DIY tool holder for inserts

I am working on a tool holder to mount commercially available inserts to machine multi angle valve seats.

I have made the basic tool holder based around standard Serdi pattern tooling, but have not yet decided on the specific insert type. The two options I am considering will both have a pocket with two shoulders for the insert to locate against. In both cases the inserts are held in with a standard looking countersunk torx screw through the insert

My question is to do with the location of the tapped hole for the screw. I have the impression, based on some of my existing commercial tooling, that the position of the screw may be biased a little toward the shoulders rather than being truly Concentric with the hole in the insert. Nipping the screw up gives the impression of the countersunk part of the screw going home and jamming the insert into the holder.

The question then is, should the screw be biased toward the shoulders or not, and if so, by how much?

Ian

Surely the screw has got to be slightly biased towards the shoulders, most probably only about five thou though.

When I make holders for carbide inserts, I get the hole position centered and then displace about 0.002" to 0.006" depending how big the screw is. Displacing the screw hole slightly towards the seat sides holds the insert tighter. The smallest screws I use are 1.8mm and the largest 6mm.

Edited By old mart on 23/01/2022 21:24:49

When the recess is machined, super glue the insert in place clamping it hard against the shoulders. Then spot through for the screw hole using the largest drill that just fits through the hole in the insert. Heat to unstick the insert, then drill and tap. I've used this approach for a t/f tool and a couple of boring bars.

Thanks for these answers, very reassuring to hear. The mounting screw is M3 so it looks like 3 thou offset toward the shoulders should do it.

Thanks again,

Ian

Just a passing idea which I've not quite thought through; this would only apply if you are using single sided inserts with a taper from the front front cutting edge to the plain back.
Rather than just gluing the insert into the nascent toolholder and spot drilling, how about adding a layer or two of paper under the insert.
This would have the affect of raising the insert up the taper a small amount, and thus also moving the hole a bit closer to the locating edges in the toolholder.
When the insert is then unglued and cleaned, the threaded hole will be offset very slightly in the correct direction, so the insert is pulled up snugly.

For parallel double sided inserts, grind a little off the side(s) of an old insert to have the same effect

Bill

If you copy a top clamp design instead of the offset hole design, you can then have the option of holding ceramic inserts with a substantially reduced risk of cracking them. The top clamp design also hold the inserts a lot better than the offset screw method as well.

Here is the tool holder finished (probably). It is a bit like a boring head but cruder in that it is set to a gauge rather than having a micrometer screw, also it needs to show a concentric bore on the bottom at all times to allow a pilot to be mounted which locates in the valve guide, hence the horseshoe shape.

The pocket for the insert was machined on the CNC machine. I glued the insert in as suggested, then clocked up the hole in the tool. I moved 3 thou toward the back of the tool holder, and drilled and tapped M3. The result was terrible for some reason and I ended up soft soldering some shims in the pocket to sort it out. Not sure why it went wrong, I suspect the tapping size hole drilling was not stiff enough somehow..... I will try harder next time.

Ian

I haven't read it yet, but MEW 312 contains an article on this very subject.

John

Different way to position holding down screw hole

Hole positioning