What is the purpose of the jig?
I had built two large custom fly-cutters of my own design. On each fly-cutter I made provision for two 10mm HSS cutting tool bits (about 3/8") located 180° apart for balance.
In this post, I refer to workshop ground HSS, or High Speed Steel tool bits as opposed to more expensive Carbide insert tooling.
I needed to grind the cutting edges of these round section HSS tool-bits.
The typical flycutters available to be purchased by hobby machinists employ square section cutting tools, however in my case, round holes would be easy to drill and ream with tools that I already had in my drill cabinet. In addition I had 10mm round HSS in stock (about 3/8").
To get going, I initially ground the tool-bits freehand on my bench grinder, but quickly realized that I needed a way to accurately form the primary grinds on the 10mm HSS blanks and then easily and repeatedly re-sharpen the tool-bits while in use. I also wanted to experiment with a number of different cutting edge shapes and angles.
This is when I put my mind to designing a jig that would provide this functionality.
The first version.
Next I made a simple jig to hold a 10mm round blank and grind a primary 60° included nose angle. I then had to grind the relief angle on the two edges by some other means; either freehand, or by some inconvenient to repeat setup. I could see this would get old really fast.
I had definitely made a step in the right direction, but I pondered the desirability of grinding both the primary nose angle and the relief angle in the same setup.
I soon came up with the current design, a jig that provides all of the required angles in one easy setup.
The JIG.
It works great!
However, the design is not perfect in that the angles are not adjustable and therefore a separate jig is needed for each different nose angle and relief angle combination.
Nevertheless, once a good grind setup has been determined for the material being fly-cut, it is plain sailing from then on because you don't need the adjustability. In many ways I prefer fixed purpose tools to adjustable or re-configurable tools because they are ready for action immediately.
A good aspect of this design is that it can be used very easily on both a small manual milling machine and also on a surface grinder table.
If you watch the video you will see that I have successfully ground tool bits in a small manual mill using a normal 115mm (4-1/2inch) angle grinder disc mounted in a home made arbor. The resulting cutting bit was more than satisfactory for what I needed.
Conclusion.
My fly-cutter was easier to make by drilling and reaming round holes as opposed to producing square holes for the typical, or more common, square cutting bits.
However, this convenience came at a price because round tool bits are arguably less convenient to hold and index for grinding, dependent of course on how well your workshop is equipped for work holding. This is because the square section tool bits have alignment surfaces built-in.
I can confirm that the jig works great, so if you feel the round holes way suits what you have, go ahead and build a jig like this for your workshop. You'll like it.
Take care and thanks for reading my blog.
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