With toggle clamps, the only way to measure tonnage is to measure the strain (stretch) of the tie bar.

Question:

How do you measure the tonnage on a toggle molding machine:

My Answer:

With toggle clamps, the only way to measure tonnage is to measure
the strain (stretch) of the tie bar. Using the strain measurement of the tie
bar, the force on the tie bar can be calculated if the tie bar material is
known. If that is multiplied by the number of tie bars, the tonnage is
determined.

Most newer molding machines use a strain gauge mounted on or in
the tie bar. The best measurement can be obtained using an ultrasonic
measurement through the tie bar to determine the exact amount of stretch in
each tie bar. This equipment is expensive, but there are many machine
evaluation consultants you can hire to do this.