Well you could grind up a bit of Carbide and put that in your holders they won’t know the difference between HSS or Carbide. Old milling cutter shanks work well enough or just buy blanks.

Do you really need the bars to be that big? For a lot of work you could simply sleeve the bore down to 12mm or 1/2″ and then buy the insert holders made specifically for boring heads. I mostly use these

Have you actually tried something exactly 3/4″ in the holes, I expect they will fit.

I just use small indexable boring bars from the lathe in a sleeve.

Or just buy a 3/4″ one intended for the US market but as that is only likely to go down to about a 1″ hole you will still need something for the smaller holes. The smaller bars also don’t unbalance the head so much which means you can run the head faster though still they generally need to run below optimum cutting speed.

While I use inserts extensively for turning, for boring heads I prefer to use HSS toolbits as they can be easily adjusted for clearance, and to reduce any chatter:

Unbalanced boring heads will not run fast enough to really benefit from using inserts.

Julie

If your new to boring on a mill Adam maybe some of this will help. Given the rigidity of Bridgeport type mills, any boring head capable of using 3/4″ boring bars is getting up there for what’s actually usable. Unless you really are needing to bore very large and deep holes, I’d go with what others have said and use adapter sleeves to allow those smaller diameter boring bars to be used. There’s hardened and ground commercial sleeves available to do exactly that. But shop made unhardened steel sleeves would still be fine. Just make sure they have either a hole or slot in them so the boring heads set screw bears directly against the usual alignment flat the better made boring bars will already have.

I’d also very much agree with Julie Ann’s point about carbide and trying to spin any larger and unbalanced head at the speed where carbide provides the best surface finish. If you don’t already know and depending on your boring heads exact design. There’s numerous Youtube videos and forum posts either saying they do or trying to give user information to others that all three set screws should be used to lock the boring heads slide after an adjustment. That’s not how there designed to be used. The outer two set screws are only used to set the gib adjustment to a smooth but almost zero clearance fit. The center set screw is then used to fully lock the slide after an adjustment. You’ll also have to double check your heads dial markings. Some are set up to show the diameter change in the hole size any slide adjustment will produce, and some will show radius which will be half of what the actual hole size increase will be.

In my opinion a mill is seriously lacking in what it can really do without a good boring head. But there not quite as easy to reliably hit your expected dimensions as it seems. Boring is much like outside turning or boring on a lathe. Ideally you want to divide the remaining material to be taken off in at least 3 equal depths of cut while measuring the exact amount removed on the first two cuts. That allows you to compensate for any deflection in the tool or mill head. With that information, you should then be able to make the proper last adjustment with a high expectation of hitting the size you want. For anything highly precise like a slip or press fit for bearings, a lot is going to depend on what you have available to even reliably measure hole sizes. Some dedicated practice with decent telescoping gauges and a micrometer can usually get you within a couple of 10ths. Those cheaper off shore sets of telescoping gauges usually don’t work smoothly enough for those levels of accuracy without some work de-burring the internal slots and parts and checking for straightness of the telescoping parts. But to adjust the boring head to it’s final size on anything that critical, I generally don’t use the dial at all. Instead I’ll use a .0001″ or metric equivalent dial indicator or dti against the end of the boring heads slide that I’m going to be moving that slide towards. In that case the indicator will only display the amount of slide offset your adjusting to as a radius change, or half of what the finished hole size change will be. And if it were me, I’d definitely practice using your new boring head on scrap material first and trying to hit exact sizes before trying to do the same with real parts that might have a much higher value.