Diamond Lap

shapeaholic · Jan 2, 2018

I have been working on the bed of my Smart and Brown 1024 Lathe, and have gotten to a point where I need to smooth the scraping marks.
The bed is VERY hard, and it just sort-of laughs at regular stones. ( makes you wonder how it got worn in the first place??)

In another thread Richard King referred to using a diamond lap to do this smoothing.

Can anyone enlighten me on how to make a lap and what grade diamond I should use to charge it?

Thanks
Pete

Richard King · Jan 2, 2018

I have used these on hardened burrs.https://www.mscdirect.co.uk/EZE-14100B/SEARCH:KEYWORD/product.html

also have made a cast iron block aprox. 1" x 1" x 3" long with 1/16 x 1/16 x 1/4 apart diamond shape sawed or milled into it and then use lapping compound.

michiganbuck · Jan 2, 2018

You can lightly tap the iron block to imbed the diamonds so they do more cutting than just rolling..

We used to make a collar device with a fine thread tightening screw to tighten a reamer in a bore to fine diamond lap the OD of very close reamers..You can lap them down as close as you can measure..a few millionths. Yes the would have set on a plate for a time for heat expansion before checking.

ironsmith89 · Jan 18, 2018

As the others have stated a small handheld piece of soft cast iron works best, brass works well too but cast iron allows you to easily reflatten it on a surface grinder as it's magnetic. I've made many and in a pinch I usually serrate them with a square file laid on its corner. If you have time to mill or saw the grooves in great, but you'll need something to break surface tension to keep the lap from riding on the film of slurry. I typically use 1200 grit diamond, I forget what micron that is, it's blue. This will cut just right, fast but not to fast to get into trouble. For a mirror finish I use 8000 grit, yellow, and I typically use Norton diamond, heavy concentration from MSC Supply. It's important to charge the lap and then wash it clean before presenting it to the work. I used to think I could just apply the diamond to the hardened surface and then wiggle the lap around over it and the grit would embed into the softer lap, which it will, however by the time it's charged, your lap won't be flat anymore as the diamonds rolling around under the lap eat vigorously at the softer entity. I've made a little handle with a ball bearing bolted to the end, I apply the diamond media to the lap, iron it in with the tool, then wash the lap with naphtha and a tooth brush, then I use mineral spirits as a lubricant on the workpiece. You can cautiously lap dry when your all done to really shine the surface but be advised, if the lap plugs it will scratch the finish and you'll have to start over. I usually don't get too crazy with eyewash and stop at 1200 grit, but this depends on the job.
Good luck.
Chris

Monarchist · Jan 18, 2018

shapeaholic said:
bed is VERY hard, and it just sort-of laughs at regular stones. ( makes you wonder how it got worn in the first place??)

Would you believe... from being lapped by a charged Cast-Iron "plate".

The underside of the carriage.

Takes longer with something less aggressive than diamond. Cast Iron constituent particles are "there" for-sure. Add also whatever workpiece alloy particles have managed to get past the wipers. I'd rather not even THINK about a Smart & Brown being used as host for grinding, but even if "never happened, here!" applies, MOST lathes do see use of abrasive cloth and papers, often, not seldom, and poorly cleaned-up-after, mostly.

We want our lap charged with Diamond so it has an advantage over the tool being lapped.

But Diamonds themselves are lapped to "brilliant cut" and other fine gem shapes with .. the dust of OTHER diamonds. In oil. Plus some of their own dust that accumulates as the process (a very slow one..) progresses.

Cast Iron & family fretting corrosion particles are doing the same thing to hardened ways, just at a far higher unit contact pressure and - thankfully - slow enough rate we can get rather a lot of years of service out of a lathe before it MUST have a rebuild.

Richard King · Jan 18, 2018

ironsmith89 said:
As the others have stated a small handheld piece of soft cast iron works best, brass works well too but cast iron allows you to easily reflatten it on a surface grinder as it's magnetic. I've made many and in a pinch I usually serrate them with a square file laid on its corner. If you have time to mill or saw the grooves in great, but you'll need something to break surface tension to keep the lap from riding on the film of slurry. I typically use 1200 grit diamond, I forget what micron that is, it's blue. This will cut just right, fast but not to fast to get into trouble. For a mirror finish I use 8000 grit, yellow, and I typically use Norton diamond, heavy concentration from MSC Supply. It's important to charge the lap and then wash it clean before presenting it to the work. I used to think I could just apply the diamond to the hardened surface and then wiggle the lap around over it and the grit would embed into the softer lap, which it will, however by the time it's charged, your lap won't be flat anymore as the diamonds rolling around under the lap eat vigorously at the softer entity. I've made a little handle with a ball bearing bolted to the end, I apply the diamond media to the lap, iron it in with the tool, then wash the lap with naphtha and a tooth brush, then I use mineral spirits as a lubricant on the workpiece. You can cautiously lap dry when your all done to really shine the surface but be advised, if the lap plugs it will scratch the finish and you'll have to start over. I usually don't get too crazy with eyewash and stop at 1200 grit, but this depends on the job.
Good luck.
Chris

Chris, longtime no see....I hope you were busy at work and not Ill. To all who don't know Chris is another professional scraper hand and maintenance tech and possibly an engineer. He has a channel on You Tube CG Precision. One of his shows is how he ground a hard way machine. A few years ago Chris made an Great analogy of a lube pump being a heart and the oil lines were the systems veins. Rich

Richard King · Jan 18, 2018

Back in the old days...late 70's to early 80's our company service contracted machine rebuilding at Midwestern Machinery in Minneapolis Minnesota who's foreman Bill Tracy and Bill Shepard who worked at Fellow Gear (Testing) Machine Company. Bill S (was a contractor like me) took over Bill T (worked for MWM) job for fellows after Bill T retired (fired) and moved to MN. They were both chain smokers and both died of lung cancer way to early in their lives.

Bill Shepard worked out of Detroit in auto factories before and after Fellows went out of business. (a good story for my book) They taught me to use small cast iron laps that we used on hardened ways on gear testers after we had them precision ground. Grinding would get the hardened ways to within .0001" and after that we lapped them using diamond paste on the CI block using electronic indicators to get them to .00002". Never had problems with the diamond embedding in the hardened ways.

You can also see how Moore Tools lapped ways if you have a copy of the Moore book "The Foundations of Mechanical Accuracy" The show in great pictures their machine builders lapping in several places. On soft metals I would not lap them unless I was using Time Savers Lapping compound. Rich

ironsmith89 · Jan 19, 2018

Not ill but very busy, I appreciate the concern.
This got me thinking of the trak mill that was done about 3 1/2 ish years ago. That mill was hand ground and then lapped, so today at work I lifted her covers and took a photo of the ways just to give a sort of update on any wear. Still looks good to me, and this machine sees heavy daily use, note how the turcite under the saddle has kind of blurred the areas around the high spots of the bed ways, seems to be common with turcite. Also I'm including a photo of an unmentionable lathe I did about a year ago, same technique, hand grinder and a lap.

Cheers!
Chris German

shapeaholic · Jan 31, 2018

ironsmith89 said:
As the others have stated a small handheld piece of soft cast iron works best, brass works well too but cast iron allows you to easily reflatten it on a surface grinder as it's magnetic. I've made many and in a pinch I usually serrate them with a square file laid on its corner. If you have time to mill or saw the grooves in great, but you'll need something to break surface tension to keep the lap from riding on the film of slurry. I typically use 1200 grit diamond, I forget what micron that is, it's blue. This will cut just right, fast but not to fast to get into trouble. For a mirror finish I use 8000 grit, yellow, and I typically use Norton diamond, heavy concentration from MSC Supply. It's important to charge the lap and then wash it clean before presenting it to the work. I used to think I could just apply the diamond to the hardened surface and then wiggle the lap around over it and the grit would embed into the softer lap, which it will, however by the time it's charged, your lap won't be flat anymore as the diamonds rolling around under the lap eat vigorously at the softer entity. I've made a little handle with a ball bearing bolted to the end, I apply the diamond media to the lap, iron it in with the tool, then wash the lap with naphtha and a tooth brush, then I use mineral spirits as a lubricant on the workpiece. You can cautiously lap dry when your all done to really shine the surface but be advised, if the lap plugs it will scratch the finish and you'll have to start over. I usually don't get too crazy with eyewash and stop at 1200 grit, but this depends on the job.
Good luck.
Chris

Thanks very much for the detailed response!! Just the info I was looking for.
The project has been on hold for a few weeks as I can't keep reasonably constant temperatures in the shop right now.
As soon as we get something close to "normal" temps I'll get back to it.

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