[AT] Drill size for bolt tap question

[charlie hill]

A 1" -8 heli-coil requires a 1 1/32" hole.

https://www.carid.com/images/heli-coil/pdf/heli-coil-catalog.pdf

-----Original Message----- 
From: John Hall
Sent: Thursday, April 13, 2017 9:54 PM
To: Antique tractor email discussion group
Subject: Re: [AT] Drill size for bolt tap question

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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