[autobot]

I have end mill holders that have a set screw on the side to secure the tools by tightening on the weldon flat of the tools.

Unfortunately most of the tools I have dont have eldon flats on them naturally and are natively cylindrical on the shank.

What method can I use to fabricate the flat on the tools so I can use the end mill holders I have?

I dont have a grinder, otherwise I might do that. Will precision fly cutting work or maybe end milling it?

The difficulty is the tool shanks are as hard as the cutting edge so its probably not good for the tool trying to cut the flat onto the other tool.

What do you suggest? (I agree grinding is best but I dont have a grinder)

[txcncman]

I have found it is usually not needed to have a flat on the tool shank.

But, if you insist, go buy a grinder.

[DMF_TomB]

you could use a portable grinder to crudely grind a flat but with carbide it is better to consider getting collet type tool holder
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many carbide tools are smaller like 5/16, 1/4. 3/16, 1/8 and it is easier to use collets.
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also what you can do is take a 3/8 ID by 1/2 or 5/8 OD bushing then saw cut it length wise 90% though then when in a set screw type holder the set screw will collapse bushing (if saw cut at 90 degree to set screw axis) and grab the end mill much better than the set screw directly on end mill.
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i have made a steel bushing and saw cut it and deburred saw cut in less than 10 minutes and been back in business when i did not have time to wait for a special collet to get ordered and delivered.