Why do indexable insert holders use a shim?

Why do indexable insert tool holders have a shim, what is it made of, etc?

I will post an even dumber question as soon as I think of one.

You mean a seat? A carbide seat?

Its so when you have an *OOPS*, you rotate or replace a $10 carbide seat instead of
spending $75 on a new tool holder..

Very nice feature on face mills.. $50 for new seats instead of $300 for a new cutter body.

Even without an OOPS, the vibration of the insert during cutting will wear into a steel seat. When using a double sided negative insert with chip breaker geometry, there isn't a lot of surface area in contact with the seat, so a lot of small pressure points exist, and these can etch into the steel. I've got a few well used boring bars that have gotten this way, due to the fact that there isn't room for a carbide seat.

The carbide seat used under a lay down type of threading insert on a lathe is actually tapered. The taper is different according to the diameter of the part you are threading.

I have always wondered why...but never gave it much thought.
Thank you

rabtrfld said:

Why do indexable insert tool holders have a shim, what is it made of, etc?

I will post an even dumber question as soon as I think of one.

Click to expand...

.
amazing is the cost for screw and shim is often over $20.

What Hu said. The carbide seat is to prevent the insert digging itself into the floor of the holder.

Gobo said:

The carbide seat used under a lay down type of threading insert on a lathe is actually tapered. The taper is different according to the diameter of the part you are threading.

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Hello Gobo
Might I add that if you want to cut external/internal left hand threads & only have an external/internal right hand tool holder & insert, you can install a negative angle anvil to cut those left hand threads.

Crash protection being the number one reason by far.
Break a tip and that shim seat will still cut a little, hopefully you notice soon enough as it cuts very badly.
Brinelling of the floor # 2.

Both talked about above but there is a third reason.
When using a negative insert your unused sharp cutting edge is down.
You can't make a pocket with absolute sharp corners so without a shim seat you have to add the cost of undercutting the floor/wall intersection to prevent chipping the downside of the insert.
This undercut is sometimes called a "dirt relief" but it's real purpose is that no endmill makes zero corners.
Shim seats have a 45 on their bottom so you do not need to add this operation that is done with very short lived and fragile tooling.
( and yes shim seats are low cost C-2 carbide with a top and bottom grind. Expensive only because of the volume they are bought in, in quantity they are much cheaper)

Threading shim seats are a different story being one sided tools and this relates to helix angle and maintaining the needed heel clearance which is why they are tapered.

Somewhat weird the terminology or correctness of "shim", "seat" or "carbide seat". Not really a shim or maybe it is.
Anyhoo, if you make toolholders you will know what the customer is talking about instantly and that is all that matters.
For a user.... they make your holders last longer and that is what really matters.
Bob

Glad I asked! I bet there's people on here write Wikipedia articles and I hope they do cuz you don't find this stuff anywhere else.

Rabtrfld and Bob,

The number one reason for a Shim in the pocket of a lay down thread bar is to control the Helix of the thread. The other reasons listed are reasonable but have nothing to do with the actual reason the shim/seat is in the pocket. The reason smaller bars do not have a shim/seat is because the smaller the thread diameter the less of an issue the Helix becomes.
Sr. Apps. Eng. Kennametal

Sr_Applications said:

Rabtrfld and Bob,

The number one reason for a Shim in the pocket of a lay down thread bar is to control the Helix of the thread. The other reasons listed are reasonable but have nothing to do with the actual reason the shim/seat is in the pocket. The reason smaller bars do not have a shim/seat is because the smaller the thread diameter the less of an issue the Helix becomes.
Sr. Apps. Eng. Kennametal

Click to expand...

Should that statement not be the finer the pitch the helix is less of an issue, .........cos it sure is on sub 1/2'' Acme and the like.

I thought it was so you could loose the bloody thing in the chip pan making you clean it out, sort of machine hygiene accessory thing, like the little torx screw,
Ok jokingly, who hasn’t lost one
Mark

Limy Sami said:

Should that statement not be the finer the pitch the helix is less of an issue, .........cos it sure is on sub 1/2'' Acme and the like.

Click to expand...

Absolutely. That guy wasn't thinking very clearly when he wrote what he did. For a given pitch, the larger the diameter the less the helix angle. For a given diameter, the greater the pitch the more the helix angle.

eKretz said:

Absolutely. That guy wasn't thinking very clearly when he wrote what he did. For a given pitch, the larger the diameter the less the helix angle. For a given diameter, the greater the pitch the more the helix angle.

Click to expand...

The Kennametal engineer’s choice of words was poor. The angle of the insert in no way “controls” the helix angle of the thread. The helix angle is “controlled” by the parameters of the thread itself - pitch and thread diameter.

The insert just goes along for the ride.

Maybe we are mixing lay down threaders and a CNMG pin lock toolholder?
Shim seats holders were around long before this type threading tool came to market.
Where is the angled shim seat on a top-notch threading tool?
Who made the lay down threader popular as the go to we all know? Hint, starts whit a C and on 8 mile in Detroit.

Standard pin lock/ top clamp holders have a flat shim seat and guess who started that now standard design.


I would be remiss If I did not do a big ass welcome to Sr_Applications to the board.
Any input is welcome, you may get hit a bit but do not take it personal when some talk back.
I am sure you know things we do not.
Bob