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View Full Version : Stupid question. what is lapping and how is it done?



triump
09-02-2006, 02:01 AM
what is lapping and how is it done?

Pat

ZipSnipe
09-02-2006, 02:32 AM
Nascar dude! Don,t ya watch TV?

Cruiser
09-02-2006, 10:51 AM
Aaron has extensive info on the lapping of the axis on his website "industrial hobbies.com" and it is to smooth the surfaces into mateing so they will slide with less friction. check out the site, His new model is pic'd now and has ground slideways so not needed the laborious lapping process.

Garage Shop
10-08-2006, 03:20 PM
Nascar dude! Don,t ya watch TV? :p


An easy definition of lapping is basically sanding. But on a flat surfase (granite surface plate). You take a piece of sandpaper. Place it on a clean surface plate. And make a flat part flatter. Usually in a figure 8 motion.

There are also "lapping machines". Many of the older styles look like record players. It had different methods to hold the part in place while the record part rotates "lapping" the part.

Generally this is used "in my shop" when the customer needs a surface flatter than .0003, or a critical finish is called out on the print.


But to make the long story shot. Its sanding a part on a flat surface making it flat.

Mike:boxing:

ViperTX
10-08-2006, 11:48 PM
Lapping also uses stuff like 'lapping compound' instead of sandpaper.....it's available in different grits....

Garage Shop
10-08-2006, 11:54 PM
Most people these days just lap stuff with sandpaper, rather than lapping compound, and a lapping plate.

I was a machinist for about 7 years before I was aware of what lapping compound was. LOL

Geof
10-09-2006, 12:33 AM
what is lapping and how is it done? Pat

What is the context of your question? To coin a pun lapping has many facets and one of them is just that; gemstone faceting where a gem is held in a precise position against a flat rotating disc with an abrasive paste or slurry on it. The abrasive wears, or polishes, a flat spot, a facet, on the gemstone which is then repositioned through a precise angle for another facet until eventually you have the sort of thing you see in jewellery showcases. All lapping works the same way; two surfaces moving against each other with an abrasive between.

Lapping is also used to get better contact between mating surfaces such as cpu chips and heatsinks. This can be done either by lapping each piece against a flat surface which can be as simple as sandpaper on a sheet of glass or by lapping the two pieces against each other. In the second case two nicely mating surfaces are produced but they are almost certainly not flat.

Lapping does not have to be done on flat surfaces; tapered valves can be lapped into their seats, spheres can be lapped between two rotating cups.

Special lapping techniques can be used to generate either spherical surfaces or perfectly flat surfaces. Two, initially flat, surfaces lapped together in a circular pattern will eventually become one concave spherical surface and one convex spherical surface. This technique has been used for centuries to make concave or convex lense and mirrors for telescopes.

Conversely, if three surfaces are lapped against each other in pairs a small amount at a time eventually, with experience and care, it is possible to generate three very flat surfaces. This is how optical flats are made; pieces of glass that are flat to within fractions of a wave length of light.

One large commercial application of lapping is that done on silicon wafers that go into chip manufacture. This is done on lapping machines that have a flat rotating disc while the wafers are held in carriers which sort of roam around in a random fashion. The random fashion is needed because this replicates the three plate effect used for optical flats; very flat surfaces are generated on both the plate and the wafer.

Another large, probably the largest, application of lapping is ball bearing lapping. Two flat discs are rotated with many balls between them and eventually all the balls are lapped down to very close to the same size with a very smooth finish.

Mcgyver
10-29-2006, 01:34 PM
Geof gave a thorough over of lapping, it is a precision operation using a LAP and ABRASIVE to remove material....However I'll take you Q about lapping in context as it is often applied to mini mills given where its posted.

People find these mills have bearing surfaces that are not accurately made and fitted and apply lapping compound between mating bearing surfaces to 'loosen' them up. This analogous to throwing abrasive grit into your crankcase to improve your engine. There are lots of threads here on the subject, please read them carefully before 'lapping' the mill (aka ruining it). I don't think its really even lapping as the is no lap involved, just uncontrolled material removal.

Proponents of lapping these mills say it is a matter of opinion, which I suppose it is, however there is just about perfect correlation between those who know what they are doing and those that say don't do it, Read and make your call. you cannot fit a machine tool by lapping to itself, if you want to fix it, you need identify what is wrong by comparison to reference and remove localized material to correct it

QSIMDO
10-29-2006, 02:08 PM
Perhaps if you consider lapping the IH mill as a secondary manufacturing step and not a final accurizing procedure?

Ennnh:rolleyes:

The latest incarnation of IH mill with ground ways should be much more accurate but I considered lapping as a way to "finish" the manufacturing process and establish a datum if you will or I'd be chasing tram & square for the next 3 years.

It certainly cured the Z stiction problem too.

X & Y were fine as delivered though. Like butta.

Allen 2
10-30-2006, 01:28 AM
Lapping is a procedure whereby using one surface is used as a reference and another surface is used as the comparison. One puts a layer of fine abrasive slurry between the surfaces and "rubs them together" so that the one becomes as smooth as the other.

Intake and exhaust valves are often lapped against their seats so that they mate well.

Any situation that calls for one surface to be mated to another could be lapped. The issue of "to lap" or "not to lap" can often be primarily about the technicalities of space and fit as much as about the results.

Perhaps the most important thing to know about lapping is that it is a proceedure intended to make things fit togther in a specific way.

BobWarfield
10-31-2006, 08:00 PM
There are a few things to keep in mind. First, the overly religious argument about lapping is based on a total misconception, which is that it is being done to improve the accuracy of the ways. In fact, lapping in this context is not done to make the ways more accurate, but to reduce friction. Those that are highly correlated as "those who know" are mistakenly arguing against the process because it makes things less straight. They want the ways to be hand scraped, which is accepted practice, and in the end, would produce a better result.

HOWEVER, this is not why the IH Mills are recommended to be lapped. The weak link in the chain is not the accuracy of the ways, but rather their surface finish. Stick slip, and friction in general, make it very hard to operate the mills with tight gibs. Lapping reduces this friction, and by tightening the gibs in conjunction one achieves greater accuracy because the machine is less sloppy on the ways. See the note about increased accuracy another fellow got by being able to torque down his gibs more tightly with proper hex bolts instead of the plastic handles.

Is this newfound accuracy as good as one would get by having the ways professionally scraped in? Absolutely not. Is it a dramatically cost effective and good "bang for the buck" way to improve the performance of the mill without the expense and required expertise to hand scrape? In this case, those that "know" are the ones who've done it, and especially Aaron, and they almost universally report an improvement especially on those machines destined for CNC conversion.

In fairness, I have not yet performed the experiment on my mill, but I do intend to do so, and I'll take a battery of accuracy measurements before and after when I do. I'm willing to take the word on faith of Aaron and others that it's a good thing for two reasons. One, they're the folks that have experience with THIS PARTICULAR MACHINE. They are successful, aside from what the textbooks say would be better, at improving machine performance, and that is pretty objective evidence. Two, I did some interesting research.

Try going out on the net and looking to see what the material removal rates are for lapping. It is an extremely slow process to remove very much material at all, even when you have optimized the lapping process for as fast a rate as possible. Aaron's recommended procedure can't possibly remove enough material to seriously "ruin the machine" as has been seen, according to the figures for material removal rates I've seen for lapping. If you read his procedure, he doesn't recommend very many strokes at all for each grit.

With that in mind, it seems clear that it is nearly physically impossible for this "lapping" process to seriously impair the accuracy of the machine IF YOU FOLLOW AARON'S GUIDELINES! Of course your mileage may vary, and you need to tread carefully, but in fact I do not subscribe to the notion this ruins the machine, you never do this on engines, yada, yada. It's a good theory, but for this particular machine, friction in the ways can be terrible, especially on the Z-axis with the heavy cantilevered head.

So I'm going to lap, textbooks and experts notwithstanding. If I do ruin my machine, I should be able to measure to what extent and provide a cautionary tale for others.

Best,

BW

QSIMDO
11-01-2006, 04:21 PM
...."you never do this on engines, yada, yada."



Ahhh, but indeed we DO lap in engines!
It's called "break in" and it occurs whether we conciously do it or not.

As for the mill; the ways WILL lap in anyway, just over a prolonged period of time depending on how much the machine is used. As I said before, I don't want to be chasing tram and square for the next several years until the tolerances settle down.
I would assume you folks with CNC would be extremely interested in starting with a machine that's well on toward bedding in.

As for smoothing the operation it truly is like chalk and cheese, night & day.