How to tell if strait edge is strait?

[Tarhead]

The is quite a clever trick based on the principle of balancing three equals. Good to a thousandth with micrometers and two accurate gauge blocks or whatever your thinnest feeler gauge.

Thanks for sharing.

You don't need micrometers just ears to hear a click.

 

[tvrgeek]

With Mile's help, I verified my strait edges are in fact strait. Now I have something to work from that is known.

 

[Gofor]

You can make a Master Bar set with three pieces of MDF the length of your straightedge or jointer and a handful of sheetrock screws. More info here:

https://www.finewoodworking.com/membership/pdf/22944/011142038.pdf

The 3-piece comparison method is how I make my winding sticks. It is also the method I use to choose a level (i.e 4' or 6'), because I use them also as straight edges. By comparing all the levels in the rack, you can usually find a couple that are within a few thousandths of straight (almost all are slightly concave in the middle, but close enough to to check the edges of panels for a glue up, like for a table, etc.) This is a bit of a gamble, though. The last level I bought at Lowe's took three trips to the store before I could find enough in the rack that came close to straight so that I could make a choice.

 

[tvrgeek]

You can make a Master Bar set with three pieces of MDF the length of your straightedge or jointer and a handful of sheetrock screws. More info here:

https://www.finewoodworking.com/membership/pdf/22944/011142038.pdf

None of my levels are strait. None of my yard sticks. A cheap H-F 4 foot edge guide clamp is actually pretty good. Anyway, the 3-point method only works if you know your tables are flat. Few are. Only with a strait edge can you properly evaluate the tool.

 

[bob vaughan]

I discovered my issue with my jointer is in fact, outfeed table slopes up a bit. Makes sense with the struggles I have had.

Now that the straightedge issue is mostly solved, keep us posted on what you did to get a perfectly straight edge on a piece of lumber.

 

[tvrgeek]

Now that the straightedge issue is mostly solved, keep us posted on what you did to get a perfectly straight edge on a piece of lumber.

Easy. Knives ( Lux head) sit about .001 higher or slightly less than outfeed. "Compression" distance.
I got pretty lucky in that snugging the gibs got me in line. No beer can shims needed. needed.

 

[ssmith]

@tvrgeek & @Mike Davis – thanks for the advice.

Tried to use cut up feeler gage stock but it was too thick (thinnest I had was 0.006). Picked up some brass shim stock from Lee Valley and shimmed the upper dovetails with two 0.003” pieces. Carefully adjusted the outfeed table height and now, two joined boards 5’ long meet with <0.010 gap at the ends. Quite happy with that.

 

[tvrgeek]