Lapping Machines - What Kind?

We make knives at our shop. I want to make the sides of the blades very polished and flat. Beyond what hand buffing can do.

If I was going to lap these blades in production what kind of a lapping machine would you suggest? I know exactly zero about lapping machines, have only saw them once in a shop I visited that made a sort of die or mold that CDs were made on.

I am looking at the ones on EBay and see Lapmaster and Speedfams.

Blades will be about 8” long

Thanks everyone.

Lapping may not be what you need, as you're dealing with a non-flat surface if you're working on a fully formed blade. I'd check into what 3M offers for large Scotch-Brite pads or buffers, and see if you can set up a couple stages of machines to run the blades through to progressively smooth/polish them up.

You'll have to take into account edge rollover and if passing the blade between two wheels, being sure they're counter rotating from spine to edge so you can't grab the blade.

You should be able to get a mirror finish with proper buffing technique.

Most lapping machines probably won't work well with a typical knive grind. They'll only work on a planar or cylindrical surface. You'd have to fixture the blades to present the ground surface to the lapping medium, and then figure out a way to get the fixture and the part of the blade with rectangular cross section out of the way.

Now, if you meant "polish" rather than "lap", there are other options. But I'll assume you meant what you said.

Still, you should be able to get a mirror finish with the right buffing wheels and compounds...

I’m only looking to lap the flat part of the blade, not the bevel.

I can get a polish by buffing but its not the look I am going for, I want something like a perfectly flat mirror.

Surface grinder? You can get a mirror finish with abrasives and buffing wheels. Are you asking because you don't know how to get there or what?

azmachining said:

Surface grinder? You can get a mirror finish with abrasives and buffing wheels. Are you asking because you don't know how to get there or what?

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I’m looking for a Jo Block type finish. Surface grinder wont do it.

I’m asking if anyone has experience with lapping machines and if they can comment.

If you're really only after the flat part of the blade, then a lapping machine may work for you. You may still have some issues with the blades tipping, but this can probably be overcome using a work carrier that places a small amount of weight on the flat part of the blade on the side opposite the lap medium.

Next question would involve blade size, and that could have an impact on whether you go with a rotary machine or a reciprocating machine. Or at the very least, the size of a rotary machine. The Engis website says their benchtop (285lb) machine will handle work up to 5.5" (based on the conditioning ring ID, which is reasonable but not the absolute limit). The next size up is a machine that weighs a ton and handles work just under 7.5". If you have a 9" blade (not unreasonable with tang), you'd need one size larger yet, with a 24" lapping platter to handle work up to 9.75". One of these, used, is currently on eBay for $24,500. You'd be quite lucky to score one for $7,500 in usable condition, I suspect.

As with many specialized processes involving dedicated equipment, you may want to think about farming this out. Operating a lapping machine properly is not a matter of turning it on and feeding it more parts.

I had 2 Lapmasters in the department that I ran. Lapping is a very slow process. We had very precise flatness specifications for some parts and those parts would spend hours on one machine and then go to the next.

IMO, don't buy a used machine. You're going to need specialized fixturing for knives. Get in touch with Lapmaster and have them do some samples for you. Make sure they give you a time for process. Just having a machine won't be enough, the fixturing will be more than the machine.
JR

I use a small Lapmaster and everything said above is correct. If you have one machine, changing grits is a pain because you have to clean up the coarser grit to prevent scratches. The process is slow, but will deliver wavelength of light flatness. It will also deliver spherical surfaces if you don't keep constant tabs on the lapping plate flatness, and correct the ring positions to compensate. Supplies can get expensive. Fixturing is critical. Parallelism between faces doesn't happen automatically unless you fixture for that as well. Because the part has to fit on one side of the lapping plate, the size of the machine goes up very quickly with part size. The rings take up space too. For an 8" knife I'd want maybe a 36" machine so I could do more than 3 at a time. IMO, wrong tool for the job. I'd surface grind coarse, followed by a specialized wheel for finish, followed by buffing. That's a guess, but based on somewhat more than zero experience with both processes.

Conrad Hoffman said:

The rings take up space too. For an 8" knife I'd want maybe a 36" machine so I could do more than 3 at a time. IMO, wrong tool for the job.

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The way that I read, he only wants the angled side of the blade done. That would mean no rings because you can't put the whole blade on. Fixture with an orbitor.

IMO, there are better ways. Lots of pump vanes were ground flat with a 4 finish using double disc grinders.
JR

We have the larger of the two benchtop size Lapmaster lapping machines where I work. The work rings are somewhere around 7" ID. Machine was somewhere around 10k new. With a gage we made (similar to the one Lapmaster sells, but more accurate and way cheaper) I find that keeping flatness to within a few microns is pretty easy.
The abrasive we use very much does not produce a mirror finish, but being very flat it is easy to polish if needed.
I agree that grinding with the right wheel the a fast polish would easily be an order of magnitude faster, and likely cheaper to operate.

jccaclimber said:

I find that keeping flatness to within a few microns is pretty easy.

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Man, you can just about cut it with an axe and still hit microns.
Most of our stuff was 2 light bands or less. That's 23 millionths. Or as we used to say "Finer than the hair on a knat's ass."
JR

JRIowa said:

Man, you can just about cut it with an axe and still hit microns.
Most of our stuff was 2 light bands or less. That's 23 millionths. Or as we used to say "Finer than the hair on a knat's ass."
JR

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That's still doable, although measuring is more work. I'm fortunate that my requirements are looser than what we keep our lapper at. It works as a "throw it on and come back in a while" solution for varying parts that are hard to hang on to, nonferrous, and need just a few microns taken off in their free state. Fixturing aside we also aren't set up to grind aluminum or carbide.
You probably had more stringent needs than I do, and this guy is talking about knives, not lenses.

Edit, it is about as easy as cutting it with an axe. Same sort of enjoyable as when I get a short shaft that only needs ±0.2 mm on diameter, almost feels like cheating.

jccaclimber said:

You probably had more stringent needs than I do,

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Just did valves. It wasn't rocket surgery
JR