My Dad was asking me what they are for and I said, well, ah, they compare the toolbit height to the desired finished plane height from the table, set via surface gage or height gage, etc.
While I think this is along the right track, I never really had instruction in proper useage of this gage.
And I have no idea about what the cylinder does......
You're perfectly correct,as the name implies, although the planer gage can be used for many other purposes.
The cylinder screws onto the gage in several places and merely extends the capacity of the gage.
I hope this helps
In the days before DRO's and digital height gages these were a pretty handy gadget. I have not used one now for years. Mine was used mostly to set cutter heights, but very handy in inspection too for checking heights from one surface to another.
Don't let "planer gage" make you think that this tool is only used on planers and shapers. I use my planer gages a lot! It's kind of a one size fits all set of adjustable parallels. It's a square, and most of them have a level. This is one handy tool. I use it for measuring dovetails in positioning tables I manufacture. Trying to use adjustable parallels is awkward compared to the increased width of a planer gage. Measuring dovetails is awkward at best since they get measured with precision balls. So you try to ascertain the distance between two balls, and you have to measure it on the crest of the ball. Nothing easier than sticking a planer gage in there and measuring it's faces.
The cylindrical tubes, normally a couple of them increase the dimensions that the gage can be used for. Measuring slots less than a 1/2 inch wide can be done with a multitude of tools, but the planer gage is just one of the handier ways to do it. Stick the tool in the slot, push the planer gage apart until it's wedged in the slot, and just measure across the planer gage with an outside micrometer. Any inside measurement with straight walls is quickly done with a planer gage.
And Of course they can still be used for setting toolheights on Planers and shapers, in case you have one of those out in your garage. I have four or five planer gages. They're available for free because people think they are antiquated. Fine by me- I just buy them for nothing. They are very finely crafted tools.
I just bought a Brown & Sharp planer gage on eBay for $15.51, didn't notice the paypal only hidden at the bottom of the page. After asking for a mailing address he pointed it out to me, but agreed to accept a M/O.
I got my Brown and Sharp planer gauge at an estate sale for $15.
I also use it to set the carriage stop on the lathe. It's a useful tool....
When using a planer gauge, start with the tool lower than necessary and jog the tool up till the gauge just slips under.
Now lock the slide.
This keeps the slide/head backlash in the up direction and you can't easily smash your gauge, or chip the tool.
Works on the mill too.
I use P. & W. 1" x1.5" x3." hardened and ground block to set tools on the cnc's this way.