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Thread: Building A Lapping Machine
11-16-2008, 09:10 PM #1
Building A Lapping Machine
If this seems simplistic to the sophisticated machinists/toolbuilders, please don't laugh.
I have been slowly working towards getting my lathe cleaned up, still have some things to do, but while waiting on parts, I've thought of a couple things I would like to do.
After reading some on grinding tool-bits in the SB forum, and one fellows response on making a lap, I thought I would do the same.
I immediately thought of a simple motor with a pulley train to bring the rpm's down to say, 60rpm? But then I thought of a gearmotor. So now I think I will buy one of a suitable rpm, and mount a disc cut from aluminum round stock, say 8 inch diameter, face it nicely, and then turn a depression and lip into it to hold the diamond slurry. I think it should look like this:
This way the slurry won't get flipped off the outside edge, and to get some fresh compound on the tool, just carefully direct the tip into the slurry held in the lip.
My idea is to mount the gearmotor in/on a safe, attractive base with the disc's being mounted directly to the shaft. If shaft mounting can't work, then a small v-belt to a shaft on pillow-blocks. I could make a plate that would be perpendicular to the disc face, and use a holder to get the correct angles on the toolbit. Then is would be a simple matter of roughing the angles on one side of a grinder, move to the side with the fine stone, then over to the lap to get a really nice sharp edge. I could have a few lap discs made up to hold varying grit sizes, and store them I would make glued together boxes out of soft pine. Each lap would have it's own box to keep things organized and not contaminate a finer lap with coarse particles.
Your ideas, thoughts and any suggestions appreciated. I have some questions:
1. Which metal would work best? Cast iron, steel or aluminum (6061, 7075).
2. What RPM should I look at? I want it to be able to hold the slurry without centrifugal force carrying the slurry off to the outside lip too quickly, but not so slow I would be taking up time.
Has anyone else done this? I did a search on "lapping" and didn't see anything about "personal lapping machines".
11-16-2008, 09:23 PM #2
Never seen a lap with a lip like that- I think it would be a PIA more often than not. If you want to make a lap for diamond paste go with CI. The paste needs to be pressed in, with something like a ball bearing mounted to a handle. Keep it wet with some light oil while lapping (you'll have a nice black line on anything in the wheel's line of sight). 60 rpm sound really low to me, unless this is a 30" lap .
11-16-2008, 09:37 PM #3
I've used optical and lapidary lapping machines. Not an authority, but some experience. Diamond lapping paste isn't a slurry. More like toothpaste. Comes in a syringe. You don't need to worry about flinging it off. There's only a tiny amount embedded into the surface of the lap. The lapping pads are generally some polymer material laying on top of the hard flat lapping disk. You just change the pads, not the disk. Machines that use slurry have a continuous stream, like flood coolant. They collect the slurry in a sump and recirculate it. They're generally big machines and have an oscillating arm to move the part back & forth or move and tilt for polishing lenses.
Look at a site that sells lapidary equipment. You can build the machine but you can buy the lapping pads and other expendables. I have a lapping/polishing machine made by "Hi-Tech". Slowest speed is around 600 RPM. The pads look like sanding disks but without any grit. I also have a bonded diamond lap that has the diamond bonded to a metal disk. Very handy. I use it all the time for sharpening tool bits. You just do a water drip.
11-16-2008, 10:12 PM #4
As others have alluded to, for a face lap, you want to roll the diamond bort in pretty well before turning it on. Though once most of the paste is worked into the face, you can turn it on and run the roller (cheap ball bearing in a holder) across under power a few passes.
There's some info on this site about making powered laps for carbide tipped handscrapers. I use shop made handlaps, myself, since it is easier when into a machine than going across the shop and back for a touch up.
However, the powered laps as promoted by Forrest Addy, and which I have used, utilize a thick CI face about 6" OD, bored & keyed to fit right on the shaft of a 1750rpm motor with a SS to fix it. But again, I use diamond wheels for shaping and sharpening, and hand laps for finishing & touch up. They are stamped on the back with "O", "R", or "G" for orange, green or red so they don't get mixed up.
Aluminum will work. Anything the diamond will press into and stick or "charge" while presenting a fairly uniform surface. I use soft steel and copper with the working face surface ground, for handlaps. You might want to experiment and report back?
11-16-2008, 10:32 PM #5
Two things: How big to you want? What kind of work pieces? Gonna use it a lot? A little?
If it's a scraper and tool lap, there's no need to get fancy. I made up a quick and dirty fixed table angle powered lap for sharpening scrapers for my scraping classes. It's cheap to build and works pretty well but there are some superior designs on the market. There's a few pictures of my powered lap floating around; maybe some kind soul will re-post one.
As was said, the diamond lapping compound is like tooth paste - even stickier. so you don't need a redustion from motor speed. Once spread it's out in a thin coat it stays put so it can be rolled in. It won't sling off unless you lay it on like apple butter so you don't neet the lip. Eventually, you may score the lap so make it easy to face if off when it's off the motor. If the lap is flat straight across, you can abrasive sheet lap it after machining - the feed marks can telegraph in the tool if your cross-feed of the tool being lapped happens to match it. The feed marks don't have to be completelly removed. 50% clean up is plenty.
There's some dandy factory made face laps for small work also; the Gledo comes to mind and theres a few with built-in lap oscillation.
11-16-2008, 10:32 PM #6
Okay, I'll do some searches using powered lap and additional search words.
I was going to make this primarily for lathe bits. As for use frequency, geez, I'll have to wait 'till I get everything up and running. But the replies have helped greatly.
11-17-2008, 01:30 AM #7
OK lathe tooling. A 5" or 6" motorized lap will do you fine. If you're gonna do lathe tools an adjustable angle will be a good feature.
Generally a diamond lapped finish is a bit overkill for GP HSS lathe tooling. A fine India stone is all I use for edge refinement and you can use it while to tool is still in the machine. Diamond lapped edges may be advantageous for some materials at low cutting speeds.
Diamond lapped carbide is not the best thing either. Dead keen cutting edges on carbide tooling actually work against you. The dead shap edge crumbles when running at usual carbide speeds. It doesn't fare much better when carbide form tools are run at HSS speeds either. There's a rule of thumb where the corner radius of a carbide tool is about 1/8 the feed rate.
So dead keen edges for machine tooling may not be what you need. I know it's a little counter intuitive.
All that said, a diamond lap is a convenient way to tune up a marginal carbide cutting edge but if you go through what ever coatiing is present, you may wind up with a restored tool that has a short life.
My suggestion is to hold off building the lap until you get into scraping and actually need it. Instead focus on a good bench grinder and some dressing equipment to keep the wheels round, straight across, and in a good free-cutting condition.
11-17-2008, 03:11 AM #8
Thank you sir for this info. I will definitely put a decent grinder and a selection of wheels with the dressing equipment on a short list.
11-19-2008, 01:12 AM #9
2. I don't know what you will be lapping or what grit you you will normaly use but keep in mind that with a coarse grit (60-240) you will be able to spin it very fast. SiC/Alox and water will stick on well and what ever spins off you can pick up and reuse. Use a removeable guard so you can lap big stuff like small engine heads.
300 RPM is OK. BELIEVE IT. 60 RPM is really too slow but may be OK for copper/diamond.
3. Aluminium stinks as a lap but you can use it as a backer plate for cast iron.
4. One cast iron plate will do for SiC/Alox. A cup of water and a brush will clean off the coarse stuff well enough. You may need a few laps for diamond though. They work differently. Diamond w/ soft(copper). SiC/Alox w/ cast iron.
5. 12 inch lap 4 inch center hole.
6. You may need a wide range of speeds since you are experimenting.
7. Mount 4-5 working laps side by side on the base,don't store them and switch. Driving them is easy and will save you a lot of time even if you only use them occasionaly.