Harris Lapping Plates

Re: Harris Lapping Plates

David Barnett

>"Would diamond at 35 and 60 microns work? The coarse plate has a diamond grid cut into it to carry away the swarf. The manufacturer recommends the sillicon carbide only for this."

Personal opinion and speculation follows:

You could, but for tool steel and wide surface lapping (plane irons, etc.), I wouldn't recommend diamond in micron sizes equivalent to SiC 325 and 600. Diamond's far more aggressive initially but has 'perfect cleavage', giving it a different friability profile than the 'blocky' (Harris' term) SiC, which tends to a more linear friability and is an almost perfect match for the Harris Meehanite*. What this means is that you'd get deeper scratches after diamond charging but that it'd take longer to bring the lapped surface to the degree of homogeneity desired before moving to the next, finer grit, whereas with the SiC, you'd progress evenly and quickly from a roughly uneven tool steel surface to the point where you're ready to move on. The diamond lapping might work alright if you began at 600 grit, but might also require an intermediary step; a sort of prepolish (perhaps around 6 micron), which could make it slower in the long run.

Obviously, there's nothing wrong with diamond lapping/honing with the lower grits, and even lapping with lower grits is fine on commercial hones or homemade laps - for gravers I start at 260, then 600, 1200, and finish on ceramic rotating laps. But you asked about the coarse Harris lap, so I limited myself to that.

Of course, if you're asking because you wish to flatten and prep carbide rather than tool steel, that's a different matter altogether. But for tool steels, whether W, O, D, A, or HSS, the SiC will get you there quickly enough, which is probably why Harris uses it.

Also, with the larger micron sizes of diamond, you'd probably be better off with a copper-bearing alloy such as bronze, which has better 'grab' for those sizes*, which would give a more consistant scratch pattern, easing the front end of friability.

Anyway, that's my best guess.

David

*There's also the issue of how grit embeds into the laps, due to the even distribution of free graphite, acting as a lubricant, in the continuously-cast Meehanite.

Re: Harris Lapping Plates

Bill Tindall, E. TN

>I don't know what a Harris plate is so I can't comment specifically on your question, but we have used 60 micron diamond on surface ground cast iron plates with great success to flatten tool steel(specifically very hard CPM 3V, Rc61-62). Our sequence is 60, 30, 15, 9, 6, 3 as recommended by metal finishing tech people. the 60 micron will take out substantial grinder scratches. It takes less than a minute per grit once flat. We arbitrary do 20 strokes on a 12" long(retangular) plate and that is plenty. We have no prior experience with this operation so i have no idea how optimum we are. But it worked, and it worked faster and better than anything else we tried. 3 dozen plane and chisel backs were done in one evening. Washing between grits took more time that actual grinding.

I have had very poor success with coarser grit silicon carbide and tool steel and the abrasive manufactures have not recommended it either. In my hands it fractured to doodle dust quickly and quit cutting. Diamond on the other hand lasts a long time.

Re: Harris Lapping Plates

David Barnett

>Yeah, Bill. I use 60 micron (220 grit) on my 6" round cast-iron laps. Fast, fast, fast. But Harris laps are designed so one can change between 325 grit and 600 grit SiC without leaving any 45 micron scratches after a short changeover break-in, so I recommend not charging with 45 micron diamond if one also intends to later switch to 30 micron. I neglected to tell Patrick that if he wished to just use the the 30 micron diamond without switching, he'd probably be fine. However, switching back and forth on the same coarse surface with diamond could give some disappointing results (the smaller Harris set comes with one plate - one side coarse, the other smooth). Choosing the 45 or especially 60 micron would leave a too-large gap to the polishing pastes, in my opinion.

For around fifteen bucks, MSC sells 6"x1" cast iron rounds, easily flattened (file, grind/sand, then scrape), which make good laps (scored or otherwise). I use these (and other substrates) for the lower diamond grits when I don't need rotating diamond plates.

More Harris Plates Questions

Patrick Gibbons, Houston, TX

>What about the possibility of contamination with the Harris plates? Two plates are used on both sides with different grits. They are placed in one holder. Do the abrasives have a tendency to leave behind and contaminate the next plate? Also, can a Veritas Sharpening Jig be used with these without the roller picking up grit and contaminating the next face? The manufacturer suggests using a jig that rides off the plate.

thanks for additional info

Bill Tindall, E. TN

>Yes, I see. The coarse silicon carbide crushes fairly easily, so the coarse grits will become finer grits with use and make switch to next finer charging grit more practical. Not the case with sturdy diamonds.

We were paranoid about cleaning the tool between grit because I presume some 60 micron diamond on a 15 micron plate would spell doom! I am assuming that it is impossible to completely remove diamond once charged onto cast iron. Is this correct?

Tell me more about Harris plates....Are they an inexpensive means to get a flat piece of cast iron of rectangular format? Where available? (I need two more plates than I currently own)

Re: thanks for additional info

Patrick Gibbons, Houston, TX

>Woodworker's Supply had them on closeout for about $150 for the two plates, the holder, two grits of silicon carbide and two of diamond and the wooden case. They are 8"x3"x1/2" and are flat to .0005" for their entire length. I also had a 10% off coupon to add to that. I believe they're sold out now. In the past I have used diamond stones and scary sharp. I don't like scary sharp anymore because of the edges of blades dubbing from the wet/dry paper. I like diamond stones and wanted to upgrade from my polycrystalline stones and saw the Harris plates. They're flatter but have less convenience than conventional diamond plates. Once charged, they must be machined to remove the old grit. That's why I'm asking the questions now before I start. What I start with is what I stay with on these.

scary sharp

Tony Zaffuto

>I've never posted here before, or left my bio (save that for a rainy day), however, with regards to the "dubbing" of edges that occurs with wet/dry SC paper, I've found that a "spritz" of WD40 helps minimize it, and a product like "RemLube" (gun nuts will already have it) just about eliminates it.

Re: scary sharp

Fred Krow

>Tony,

Thanks for posting, I have some Rem Oil (WallyMart) and will try it on the glass/abrasive paper. The oil may float the steel above the paper surface and minimize tear out of the SiC particles.

Regards,

Fred Krow

WOW$$$$

Bill Tindall, E. TN

>That seems like a lot of bucks to meet the need. I paid $50 to have 3- 3" x 12" cast iron pieces of bar stock surface ground, both sides for a total of 6 good surfaces. Cast iron can oft be found at local scrap metal yard.

I like the 12" length, especially when hand holding. Once I get set with the bevel indexed against the plate, the long stroke is useful.

If you ride a jig on the plate, the jig is going to use up a lot of the 8" length(my complaint against water stones).

I have no doubt that what we are doing is not optimuim and might even be judged naive by someone who really knew about lapping, but it is cheap and successfully flattens and sharpens.

Our criteria for flatness is empirical. When a plane or chisel back is not flat after the coarser grits the finer grits don't abrade everywhere (the low spots). If we can progress through the grits and everything abrades uniformly I presume the surfaces with the diamonds are sufficiently flat. I doubt that this means they are optically flat, but then it probably doesn't matter. Anyway, I thought it useful to precisely define what we are doing in case someone with more expertise sees a serious flaw.