In the GB-22 image you can see here, there are three pieces screwed together in a stack to make the slide. I've previously cut them to the proper size, so it was time to get the through holes drilled in two pieces and the other piece drilled and tapped. First problem: I realized that I cut the middle of the three slide pieces too short and need to start it over. Moved on to the two side pieces, which are thicker and longer. Drilled the first one out to clear the screws, then drilled the opposite side with a smaller drill so I could tap it... and broke the tap off in the first hole. After some time trying to drill the 10-32 tap out, first with a carbide drill bit then with a diamond coated bit, all I can say is... dang that stuff is hard! No success - I may have put a 10 or 20 thousandths dimple in it. Can't use alum because that only work when it's a steel tap in aluminum; this is steel in steel. I could build an EDM tap burner. Gee, that commercial machine is $1500, I'm sure I could build one for a few hundred $. Lessee... $250 or $500 depending on what I can scrounge, and the time to build it, to save about $3 worth of steel...no, I think this one is scrap, too.
So I'm 1 for 3 on simple, minimally machined parts.
While trying to drill the tap out, I needed to raise and lower the mill head, so I'm using Mach3. Out of the blue, my keyboard input stops working. It's a wireless keyboard, and changing the batteries didn't fix it. At one point, turning the computer off and on seemed to help, but I'm not sure.
So, all in all, an annoying day. One thing worked out well: I came up with a mount for my Fogbuster system. Sort of a wooden hat rack.
Well, tomorrow will bring more time to mess-up some more metal pieces. I'll be smoking a turkey in the afternoon, so plenty of time to play in the shop.
The broken tap. Annealing the tap usually doesn't work. A longer explanation of why is found here.
ReplyDeletehttp://www.practicalmachinist.com/vb/metrology/what-makes-hss-taps-resistant-annealing-294894/
Depending on how much of the tap is sticking through the hole, try holding the piece in a vise and shearing the tap off with a chisel and if you are lucky the fracture will free up the tap enough to turn it out. If not then grind the tap flush on both sides, put your piece on a solid surface and use a center punch to attempt to chip out the tap. (eye protection obviously) If the tap is still there, then try heating it red hot and punching it out with a center punch or a pin punch. Then redrill and try tapping, if that doesn't work then drill the hole out, silver solder in a steel bushing and drill and retap.
None of the above makes sense on a time/money basis but it would all be good practice for that time in the future when you really, really, have to fix something that cannot be replaced.
I forgot to mention tap extractors. But when you figure out that they are specific to tap size, and specific to the number of flutes on the tap, and you can't use them on spiral taps, it is wishful thinking. Still interesting though.
Deletehttp://www.waltontools.com/products/extractr.htm
Good point about the tap extractors and this being a spiral tap. I start running the tap into the hole and in my mind I'm going "the guy who invented these really needs to get a Nobel prize or something". Then I decided to back it out a half turn and heard that stomach turning, very quiet "click".
DeleteI hadn't thought about trying to use the tap's hardness and brittleness against it, though. It's worth a shot. I have a center punch and a 2 pound hammer.
Why did it break, and how will you prevent it next time?
ReplyDeleteMy common mistakes have been drilling the hole too small- full thread engagement is a sort of curve up with regard to breaking a tap, and down, with regard to thread strength. The machinists handbook may have charts showing thread strength to thread depth-it is always tempting to get "full strength",but the actual thread engagement necessary for that is much less than we might imagine.
If you have a small, hard punch, you might be able to turn the broken tap by lightly hitting the flutes at an angle.Jarring it loose, in effect.
Is it a blind hole or a through hole? What sort of tap was it? I was taught with a blind hole, to use a standard taper tap, then a plug tap, then a bottoming tap.
Why did it break, and how will you prevent it next time?
DeleteThe big question! I originally thought it was from drilling too small a hole, but now I'm not sure. The hole being tapped is a through hole in 1/4" thick, 1018 steel (called low carbon, cold roll). The tap was this spiral tap.
The drill I used is from a set of taps and matching drills that I picked up from Sears 30 years ago; I just grabbed the drill it had for 10-32, a #21. As I was starting to tap, part of my brain said, "is it OK to use the same bit you used on aluminum on this steel?" and I told myself that the set didn't differentiate, so it was probably a common size that would work for home use. In the places I've since checked, it's listed as the size to use.
The spiral taps thread in very easily, but I believe it broke while backing it out. I wanted to back it out completely, which I've done on aluminum. The thing is, there was no increased resistance telling me I had to back it out.
Now I'm not sure if I didn't apply bending moment or something like that.
I do have a pointed punch, so I'm going to try whacking it with a 2lb. hammer. It can't get any more scrap than it is.
The choice of tap handle can really make a difference with regard to the bending moment- I like the Tee type with a sliding handle, shank and chuck on the end, rather than the type that is short with one side of the handle acting as a clamp for the tap- the reason being there is room so it can be used held like a corkscrew, with the shank between the second and third fingers, so each side of the handle gets even pressure.
ReplyDeleteIf you can get that busted off piece even a little loose, it might be able to be worked out far enough to grab it and spin it out the rest of the way. Maybe sharpen the ends of a pair of needlenose to get in on each side- if it was a two flute tap-probably not enough room on a three or four flute.