Cutting Vs Forming

I've been slowly retrofitting a MaxNC mini mill that was all clapped out and had V screws over to acme screws with anti-backlash nuts. Of course nothing
is easy about it. I just finished milling the Z axis slide for a mounting block for the nut and making the mounting block. Due to limited space I find myself having to use 6-32 cap screws for a couple things. I just happen to have a box of 6-32 stainless screws leftover from another project. Some of the holes tapped perfectly the first time, but that nice little mounting block I spent most of the day figuring out how to make and cutting between real work grabbed ahold of the tap and snapped it over its proverbial knee like a pretzel stick.
It only took me an hour to make another mounting block.
Now I'm paranoid about snapping another tap. I know 6-32 is tough to do by hand so I was wondering about options. Maybe going with a forming tap instead of a cutting tap. I looked up a table or two and there is a big difference. #36 drill bit for a regular cutting tap at .1065" diameter vs .125" according to one table when using a forming tap. That's a lot more material removal. I know. I know. Its just aluminum threads, but what kind of difference in strength are we talking about for the finished threads?
The holes that mount the block to the slide are done, now I need to tap the holes that mount the nut to the block.
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I've found that you've got to be really selective as to what you use for tapping fluid when doing aluminum, that stuff can weld to a cutting edge in an instant. Grade of aluminum makes a huge difference as well. 6-32 purely sucks for tapping, I've heard it called one of the weakest threads, too. Best aluminum tapping fluid I've got is called Alumicut, don't know what's in it or if it's even still available, but leaves polished threads instead of bubblegum on extruded material. A tapping block helps, too.
Stan
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One of these: http://www.use-enco.com/CGI/INSRIT will greatly improve your results over doing it entirely by hand. Last week, I busted a 6-32 in a piece because I was too lazy to pull out the tapping machine for just one hole. Then I got to make the widget* over from scratch. Tapped it with the machine, no problem.
* said widget is an adapter for my Pace desoldering station to hold the smaller tips. Pace wants $70 bucks (+ shipping) for something that took me maybe 15 minutes (the second time) and a couple of inches of 7/16" drill rod. It's very nice to be able to make this stuff.
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rangerssuck wrote:

"The Enco Model Number that you entered could not be located."
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    What kind of tap were you using? The standard 3-flute ones which you have to back up every quarter or half turn -- or a 2-flute spiral point "gun" tap -- designed to tap on through chasing the threads ahead of it so they don't clog the flutes?
    Did you have the workpiece in a vise, or hand-held? In a vise is better, so it at least won't shift on you.
    And 6-32 is almost *designed* to fail. It is the deepest thread relative to the root diameter of any common thread. Really too coarse a pitch for strength at that size.
    I'm personally more comfortable tapping by hand with a 2-56, or even an 0-80 than with 6-32.
    Also -- what cutting lubricant were you using -- if any?
    What grade of aluminum? Some grades, including pure aluminum, are so soft that they gum up the process. Something harder, like perhaps 6061-T6 are more forgiving to tap with a thread cutting tap. If it is a soft alloy, you are almost certainly better off with the thread forming (roll) tap you mention below.
    You say "by hand" below -- did you have anything to make sure that the tap started square to the workpiece? If not, it is even more likely to break.

    Actually -- the tapped hole will be stronger with the formed threads. The hole has to be larger because metal is displaced from the root of the threads and migrates to form the crest of the threads. Drill for a normal cutting tap and you have metal with no place to go -- and again a broken tap. The forming work-hardens and makes stronger threads which will last longer from repeated removals and replacements too.

    Make up some kind of fixture to make sure that you start the tap parallel to the hole and on axis.
    Use a good lube. (Actually -- even WD-40 works pretty well with a cutting tap in aluminum. For a thread-forming tap, I'll let others suggest the proper lube. I might try the Castrol stick lube used for bandsaw blades and the like, or perhaps the Molly-Dee.
    Good luck.         DoN.
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DoN. Nichols writes:

I form 1/4-20 and 10-32 holes every day in 6061, using perfumed kerosene (aka WD-40).
Formed threads have a very poor quality. The crests look like little opposing bulldozers pushed snowbanks together, with a thin gap where the tops of the flow meet, so you have a potential false thread engagement halfway out of "phase" with the true thread. For some cursed reason this false thread start is the one that most easily engages when you casually insert and start a screw. They are OK if you are doing the assembly (it takes a certain feel to get a screw started, getting the false start, and then backing up a half turn with a little pressure), but I avoid presenting them to customers who have to start screws into them. But forming certainly is faster and less prone to breakage in soft gummy material like aluminum. The lube is important because if the metal starts galling onto the surface of the tap, which it will after a few dry holes, the tap will seize up and break.
One of these days I want to mill apart a cross section of a formed thread and post some photomicrographs explaining this low quality.
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Another hand tapper tool.
http://www.harborfreight.com/hand-tapper-46502.html
I don't think I would buy this one over the internet as it looks like it could have some issues with the rear clamp on the rod, but that's ok. Its listed as in store purchase only. LOL. Now if only the local Harbor Fright store actually stocked much...
I like the Speed Wrench idea. Even with a tapping tool the speed wrench might be a good modification.
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I've had a US made one of these for years. It works well if you have a lot of holes. The speed wrench handle is a help, but if you use it, you want to bolt it on. It slips off pretty easily.
http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO01846&PMT4NO045824
Just a bit more spendy than the Harbor Fright version...
I still tend to use my $27 Brownells tapping guide for most stuff (see earlier post).
Doug White
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Well, Harbor Fright came through again. Listed as store purchase only on-line, and deleted from their inventory in the local store. I wonder if they are going to survive. The have less selection on-line and less inventory in the store. I'm beginning to think the old man was right. I hope he wins his lawsuit.
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