Tap use limits

by and large I don't break taps unless I do something stupid, like forget to tighten the work in the vise, as I drill/ chamfer/ tap in one set up. Uncoated spiral point for through holes, uncoated spiral flute for blind holes. For blind holes I'll start the thread until the lead in of the tap is a couple of threads in, then I'll finish off by hand, either in the mill or the bench vise. Tapping fluid or kerosene is really important too, stops the chips sticking to the tap.
 
I have broken more taps off in aluminum than any other material. A guide to hold the tap straight is a good idea. Nothing worse than getting to the last few holes then busting the tap and not being able to get the broken tip out!
 
MikeInOr said:
I have broken more taps off in aluminum than any other material. A guide to hold the tap straight is a good idea. Nothing worse than getting to the last few holes then busting the tap and not being able to get the broken tip out!
Click to expand...
There is at least the consolation that alum can be used to remove the tap from aluminum. No such option in steel.
 
There was an almost identical discussion on a professional board a few months ago. Of the hundred or so responses most people were saying they got 1,000 tapped holes from a 4-40 tap before retiring it. Keep in mind the 1,000 holes were drilled and tapped under almost ideal conditions in a CNC mill with flood coolant. A quality tap will last at least 1,000 holes when the proper size hole is drilled, the tap is dead on in the hole, it's fed at the optimum speed, it's lubricated properly, and the swarf is cleared. If you can duplicate those parameters a single tap should do the job.

In a past life the company I worked for made food processing and packaging machinery for their own use. One style packaging machine used steel inserts in aluminum plates as a back plate for rotary knives to cut plastic packaging. The inserts were held to the aluminum plates with 4-40 machine screws. Each plate or platen had 8 holes drilled and tapped to secure the inserts. There were between 220 and 240 platens on the machine depending on the product being packaged. So for each machine there were between 1,760 and 1,920 drilled and tapped holes.

All the prototype machines were made in house using manual mills and lathes. The holes were drilled and tapped using Tree 2UVR milling machines with flood coolant. The companies preferred taps at the time were Triumph. In this case they were bright uncoated finish. Being conservative the taps were changed out after tapping between 800 and 900 holes.

Personally I would use an uncoated tap for this operation. Here's a quote from Regal cutting tools referring to taps used in aluminum. Regal is a well respected name in the field and I personally would follow their recommendations.

"Coatings are generally not necessary in aluminum operations and can actually be detrimental if they contain aluminum. Recommended coatings include Chrome, CRN – Chromium Nitride. Titanium CarboNitride (TiCN), and Titanium Nitride TiN for use with taps and drills designed for aluminum. CrN is similar to TiN but has added high temperature resistance and more corrosion protection."

Here's a link to the full article:

As mentioned by MrWhoopee, if you're machine tapping the holes this might be a perfect application for a thread forming tap.
 
I'd personally buy a couple sharp ground HSS taps. I wouldn't be worried about the tool life, but having a few extra sitting on the bench makes it easy to switch to a new one if you start to question the sharpness of the current tap.

Do not use home depot taps of any kind, they will break
 
While you are at it, order some fuming nitric acid. (but do not store it in the shop!)
Robert
 
If not already mentioned, use plenty of lubricant, something like Tap Magic. Aluminum is soft and grabby, and 4-40 is a rather small tap. You should use a guided tap handle arrangement of some sort, in the mill.
 
if you regularly break taps then consider something like this
www.hobby-machinist.com

Keyed Washer/spacer

Hey Sav, i have a win-win for you... tell swmbo, that you are working on a gift for her or someone she adores, and can't be disturbed because it's a surprise. she won't mess with you, you get to play in the shop for double the length of time because "i want it to be extra special and...
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I've used it to remove many broken taps.
 
New twist (pun) to this question. The drawings call out a #45 drill for the tapped holes instead of the standard #43 drill. This is a 0.007" smaller hole. Mr. Hodgson indicates it was the standard used by Chance Vought where his father worked many years ago. Seems to me this means greater torque is required. Does this mean I shouldn't use a thread forming tap? Should I get a "looser" fit (H3 vs H5) tap? Tap specifications can be quite complicated.
 
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