tstageI've broken a few taps off in blind metal holes. Most I was able to remove with a tap extractor. There were a couple though that just couldn't be persuaded.
I was taught that the periodic reversal breaks up the chips being 'machined' by the thread edges. On a lathe or with a boring bar the chip can just 'spiral away' but in the confined space of a fluted tap it cannot curl and displace. So you reverse to break it and, if necessary, turn it in the flute.
I was also taught the value of keeping the tap at exact right angle to small holes when working -- just as with fine drills. In smaller sizes, the surface hardening of the tap necessarily extends through the tool and makes it brittle. In some cases, using a drill-press-like or bushing arrangement might help reduce tap breakage.
ChrisVAI'm just learning how to tap holes in metal. Broke my first tap! But I also successfully got a hole tapped properly. Slow is the key. One turn forward, one half back, stop at resistance. Tapping seems to be an essential MR skill.
Actually, I was taught to turn the tap a half-turn in and back off at least one, or even two, turns, to clear the chips, once the tapered tip of the tap is well-seated and going in at 90 degrees. It's also a good idea to lubricate the whole shebang, either with oil or even with bar soap. I haven't broken a tap in a long time. I'm in O scale, so I added larger taps--and threading dies--for the original four I used in HO (00-90, 0-80, 1-72, and 2-56). I now have 4-40, 6-32, 8-32, and 10-32, and have found them all useful. BTW, I've often been told that 1-72 is obsolete, useless, but as my mentor in all things, my eldest brother Dwaine, told me, if a smaller screw size hole is stripped--something that used to happen fairly often in Zamak locomotive castings, where a loco is torn down and reassembled a lot--you can usually re-tap the hole for the next size up. I don't know about now, but in the '50s and beyond, 0-80 was a popular choice for manufacturers for mounting cabs and such. If the threads wore out from trouble-shooting mechanisms, you could usually just tap the hole 1-72 and use that size screw (and the same for stripped 00-90 holes, just re-tap and use 0-80 screws). Of course, you could clear the oringinal hole with the tap drill first, just to be sure. Tapping is a good skill to develop. Brother Dwaine prompted me to buy the drills and taps, plus a good supply of various screws, nuts, and washers, when I was 16, bless him.
Deano
While wax can be an excellent lubricant, it can be difficult to remove from the work piece. Most any oil will work instead, and can be removed with lacquer thinner and other solvents.
Ed
ChrisVASlow is the key. One turn forward, one half back, stop at resistance.
I've hand-tapped a few holes in my time machining at a prior job. If tapping into metal: Rotate the tap CW no more than a 1/4 turn (90o) then rotate the tap back 'n forth the same distance 3-4 times to help clear out the chips. (That what the flutes along the side of the tap are for.) If the tap starts to bind sooner than 90o, rotate the tap only 1/8 turn then clear the chips. Clearing the chips is crucial because the chips cause binding, which leads to flexing then the dredded *snap* of your tap.
Plastic is generally way more forgiving than metal because it's less hard. Even so, I still use the "no more than 1/4 turn" adage with plastic just to be consistent. Better to be too cautious when tapping than over-exuberant. I've broken a few taps off in blind metal holes. Most I was able to remove with a tap extractor. There were a couple though that just couldn't be persuaded.
In regards to lubrication, I use cutting oil for metal. For plastics it's generally not necessary. In fact, some lubricants will attack certain plastics (e.g. lucite or plexiglas) and cause micro-fissures, which can exacerbate and lead to eventual failure around the hole.
I've even found that tapping isn't really necessary for certain MMRing steps - e.g. tapping the truck bolster or draft gear box holes on rolling stock kits. The hole in the underframe is generally the correct (or close to correct) tap hole size and I just use the metal screws to tap the threads inside the hole.
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
In my early days on this forum, one of the contributors suggested using Mr. Zog's Surfboard Wax as a lubricant on taps and drills.
I tried it, and it worked wonderfully. It works better and makes less mess than other tapping lubricants I have used.
If I could remember who suggested this tip, I would give them proper credit.
I had to edit the picture so moderators do not remove it.
-Kevin
Living the dream.
Unless you're bolting engines on to jet aircraft wings most of our model RR tapping doesn't have to fall into the upper ranges of theoretical thread engagement. You can save some aggravation and a few taps if you keep your percentage around 50-65%.
My faded, old tap drill chart§ shows some alternate sizes for less thread engagement:
Tap_drill-W by Edmund, on Flickr
Of course you can find neater charts and frames without cobwebs around it.
Tap_drill by Edmund, on Flickr
Theoretical thread engagement. More than you'll ever want to know (but neat to look at).
https://www.fastenal.com/en/78/screw-thread-design
§ I do have a much cleaner little chart in the model workroom
Good Luck, Ed
Dave,
Thanks for that chart! I was wondering if something like that existed. How the heck do you know what size hole to drill first? Thankfully now that's answered.
I think that is what happened - I drilled a pilot hole that was too small and then too much resistance turning the tap! A rite of passage.
"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."
Hi ChrisVA,
Another cause of broken taps is drilling too small a pilot hole. Here is a chart showing tap sizes and appropriate drill sizes:
https://www.fastenersinc.com/wp-content/uploads/fasteners_tap_drill_chart.pdf
You should be able to click on the chart to blow it up.
Cheers!!
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
Joe is quite correct. Straight out like that, and it locks the ratchet. Positioned on either side, it determines the ratchet directionality.
Congratulations on breaking your first tap. It's a compulsory milestone.
Lubrication can lessen/eliminate the problem, though then you have to clean out the lubricant.
Zamac seems to be the worst for tap breaking, of the metals we run into.
And your forward and back is proper technique.
It actually says lift and turn. It might be a ratcheting tap, and that is the lock release.
Joe
I'm just learning how to tap holes in metal. Broke my first tap! But I also successfully got a hole tapped properly. Slow is the key. One turn forward, one half back, stop at resistance. Tapping seems to be an essential MR skill.
My question:
I got this tap wench with a small part at the top.I don't know what its purpose is. One the tap it says "Lift and turn". What is this thing supposed to do? It would seem to be a lock or something?