I conducted some tests using similar setup conditions
A piece of softwood and a piece of hardwod was used,
The gap between the auger and Hollow Chisel (HC) was 2.5mm in each test
A test instrument for automotive testing was used to measure temperature with a K type thermocouple
A luggage scale and chain was used for the force tests.
The test results are approximate only and are subject to human variations but I did my best to replicate identical conditions as much as possible.
So here we go.
Test 1 is with an unlapped and unpolished 3/8 HC
Test 2 is with a lapped and polished 3/8 HC
I used my phone to take the pictures as I ran the tests so they are all raw data shots so please bear with the blurring and framing.
Test 1 softwood
Two mortises were cut
The effort in kgs to do this varied from 9.5 kgs to 12 kgs
The temperature at the tip of the HC at completion was about 67deg C
Test 2 Softwood same material
Two mortices were cut
The effort in kgs to do this varied from 9.2 to 9.7 kgs
The temperature at the tip of the HC at completion was about 50deg C
Test 1 Hardwood
Only one mortise was completly cut due to the HC blocking with chips, To remove the chisel and reset everything was not considered worthwile
The effort in kgs to do this varied from 14.2 to 16 kgs
The temperature at the tip of the HC at completion of only one complete mortice was about 70.4 deg C
Test 2 Hardwood same material
Two mortises were cut
The effort in kgs to do this varied from 15.8 kgs to 17.4 kgs
The temperature at the tip of the HC at completion was about 59 deg C
The effort to cut the mortices appeared to be less (by feel alone) however the results for hardwood show differently.
The effort to withdraw the HC was certainly improved to the extent no jambing in both materials at all occured after lapping the sides.
Apperance of the finished mortise.
As the mortices were individual cuts only this was not assessed.
Temperature: There is a measurable reduction in temperature after lapping.
Was it all worthwile? I would say for somebody who does mortises on a regular basis most certainly.
For those of us which the use is less fequent I would also say yes, conditionally on how much pride you take in your work and maintenance of your tools.
I should try and get Mathias to rig up some proper test rigs to accurately validate the work, he would possibly get a real kick out of it too!
-- Regards Robert