Pedestal Grinder Modification
for Grinding Single Point Tool Bits
By Jim Hanrahan
Here is a quick and easy modification to a standard pedestal
grinder that will make grinding single point tool bits with nice
straight angles. It is especially helpful when grinding thread
cutting bits for the lathe. This idea is not new and I forgot
where I learned of it. It is very similar to a feature that
comes with a standard "carbide" tool grinder. However, if you
have not already done this, you might benefit from this tip.
Figure 1. - Pedestal Grinder
Figure 1 shows my Baldor pedestal grinder. This type of
grinder has simple, flat, tool rests which can be adjusted,
although tediously, to give an approximation to the desired end
relief angle (let's use 5 degrees), see figure 2.
Figure 2. - Tool Bit Nomenclature
This means that the surface
of the tool rest and the tangent to the wheel is more than
90 degrees, in this example 95. The wheel is circular, therefore
the tool bit is cut with an arc instead of a straight line, but
my 10 inch wheel reduces that somewhat. The problem comes when
trying to put the side cutting edge and the end cutting edge
angles on the tool bit. Especially, when grinding thread cutting
bits, this must be a straight line at the desired angle. Holding
the tool bit by hand made this task difficult.
Figure 3. - Using the Jig
Figure 3 shows one solution. It is composed of a simple jig that
slides in a slot milled in the grinder tool rest.
Figure 4. - Slot in Tool Rest
Figure 4 shows the
slot in the tool rest which is parallel to the surface of the
grinding wheel. When dressing the wheel, I recommend using the slot
for reference to maintain this alignment. I sized mine for 1/8 by
3/8 flat stock. Leaving some room for clearance and to allow the
flat stock to slide back and forth in the slot even when dirty,
I made it 0.150 deep and 0.385 wide. In use, press the jig against
the side of the slot closest to the grinding wheel. The jig rests
on top of the tool rest, so the 0.150 dimension leaves clearance
at the bottom of the slot for grinding dust.
Figure 5. - Setting the Angle
Figure 5 shows how I
set the angle using a protractor. I used oil hardening flat stock
(I used it unhardened) for the part of the jig that slides in the
slot since its dimensions are nice and square. The set up shown is
very simple and only required drilling and tapping the hole for
the 10-32 screw. The block that the tool bit rests against has a
clearance hole and the 1/8 flat stock is tapped.
Simple, but it works!
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