advice on how to machine a back side counterbore.

[empower]

howdy fellas.
got a job to do, 8 parts. each one has 2 'hinges' with each one having a counterbore on both sides.
i can access one side, but back side is problematic. even if i got a 90* angle head, would have to be less than 1" diameter to clear the part.


so this is the feature. the main bore is .382" so i'm thinking maybe a back spot face tool 5/16" diameter with 5" reach.
counterbore needs to be .5000-.5007" .515" deep.

or maybe a custom reamer with the shank maybe .001" under the bore size, put it in manually from the back side and either use a drill or bridgeport to back face it using the main bore as the guide bushing?

cant show the part in full but its about 40" long, so really can only come in from 1 side.

any advice is much appreciated!

 

[Donkey Hotey]

The search term you're hunting for is Back Spot Facing Tool. Multiple vendors. Probably one that you already deal with.

 

[SteveEx30]

No EDM? Only 8 pcs. Can side burn it no problem.

 

[guythatbrews]

You'll play hell holding .0007 with a back spot facing tool. Maybe a special piloted back reamer as you suggest, but you still have to have a back spot facer or another way to rough it out.

It must be a high value part. Maybe edm is the way to go. Maybe some edm guy will chime in.

 

[SteveEx30]

Maybe some edm guy will chime in.

Ummm

 

[JonFinn]

Fine boring head with a back boring bar. Orient the spindle and enter off-center.

 

[empower]

No EDM? Only 8 pcs. Can side burn it no problem.

no EDM.

 

[empower]

Fine boring head with a back boring bar. Orient the spindle and enter off-center.

would have to be like .25" shank thats 6" long... are you sure about that?

 

[guythatbrews]

Fine boring head with a back boring bar. Orient the spindle and enter off-center.

5" reach? Don't think so.

Empower beat me to it!

 

[crossthread82]

Get harvey or someone else to make one of these (below) with a 3/8 major cutting diameter:

https://www.harveytool.com/products/keyseat-cutters---square---reduced-shank

And hold it in one of these:
https://www.harveytool.com/products/tool-holders---extended-reach-tool-holders

basically a 1/2 od extension holding onto a 3/8 key cutter that has a 1/4 shank.

Edit: Or if getting a custom tool made get a 1/2 shank solid carbide tool to replace the extension entirely.

 

[Mark Rand]

Turn and thread a mandrel .381" dia, with a flatted pilot, out of drill rod to take a button .5" back-spotfacing cutter. Put a small flat or groves on the mandrel to carry lubricating oil. Run it fairly slow to avoid the mandrel galling in the hole (you aren't looking for seconds with only 16 cuts). If you can't find one of the correct diameter, cylindrical grind or hard turn it to what you want. It isn't rocket surgery.

Even I managed this for a dozen holes on a job.

 

[mhajicek]

Get harvey or someone else to make one of these (below) with a 3/8 major cutting diameter:

https://www.harveytool.com/products/keyseat-cutters---square---reduced-shank

And hold it in one of these:
https://www.harveytool.com/products/tool-holders---extended-reach-tool-holders

basically a 1/2 od extension holding onto a 3/8 key cutter that has a 1/4 shank.

Edit: Or if getting a custom tool made get a 1/2 shank solid carbide tool to replace the extension entirely.

I've done similar things with tools like this. Keep the length of cut small, to minimize deflection force, and step down about half to two-thirds of that.

 

[empower]

Get harvey or someone else to make one of these (below) with a 3/8 major cutting diameter:

https://www.harveytool.com/products/keyseat-cutters---square---reduced-shank

And hold it in one of these:
https://www.harveytool.com/products/tool-holders---extended-reach-tool-holders

basically a 1/2 od extension holding onto a 3/8 key cutter that has a 1/4 shank.

Edit: Or if getting a custom tool made get a 1/2 shank solid carbide tool to replace the extension entirely.

not sure if that would work. would need 2+ inches of shank thats .25" diameter. is that reasonable?

 

[13engines]

Bore it straight thru and deep freeze a bushing into it so you don't leave any press fit scarring. Could even revert to some bearing material if it has that function to any extent. You'd have to sell it but... just a far flung idea to get you where you're going.

 

[mattthemuppet]

I thought you might be able to do it with a dovetail cutter and something like a BP quill master head - angled so that there was just enough clearance for the dovetail cutter to cut only on the end. But, that would produce an oval hole on a manual machine and you probably can't fit a quill master head to a CNC.

I think the bushing approach has the most merit

 

[dandrummerman21]

Bore it straight thru and deep freeze a bushing into it so you don't leave any press fit scarring. Could even revert to some bearing material if it has that function to any extent. You'd have to sell it but... just a far flung idea to get you where you're going.

I like this idea.

Looking at the part, this is the kind of design that crushes me. Because you know that this hinge is designed to have 1/2" od oilite bushings put in it, probably for a 3/8 diameter pin. They could have spec'd the whole hole to have a single long bushing in it that itself has a relief bored in the middle.

 

[crossthread82]

View attachment 432750

not sure if that would work. would need 2+ inches of shank thats .25" diameter. is that reasonable?

It's not going to be fast but it is doable. What material are you cutting?

 

[empower]

I like this idea.

Looking at the part, this is the kind of design that crushes me. Because you know that this hinge is designed to have 1/2" od oilite bushings put in it, probably for a 3/8 diameter pin. They could have spec'd the whole hole to have a single long bushing in it that itself has a relief bored in the middle.

thats exactly what it is! lol

 

[empower]

It's not going to be fast but it is doable. What material are you cutting?

7075

 

[Vancbiker]

Bore it straight thru and deep freeze a bushing into it so you don't leave any press fit scarring. Could even revert to some bearing material if it has that function to any extent. You'd have to sell it but... just a far flung idea to get you where you're going.

Or sell them on the idea of a couple grooves for internal snap-rings to provide the shoulder for the bushings.