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Taper Boring

AzulShiva

Plastic
Joined
Jan 11, 2022
hole.jpg

I need to make a 20mm deep hole into a round steel bar that is 8.5mm thick at the entrance and 8.0mm thick at it's deepest end. My carbide inser tools are too thick to fit inside the hole. I have extra thin bits that would fit inside the hole but they are not stiff enough (PAULIMOT HSS-Drehmeissel-Set 5-teilig 3,5 mm : Amazon.de: Gewerbe, Industrie & Wissenschaft). So the idea is to get drills from 8.0mm to 8.5mm in 0.1mm increments and drill with each at its respective depth to form the taper. Instead of a steady slope I would have several steps 0.05mm each. What tool could I use to smooth it out further? Unless someone has a better idea how to do it.
 

Clive603

Titanium
Joined
Aug 2, 2008
Location
Sussex, England
If the material isn't exotic a tapered toolmakers reamer or D bit would probably do it. Far as I know not something that can be bought.

Basically put the taper on a round tool blank.

For a toolmakers reamer grind the end off at 30° to produce a sloped cutting edge. Chop the end off a little below here it hits half thickness if thats going to get in the way.

For a D-bit grind a touch over half the thickness away to create a D shape tool with a cutting edge.

D-bit a little less stiff but more room to accommodate chips.

Removes a couple of thou or so.

Traditionally filed from silver steel then hardened and tempered but grinding from HSS or carbide works fine. Whatever the material they need to be run pretty slow and kept razor sharp. Generally not for high production jobs but when you just have to git'er dun.

Clive
 

Tony Quiring

Titanium
Joined
Nov 5, 2008
Location
Madera county california usa
Custom cutter?

Drill hole straight

Maybe a couple steps as you described.

Get some braized carbide boring bars or normal side cutting lathe ones that are larger body than boring head socket and turn to fit?

Modify side of cutter to be same angle as the taper and use boring bar to make final cut.

Sent from my SM-G781V using Tapatalk
 

AzulShiva

Plastic
Joined
Jan 11, 2022
I don't follow this. Let me ask for three dimensions:

(a) Diameter of steel bar?
(b) Diameter of hole where it enters the bar?
(c) Diameter of the hole at its deepest point, 20mm into the bar?

Cheers,
Bruce

a) 20mm
b) 8.5mm
c) 8mm
 

ballen

Titanium
Joined
Sep 25, 2011
Location
Garbsen, Germany
a) 20mm
b) 8.5mm
c) 8mm

I think this is straightforward. Look in a catalog of boring bars, for example: https://www.sumitool.com/en/downloads/cutting-tools/general-catalog/assets/pdf/GC_E_en.pdf. If you look on page 5, in the rows labeled "stop boring" and the column for "minimum diameter 8mm" you will see for example the S-STUB and S-STUP bars, which in steel are rated for L/D of up to 3, meaning a depth of 24mm. This is suitable since your L/D is less than that. There are additional bars in the "face boring" section.

So you can bore this taper hole on a lathe. Drill a 7mm hole to 20mm depth, then set up the correct boring bar/insert on a compound, and bore away. There are several methods that can be used to set and tweak the angle of the compound rest. Let us know if you need details. Alternatively, use a lathe that has a taper attachment, which will make it possible to use gage blocks to set the correct angle and hit it the first time.

If it's important that the bottom of your taper hole be flat (as in your sketch) then you need to finish the bottom of the hole first, for example using an end mill or square faced drill bit in the tailstock. If that's not important, so that the bottom of your taper hole can (for example) have a bit of drill-point profile sticking through, then that's easier.
 

AzulShiva

Plastic
Joined
Jan 11, 2022
I think this is straightforward. Look in a catalog of boring bars, for example: https://www.sumitool.com/en/downloads/cutting-tools/general-catalog/assets/pdf/GC_E_en.pdf. If you look on page 5, in the rows labeled "stop boring" and the column for "minimum diameter 8mm" you will see for example the S-STUB and S-STUP bars, which in steel are rated for L/D of up to 3, meaning a depth of 24mm. This is suitable since your L/D is less than that. There are additional bars in the "face boring" section.

So you can bore this taper hole on a lathe. Drill a 7mm hole to 20mm depth, then set up the correct boring bar/insert on a compound, and bore away. There are several methods that can be used to set and tweak the angle of the compound rest. Let us know if you need details. Alternatively, use a lathe that has a taper attachment, which will make it possible to use gage blocks to set the correct angle and hit it the first time.

If it's important that the bottom of your taper hole be flat (as in your sketch) then you need to finish the bottom of the hole first, for example using an end mill or square faced drill bit in the tailstock. If that's not important, so that the bottom of your taper hole can (for example) have a bit of drill-point profile sticking through, then that's easier.

I will try and get a new smaller boring bar but I doubt it will be stiff enough. If I undershoot by 0.1mm and have to correct the tool will not bite, and when it does it will shave off way too much. I have ordered the drill bits in 0.1mm increments and will try using this file to even it out (Pferd Schleifstift Diamant Spitzbogen 3 x 7 mm Schaft O D126 zur Gravur und Schleifen kleiner Offnungen (126) - Galaxus)
 

ballen

Titanium
Joined
Sep 25, 2011
Location
Garbsen, Germany
I will try and get a new smaller boring bar but I doubt it will be stiff enough.

As I wrote, the manufacturer rates this bar for an L/D of 3, meaning a 24mm deep hole.

If I undershoot by 0.1mm and have to correct the tool will not bite, and when it does it will shave off way too much.

This does not make sense. What material is the 20mm round? What type of lathe do you have? What cutter or insert are you using?

On standard materials (steel, brass, aluminium) with a reasonable lathe (decent spindle bearings and ways) and the correct cutter or insert (ground edge would be good here) you should be able to shave off a few microns.

I have ordered the drill bits in 0.1mm increments and will try using this file to even it out

This won't work.
 








 
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