Drilling and tapping into the cross slide for a DRO

daveog

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Hello,

Well, I'm in a pickle. I bought a DRO to mount on my Logan Powermatic 12x36 lathe. I made a couple of spacers to get the scale out away from the cross slide easy enough, but tapping holes to mount them to is a nightmare. I put the cross slide in my mill so I could keep all the holes on the same line and bought some #36 stub drills to drill for the 6-32 holes. I broke 4 bits drilling two holes and broke the tap off in the first hole. I'm guessing the cross slide is hardened cast iron and it doesn't like to be poked at. So now I have a hardened tap in a hole in hardened iron that I may never get out. I'm not sure tapping these holes is even feasible. I'm thinking of abandoning the DRO project altogether, which is going to hurt my pocketbook. Does anyone have any ideas on how to: A) get that darn broken tap out and B) how to tap at all. Even if I get these done, I have to do the same in to the saddle, which can't get mounted in my mill to drill, so I'm not sure that's doable at all.

Thanks guys,
Dave
 

RJSakowski

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Cast iron can be finicky. When I drilled and tapped holes in my G0602 bed for the z axis, three of the five holes went well. The fourth protested a bit and in the fifth hole, I broke the tap. Using a combination of carbide drills, a punch and a pick, I managed to get enough of the tap out so I could use a shortened screw.
Cast iron often has inclusions which are quite hard. You can tell when you hit one because drilling progress slows way down. You need to be very careful tapping those holes. 6-32 taps are notorious for brealking. An 8-32 tap would be a better choice. Also, drill the holes oversized. The recommended tap drill is for a 75% thread but you can go with a 60% or even 50% thread. There won't be a lot of stress on the screw. For an 8-32, the tap drill for 75% is a #29 and for a 50% thread, a #27. (#36 and #32 for 75% and 50% for a 6-32, if you still want to go that route.) Cast iron can be tapped dry but back it out frequently to clear the chips.
 
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Bob Korves

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Yep, 6-32 is the tap most likely to break, due to deep threads in relation to the diameter of the fastener. Bending them, even a little, also helps them to break. A tapping block to help keep the tap straight while tapping makes the entire process go much better. +1 on the oversize pilot drills. Good quality taps are also a major help when tapping difficult stuff.
 

Richard King 2

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Also if you can use a fine tip torch and pin point the tap to take away the hardness that should help in drill it. But if you have in the mill see if you can use a carbide end mill. Also tap it in the mill to keep the tap straight. Put, spindle in neutral, put it in a chuck and carefully turn a "plug" tap. I also would drill it oversize as RJ said. I do that on all no pressure bolt holes. Be sure to get some Taping Magic too. If more folks knew about drilling oversized holes, there would be a whole lot less broken taps. A Moderator should post that in the "tip section"
 

Flyinfool

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What kind of tap is it? If you used a cheapo carbon steel tap, they are very hard and brittle, but that is also their saving grace, when you break one off they can be shattered with a good center punch and then pick the pieces out of the hole. If you used a good HSS tap then the center punch can be used along the edges to try to turn the tap out. You can try to insert some pins into the flutes of the tap to get enough leverage to turn the tap out of the hole. Since it is already in the mill, you can cut the tap out with a carbide end mill.

As mentioned, open up your holes a bit, this is not a strength application and a full form thread is not needed.
 

wa5cab

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One other comment - most good taps come in sets if three, taper, plug and bottoming. For something like this case, always use the taper first, and then deepen the threads slightly with the plug. The bottoming one shouldn't be necessary. Also, if possible, try to make the hole drilled through, not blind.

And for removing the broken tap, the best way is to take it to a machine shop with an EDM broken tap remover set up. Unfortunately, that's also the most expensive way.
 

daveog

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Thanks for all the advice guys. This is a great forum where I can always go to get positive support and I appreciate that!
 

daveog

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The tap was a decent tap, but it crumbled when it broke. I'll try to break it up and remove it. It didn't get very deep before it snapped.
 

Bob Korves

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Quite a few formerly well known and respected tap and die manufacturers no longer make high quality products. It is not just where they are made, it is that they are made to a price point, with quality taking a back seat to profit in the shorter term. The ones you will find in most hardware stores today are not too special, regardless of the brand names on the package. If they are made in China, go on even higher alert. If you are using them on softer materials that are not tough, they may work just fine, but on harder and tougher metals, use truly high quality taps and dies. Learn good practices for using them, and then do the work carefully and not in too big of a hurry. A broken tap can take much more time to deal with than being careful in the first place, and it can ruin the work, often after considerable time, effort, and money has been invested in it.
 

Happycamper

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I didn’t have any problems drilling and tapping my cross slide. I did the same thing with spacers to try and give some clearance to get to the gib screws. A couple of months ago I bought the casting from MLA for a new cross slide and machined it so the gib and gib screws are on the left side. Then I mounted my scale to the right side and it works perfectly. While I was machining it I drilled the holes and tapped them for the scale mount. I used a piece of 1/4” flat bar, mounted that to the cross slide and the scale to the flat bar.
 

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ericc

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Is it possible that you could just glue a block on for the DRO? It doesn't need to be very strong.
 

pontiac428

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Sounds like it's too late for the OP, but a set of Walton broken tap extractors would have worked. Once you try to shatter a broken tap by beating it with a punch and hammer, this reliable method is hopeless, but it's something to think about for next time. My set doesn't come out very often (thank gawd), but when it does, it has the last word on that tap.
 

bobl

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I always use aT type home tap handle at the bottom of the tap shank then all I can get up in the drill chuck to support the tap you will save more small taps this way cast is always fun I agree drill a little bigger and always clear shavings make tap handles from 1/4 square stock put in lathe turn a small diameter one end to hold then tap through in one each side of where the tap goes droid clearance on second part then use to cap screws I also file where the tap go


Sent from my iPod touch using Tapatalk
 

ezduzit

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A carbide end mill is what I use for removing broken taps.
 

Moper361

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Is it possible that you could just glue a block on for the DRO? It doesn't need to be very strong.
I am actually contemplating doing this myself on a boxford as its pretty much the same set up as a Logan. Some of the epoxys now are very strong.
 
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