# Turn down shaft of tap



## BarryMcM (May 14, 2021)

Hello folks,
I am new to machining (7x12 benchtop), and I am looking for advice.
I have to tap a hole 35mm deep.
The hole is deeper than the threaded part of the tap's shaft (~25mm).
The shaft is of a larger diameter than the hole.
I want to turn down part of the tap to fit into the hole.



The very first piece of anything I turned was a hardened steel shaft out of a plunge router base.
I used a parting tool to trim off the end with the rough cut.
It was ugly sounding, but worked.
I made the cut dry at a few hundred RPM (no indicator as of yet, so I am guessing).
On this tap shaft, however, the flute passes through the cutting path.
This sounded very ugly, and as you can see, I have barely made a mark in the shaft before chipping the carbide insert.

Any suggestions on how I could turn this down would be appreciated.

thanks,
BM


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## cathead (May 14, 2021)

You might get by with a carbide but a grinding wheel would take the meat off it for sure.


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## RandyWilson (May 14, 2021)

I would think the interrupted cut of the flutes in the extremely hard tap will just shatter the tip of the carbide insert. 'Specially the pointy D profile. Rather than damaging the tooling, I would think it cheaper, and possibly quicker, to just buy a reduced shank tap.


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## stupoty (May 14, 2021)

I have some taps that have been ground on the shank for clerence , you can get longer taps though so might be worth looking for a cheepy on ebay 

Stu


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## RJSakowski (May 14, 2021)

A pulley taqp has a turned deown shft as well s longer length for threading deeper holes.  I woulde modify a conventional tap by grinfing.  It oesn't have to be precise.  You just need a smaller diameter than the tap drill.


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## mmcmdl (May 14, 2021)

They sell undercut taps .


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## markba633csi (May 14, 2021)

Grinding is the only practical way to reduce the shank- it's way too hard to machine, as you discovered
-Mark


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## macardoso (May 14, 2021)

I too have done as you described. A wider included angle insert will be stronger for example a square insert with a 90 degree corner will be stronger than the 50 degree included angle on your DCMT insert.

I recommend NOT reducing the shank smaller than the tap drill. The tap can cut crooked more easily if the shank is reduced too small.


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## macardoso (May 14, 2021)

macardoso said:


> I too have done as you described. A wider included angle insert will be stronger for example a square insert with a 90 degree corner will be stronger than the 50 degree included angle on your DCMT insert.
> 
> I recommend NOT reducing the shank smaller than the tap drill. The tap can cut crooked more easily if the shank is reduced too small.
> View attachment 365836



I machined my tap with a SCGT09T0304 insert. Spindle speed was adjusted to give a light blue chip. Dark blue or black is too hot/fast. No Coolant


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## Jimsehr (May 14, 2021)

You might try eBay and look for a nut tap or a  pulley tap.


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## 682bear (May 14, 2021)

I just bought a couple of deep hole taps on ebay a couple of weeks ago...




$9.95 for two taps, that is cheaper than ruining inserts trying to machine them...

-Bear


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## Flyinfool (May 14, 2021)

With the interupted cuts and what I have to work with, I would chuck the tap in a drill and head over to the bench grinder. Dont over heat it. Don't take off any more metal than you have to, and dont leave a sharp transition up to the original diameter or that is the spot it will break. The other option is a tool post grinder.


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## Dave Smith (May 14, 2021)

use a hand drill in a vise and use your angle grinder. easy and take your time not to heat it up.
Dave


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## Lo-Fi (May 14, 2021)

Machining an HSS tap should not present a major problem. I quite like hard turning, even on my little benchtop lathe.


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## mattthemuppet2 (May 14, 2021)

buy a different tap. a) that one is a basic hand tap and you'll have more success with a spiral point (through hole) or spiral flute (blind hole) tap b) thinning the shank weakens the tap which may = broken tap. Presumably the pros have thought about this and accounted for it in their deep hole/ reduced shank taps. Taps are cheap, getting one out of the last hole in a piece you spent hours working on is not


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## mmcmdl (May 14, 2021)

mattthemuppet2 said:


> Taps are cheap, getting one out of the last hole in a piece you spent hours working on is not


I try to teach my guys , if you're gonna F it up , do it in the beginning of the job . That way you don't waste much time . 

Back to the point though , they sell the relieved taps or mount in a whirly jig and grind them on a surface grinder .


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## Larry$ (May 14, 2021)

Try not to use carbide on an interrupted cut, especially on harder material. Grinding is the 2nd best solution. Buying the correct tap is the best.


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## NC Rick (May 15, 2021)

All i can think of is that if in a similar situation, i would likely need the tap right away so ordering a pulley tap wouldn’t be an option i would want.  Any of these expeditious means would be cool.  Hard turning is really a fantastic thing but not something i have learned enough about to get consistent results.  your guys photos and advice are registering with me.  Thanks.


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## epanzella (May 15, 2021)

I have a number of taps in my favorite sizes that I ground down the shafts on for deep hole clearance. Don't make this any harder than it has to be. Three minutes with a bench grinder and you're done.


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## extropic (May 15, 2021)

macardoso said:


> I too have done as you described. A wider included angle insert will be stronger for example a square insert with a 90 degree corner will be stronger than the 50 degree included angle on your DCMT insert.
> 
> I recommend NOT reducing the shank smaller than the tap drill. The tap can cut crooked more easily if the shank is reduced too small.
> View attachment 365836



I see IRWIN brand on the tap shank.

That must be a carbon steel tap (not HSS).


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## Buffalo21 (May 15, 2021)

All of the pulley taps, I have have larger shanks than the tap size, but the nut taps are all smaller shanks than the tap size.


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## Illinoyance (May 15, 2021)

A nut tap has a shank smaller than the minor diameter of the thread. A pulley tap has a shank equal to the major diameter of the thread.  T tap a hole longer than the flute length requires either a reduced shank tap or a nut tap.


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## ericc (May 16, 2021)

This solves a mystery for me which has been bugging me for quite a while.  I have a commercially made hand tapping block, and the reduced shank taps are unpleasantly wiggly.  I think the block is made for full diameter shanks.  When I tap with those fat-bottom taps and they wobble on me, I just sing to myself "fat-bottom taps you make the rocking shank go round."


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## macardoso (May 17, 2021)

extropic said:


> I see IRWIN brand on the tap shank.
> 
> That must be a carbon steel tap (not HSS).


Accurate, HSS would probably be a whole lot less fun. CBN tipped inserts would machine it fine though.


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## extropic (May 17, 2021)

macardoso said:


> Accurate, HSS would probably be a whole lot less fun. CBN tipped inserts would machine it fine though.



True, except the OP says he's got a 7" x 12" lathe.
I've never seen such a machine stiff enough to hard turn.


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