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Old 02-03-2017, 12:56 PM   #1
Gordon
 
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Default Tap lesson

I recently purchased a couple of spiral fluted taps because others here have said that they worked much better than the standard straight flute. They are correct. I tapped some blind 8-32 holes in aluminum and they indeed go in much easier. That was the problem. I am used to feeling when the tap reaches the bottom of the hole and then backing out. I was not careful enough and when I reached the bottom I thought that I just had to back up a bit to clear the chips and keep going. Unfortunately I kept on going and stripped out two of four holes. Fortunately in this case it is easy to just retap to 10-32 but it could have been a costly lesson. I will continue to use the taps and will probably buy more but I will be more cautious next time.


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Old 02-03-2017, 03:30 PM   #2
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Is amazing that you stripped the thread instead of breaking the tap.
That makes for a darn strong tap or perhaps a shallow depth of thread.


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Old 02-03-2017, 05:19 PM   #3
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The hole was .300 deep to be tapped to .25 depth. I did not think that I had applied an abnormal amount of pressure. It was similar to what I would have used with a straight flute tap. The spiral flute tap was new and it was the first time I used it.
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Old 02-03-2017, 07:38 PM   #4
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I'm wrestling with this issue myself on a (broken tap) project. I'm no expert but seems like there several factors in the mix. Even by saying 'spiral flute', there are different degrees of spiral helix. This link suggests angles as a function of material.
http://www.natool.com/engineering-data/tap-style-guide

And there are different flute counts among similar spiral flute varietals. I've also heard that for tapping aluminum, uncoated/bright is better than coating & is more important than angle. I've heard that 2-turn-in + 1-out routine is not what you are supposed to do with topside ejecting taps, that actually pinches off the swarf & causes chip jamming.

My machinist buddy deals with deep hole copper which is another bugger material & his tap salesman said he was tapping too slow which was counter-intuitive to him. He sped it up & breakages decreased. Who knows, I have great runs & then the tapping gods decide to wreak havoc (proportionately related to how much work has gone into the part!).

Bottom tapping to me now means trying really hard to avoid ever touching physical bottom with the tap. That seems to be the root of most evil. I would rather deepen the pilot hole a bit or decrease the thread count a smidge. There is some calculation out there that says anything more than X threads provides diminishing returns on holding power anyway.

I'll think more positively about this subject when I get this fr*ggen tap out
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Old 02-03-2017, 08:06 PM   #5
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I have never given it much thought before. I have mostly just tapped until I felt the tap hit bottom on blind holes. I have never counted turns but it should be possible to calculate the number of turns required. Obviously on a 32 pitch tap the tap will advance one inch after 32 turns so eight turns would have advanced the required .25". If I am going to use spiral taps I will have to pay more attention.
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Old 02-03-2017, 09:49 PM   #6
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A common overlooked issue is the use of the term
Spiral tap
There are spiral flutes taps - The tap looks like a threaded twist bit with dense helix
The flutes act to lift the chips out like a twist drill bit. They are great for blind holes.

There are spiral point taps - looks like regular tap but the tip are angled opposite to a spiral helix tap The point is designed to push the chip forward they are recommended for though holes, although with a bit of attention they work well for blind holes

Spiral point taps always have only 2 flutes, therefore they are very strong compared to spiral fluted taps.

I stop breaking taps after stopping making two mistakes:
1st the insistence of drilling a hole based on 100% tread
2nd quit aligning bi eye-hand and use the mill/late/DP to keep the tap on axis and location.

When a tap enters even a bit crooked, it can not straighten and will progressively attempt to tap into more solid material. On a tin piece you can get away but on a deeper hole it will bind. Good lubrication is a given.
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Old 02-04-2017, 12:28 AM   #7
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For softer materials like aluminum I'm gravitating towards using only thread forming taps - acquiring them slowly. No chips = less chance of breakage, as long as you tap the hole as recommended for the particular tap you are using (not your standard tapping chart!). Just lubricate and tap, no chips to clean out of the bottom of a hole. Another plus is that the threads are stronger having being formed rather than cut.

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Old 02-04-2017, 06:37 AM   #8
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One thing I've learned is don't tap tired! Fatigue will cause you to loose focus and that leads directly to bad technique. This may very well be why taps break at the end of a long project.
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Old 02-04-2017, 02:09 PM   #9
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Quote:
One thing I've learned is don't tap tired!
Good advice for most operation in the shop.
All I do when too tired, clean up, label stuff, organize, anything safe and relaxing.
Order in the shop pays big dividends in time saved.
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Old 02-06-2017, 05:09 AM   #10
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Quote:
Originally Posted by tornitore45 View Post
Good advise for most operation in the shop.
All I do when too tired, clean up, label stuff, organize, anything safe and relaxing.
Order in the shop pays big dividends in time saved.

Good that you mentioned the hazards involved in working tired. When i responded i was think about my ability to think well and not let my quality of workmanship slip. This has happened a few times to me with both wood working and metal working projects and the next day you end up disappointed with your self ask why didn't i just hit the sack.


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