[AT] Drill size for bolt tap question

[Stephen Offiler]

I've been using the same tap chart since college and it's from Greenfield.
At some point I ran it thru a laminator so now it'll outlast me.  It
actually lists "probable hole size" acknowledging that drills usually drill
slightly oversize, and then it gives % thread based on that probable hole
size.  I'm a lot newer to CF taps and I tend to use the mfg recommendation
for tap drill.

But really I'm responding to say M30, wow!  What material were you tapping?

SO


On Thu, Apr 13, 2017 at 9:54 PM, John Hall <jtchall at nc.rr.com> wrote:

> Biggest form tap I stock is about 1/2" or 12mm. I wasn't suggesting
> using one, just pointing out to use the correct calculator. I posted
> this link because it actually works.  There are a LOT of tap charts with
> bad info--they round the hole sizes to aprox. 5% increments. I have even
> found charts that just flat out missed the hole sizes. The best chart I
> have ever found was one put out by Cleveland 30 years ago--it lists
> actual percentages. I'm anal about threads as most everything we do has
> to be gaged.
>
> Biggest cut tap I have ever used was an M30---hit it heavy with
> Moly-Dee, shut the doors and backed way up. We had enough HP fortunately
> to get it done.
>
> John
>
>
> On 4/13/2017 8:15 PM, Stephen Offiler wrote:
> > OSG is excellent stuff.  We use a lot of OSG and Emuge.  But for CF taps
> > (CF stands for "cold-forming" meaning they force metal into the shape of
> a
> > thread without creating any chips) I can't imagine a 1"-8 even being a
> > thing.   Judging from experience up to 3/8" CF taps into aluminum, a 1"
> CF
> > tap would take a stunning amount of spindle HP.
> >
> > SO
> >
> >
> > On Thu, Apr 13, 2017 at 4:32 PM, John Hall <jtchall at nc.rr.com> wrote:
> >
> >> This is what I use all the time at work--be sure to use cut taps, not
> >> form taps!!!!!!!!!!!!!
> >>
> >> http://www.osgtool.com/Technical.asp?tid=1&id=1
> >>
> >>    For most threads 67-75% is about average. A couple notes--drill bits
> >> almost always drill oversize--especially being used in a hand drill.
> >> Stainless steel and tool steels can be hard to tap so use a lower
> >> percentage. Some thread sizes calculate to the point there is no "good"
> >> drill size--sometimes metric/english drill is substituted, or the hole
> >> is milled or bored as needed. Having said all that, a 1"-8 hole with a
> >> 15/16 drilled hole will produce only 39% of thread--absolutely horrible.
> >> No amount of Loctite will remedy that. You will need some sort of epoxy
> >> to take up the slack.
> >>
> >> There are some threaded inserts that use a larger than normal drill
> >> size--I think McMaster Carr carries t
> >> hem.
> >> John Hall
> >>
> >> On 4/13/2017 2:23 PM, Al Jones wrote:
> >>> I've got some wallered out holes on a Farmall Super A torque tube that
> I
> >>> want to fix.  They need to be 5/8" coarse thread, but somebody stripped
> >> 'em
> >>> out, then did some sort of funky drilling, and now my holes are 15/16"
> in
> >>> diameter.  I'd like to fix it myself but the longer I look at it the
> >> more I
> >>> fear a machine shop visit is in order.
> >>>
> >>> My notion is to try to tap the holes for a threaded insert.  If I drill
> >> the
> >>> holes out any larger for a larger tap, it's going to eat away the
> square
> >>> boss on the side of the torque tube, so I want to avoid that. Now, the
> >>> chart I am looking at says you need a 7/8" hole for a 1" tap. That
> means
> >>> the existing holes are 1/16" too big.  Can I still tap it with the 1"
> >> tap,
> >>> Loctite the daylights out of the insert, and get it to hold? These
> holes
> >>> are on the left side, and there weren't many implements that used these
> >>> holes, plus it's going to be mainly a show/plow day tractor.  But I do
> >> want
> >>> them functional and they have to "look good."
> >>>
> >>> I have a good torque tube, but this is an early (probably built the
> first
> >>> or second day of 1950 production) white demonstrator tractor, so I'm
> >> trying
> >>> to keep the castings original.
> >>>
> >>> thanks!
> >>> Al
> >>>
> >>>
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