Dimensioning Twist Drills

[ArchusDsr]

We are designing a custom drill for orthopedic use and need to dimension the flutes and the tip including the web, land, chisel edge, etc.

ASME B94.11M-1993 does not show helix angles, web thickness dimensions and the like; nor does the Machinery's Handbook.

Does anyone have an example of an accepted method for dimensioning (custom) twist drills?

Thanks in advance,

Pberg

 

[Metalfixer]

Usually the company making the drill will also provide you with a drawing based on your requirements. You can receive information on special drills from

http://www.empiretool.com

 

[EnglishMuffin]

There is one thing worth mentioning that not many people are aware of, which is that twist drills have a very slight taper (ie the diameter decreases as you move back from the tip), which is done to lessen the chance of binding. Don't ask me what the amount of the taper is though.

 

[diamondjim]

I do not think that they taper back.
That is a new one. Too expensive.
The tip automatically gets larger
as it heats up.

 

[BillPSU]

The OD taper on drills is in the .001"/inch of length. Helix angle vary depending on the drill (high helix, slow helix, standard). The web of a drill is also tapered which as the drill is resharpened causes a wider point. All of these features are the trade secrets of the drill manufacturers. You can reverse engineer but I have been unable to get this information from any maker.

diamondjim
Get a drill and a micrometer, check the land diameter close to the cutting edge and then close to the shank. You will see the taper.

 

[EnglishMuffin]

I would rather doubt that every manufacturer uses .001"/inch back taper on every drill they make. Did a bit of web research. According to this,

http://www.mmsonline.com/articles/069902.html,

it seems as if it can vary in some cases from .0005"/inch to .004"/inch at least. This reference also discusses many other aspects of drill geometry, which like everything else is far more complex than one might suppose when one studies it.

 

[diamondjim]

English Muffin,
Thanks for the url on the twist drills.
My apology for not picking up this new
technology. We have been coating the
drills for years, but have never seen
this back taper. I have relayed this to
the shop guys to check this out.
Drilling and grinding has been our
greatest hindrance in getting parts
thru the shop on time.

 

[MikeHalloran]

Diamondjim,
If your guys are watching the drills and patiently picking off bird nests of long stringy chips, they may be doing it wrong, or using the wrong drills. Of course it depends on what you're drilling, but in readily machinable materials, a drill should spit out small, hot, fragmented chips, so fast that it can't clog.

Sometimes increasing feed or speed or both is all that's necessary. Sometimes premium or special drills are required to make that possible. Even experienced machinists usually run drills too slow and with too little feed, especially small drills, "so they don't wear out".

Hint: Drill bits are consumables. If you're not buying them on a regular basis, you could probably run them harder and save money and increase production, even if you have to buy the most expensive ones.



Mike Halloran
Pembroke Pines, FL, USA

 

[EnglishMuffin]

Diamondjim : Are you implying that back taper is new technology, or just that some of the stuff mentioned in the link I posted is new technology ? Back taper has been around for as long as I can remember - I first learned about it from a fellow student 35 years ago and it was old technology then - although as I said, not many people are aware of it - or if they do notice it they probably think it is random manufacturing inaccuracy.

 

[diamondjim]

Mike,
I do not think the shop can be accused
of drilling too slow. We drill either
alloy steels or aluminum.

English Muffin,
I guess we never measured any drills
before to check out back taper.
Thanks for the info.

 

[strokersix]

Every twist drill I've seen in the size range from numbers through 1/2 inch is tapered. Typically about .001 inch smaller at the shank end.