tapping a hole that comes through on one side but not the other

Posting here rather than in a general forum, because I trust the people here.

I'm making eight M10 threaded holes on the bottom side of my lathe bed, for fastening a taper attachment. The material is high quality cast iron.

Because of the shape of the casting, I have two choices:

(1) Make blind tapped holes about 20mm deep, or

(2) Make through holes, and tap them.

I would like to do (2), but the problem is, the shape of the casting is irregular, so the hole will pierce "through" on part of the boundary circle, and remain in the casting on the other part.

I think the last time I broke a tap was in exactly this situation. Once the tap got into the region where half the hole was in metal and half was in air, it twisted and broke.

So is a blind tapped hole the right thing?

Cheers, Bruce

Tapping a half hole.

ballen said:

Posting here rather than in a general forum, because I trust the people here.

I'm making eight M10 threaded holes on the bottom side of my lathe bed, for fastening a taper attachment. The material is high quality cast iron.

Because of the shape of the casting, I have two choices:

(1) Make blind tapped holes about 20mm deep, or

(2) Make through holes, and tap them.

I would like to do (2), but the problem is, the shape of the casting is irregular, so the hole will pierce "through" on part of the boundary circle, and remain in the casting on the other part.

I think the last time I broke a tap was in exactly this situation. Once the tap got into the region where half the hole was in metal and half was in air, it twisted and broke.

So is a blind tapped hole the right thing?

Cheers, Bruce

Click to expand...

I would drill through and tap the hole. Since the material is cast iron, tap without oil. Drill a hole for the tap drill in a scrap block and clamp it to the lathe to start the drilled hole square. Use a square to start the tap square or remove the scrap block and open the hole for the tap. When you start to break thru use care, reversing the tap often.

Roger

I don't think the previous poster understood your query.

Drill the holes 20 mm DP, blind.

Bruce:
Your problem is two fold....Because the drill exit the casting as a partial hole if you drill through, the drill will wander toward the open side when it breaks through....
Likely not practical, but in some cases its possible to clamp a piece of scrap stock that becomes a filler so the drill continues cutting on its full diameter...partly in the casting and partly
in the scrap.

Further as you tap the hole, as you already know, there will be a tendency for the tap to wedge toward the open portion and the results often are less than pleasant. The packer (scrap) will help eliminate this..

My suggestion is to go with the 20mm depth as "RC" states above...If you are worried about the engagement of the fastener, a good solution would be to use studs in the casting...that way you can run them to full depth
Having of course first finish tapped the holes to full depth (almost) with a bottom tap)

Cheers Ross

Drill and tap your holes 20mm deep. It's what you need and will be a neat looking job.

To all who replied, thanks for the prompt and useful advice.

Ross, you are correct. In addition to tapping the hole, there is also the issue of drilling it first. And now that I think about it, the last time I broke off a drill in work was a similar situation, where the drill exited on one side of the OD but not
on the other, began to wander, and then fractured.

The idea of clamping some scrap "packing" stock as backing is a good one. The area is accessible enough to allow this, but unfortunately the surface I would need to clamp against is in between the cross ribbing on the bottom side of a lathe bed. It's got a curved, rough and uneven surface. The only way I could get a decent fit to it would be to use epoxy to fill the gaps, and epoxy is so much softer than cast iron that this would be almost like air. Bottom line: not practical here.

I would rather not use a stud because a bolt will allow for better clearance. But I can tune the bolt length carefully, and 2 x diameter depth in cast iron is more than enough to achieve full fastener strength, provided I use a bottoming tap to thread (almost) the full depth.

RC Mech, SIP6A, thanks, you are right, will do.

If you drill all the way thru the casting, can you access the back of it with a piloted counter boring bit to even up the exit hole?

Hi All,

Bruce, Did I miss your post that named the lathe that you eventually chose to buy?

Alan

I was able to go 25mm deep without breaking through the casting, so the blind holes worked out nicely, they are the 8 threaded holes in between the two clamping bars. The 8 outer threaded holes are original, where the bed attaches to the "foot" supports.



Alan, I am still shopping for a larger lathe, am waiting for the right one to show up. But I have decided that I will keep my smaller Logan lathe as well. Right now I'm building a taper attachment for it, using a linear slide. The 8 threaded holes are to fix this to the lathe bed.

Cheers,
Bruce