Deep Counterboring Tool Selection

Hello everyone.
I need to make the attached part, and one of the features on it is a counter bore which will require a long tool. Part will be done in a mill/turn center. Can someone please point me in the right direction regarding tool selection?
Thank you in advance.

How many you have to make? It is important, will change the answer from "just drill it and use a long hss endmill in an extension on a bridgeport" to "have someone make a long cbore step drill"

I think you make it in two parts. The original looks like it might have been welded then turned.

Use a standard piloted C'bore tool with an extension welded onto the shank of the c'bore tool.

If running the 4 screw holes C bores you might drill the but end first with a simple tool so getting a more precise location, then with a piloted counterbore for the difficult end with having an extension shank on your C bore tool.

Having the 4 screw holes in place will guide the C bore tool when it comes in from the long nose end.

Maybe try a back counterbore tool?
.500 - 1.187 Dia. Back Counterbore Tools - K-Tool, Inc.

michiganbuck said:

If running the 4 screw holes C bores you might drill the but end first with a simple tool so getting a more precise location, then with a piloted counterbore for the difficult end with having an extension shank on your C bore tool.

Having the 4 screw holes in place will guide the C bore tool when it comes in from the long nose end.

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I like the idea of drilling the holes from the backside first.
Is there anything available that will allow me to extend a counterbore tool? I could weld a shank to it but I am wondering if there are other options.

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dandrummerman21 said:

How many you have to make? It is important, will change the answer from "just drill it and use a long hss endmill in an extension on a bridgeport" to "have someone make a long cbore step drill"

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I only need to make 3 for now.

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jephw said:

I only need to make 3 for now.

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Then I'd just do the counterbore as a separate operation.

You ask about extending a counterbore tool. You can get a standard one and find an extension for the shank. A long endmill holder extension should do. If nothing comes up, you could always turn one up and press it in (I would put a set screw in the side even if you press/shrink it in).

In addition to the type of back counterbore that mtndew linked, there's a couple others. Cogsdill makes one called a "flipcut" and I'm sure other companies do as well. Flipcut | Deburring Tools by Cogsdill

And then there's the removable head one. I've used this before on a really stupid small job. It worked a treat. Here, I found a quote of someone who linked it on PM a long time ago:

S_W_Bausch said:

I have a few back spotfacers in a drawer, but plenty on ebay

METCUT 150 5237 HSS BACK SPOTFACER BALANCED DRIVE | eBay

back spotfacer | eBay

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Be careful to get the matching shank; piecing the system together may result in stuff that won't work. I have a shank for a back spotfacer, it locks up, the edges will rotate in the correct direction, etc....But the shank has a Morse Taper, useless for back-side work.


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I realize you need something to pass through a 3/16" hole, and the stock catalog items don't go that small (only .3125"). However, you now know the stuff exists, you may need a custom item, or another vendor.

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Personally for 3 pieces, I'd do it on a bridgeport with just a standard HSS cutter. No reason to spend a ton. I will say the back spot face tools are relatively cheap too. Perhaps on ebay you'd find the size you need for less.