Blocking a log cabin block
 

Have finished my log cabin blocks getting ready to sew them together. How do I l
Block each square, they are supposed to come out as 15 inch squares.

I strip pieced the one and only log cabin I made. I then pressed w/steam; let dry thoroughly and trimmed to the correct size. Probably not the 'correct' way but it's what I did and it worked for me.

Log cabin blocks are one of those that look deceptively simple. I mean it's just strips, right? I use the method where you cut your logs to size before you begin. This way you know if you're off as you add each one. I also press and square as I go.
Sounds like your blocks are all done and they aren't all the expected 15" finish. At this point, the only thing you can do is find the smallest one and cut them all down to that size. If there are just a couple that are a lot smaller than the rest, make new ones or unpick and re sew those. The problem with waiting until the end is if you have to trim, some of your outside logs will be narrower than the rest, however, hopefully not enough to be too noticeable.
Another tip for next time is: if possible, cut your strips along the length-wise grain of fabric. It stretches a lot less. Easy to do if you are starting with yardage, if you are using scraps or precuts, not so much...

I cheated. I cut my outside strip an 1/8" wider, then trim them all down to size. Fortunately, my seams did not have to match up in the pattern layout.

may i ask which layout that is?

I cut accurately, and make sure I sew accurately. When I finish and I'm pressing, I spray w/ spray starch and press. The block is wet because of the spray starch and i can straighten anything out that might need it.

I too make the outside logs larger than the inside ones. I make them about 1/2 in larger and cut them down to size. Better too big than too small.

The main problem with the regular log cabin block (as opposed to court hose steps log cabin, for instance) is that the block is pieced in concentric circles -- you keep going around and around the block in the same direction, clockwise or counter-clockwise, with each strip (log) you add, whether you've precut or not. Anyone who has done needlepoint knows this can easily make the finished block/piece skew on the diagonal.

When I teach the log cabin, we make it with long strips, not precuts, and add a 'round" - or 4 logs - then square up using the 8" Bias square Ruler from that Patchwork Place.
In squaring, we make as many blocks as we need, stopping each after the first "round" has been sewn(or you could think of this as after 1 log has been placed on all 4 "walls" of the cabin).
Then we square up all at one time, using the smallest block as the guide for all of them.
We shave off the tiniest amount of fabric to make each block perfectly square.
Then we add the second "round" of 4 logs, and square up again.
This seems time consuming at first, but it saves time at the end, when setting the blocks all together, because they ALL match perfectly and piecing the top into rows goes very quickly.

Jan in VA

Thanks so much for this information. I have a log cabin in my future someday and it would make me crazy if the finished blocks didn't match up halfway close.

I "cheat". As long as I don't want a finished block larger than 7", I paper piece.