Have a problem tapping

4435

Member
Working on front bolster on a tractor. A casting of some type cast steel is my assumption. Started out with a twisted off bolt drilled off center (not me) 10X1.50MM. Tried the washer and nut weld method numerous times. Decided to drill and try easy out have some of the fluted ones, no success there.
Next was drill for helicoil, did not seem to have troubles drilling but now am unable to tap. As I start tap it is breaking the teeth off the tap. Have I hardened the cast some way and how can I get around it. TIA Tom
 
Well I have to ask... are u using a taper tap? Is the hole drilled the correct
size? Are u using rapid tap oil?need to start someplace .need info.
 
Is the hole you drilled straight? A piece of old bolt can wedge and cause real issues. Another Idea is tramp metal in the casting as hard as the
hubs of a NASCAR wreck. Jim
 
You are limited in tap choices with a helicoil tap.

It could be the hole is drilled crooked, trying to get the bolt out welding could have hardened the steel or left some weld in the hole, or a part of the old bolt is still in the hole.

You are lucky just the teeth are breaking off. I would stop while ahead.

If there is room, I would go with a thread insert bushing.

You can make a drill guide with a thick block of steel and a known straight hole to keep the drill in the right location and straight. It is bolted down with an adjacent hole. Sounds like a lot of trouble, but you only get one more chance to get this right without making it worse.

The bigger tap will be more robust, and the bigger hole will clean up any remnant of bad threads, left over pieces of the bolt or work/heat hardened material.
 

Unlikely to be cast steel . More proprobably it is Spheroidal Graphite cast iron . This really does machine well in normal circumstances , has the hole been welded up and re drilled before ?
 
Dennis, Four flute tap. Have not seen a two flute helicoil tap. Part number on tap is "570-10". Kit package calls for 13/32 bit. Checked with friend of mine has same kit, calls for same bit. Tom
 
Jim, Being a casting and thinking that we don't know what goes into the crucible you may well be right. Those old cars have some special steel makes me think of drilling a T-post, never know what will happen then. Tom
 
Steve, You unfortunately probably have the only way out. Searched for an insert yesterday, no success locally. Will study up on those more. Have done many helicoils but no inserts. Tom
 
McMaster Carr has them.

Also once the hole is drilled, you can drill out the guide and use it to guide the tap straight.

Start with a taper tap, finish with a spiral bottom tap to pull the chips out if it's a blind hole.

Some insert bushings have red locktite already on them. It is serious stuff! be sure the hole is tapped deep enough to get the insert all the way in. If you put it in and it bottoms, get it out immediately! Once the locktite sets it's near impossible to get it out!
 

It is POSSIBLE there's it's just the surface of your cast iron that has gotten really HARD and that's what is tearing up your tap, once you get past that the rest of the hole MAY tap easily.

If there's enough depth to the hole to allow it I would take a die grinder and enlarge the hole one thread's depth down, then try again to tap.

HOPEFULLY the casting will be "softer" at that depth.

I have experienced what I wrote about above with the head bolt holes in Wisconsin air cooled engine blocks.
 
(quoted from post at 10:35:21 01/30/21) McMaster Carr has them.

Also once the hole is drilled, you can drill out the guide and use it to guide the tap straight.

Maybe instead of drilling out the guide, tap it also, and keep going.

Dusty
 

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