minimum flatness requirement prior to lapping plates? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: June 21, 2019, 02:54:47 am
content: If you just want to make flat faces on parts, what is the minimum machining process required before lapping can occur?Like, typically stuff is milled, ground then lapped.Can you go right from milling to lapping if you use abrasives typically not used for lapping in terms if particle size? Or maybe do pre-processing with ghetto lapping using sand paper and glass before going to a real lapping plate?Can the surface grinder be removed from the equation if you have a end-mill finish with a non conventional step? I mean if you do it right and get different lapping plates sets.. or do lapping plates just take too much damage doing heavy materials removal?Can it be tuned up some how, i.e. using different plate materials which are less flats for pre-finishing steps that wear less?I am talking small parts for thermal interfaces

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2019, 11:17:51 pm
content: How long you want to lap for, and what's the grinding rate of the medium used?Tim

author: Dubbie
author_rank: Unknown
timestamp: June 22, 2019, 12:59:49 am
content: if it is hard steel you are lapping, then I prefer to have a ground surface. A milled surface would take absolutely forever. A finely ground surface takes long enough to get to mirror as it is. Its a really good way to see what your grinder spindle bearings are like!I lapped an aluminium part flat a few weeks back, I was a shell milled surface about 250mm square. About an hour into it, I was cursing how long it was taking!

author: coppercone2
author_rank: Super Contributor
timestamp: June 22, 2019, 01:43:58 am
content: is there a affordable SMALL benchtop surface grinder out there?Size is the main concern, Like a proxxon size tool.I see you can get beasts cheap on ebay sometimes, but I just don't have the room for that.It would be copper or aluminum. For electronics work steel is not really used ?Like this thing that is meant for leather belts, but for metalshttps://www.hobby-machinist.com/threads/id-this-miniature-surface-grinder.4779/

author: Dubbie
author_rank: Unknown
timestamp: June 22, 2019, 09:24:30 am
content: Not really. The physics of machine tools means that some things just can’t be shrunk easily. My grinder is pretty small and lightweight, yet it still weighs ~800kgs. I can only achieve about 3-4 microns of flatness over a 100mm surface, so to improve on that, I have to lap.

author: coppercone2
author_rank: Super Contributor
timestamp: June 22, 2019, 02:04:03 pm
content: is it the stiffness requirement of the spindle that leads to heavy construction? I have seen figures in KSI for spindle stiffness.So is 200kg considered microscopic for a surface grinder?

author: David Hess
author_rank: Super Contributor
timestamp: June 22, 2019, 03:34:07 pm
content: Aggressive hand lapping also risks making the surfaces more nonparallel.

author: coppercone2
author_rank: Super Contributor
timestamp: June 22, 2019, 07:37:39 pm
content: do you mean using too high a slurry grit on the laps?If you go high micron can you get away with it by more frequently truing laps?I was thinking making sets of 3 lapping plates for each slurry mixture so they can be flattened on top of each other frequently despite heavy abrasion?

author: Dubbie
author_rank: Unknown
timestamp: June 22, 2019, 08:14:02 pm
content: Using the three plate method is the only way you can be sure your laps are flat. You need multiple plates anyway for the different grits, so it’s not such a problem. You should make round or square laps. Rectangle shaped are much harder to get flat.There is a really good book called “The foundations of mechanical accuracy” that you can find online as a PDF. That goes into a bit of detail on the subject. Also check out some of Oxtools and Robin Renzetti’s Youtube videos on the subject of lapping.

author: coppercone2
author_rank: Super Contributor
timestamp: June 22, 2019, 08:34:58 pm
content: I have that book but I have yet to much more then gloss over it.As for oxtoolsco, I just recall him using really fine compounds. But the diamonds come in way bigger grits then he uses. I never really saw a video on that, I assume its either because the laps don't work at those grits, or because the surface grinder wheel grit is fine enough that using them is pointless. I think those grits are typically used in other processes which are not friendly for grinders (i.e. bowled glass optics)What I imagined is a nice lap cabinet that has 3 plates each for different grits, so you don't have to clean them so much before switching grits. Like a bunch of really shortly spaced shelves.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 22, 2019, 09:39:43 pm
content: Can´t help you with the question for where to start, i´d probably try the sand paper on glass/surface plate to get it reasonable before lapping. The question is how long it will take or how long will be acceptable for you. You can get mirror finishes from a milling machine, but that depends on tool diameter and surface area, plus machine setup. It will be a tradeoff between setup work for that and lapping required.You would need to be concerned about changing grits if you expect the abrasive to embed into your tool surface.The thing is as much as this might happen, it might embed into your workpiece and cross-contaminate your tools, so you need to solve that first.Robin Renzetti showed how to work around that problem.Without possessing any experience on it myself, these are the videos i would recommend to watch:and the series of how to make the plates yourself, starting there:

author: Dubbie
author_rank: Unknown
timestamp: June 22, 2019, 10:12:28 pm
content: Depending on how flat you are shooting for, sandpaper causes round surface, not flat. Think of a piece of sandpaper laid on a blanket. The paper backing acts like the blanket and tends to cut more on the edges. On a 50mm square, you’ll only end up with a 20mm flat area in the centre. This is assuming you have your sandpaper on a surface plate.

author: coppercone2
author_rank: Super Contributor
timestamp: June 23, 2019, 12:12:59 am
content: how fast does a surface plate get damaged doing this? Should I get two surface plates?If I am using it as a presanding step for laps, can I get the cheapest surface plate grade (vs a measurement one I plan on getting) since the paper will mess up the workpiece geometry anyway?I guess I need to try it with glass, surface plate and get a tool for measuring flatness more advanced then surface plate caliper to see how I can divide up the process for minimum ware to expensive granite

author: coppercone2
author_rank: Super Contributor
timestamp: June 23, 2019, 12:18:48 am
content: Also, is it OK to lap ceramic surfaces on a surface plate? If I were to use aluminum it would get anodized for a heat sink. Not sure if I should mask bare aluminum for the thermal interface before anodizing or anodize it and then lap it or lap it then anodize it or lap it , anodize it and try lapping it again.I feel like the whole thing should be anodized so it does not bow when its screwed down as much since the surface will be stiff.

author: coppercone2
author_rank: Super Contributor
timestamp: June 23, 2019, 12:21:57 am
content: Also, is it possible to make linear surface plates, say you make a custom enclosure that is ALSO a heat sink (think like a sealed RF amplifier), so you need to lap a small flat inside of a weird shaped chassis. I was imaiging putting a lapping element ontop of a surface plate with some plastic to slide the work piece on the surface plate at the right height, but it seems really complicated if its just a small protrusion.Just thinking of what the OPTIMUM impractical thermal design would look like for miniaturizationor, what is the correct method for getting a flat on the side of a milled enclosure so a transistor can be mated to itWALL_______||WALL where || = transistorLike, can you (temporarily glue?) a right angle bit (maybe a gauge block) to a surface plate with a lap fixed on it that you can put a box over it and get a protrusion from the boxes wall flattened? Or, at least flatten any interior box surface (upside down box with a flat bottom), it would depend on the type of transistorIt seems that unless it was in the dead middle of the box, you would have moment that is difficult to 'level' with hand pressureI know you can make tapered flat bits that you screw into the chassis, but what if you wanted a solid work piece?? (think insane thermal ASIC with minimum resistances and size)

author: KL27x
author_rank: Super Contributor
timestamp: June 23, 2019, 01:19:40 am
content: How the bleep are you going to make the rest of that enclosure, let alone flatten the heatsink?

author: coppercone2
author_rank: Super Contributor
timestamp: June 23, 2019, 01:37:18 am
content: Quote from: KL27x on June 23, 2019, 01:19:40 amwell I suppose the flanges can be welded on after wards but my little mill? (did not think of this before I drew the picture)I think you can do those if you use the blue thermal absorbant compound while welding, but I think if those protrusions are near the corners you cannot safely weld them after lappinglike the size of a deck of cardsOr, is it safe to weld and it will go back into the correct shape?like, fill it with this stuff (don't have the same brand but similar)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 23, 2019, 04:07:00 am
content: Anodize is fine for heatsinking. Dimensional change is small.Tim

author: Dubbie
author_rank: Unknown
timestamp: June 23, 2019, 04:22:37 am
content: I think this is an x-y problem. Flatness is not your problem. Engineering a proper enclosure is.

author: coppercone2
author_rank: Super Contributor
timestamp: June 23, 2019, 04:36:59 am
content: if you want to use the enclosure as a heat sink it needs a low thermal resistance. anything that compensates for thermal resistance changes the enclosure and increases weight or size or adds complexity etc. am i missing something?its theoreticalI want to know how a construable tool to build that would look like. A perpendicular lap with 1 axis of freedom to true might be a strict definition of the problem but I am not sure

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 23, 2019, 02:04:00 pm
content: Lapping within a pocket will get you the problem that at the end of the stroke the abrasive will not get out of the corner, the surface is bound to not become flat. You would need to clean after each stroke. That´s why in the 3-plate-method you should rotate 90°. It not only works faster, it evens out a slope that might form by unequally applied forces (downward pressure might change from start to finish of the stroke).Of course people lap surfaces to get them as flat as possible and therefore improve heat transfer, but seriously, under a certain flatness just use thermal paste. If you get this under a certain thickness it will do the job quite well and the contact surfaces of most parts are heat collectors for an unevenly generated heat anyway (there might be hotspots under the heatsink-pad). In other words there is not much to be gained from getting the non-evenly generated heat transferred as evenly as possible. These parts deform under the force of their tensioners anyway, creating new gaps.It might be more worthwile to put the effort into a thermally stable environment, i.e. reduce the amount of temperature variation in operation as much as possible. I guess that when your application would fail because of trying to avoid thermal paste you have other problems, like a too steep temperature rise.

author: coppercone2
author_rank: Super Contributor
timestamp: June 23, 2019, 03:37:33 pm
content: what is the thermal resistance of a wrung surface anyway? like if you had two gauge blocks. I think modern methods actually test electrical resistance of wrung blocks to see if the gauge blocks are within specI can't test it because my gauge blocks are too old and they don't work in that regardand, can you even get aluminum and copper to do it?

author: David Hess
author_rank: Super Contributor
timestamp: June 23, 2019, 08:17:23 pm
content: For heat sinks, deformation from mounting stress has a much greater effect than surface flatness beyond an easy to reach point so sandpaper on glass is completely adequate and a mirror finish offers no benefit. The only reason I use a granite surface plate is because I happen to have one.This goes double when the typically relatively thin metal of an enclosure as a heat sink. In the past where high power was required and mounting was a problem, I have seen the power devices carefully mounted to a massive heat spreader which then has much easier mounting requirements to the enclosure because the surface area is so large. A lot of thermal compound was used between the heat spreader and enclosure surface. This brute force method can be very reliable.With common mounting conditions where fasteners at the edge of the device, lapping to create a rounded surface is actually advantageous because deformation results in continuous contact while if the surfaces were flat to start with, the center will cup away. Motorola's classicapplication note 1040covers this to some extent.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 24, 2019, 08:41:50 pm
content: Yeah you can't lap two surfaces flat then put screws in to pull them down. They won't be flat anymore. Better would be to apply spring pressure above the top surface. Can't weld anywhere on the structure near where the lapped piece is attached either unless you will be lapping afterward not beforehand for the same reason. Best approach is to get the parts reasonably flat then use thermal compound. Even parts lapped to a high level will get higher thermal transfer with compound, as no matter what the surfaces still won't be perfectly flat.

author: duak
author_rank: Super Contributor
timestamp: June 24, 2019, 11:35:17 pm
content: It's not impossible to grind copper or aluminum, however the proper abrasive wheel, coolant and technique has to be used. If a standard wheel for ferrous metals is used, its surface will load up and get clogged preventing it from cutting further. If the wheel gets too hot, the metal in the pores of the wheel expands and can split off pieces of the abrasive. Sometimes the wheel comes apart, a Very Bad Situation.Copper is gummy and is a pain to machine, little alone grind. A proper milling center should produce some pretty doggone flat surfaces. I have some aluminum samples that were milled with a face mill rather than an end mill and the final surface is almost mirror like. I have a small manual milling machine at home, and I would use a flycutter (single point) to flatten the surface.Since most abrasives are ceramics, If you're going to lap ceramic, you'll have to find something harder than the material.

author: coppercone2
author_rank: Super Contributor
timestamp: June 25, 2019, 03:18:34 am
content: well I use some kind of lubricated disks for grinding aluminum but I don't know if you can get a wheel for a surface grinder that does it (norton sells em for angle grinders).OK, but in regards to the previous question, if you get a gauge block level surface, where there is 'wrining' bonding effecting the two surfaces, what does thermal transfer look like at this interface? Will it still be superior to grease?A wrung surface has a high holding force if what I see about gauge blocks on youtube is true. I can't get my old gauge blocks to stick very much because they are like 40 years old and unmaintained.

author: duak
author_rank: Super Contributor
timestamp: June 25, 2019, 05:13:46 am
content: One term that's used is "contact thermal resistance" Here's an introduction to it:https://www.lytron.com/Tools-and-Technical-Reference/Application-Notes/Reducing-Contact-Thermal-ResistanceIt gives some numbers for surface finishes with various techniques.The second paragraph in this app note is interesting:https://www.lytron.com/Tools-and-Technical-Reference/Application-Notes/Thermal-Interface-MaterialsNote the percentage of actual metal to metal contact. I suspect that even the best gauge blocks still have a limited contact area.Regarding wringing gauge blocks together - have you tried cleaning with acetone and drying with lens cleaning paper? Oils are removed by diluting them off the surface, but even so, can leave a monolayer. I don't know how this affects wringing, but if I want paint to stick even an oil monolayer is not good.There is something called tooling plate (MIC-6 is one product) that is flat and stress relieved and is used by machinists when they want a flat piece of aluminum. I've got a few small pieces and when it's clean they'll stick together, although not as well as gauge blocks will. This stuff might make a good lapping surface.I worked with optics where the lens surfaces were specified to have a maximum amount of scratch and dig as well as being flat or curved as required. AFAIK, semiconductor wafer surfaces and optical surfaces are the smoothest and flattest surfaces commonly available and make typical metal surfaces look pretty rough by comparison.I don't know what type of abrasive wheel is used in a surface grinder for Cu or Al but I sometimes use a really friable Silicon carbide wheel (bits come off easy) with aluminum. The manufacturer says it's OK. A disk grinder is a whole 'nother thing; it's usually flexible and it's not expected to be accurate to 1/10,000 of an inch.

author: coppercone2
author_rank: Super Contributor
timestamp: June 26, 2019, 01:44:24 am
content: I thought with the gauge blocks you are supposed to just wipe them on your wrist and coat them with wrist oils before mating them.I think I tried cleaning them with acetone, then wiping them on my wrist, then wriing them. It did not work but I kind of wondered what would happen if I gave them a spray down in de-oxit or something before hand and cleaned with lens wipes and lab grade alcohol. The store acetone has a high reside contents.Maybe a ultrasonic cleaner too. It would be nice to rescue them lol

author: duak
author_rank: Super Contributor
timestamp: June 26, 2019, 02:45:18 am
content: If Starrett doesn't know how to clean gauge blocks, no-one does:http://www.starrett-webber.com/GB45.htmlI don't have a set and never had to use them although I spent time in various machine shops.Skin oils are bad news because they carry salts. If you take a piece of freshly machined and cleaned 10L14 steel and pick it up with your fingers, there's a good chance you'll see your fingerprints highlighted in rust in a few days.Gauge blocks are pretty amazing things. If I remember the story correctly, Johansen (the developer) used his wife's sewing machine to make a set to prove the principle. Perhaps he hacked it into a surface grinder.

author: Dubbie
author_rank: Unknown
timestamp: June 26, 2019, 06:59:21 am
content: Not a surface grinder but a lapping machine.He had mounted a cylindrical cast iron lap where the hand wheel usually is on the sewing machine. He would hold the blocks up to them to lap away material. I have no idea how he measured what he was doing. Maybe some sort of height gage on a surface plate.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 26, 2019, 03:37:02 pm
content: He had quite the contraption for measuring them. There's a publication about it somewhere. He was a resourceful fellow.For wringing gage blocks the surfaces should be cleaned first, then wiped on something like a chamois lightly wetted with oil. If the blocks won't wring check them for burrs. Otherwise they are probably out of flat. It doesn't take much.A wrung lapped surface without thermal compound won't have superior thermal transfer over a merely relatively flat surface that has thermal compound applied. The surfaces absolutely do have a considerable area not in actual contact even when finely lapped. The act of lapping itself produces microscopic peaks and valleys.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2019, 10:49:32 pm
content: Indeed it can be worse, as in the 10s of nm range of average distance between surfaces, not only is the air trapped between them very much unable to move (the reason fluffy materials are effective insulators), it's trapped in a space smaller than the mean free path length, making it effectively a vacuum filled space, and radiation dominates over diffusion (convection is nonexistent because of the small space). So it's worse than a modest gap where the air has somewhere to move. Or something like that.Whereas with a substance filling the gap, even if it's just oil let alone a thermally-conductive-ceramic-filled one, it's much better.To do better, you'd need the surface flatness in the range of, I guess, a tenth of that distance, where tunneling between surfaces can take over, and if clean enough, the surfaces can diffusion weld together as well.Tim

author: duak
author_rank: Super Contributor
timestamp: June 27, 2019, 11:12:22 pm
content: Coppercone2, there's a section on cleaning and deburring gauge blocks on p. 68-70 of this doc:https://emtoolbox.nist.gov/Publications/NISTMonograph180.pdfThe above doc talks about using optical flats to check block surfaces. I understand a glass microscope slide is quite flat. You might be able to get an interference pattern between a slide and a gauge block to see where the high points are.

author: coppercone2
author_rank: Super Contributor
timestamp: June 28, 2019, 12:10:11 am
content: i have optical flats (very old east german)will test eventuallythe gauge blocks themselves are somewhat 'tarnished' looking but not rusty

author: eKretz
author_rank: Frequent Contributor
timestamp: June 28, 2019, 02:54:23 am
content: If there are any surface defects at all they will start to get difficult to wring. Still good for most metrological tasks that don't need millionths level precision though.