Perfect planing

[Andrew DeMarco]

Hey,

I'm planing down the top plate of a flat top instrument and I am just nearing the correct thickness. However, I don't know how the heck to make it perfectly flat. Any ideas? Just plane and caliper and plane and caliper?

[sunburst]

Straight edge lengthwise, straight edge crosswise, straight edge diagonally, calipers, sight along the surface, side light in a dark room, winding sticks, scrapers, any tool that helps.
If you're real close, it might be time to switch to a sanding block.

[joec]

Straight edges and proper lighting are key...and here's a cool low-tech but very accurate planing aid: Plate glass, dimensioned for the stock you use often, frosted on one side.

Chalk the frosted glass liberally and then place your stock on it and rub gently. Chalk marks the high spots and you plane 'em away. Keep in mind that this really works best with a sharp plane which is well tuned. For a large flat surface, it helps to have a little radius to the business end of the iron so the corners don't dig, and a flat sole. This is also a great way to flatten the eccentric shape of a rib-structure or sides of a neck blank with a plane. Tapping the stock also helps you see and hear exactly where you've got a rock or twist. Combined with straightedges and a good eye, you'll find any little hump, bump or wind. I prepare lots of stock this way, flattening and squaring, before ripping on the bandsaw. You'll also feel some suction developing as the stock is gradually flattened.

The main thing is to actually target your cuts to the exact high spots, rather than just planing across the entire piece. Sometimes I mark with chalk on the glass, then cross check with a straight edge and mark with a pencil to really make a conscious cut.

If you're having trouble making the cuts land where you think you're cutting, stop and sharpen your plane iron, and re-set for a very light cut. Remember, the depth of cut depends both on the set of the iron, and, if it is sharp, downward pressure on the plane. It can also be helpful to have several planes going at once, with different depths of cut, or one set with a toothed blade when working maple.

For me, planing is an incredibly satisfying way to flatten and prep stock, with a miniumum of dust, leaving beautiful glue ready surface.

Joe

[alt_2ooning]

Check for any wind as John instructed. This can be done by the following .

1. Set up a 3/4 inch offset at each corner of the rectangular plate. Use a common stock 3/4 inch thick block by an inch or so square.

2. Stick each block with double sided tape to each corner of the plate.

3. Run (tack it to the top of the block) a light string/thread DIAGONALLY from the top of each block to it’s corresponding diagonal mate.

4. Where the two string lines cross one another #on a perfectly flat plain then the two strings will JUST TOUCH one another.

5. If there is a gap then apply the logic of how to correct the gap. If there is 1/4 inch gap then the TOP string must be LOWERED a 1/4 inch at EACH end ….therefore shave off a quarter of an inch at those points on condition there is enough thickness remaining in the plate for the necessary thickness.

6. Reposition the 4 offset blocks and run the string over the blocks once more.

7. When the strings just touch then the corners (where the blocks were positioned ) are now the established benchmarks.

8. Use a #STRAIGHT EDGE and place it on ANY #two of #the 3/4 inch #blocks (@ established benchmarks) and gauge the depth from the underside of the straight edge to the top of the plate ….hopefully the readings indicate material to be removed.

9. If #there is a hollow with readings greater than 3/4 inch #then you will have to re-adjust #ONE or MORE of the corner points to lower them in order to negate the hollow ...... then repeat the drill......remember, the necessary plate thickness at the perimeter will govern how much you alter the four corner points.

10.Using double sided tape then stick the #sandpaper to the milled edge of a shooting board or a 30 inch hand level in order to #correct any minor flaws.

Mike

[dunbarhamlin]

Note that this is for flat top plates already approaching final thickness (probably around 0.120" top, 0.080" back?) so wind isn't really going to be measurable with sticks - they'll just read the bench. Similarly, unless the bench is flat, you'll have difficulty getting the plate flat as it will conform to the bench under the weight of the plane.

So I'd say firstly, make sure the bench surface you're planing on top of is flat by using your straight edge/winding sticks. If it isn't, either resurface or try using a piece of mdf on top of the bench. If the mdf in situ reads out of flat, shim underneath with paper to fix it.

Then (if you have a choice of planes, I'd go long - a 5 1/2 or even 7 (but be particularly careful of 'snipe' at the edges with a heavier plane) and) use a very lightly cambered or flat-with-softened-corners blade to take a set of extra fine (~ 0.001") full length shavings (use freshly sharpened blade, plenty of pressure and a tight mouth to avoid tearout.)

This'll give you a smooth and even surface, so now spot check your thickness, marking any thick areas in pencil/chalk and take stop shavings over just these areas.

Then a single set of full length, and test again.

If backs are being a beast with tearout, and rehoning the blade doesn't help, switch to a scraper but don't concentrate in one spot - it'll give you a hollow real quick.

Any slight warp/wind isn't going to be a problem with stuff this thin - it'll be just fine when glued to the rim.

Cheers
Steve