small a diameter to +/- 0.0001. .

you can't. With skill telescoping gauges are definitely good to less than a thou, but measuring anything to a tenth is a challenge. imo you'll need a dial bore gauge. How did you decide on the tolerance?

One of my biggest issues is whether or not a similar process exists for cylindrical laps ?

as for whats possible and original generation of geometry yes if its loose slurry. With care you can produce a bore to a micron or two of consistent diameter. As I said, the lap becomes a perfect cylinder, and you can feel tight spots. However also as described, I wouldn't do with with brass and would use a charged lap
 
you can't. How did you decide on the tolerance?



as for whats possible, charged lap, or loose slurry? The second, with care can produce a bore to a micron or two of consistent diameter. As I said, the lap becomes a perfect cylinder, and you can feel tight spots. However also as described, I wouldn't do with with brass and would use a charged lap

The ideal clearance on professional horns is apparently 0.0003. Another respondent suggested 5 tenths. Irregardless, that means I'm going to have to get down to a tenth on my measurements. Thanks for the advice on the slurry, but I don't understand what difference it makes as a slurry is just particles suspended in oil. I had good experiences with Clover brand lapping compound and I'm thinking a 1200 paste. I guess it wouldn't hurt to experiment on some old junkers with cast laps. I didn't quite understand your comment "I wouldn't do it with brass". I of course have no choice when dealing with brass instruments and wasn't going to make a lap out of brass. My wildest guess, if having a lap of softer material than that being lapped, would lead me to aluminum, but I've never heard of such a thing. I'll take your post to mean "Proceed with cast, even when lapping brass".
 
The ideal clearance on professional horns is apparently 0.0003. Another respondent suggested 5 tenths. Irregardless, that means I'm going to have to get down to a tenth on my measurements. Thanks for the advice on the slurry, but I don't understand what difference it makes as a slurry is just particles suspended in oil. I had good experiences with Clover brand lapping compound and I'm thinking a 1200 paste. I guess it wouldn't hurt to experiment on some old junkers with cast laps. I didn't quite understand your comment "I wouldn't do it with brass". I of course have no choice when dealing with brass instruments and wasn't going to make a lap out of brass. My wildest guess, if having a lap of softer material than that being lapped, would lead me to aluminum, but I've never heard of such a thing. I'll take your post to mean "Proceed with cast, even when lapping brass".

I mustn't be explaining it well. You can lap with a slurry or you can lap with a charged lap. Its the former that I wouldn't do with the brass. The former self corrects the lap but will charge the work if the work is softer than the lap (i.e. brass). The later is a cutting tool, no loose abrasive, nothing to embed in the brass. I described in an early post how to make on from drill rod that will work, but its slow work.

That's a tight clearance, but doable....what a perfect fit of tailstock barrel to bore is suppose to be. SOP is make the bore, then make the mate to fit - is that an option, or do you have to make the bore to fit something existing? If so, try the charged tapered drill rod idea (tapered a few thou), its quite accurate, just slow.
 








 
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