Well Tracy G has been bugging me for awhile to explain, with pictures, my marking jig. So I'll make him happy by posting it now. (About time, eh Tracy?)
My first jig I made out of a strip of spruce lath and a finishing nail. The one I use now is made from a strip of rosewood (its' hard) and a HHS drill bit (sharper), about 3/32" diam.
Waiver: if this helps anyone, or if it is of value and with the basic idea, can improve on it, go hard!
Here is what I did: (forgive me if I explain too much: if this helps someone who may have just a little experience at the moment in jig making, good!)
Overview: This jig helps maintain a proper cutter angle (in this case, 26 degrees, which is roughly halfway between the ideal range of 22.5 to 30 degrees). This allows for some leeway in cutting angle. It works by scribing a line in the throttle body downstream from the plate, so that when the top of the large cutter is touching the throttle plate and the bottom back edge of the cutter is touching the scribed line, the cut will be at the desired angle. The finished jig is shown below:
I needed to determine the original angle of the throttle plate for the throttle body I was working on. To do this, I took a piece of stiff cardboard (non-corregated) and with the longer straight edge against the throttle bore (parallel to the sides), I cut the short side edge which rides against the plate itself, until it matched the angle of the plate. (As some bores widen under the plate, and are therefore angled in the bore where you measure, I used a small square at the base of the TB to make sure the long edge of the cardboard was parallel to the bore.)
Here you can see the angled edge which matches up with the throttle angle.
Once I was sure that the angle was correct, I used a compass to determine the degree. In this case I am presenting, the angle was 16 deg. (It may be 12 deg, etc: check and determine)
Next, I transferred the angle to some stiff cardboard
Next I transferred the cutting angle that I wanted (26 deg), but bisecting the first marked angle at the point about 5/16" above the bottom (which corresponds to the diameter of the larger cutter). This intersection is where the top of the cutter would be touching when first starting the cut.
The bottom edge of the cardboard represents where the cutter actually cuts into the throttle body wall. The difference in the angle is where the rear bottom edge of the cutter would rest on the throttle body wall to start the cut. In this case it is about 3/32" back from the plate. The scriber is a sharpened drill bit tapped into an undersized hole in the wooden strip, with about 1/64" of the sharpened tip showing, with a drop of CA glue (crazy glue) to secure it from pushing through when scribing. The front bottom edge of the jig rides up against the bottom of the throttle plate, and the scriber is set back 3/32" (determined again from the angle difference between the original throttle plate angle and the desired cutting angle).
Here are some other shots of the jig:
Here is the picture showing all the angles:
When the jig is used, make the scribed line from throttle plate axle to axle. The line scribed should be parallel to the throttle plate. Once the cut is started, with top edge just touching the plate and the bottom edge starting the cut on the scribed line, maintain the angle and cut straight down. The top of the cutter of course will be no longer touching the throttle plate as per the angle difference.
This jig may be used with various different angles of throttle plates, but I always check the throttle angle first and measure back so that I know for sure that the cutter angle is within the proper parameters of 22.5 and 30 degrees. Know also that this jig is a guide only and practice and mods are the key.
This, for me, helps in establishing a correct angle without guessing, and after a while, the angle will come more easily as it becomes established in your mind.
Hope this helps.
Regards,
Luke
