# Tapping lots of 1/4 x 20 blind holes in 6061



## arvidj (Feb 15, 2017)

Here is the use case:

I have to tap 48 blind holes 1/4 x 20 in 6061 aluminum. The holes are drilled about 25% deeper than what needs to be tapped.

Tools available:

I have a drill press with a Tapmatic head, A9 tapping fluid and both an OSG two flute spiral point tap and an OSG three flute spiral bottoming tap. The point tap is not long enough to put threads as far down the hole as are needed. The bottoming tap does create threads far enough down the hole.

Conundrum:

It seem like there are two possible ways to move forward.

The first is to tap all the holes using the point tap, clean the chips out the holes as best I can and then go over them again with the bottoming tap. The advantage would be considerably less strain on the bottoming tap as it only has to cut a small percentage of the threads. The disadvantage is the extra work of cleaning the holes and going over all of them a second time along with the concern that the Tapmatic and the bottoming tap will actually pick up the existing threads and not try to create new ones. [side note: a few experiments with scrap suggests it actually will pick up the existing threads but the research was not exhaustive enough to determine if that was the norm or just luck]

The alternative is to skip the point tap and just go with the bottoming tap. The advantage is a lot less work. The disadvantage is the stress on the bottoming tap as it cuts 100% of the threads for almost 100% of its length. The thought of having to remove a broken bottoming tap and\or scrapping the workpiece is not high on my list of things to do.

Suggestions? Thoughts? Concerns? Advice?

Thanks,
Arvid


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## tq60 (Feb 15, 2017)

We believe normal process is multi step.

Cut with taper then finish with bottom.

Yes takes time but blind hole is going to require lots of peck motion to remove tap from hole and clear chips.

Tap will catch the thread if carefull. 

Take your time as clearing a broken tap will be lots of work.

Be certian it is secured well as the tapping via machine can lift the part causing angle on tap that breaks it...been there.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk


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## JimDawson (Feb 15, 2017)

arvidj said:


> [side note: a few experiments with scrap suggests it actually will pick up the existing threads but the research was not exhaustive enough to determine if that was the norm or just luck]



I have had pretty good luck doing that in hundreds (thousands maybe) of holes.

@tq60 pretty much said it all.


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## Uglydog (Feb 15, 2017)

I had a job this summer where I needed to tap 280 3/32 holes 2inches deep around two 6inch 6061 cylinders as part of a vacuum assembly.
There were also multiple taped holes.
I broke two 3/32. Try getting those out! Buyer was pleased enough with the rest that he didn't care. 
All I can suggest is go slow and peck. 
Always clear the chips. 
Use appropriate lube.

Daryl
MN


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## T Bredehoft (Feb 15, 2017)

I would suggest using a "Spiral Flute" tap. they are engineered to direct the chip up the flutes and out of the hole. They work well in aluminum if kept lubricated with either a tapping fluid or a light oil. They will leave the same thread in the bottom as a bottoming tap. This eliminates the ball of chips in the bottom of the hole and the necessity of changing taps and tapping each hole twice.


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## mcostello (Feb 15, 2017)

Done in the hundred of thousands in a screw machine shop. Can You grind the point off Your tap to gain the needed threads? Never 2 tapped threads, Boss would have had a breakdown if that much time was wasted.


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## jbolt (Feb 15, 2017)

Another option is a thread form tap. Taps are stronger and no chips to deal will.


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## wawoodman (Feb 15, 2017)

A spiral POINT tap pushes the chips forward, and is recommended for through holes. A spiral FLUTE tap pulls them back.

But I agree, a thread forming tap might be a better choice. I've never used one in a Tapmatic, though. Any people who have?


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## willthedancer (Feb 15, 2017)

Go with the spiral flute tap. A9 is good stuff. Use it generously. You might back the tap just enough to let the fluid back into the cutting surface, but not so far as to break the chip.


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## Fabrickator (Feb 15, 2017)

I would proceed with your Option #1. That's how I do a project with multiple 1/4"X 20 holes.  Pointed tap followed up by a bottom tap.

 Question:  Are all 48 holes in one piece, or is that the total for multiple pieces.  If it's multiple pieces, you can make a few extra and try both Option/methods.  If you break a tap, just scrap the single piece and move back to Option #1.


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## Billh50 (Feb 15, 2017)

Spiral Flute Taps are best way to go for holes that don't go through. I have also relieved the shank slightly to get a bit more thread length in the hole.


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## Wreck™Wreck (Feb 15, 2017)

Drill to the maximum minor diameter, use a spiral flute tap and machine tap in one shot. If it takes longer then 2 hours you are doing it wrong. Set a stop so as not to bottom out the tap in the hole.

Above all buy a good tap, steer clear of anything from the hardware store, HD, Lowes, Ace and so on, it will be expensive by hobby standards but well worth it.


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## David S (Feb 15, 2017)

I had to do something similar recently with smaller thread size and did indeed use a spiral flute, but also managed to get away with a plug tap, rather than the starting tap and didn't need the bottoming tap.  The plug tap worked since I was using it in the drill press, so straight starting wasn't an issue.

David


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## Jimsehr (Feb 15, 2017)

Use a form tap. No chips. Be sure to drill hole to form tap size.
Jimsehr


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## coherent (Feb 15, 2017)

Looks like the advice given already answered your question. Type of tap is important. Like others said a spiral tap that pulls the chips out is required. Prob obvious and you already know, but don't use a hand tap in the tapmatic. I have a different brand tapping head and have used it for hundreds of blind holes in aluminum down to 8-32 or so without issue. Blow all the holes out well after drilling. Be liberal with the lube and take your time & 1/4 shouldn't be a problem at all with a quality OSG flat bottom tap.


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## jbolt (Feb 15, 2017)

wawoodman said:


> But I agree, a thread forming tap might be a better choice. I've never used one in a Tapmatic, though. Any people who have?



I have tapped thousands of holes with a tapping head and form tap. First time I ever used one was when I was the "New Guy" at a shop. They set me up on a drill press with a tapping head and I tapped 8-32 blind threads into aluminum extrusions, 8 hours a day, for three weeks. I wish I could remember the name of the greenish tapping fluid we used. I'm sure it has long since been outlawed.


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## arvidj (Feb 16, 2017)

Thanks for all of the great responses. To answer a few of the questions ...

There are three pieces of material each with two rows of 7 holes that need to be tapped. I guess that is only 42 holes, not 48. Duh.

The taps are OSG. Maybe not THE BEST but I believe them to be better than the hardware store variety.

The point tap is a 'gun' tap and therefore pushes the chips into the hole. That was the reason for the extra hole depth. It also has the advantage of having 5 - 7 starter threads, which make it easer to turn but does decreases the effective depth to which the hole can be tapped.

The bottoming tap is a spiral tap. The advantage is ejecting the chips out of the hole. The disadvantage is that it only has 2 - 3 starter threads and is therefore much more difficult to turn ... and the follow on ... much easier to break.

Given that this is a hobby and not production I'm leaning towards spending time rather than money on a possibly broken tap and go over then one with the point tap, clean, then bottom tap.

Thanks for all the suggestions and wisdom,
Arvid


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## ch2co (Feb 16, 2017)

I want to thank you guy for raising my awareness to taps and tapping. I've tapped a lot of holes in my amateur life and usually use taps with ground off points for bottom tapping, this method emerged just by trial and error using the taps that I had on hand. I have seldom broken taps and those that did break was carelessness hand tapping smaller holes, 2-56 and 4-40.  I was not aware of the difference between spiral point and spiral flute taps. I wasn't aware of form taps although I have seen them many times, never knew that they were.  Gotta try them out.  
So what is the purpose of a spiral point tap? 
Thanks guys

CHuck the grumpy old guy


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## wawoodman (Feb 16, 2017)

Spiral point taps are great for tapping through holes. They push the chips ahead of the tap, and out the hole in the bottom. So you don't have to keep backing up to break and remove the chips. I've had great luck running smaller ones (6-32 up to 5/16) in my cordless drill.

Once you've tried it, it'll be a real head-slapper!


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## ch2co (Feb 16, 2017)

Thanks Mike. I have a few spiral point taps. How do you visibly tell the difference between spiral point and spiral flute taps.


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## woodchucker (Feb 16, 2017)

T Bredehoft said:


> I would suggest using a "Spiral Flute" tap. they are engineered to direct the chip up the flutes and out of the hole. They work well in aluminum if kept lubricated with either a tapping fluid or a light oil. They will leave the same thread in the bottom as a bottoming tap. This eliminates the ball of chips in the bottom of the hole and the necessity of changing taps and tapping each hole twice.


I agree.


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## JR49 (Feb 16, 2017)

ch2co said:


> So what is the purpose of a spiral point tap?



Spiral Point =  pushes the chips in front of it- best for through holes
Spiral Flute = pulls the chips out behind it much like a drill bit- best for blind hole
                                 Happy Machining,  JR49


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## David S (Feb 16, 2017)

Mike Spiral point on left, Spiral flute on right.  I still think a spiral flute plug tap may do your job in on shot if you are using them in a drill press.



David


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