How to Save Money When Buying square cast iron lapping plates

We had a discussion about this a while ago. Here in no particular order :

Link to Hengfeng

I purchased a kitchen top off cut. Aprox 450 by 320mm, 30mm thick. Was mirror polished and they cut it rectangular for me. 

As it was, chalk would not stick to it and it was out of flat by around 0.03mm mostly within 2" from the right edge. I prepared an Al lapping disk 180mm diameter , 20mm thick with a 40mm central hole. I lapped the plate with 120 grit carborundum in three stages for approximately 30-40  minutes. Meticulous cleaning between stages with Windex.  I used circular motions overlapping the edges to half lap and constantly paying attention to the drag of the lap. The fluid was water with a bit of dishwashing detergent. Only a small amount of carborundum needs to be sprinkled on the plate - less than a teaspoon. Plate was supported on 3 feet of hard foam at 1/6 L/W ( Airy points ).

The end result as measured by my plan-o-meter calibrated to my large lab grade plate was better than 9 micrometers whichever way with an incertitude of 1.6 um which is the diagonal error on my large plate ( 4by3'). A decidedly better result could be obtained with a heavy cast iron lap of the same size (!) and I believe a small brake disk of decent brand will work as well. As it can be seen in the picture attached the surface of the plate has become almost completely mat and uniformly so - chalk sticks to it beautifully. This particular plate has a very high quartz content. A flat gages floats on air when pushed over it. It is well within toolmaking tolerance and I suspect it'll take maybe an hour of polishing with 100 grit to bring it into lab grade territory.

The abrasive must be well graded not a mix of various grits - that makes a substantial difference. One does not want to polish it - one wants to see a MAT surface.  Total cost around $20.

2 hours ago, JacksonMaberry said:

What's wrong with glass? 

I expect the post was in repose to my asking how to get a cheap flat surface a couple weeks ago, and do appreciate the extra information!

The flat I am looking for would need to be large enough to sand/flatten the ribs on a cello before mounting the back or front. Some do and some don't do this, I am learning and following the "Davide Sora Online School of Violin Making" and he does
 

So the size is larger than if you are only using the flat to sharpen tools. I expect in the 30" x 40" range would be easiest - smaller may work. And glass is very good, I have some 12x48 sheets of 1/4 in plate and a couple 4x6 in blocks of 1/2 in that I found laying around in the shop I bought years ago. But, I expect I would need 1/2 in thick at the larger size, and that would run around $100 or more depending... As I am trying to buy a cello and tool up a shop to make cello's all on my Social Security, I try to always think about less expensive alternatives where they exist and spend money on the "best" when recommendations here say it is not a good idea to cheap out on something. Hence my asking about flats - my searching resulted in many options, but all seemed to be $100 to $300 and that is in my pain threshold to at least see if there is an alternative people are happy with before buying the best.

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Frank

23 minutes ago, FrankNichols said:

I expect the post was in repose to my asking how to get a cheap flat surface a couple weeks ago, and do appreciate the extra information!

The flat I am looking for would need to be large enough to sand/flatten the ribs on a cello before mounting the back or front. Some do and some don't do this, I am learning and following the "Davide Sora Online School of Violin Making" and he does
 

So the size is larger than if you are only using the flat to sharpen tools. I expect in the 30" x 40" range would be easiest - smaller may work. And glass is very good, I have some 12x48 sheets of 1/4 in plate and a couple 4x6 in blocks of 1/2 in that I found laying around in the shop I bought years ago. But, I expect I would need 1/2 in thick at the larger size, and that would run around $100 or more depending... As I am trying to buy a cello and tool up a shop to make cello's all on my Social Security, I try to always think about less expensive alternatives where they exist and spend money on the "best" when recommendations here say it is not a good idea to cheap out on something. Hence my asking about flats - my searching resulted in many options, but all seemed to be $100 to $300 and that is in my pain threshold to at least see if there is an alternative people are happy with before buying the best.

Frank

Frank, glass will not do. At 30by40" you'll need around 1 1/2" for any resemblance of stiffness. I think (?) that commercially glass stops around 19mm and it's laminated past that. I worked a bit  with float glass to see if a decent mirror can be made from it. In that case it wasn't worth the trouble. What you really need is a plate of whatever on which you can test a piece of wood for flatness ( I test fingerboards ) by seeing if it picks up chalk. The longest it needs to be is 150% the glued length of a cello fingerboard. And that might flatten a violin, too. You chalk a small area of the plate, lay the fingerboard on it and gently push it ( not downwards !) over the small chalked area - where it picks up you plane or scrape off. You keep doing this until you are ready to do a final flattening with a plane in a couple of very light strokes. A second use for a flat plate is to check the flatness of planes and here the plate needs to be only 2/3 of your longest plane. You dab a THIN layer of pre-dried engineers blue on the plate, spread it uniformly with your fingers , place the plane on top and give it a 2mm push under it's own weight. The high areas of the plane will pick up the blue and can be scraped away, or filed or abraded etc until is good enough. A third use for a flat plate is to determine the center of plates on an assembled instrument - that's involved. And of course, you can use a flat plate to check your straight edges and your squares - you'll never need that in violin making. As an aside, checking flatness is easy, checking squareness is a bloody difficult.

You'll NEVER need a "flat plate" to flatten ribs - that's a nonsense. The reason is that the pressure you apply on the instrument to do the flattening far exceeds it's stiffness - in oder words, you'll only distort it and LOCK it into a distorted state once you glue the top. The back and ribs become very stiff once the top has been glued on - this will take care of a light twist. A well sharpened plane set for a thin cut will eat endblock grain like butter and keeping it square to the ribs is easy even for a moron such as myself. Some people are concerned that the gluing surface of the rib edge/lining might not be dead square to the plane of the ribs - sanding will ensure that. But application of brain power tells us that unless you hold your plane particularly crooked, the difference is minute - maybe 1/10 of a mm. Again, the clamps will distort the assembly way more than that. In "fine mechanical arts" precision is always obtained by working on small areas which are compared amongst themselves - not by doing everything at once.

I suggest you make yourself a small 300 by 400mm granite plate as I described. If you work carefully and with low pressure you'll have a bonus : the lap will become flat too and aprox 2-4 times better than the granite plate. It can be used ( particularly if it's cast iron ) to check small planes , or the back of plane irons or chisels.

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