Tapping 1/4-28 to 1/4-20?

[tarbellb]

Quick question:

Im looking into if tapping a 1/4-28 hole to 1/4-20 would be OK?


Application:

Its a 1 1/4" OD pipe that I am applying a steel single split shaft collar onto, it will be acting as pinch clamp for telescoping pipes.

I will be cutting a single split into the pipe, approx 1/8" x 2", then adding the
shaft collar (welded) onto the upper most part.

The shaft collar comes standard with a 1/4-28 pitch cap head screw, but I want to add a steel 1/4-20 threaded knob for tightening purposes.

The load will be the weight of the pipe, maybe 5lbs, plus signage, 10-15lbs.

Thoughts?

Heres a quick sketch-

 

[Thumper68]

Why not just order the proper knob?

http://www.mcmaster.com/#machine-component-knobs/=145jc1f

 

[MP&C]

....or if you must re-tap to an alternate thread of the same diameter, install a free-running heli-coil

 

[txvwnut]

....or if you must re-tap to an alternate thread of the same diameter, install a free-running heli-coil

Do this. ^^^

I think fine and coarse thread holes use a different drill diameter for the tap so you might end up with a loose fitting set knob.

 

[rlitman]

Do this. ^^^

I think fine and coarse thread holes use a different drill diameter for the tap so you might end up with a loose fitting set knob.

It does, and the fine thread uses a larger drill diameter than the coarse, so retapping the fine with a coarse thread is a recipe for failure (even more so than the other way around).

However, I'd suggest a third option. Redesign and forget that knob entirely.
Buy bicycle seatpost quick release clamps. They can be had for just over $1 if you're willing to wait a while for shipping from China. Or your can buy a dollar beat up razor scooter at a garage sale and pull two or three of these from it.

 

[tarbellb]

Why not just order the proper knob?

http://www.mcmaster.com/#machine-component-knobs/=145jc1f

I did in fact order a knob that has the oe pitch, but the dimensions arent as ideal (bigger).

The Helicoil is certainly a option, but I was looking for a cheaper in house solution.

So.... its the strength I am most worried about. Thoughts?

 

[larry_g]

You will end up with a ~50% thread with cross areas at zero % so I would not recommend it.

http://www.guhring.com/documents/Tech/Charts/CutTapDrill.pdf

lg
no neat sig line

 

[lazer50]

It will definitely be weaker if you do it your original idea.

 

[tarbellb]

It does, and the fine thread uses a larger drill diameter than the coarse, so retapping the fine with a coarse thread is a recipe for failure (even more so than the other way around).

However, I'd suggest a third option. Redesign and forget that knob entirely.
Buy bicycle seatpost quick release clamps. They can be had for just over $1 if you're willing to wait a while for shipping from China. Or your can buy a dollar beat up razor scooter at a garage sale and pull two or three of these from it.

Great idea riltman!!! I am always using the axel quick release mechanism for projects!

Totally didnt put together that standard seat post are the same OD. Stoked!

 

[seber]

why not just stick an allen wrench in the screw and soft solder in place.

 

[Jim Johnstone]

Great idea riltman!!! I am always using the axel quick release mechanism for projects!

Totally didnt put together that standard seat post are the same OD. Stoked!

Yep, 1 1/4" is 31.75mm. The bicycle industry is 100% metric, order or go to a bike store and ask for a 31.8 seat collar and quick release. 31.8 is probably the most common size.

 

[rlitman]

Great idea riltman!!! I am always using the axel quick release mechanism for projects!

Totally didnt put together that standard seat post are the same OD. Stoked!

I'm not sure that they are, but from the looks of it, I think it will fit through. The nice thing about seatpost clamps, is that the come with a matching nut. That way you can drill out the threads, and not worry.

 

[rlitman]

Yep, 1 1/4" is 31.75mm. The bicycle industry is 100% metric, order or go to a bike store and ask for a 31.8 seat collar and quick release. 31.8 is probably the most common size.

It's not 100% metric. Headsets and tubing are based on imperial numbers, even though they'll often be listed by weird metric sizes.

Anyway, those ears may be less than 1.25" from flat to flat. But with a seatpost clamp, so long as the threaded rod is long enough, you can always run a tap down the threads to allow the nut to go a little further.

 

[Jim Johnstone]

It's not 100% metric. Headsets and tubing are based on imperial numbers, even though they'll often be listed by weird metric sizes.

Anyway, those ears may be less than 1.25" from flat to flat. But with a seatpost clamp, so long as the threaded rod is long enough, you can always run a tap down the threads to allow the nut to go a little further.

The only thing that will ever be called out on a bike in imperial units, is the pedal thread. All the weird metric conversions are still called out in metric. You'd be hard pressed to find a bike mechanic who had any idea what you were taking about if you asked for a parts in inches.


I would know, I was a bike mechanic for 10 years.

Edit, and headsets as you mentioned.

 

[rlitman]

The only thing that will ever be called out on a bike in imperial units, is the pedal thread. All the weird metric conversions are still called out in metric. You'd be hard pressed to find a bike mechanic who had any idea what you were taking about if you asked for a parts in inches.


I would know, I was a bike mechanic for 10 years.

Edit, and headsets as you mentioned.

Yep, and isn't it funny how so many things jump in 1.6mm increments of size? Who came up with such a weird number?

But yeah, seatposts are all called out in metric numbers lately. The last time I was a bike mechanic professionally was in the 1990's.

 

[Jim Johnstone]

Yep, and isn't it funny how so many things jump in 1.6mm increments of size? Who came up with such a weird number?

But yeah, seatposts are all called out in metric numbers lately. The last time I was a bike mechanic professionally was in the 1990's.

A few things are going truly metric. 35mm has become a new handlebar standard. Most of the new bottom bracket standards use metric bearings press fit in.

 

[tarbellb]

Yep, 1 1/4" is 31.75mm. The bicycle industry is 100% metric, order or go to a bike store and ask for a 31.8 seat collar and quick release. 31.8 is probably the most common size.

Exactly what I found, totally stoked that the tubes I have spec'd will be a very close match - 1.25" = 31.75mm and the seat post qr is 31.8.

Thanks guys, you just improved my design vastly. My bid on this is looking even better.

 

[kerrynzl]

drill the threaded part out, then press in a nutsert from behind [ heat it, press in. and let it shrink]

 

[bullnerd]

For a clamp, wouldn't the finer pitch be better?

 

[Kevin54]

If you need the 1/4-20 thread, then tap the hole for a 1/4-20 heli-coil. IIRC, the drill size for a 1/4-20 heli-coil is something like 5/16". I would have to look it up to be sure.

 

[MP&C]

if you need the 1/4-20 thread, then tap the hole for a 1/4-20 heli-coil. Iirc, the drill size for a 1/4-20 heli-coil is something like 5/16". I would have to look it up to be sure.

17/64"