When and how often to trim?
 

I'm working on a Sunny Lanes quilt top (http://www.patspeth.com/Quilts/sunny-lanes.html), so I am making lots of 4-patch units and lots of half-square triangle units. They rarely finish at the same size.

Should I trim each of these small blocks before sewing them into the larger blocks (made up of 16 small ones)? Or can I wait and trim only the large blocks before sewing them together to make the top?

What are the pros and cons of each method?

Thank you in advance for your advice!

I'll leave that answer to the experts but I can't wait to see your finished quilt.

I guess it would depend how far "off" your 4 patches are. Just for the essence of time, I would just square up the 16 patch block, not the 4 patch. Technically you should not have to trim or square up at all. If your 4 patches are off a significant amount, I would suggest you slow down and really examine what is causing your problem---is it inaccurate cutting or inaccurate/inconsistent seam allowances? Once you get the problem corrected you should be off and running and not have to spend a lot of time trimming.

it looks like lots and lots of pieces. i would square up as often as i could. if each piece was off even 1/16th inch, each row could be off by 2 or more inches.

I can't think of any pro's for putting wrong-size units into a block. If you're doing your HST's with a trim-down method, you definitely need to trim each one before putting it into a block. If you're not using that method then you need to check each HST to be sure it is the right size and trim if necessary. (If it is too small, figure out what's going wrong and correct it.) Same with the 4-patches. If each unit is the right size before being added to the block you will have far fewer problems later on. With the 4-patches, if they are not coming out the right size, you probably need to be sure the pieces are cut correctly and look at your 1/4 inch seam.

I made a similar quilt, with lots of HST & 4 patch units. I trimmed the smaller units first. If you trim a 4 patch, you may need to cut just a sliver, which will not be noticed. If you wait until you join into the 16 patch, any imperfections will be magnified and you may need to trim a larger piece which will be noticed.

I trim each unit as I go if needed. As others have said, makes the end result far more accurate and less apparent if still off at the end.

I agree with Dunster's answer, although I don't always follow best practices on family quilts (trying to be better about that).

If you trim earlier, you will need to re-work your math across more steps unless every single time you're trimming it's because your block ended up larger than what it was supposed to be. The upside of that is that once you do the math, you're likely to end up with blocks that are proportional & square as long as they're pieced properly. The downside is that you are spending a lot of time between cutting every single block to the new target size & refiguring the sizes of the setting triangles & sashing.

If you trim later, you're likely to have crooked blocks because some will be bigger than others (if I understand right. If they're all the same too-large size, the only issue will be that the outer blocks in the set of 16 will be smaller than the inner blocks once you trim a 16-patch). The math will be easier to calculate since there are fewer steps to the end, but if there's a problem with cutting/piecing that is causing the blocks to not come out a consistent size, that will be compounded by piecing together pieces that are not the right size to start with.

I'm with QuiltnNan, I trim often between each step. For me it's the process more than the product. But then, I don't get bothered if a block ends up an odd size like 8 1/8 instead of 8 1/2. As long as they are all the same size, it works. But having said that, there are some blocks that can't be cut down, you loose your 1/4 inch seam. So I avoid those blocks.

I recommend checking the size often, trimming each unit if necessary and then moving on to the next unit. I feel sad when someone writes of problems because of poor cutting, inaccurate seam allowances and poor pressing (frequently only finger pressing). Then, they are disappointed with the outcome and have another expensive UFO. Great results come from careful work along the way. Enjoy the whole process.