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Tool Selection Guidance

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Plastic
Joined
Mar 18, 2022
I have access to an old Hartford 2S. I have it cleaned up and have made a jig to mount a Northstar upper block to the table. I have a broken and mangled head bolt about 1.5 inches down the hole. I want to machine the top of it smooth enough to be able to drill it out on center. The bolt is partially hand drilled off center, the main hole is .510" the bolt is metric 10.9 about .400 OD and it is locked into a Time Sert. It looks like a bull nose end mill brought down with the quill about.002 at a time might get it out without tearing anything up. I know this would be slow, but I have much more time than skill. Any advise on tool choice and operation, RPM etc.? I have tried local machine shops and they won't touch it (I assume to tedious for the cost). Any guidance will be greatly appreciated.
 
I don't suppose you have any Kennametal TX drills laying around? You can drill a cloverleaf in steel with those things. Won't really matter what the top of the bolt looks like.
 
Don't have anything like that currently, just have boxes of old somewhat rusty end mills. All sizes, some 4 flute, some 2 flute, with different style tips. Some are broken, some appear well used, but a good portion seem to be sharp and pretty solid. However, I'm not sure any of them are ideal for the job at hand and will buy whatever I need (within reason). This is likely a one shot deal so don't need the best just sufficient.
 
About .400 OD..
I would make a slip in the hole bushing for a drill size, or end mill line bore spotting the broken bolt.

The slip in bushing long enough so be able to pull it out of the hole.

likely file feel one of the other bolts to know about how hard it might be.

Good to fill it up with WD or other penetrating liquid while you get ready to work on it.

If still time you might change the title to "Bolt broken off in Hole."








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Sadly, I don't know how to change the thread title. You are saying to make a drill guide to keep a bit centered? Could this be made from aluminum or would that be too soft and need to be mild steel. Also how much difference between guide hole and size of bit. Would 1/4" be a good size to try?
 
Drill guide bushings are usually very hard steel, available from McMaster Carr, MSC and others. I usually just use a carbide square end ( with small corner rad.) endmill to flatten the surface for center drilling.

Sometimes I have to do the drilling with carbide if the bolt( or tap) is hard enough to wreak havoc on an HSS bit.

Patient is key on those jobs, a shop vac will help too!

Good luck!
 
It sounds like you are not experienced, so don't use carbide tools. If you break one of them in there it won't be drillable. Carbide chips and breaks easily.

It sounds like you have the head set up on a mill?

So any sharp centercutting endmill can face off the existing bolt by centering over it, and pecking carefully down, until the mill runs true. Without moving anything, substitute the ball end mill you mention into the spindle, and peck a pocket in the top of the bolt. Then use a center drill, and drill it enough to include the chamfer. Vacuum chips out as you go. Then use a split point 5/16" drill and drill right through the bolt. Now you can decide whether to try extracting it, or whether to put an EM back in and mill it off flush with the threadsert. (Through the threadsert?) I can't quite picture why you want to do this, but assume you will replace the threadsert? or?

Sorry, can't quite picture what your end goal is.
 
Sadly, I don't know how to change the thread title. You are saying to make a drill guide to keep a bit centered? Could this be made from aluminum or would that be too soft and need to be mild steel. Also how much difference between guide hole and size of bit. Would 1/4" be a good size to try?


QT:[Would 1/4" be a good size to try? 1/4" what? end mill? I guess that would be Ok
The bushing should be .003 smaller than existing hole

(X) Aluminum would be too soft,
CRS would be OK, any steel Ok, drill rod and you could harden with it with water quench.
.003 clearance would be OK as you dont want the bushing stuck, or the cutter stuck.

Cutter flatten the broken bolt or put a centered dimple in it. Feed the cutter slowly to not break it, as Stephen said avoid carbide if the bolt is file-soft. Still take care with a nibble feed rate because you don't want a broken HSS cutter in the problem.

Getting the center then you might drill all the way through and fill it with WD or the like for a few days soaking.

With the broken not becoming lose and having some still good thread at the top of the hole, you might drill the broken bolt out to near the hole wall diameter ..and then go in with a tap.

*Oh you don't want the guide bushing to spin out of control and wreck the top of the hole, so put on a wrench flat..or a vise grip on it so not to spin. You can deliberately turn the guide bushing by quartes to maintain the ID from the effect of any bushing cutter wear.

*Sounds like Stephen is suggesting to just carefully (slow feed) come straight down with an end mill to square off the top of the broken bolt...That should work with no problem. Perhaps a 1/4" or 3/8" end mill and a slow down feed.
 
Thanks for the tips, and yes I have no real experience with milling so patience and going lightly is key. Some of the other inserts failed so now they all have to come out and be drilled larger and re-tapped for larger inserts or studs. The broken bolt is in an aluminum engine block. I have secured the block to the table on its base and have to access the hole by rotating the mill head 45 degrees to make it perpendicular to the hole. The stud repair kits use guide jigs and hand drills to re-do the bolt holes so hopefully I will only use the mill for the broken bolt - then I can proceed with normal repair procedures.
 
A good experience to set your mill to an angle job.

Sometimes a level mill..and then use a good level to check a part face.

With an angle plate, one can even check a verticle line of a part using a level.
 








 
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