View Full Version : Rectangular cast iron lap

07-27-2003, 08:22 PM
I bought a cast iron lap that is about 4 inches thick and 18" x 20" with 1/8 grooves on about 5/8" centers. Not exact measurements but typical lapping arrangement of cross grooved surface. The lap is built into a bench and has a nice wooden cover. I got it from an R&D lab and it looks like it has never been used or at least has had very, very little use. I got nothing else with it and I am interested in how to use it and maintain its flatness. I have an idea that it orginally had a round hand lap along with it that could be used to correct the flatness when necessary. I have never used a lapping bench like this; most of my experience has been with optical lapping and polishing machines that use round tools. Any lapping gurus out there?

Forrest Addy
07-27-2003, 10:50 PM
The flatness of a cast iron laps have to be maintianed as you go. As you wear away the surface you have to shift your lapping to high areas.

For that you need a precision straight edge. Everytime before the lap is used the surface should be schecked all over for flatness using the straight edge and note should be taken of the high spots.

Go about it right and the lap should never have to be machined.

Lapping is a time consuming process and it takes skilled people to do it right. There's still a few operations that require hand lapping like precision conditioning of small parts for fine apparatus.

07-28-2003, 11:14 AM
The only way to achieve true flatness is to use three laps. Laps A, B and C. A is used to grind B, B is used to grind C. C is used to grind A and so on. The only thing all three have in common is a plane surface. This is standard practice in making optical flats.

07-28-2003, 01:41 PM
I have several optical flats to check the flatness of lapped and polished parts. I have an idea I can check the cast iron lap with an optical flat but in order to do that I would have to polish it some and that is a lot of trouble if I am using a grit that is more course. I envision using this lap to lap ground surfaces flat so there would be a significant amount of stock removal relative to polishing and therefore I would use a course grit. It seems like it may be difficult to keep this lap flat in the area of the corners. I may make another smaller round lap for use in maintaining this big surface. I am hoping someone out there has had years of experience pushing parts around on similar surfaces. Most lapping is done on SpeedFams or Spitfire machines where everything is round so there are probably not that many hand lappers. I am familiar with the ABC permutation technique. Moore Special Tool Co. now Moore Tool uses that procedure to make their ultra precision large cast iron surface plates by hand scrapping. I had the pleasure of watching that operation done at Moore a few years ago. It is has been used on optical flats but these days checking with an interferometer is much faster, more accurate and cheaper. It is more accurate because you get a clear and detailed indication of the surface under test, not because it is inherently more accurate. The resolution of a phase measuring interferometer is nominally 1/20 wave visible and can be 1/00 wave and that is far better than an optician can do by eye. Of course you have to buy the interferometer which might cost more than the average small time machine shop.

07-28-2003, 01:58 PM
It's true that in production it is no longer necessary to use the ABC technique. They now have single diamond point lathes that are capable of turning telescope mirrors to 1/4 wave of red light accuracy with no finishing steps at all. These are used to machine solid aluminum mirrors for lasers from 6061 billets. The ABC technique is still the only one available to the home machinist or amature astronomer.

07-28-2003, 03:12 PM
Phase measuring interferometers themselves are a small part of total price. The tooling & accessories raise the $$ amount by a factor of 10-15. As an ex-optiker I don't see why you need to use an optical flat to check your lapping plate as long as the parts you lap are OK. Small parts on a big lap, with long figure-8 strokes all the way around aren't going to change the lapping plate much. Also the ratio beween the diagonal/diameter of part & the lapping plate will allow you a lot of tolerance on the plate unless the part is nearly the size of the lapping plate. That's why you got a big one. Fine lapping compound will put a shine on the plate surface & cleans up rapidly. If you need to check with an optical flat, you can see fringes easier at an angle( reflectivity is better ).

07-28-2003, 04:02 PM
Optics and diamond turning is what we do. Often times it is necessary to lap the back of a part flat to about the same tolerance as desired from the diamond turning of the other side. To determine if the parts are "ok" means checking them with an interferometer or an optical flat. If they are lapped with a grit that doesn't yeild a sufficiently specular surface for testing it is difficult to judge them ok and therefore the same applies to the cast iron. I hope you are right about the lap not going out of flat very quickly if a good technique of figure 8 lapping is used. I so far have not checked the surface as it is now so I will try to polish it some and do that. I can probably mark it up with a crayon and get fringes without too much work. I will be surprised if it is optically flat right now as I am a big beliver in Murphy's Law.

Forrest Addy
07-28-2003, 04:28 PM
Figure 8 lapping if blindly used is responsible for the tons of lapping plates I've machined over the years. Figure 8 lapping looks impressive and professional but if I don't see a straght edge nearby, and I don't see the work moving all over the plate I know the guy on the lap is a greenhorn trying to look professional.

It isn't the fancy figure so much as the quasi-randomized work motion that focused on the high surfaces of the lap that ensures both flat work AND a flat lap. The work preferentially wears in the lapping process but so does the lap. If you don't consider the lap's flatness as you work your precipitating a problem that only machine work will solve.

High corners certainly evolve but that's where you rough small work, isn't it?

For very accurate work the three lap in rotation process is necessary to ensure initial flatness. 3 micro inches is about the best one could hope for when it comes to surface flatnees on all but very small workpieces. For anything larger temperature controls are necessary.

I recall when I was an apprentice I lappedg some brass bump barrels to 2 light bands on the porting face. When the barrel got to the inspection station the face had turned concave. Why is that?

The inspector was kind enough to explain how the heat from my fingers warmed the cyinder end expanding the metal. The port face was cooled by contact with the lap and so what was made flat when un-uniformly heated turned convex when thermal equalibrium was restored. He let me take them back and rework the barrels off the sheet this time taking care not to let my hot little hands screw things up. Earl Mosbarger. Great guy.

[This message has been edited by Forrest Addy (edited 07-28-2003).]

Forrest Addy
07-28-2003, 06:53 PM
Accidental double post

[This message has been edited by Forrest Addy (edited 07-28-2003).]

07-29-2003, 05:52 AM

Forrest is right abot taking due care and attention while hand lapping. Two bronze laps are used to flatten the plate - a ring and plug. You do not need to use ABC methods with these laps. For tool work the ring and plug is adequate. Thorough cleaning of the laps cannot be over emhasized. The plug (a plain round disk) is used to remove high spots. The ring is to even out the low spots or "flatten" the plate overall.

It is interesting to note that the cast iron laps used to facet diamonds wear very little and experience very long service life.

07-30-2003, 12:07 AM
Well Forrest, maybe you fixed some of the laps I screwed up. I don't remember any of them wearing a great deal however. The figure 8 thing only works if you do it like the pedals of a large flower so you cover most of the lap surface. We used to check the flatness of things like laps by rubbing them dry a few strokes on a surface plate. The surface plate was on cal cycle for the site, so we didn't pay to have it trued up, which happened every few years. Sorry for having been a knucle dragger.

07-30-2003, 12:53 PM
How you define acceptable flatness is dependent on what you are doing. Lapping on a surface plate works pretty well for getting things flat to about one fringe visible which is about 12 millionths of an inch. That is very good flatness for applications such as seals and other mechanical applications but just ok compared to optical surfaces. I once gave an optician a brand new AA Lab Grade Starrett pink granite plate that was about 12" x 18" to use in lapping a particular mounting surface. He managed to get the surface as flat as needed using other optics shop techniques but then worked on the surface plate which he said wasn't very flat. He got it flat to about half a fringe over the entire surface allowing for some roll off at about 1/2 inch from the edge. I intended to retrieve that plate from him but never got around to it. It was used in the optics shop for pressing pitch laps once in a while. I expect we will have this cast iron lap a lot flatter than the new condition after using it a while, if we use it at all. It looks pretty heavy at 4" thick unless it is ribbed underneath so it may be hard to lift out of its catch pan for any rework. First thing will be to clean it and then rubbing it a little with a known flat should show the high spots. There are techniques for lapping to very tight tolerances and there are techniques for getting a rough surface mechanically flat. The middle ground between super flat and nominally flat as from a surface grinder is where it gets lonely. Rapidly and cost effectively taking a surface that is flat to 0.001" to a flatness of 0.00001" is tricky and it gets more tricky when the surfaces get large. The earlier comment about temperature control becomes exponentially more important with size increase. I have discovered that an additional complication of corrosion pitting and galling starts to get important when sizes get above a couple feet in diameter. This causes unacceptable scratching so you become a chemist.