I have to confess that I am not a craftsman. So I don't do things beautifully. Please forgive me for my unskillful movements.
We sometimes have to perform “shinai adjustment”.
What I mean by "shinai adjustment" is to adjust the length of a shinai, not the weight of a shinai.
If you are an adult, you are using a shinai for adults so you should not worry about making your shinai shorter.
However, sometimes, young students cannot get an appropriate shinai for them because...
they do not know what kind of shinai they have to buy.
As a result, they buy a shinai that is inappropriate for their age.
I think this is the case most of the time.
I would like to show a table of the shinai length for different age groups.
Click the table to enlarge.
For elementary school students, there are no specific rules about shinai, so the length and the weight are reference only.
If you have students between 7 and 12 years old, this is how you should choose an appropriate shinai length for them.
That is a length for your young students.
I used to buy 38 or 39 when I was a junior high school student and make them shorter. So I could have some heavier shinais for training.
I listed a brief procedure for shortening a shinai. I have a video ready, too. Please refer to the video for more details.