Lapping a cast iron surface

[vulrath]

I wasn't sure where to put this but I figured this would probably be the closest place.

I'm working on a project that requires a I somehow level a cast iron surface (table saw top is way out of flat - it dips in the middle). I wondered if lapping would suffice - with the amount I need to remove from the edges this would probably (at least in the beginning) I thought it might be a simple way to start. Now, I know cast iron is frequently used as a lapping surface, so if this is a good way to proceed I wondered what the best material to lap with would be.

Or should I just go about scraping it and be done with it? For reference, I don't have access to much in the way of machine tools, and I don't think taking an angle grinder to the table is an appropriate way forward.

Thanks in advance for any tips/information you can give me.

 

[benmychree]

I would advise scraping, lapping usually uses a lap material softer than what is being lapped, whatever that may be.

 

[vulrath]

Thanks, the more I thought about it, the further that way I lean.

 

[woodchucker]

I wasn't sure where to put this but I figured this would probably be the closest place.

I'm working on a project that requires a I somehow level a cast iron surface (table saw top is way out of flat - it dips in the middle). I wondered if lapping would suffice - with the amount I need to remove from the edges this would probably (at least in the beginning) I thought it might be a simple way to start. Now, I know cast iron is frequently used as a lapping surface, so if this is a good way to proceed I wondered what the best material to lap with would be.

Or should I just go about scraping it and be done with it? For reference, I don't have access to much in the way of machine tools, and I don't think taking an angle grinder to the table is an appropriate way forward.

Thanks in advance for any tips/information you can give me.

Lapping is a slow process.
I did my top with a random orbital sander.. I don't recommend a good sander, it will get destroyed, either that or use solid withOUT holes to avoid destroying your sander.
Start out with 80 where it is high and work toward finer. Or use an angle grinder, but test frequently. If you have a belt sander you might try that.

Just realize the casting may have voids in it, and you may open a good spot, and find a void underneath.
How far is your table out by? For me it was to remove the Delta milling marks. I don't think they blanchard ground it.
I wanted a smooth top.

 

[FOMOGO]

Are you using it as a table saw or for something that requires more accuracy? If you need to remove a substantial amount of material it might be easier to mill it flat and sand it smooth for a final finish. Mike

 

[vulrath]

Lapping is a slow process.
I did my top with a random orbital sander.. I don't recommend a good sander, it will get destroyed, either that or use solid withOUT holes to avoid destroying your sander.
Start out with 80 where it is high and work toward finer. Or use an angle grinder, but test frequently. If you have a belt sander you might try that.

Just realize the casting may have voids in it, and you may open a good spot, and find a void underneath.
How far is your table out by? For me it was to remove the Delta milling marks. I don't think they blanchard ground it.
I wanted a smooth top.

I'm not sure how far out it is right this second, but I can say it's out enough that I suspect it's messing with the cut. It's been...painful...to get the blade dialed in. I too have a Delta saw (36-725).

Are you using it as a table saw or for something that requires more accuracy? If you need to remove a substantial amount of material it might be easier to mill it flat and sand it smooth for a final finish. Mike

I'd like to use it for both a saw and as a reference surface. I looked into getting it milled and/or ground, and it would cost almost as much in labor as the saw itself. It'd be one thing if I had the machines to do it myself, but I don't.

ETA: When I say reference surface I'm talking about woodworking, not metalworking. I'm not building rockets here, just furniture and some occasional weld art.

 

[matthewsx]

Check around for local shops with a Blanchard grinder. If you can get a price then you can compare what your time is worth vs. money to have it done.

If it turns out to be more than the saw you really want you have your answer....

John

 

[woodchucker]

That's a new or relatively new saw, mine is much older. Some people get too wrapped up that it must be dead flat. It should be flat, but if it's dished even up to 20thou, I would not worry. But if you want it less, get ready for some dirty grinding/sanding. It is definitely a power job. I had a Record jointer that I bought new a few years before Record went out of business. I spent weeks hand sanding it on a glass plate with 80 grit sandpaper both self stick and cloth backed AL oxide. I would never do that again. Fortunately I don't have to. Good luck, slow, check often, less pressure. Too many think more pressure is mo better, it's not.

 

[Dabbler]

I just want to put this out there. Lapping is used for very small changes, such as a small part of a thousandth of an inch. For small surfaces, you can lap a couple of thou, tops.

First you need to measure how far you mist go. If you are removing .010 is a lot different than adjusting .003... If you are out .050, then you have to start in a completely different place. Feeler gauges and a straight edge will do.

Second for you to measure you need a flat reference surface, such a s a cammelback straight edge - something with at least 1-2" wide. This will also help you in subsequent operations. If you have a mill or get someone to help you, for a table saw, taking a 2X2" piece of steel, milled flat on one side, the front-to-back size of your saw will get you close enough without spending hundreds of $$$ on a precision straight edge.

For coarse adjustments, you can use a mechanized orbital sander with silicon dioxide abrasive. You will find the dust and carbides in white cast iron will dull and clog regular AO paper very quickly. For minor adjustments to get close, sand by hand using wet/dry paper with varsol as a lubricant to remove the waste. Use a steel backing, such as a 1/2" thick, 3X3 sanding block. Go slowly and don't over do it.

This can easily get you under .010 flat. To get closer you have to scrape, and use a precision straight edge with prussian blue or Canode.

 

[C-Bag]

+1 on what Dabbler said. The very first absolute necessity is a truly flat bar or camelback and some feeler gauges. My first go to was a SnapOn straight edge that I used to test engine heads with. My next was a 24x18 granite plate and soon after I bumbled into a 36” B&S camelback. I use all 3 when I’m trying to get something flat to crosscheck as it’s way easy to make things worse. I also bumbled into a 12” round Shamrock cast iron lapping plate on eBay that is perfect for the final finish on precision things. Like all before have said all of this is mucho manual labor but by far lapping is the most tedious.