Manual Horizontal Drilling over 1" Dia.

Have a repair to make in cast iron (fortunately) 6 threaded holes need repaired, drilled out and tapped to accept thread repair inserts. Original thread size is 1"-8, the inserts will require a 1-3/8-12 thread. Holes are in a horizontal orientation and due to the size, the part is not moving. I.E. The machining will have to be done in place.

It started out as one of those, just drill out the holes, helicoil them and be done. But then the reality hits.

My first thought is a mag-drill with an annular cutter, but I'm having a hard time coming up with one in the tap drill size for 1-3/8-12. Next thought was drilling undersize with an annular cutter and then single point boring to final tap drill size, but I'm not sure how well an unsupported mag-drill would do in that scenario. Holes are blind, can't line bore to size.

Only other thought is to rig a Bridgeport head on some kind of rigid stand and use it as a 'horizontal drill press'.

Tapping will be done by hand, I'm not concerned about that part.

Would prefer not to drill by hand just due to the hole size and enlarging an existing hole, I'll need my wrists for the tapping.

Also, I'm not looking to break the bank on this, couple hundred dollars (I can come up with a mag-drill or Bridgeport head) for tooling max.

Thoughts?

my guess is there blind holes or you would just run longer bolts with nuts on the back side if there's room . the mag base with piloted drills or reamers in two or three steeps to keep things centered and round

Is this a job for a vera-mil?

I had a similar challenge, and invented a very cheap solution.

I needed to bore round, correctly oriented 30mm pivot pin holes for my tractor front end loader.
The old holes were very, very egg shaped.

I used a carbide burr in a small woodworking router. the router was way, way too fast and it would over heat the burr in 10 seconds.
I did the job in many 10 second bursts, letting it cool between. That could be improved on a lot.

I slid a ball bearing into the shaft of the burr, and made a correctly sized template on my lathe. The bearing OD is not the same as the cutter ID, so there's a correction factor.

The template is wide enough to hold the router against by hand.

It went really easily, and came out better than I hoped. The pin slides through both holes easily but with no detectable slop.
In my case, I had to get 4 holes in a line. In actual use, it's 2 short pins but I tested with one long one to get lineup.









Not relevant for this post, but if anyone wants to know, I used a smaller 25mm straight drawn rod and bushings to get the template lined up before cutting.

How could you drill undersize with an annular cutter if they are blind holes ?

You can generally break off the core with a wedge in cast iron....A lot of big castings the iron is so soft it can be worked easily with chisels......which is how it was done 300 years ago.

I'd get a custom diameter annular cutter (or end mill) if at all possible.

Borrow or rent a mag drill with a Morse taper.

I have a Milwaukee with an MT 3 spindle. Has a drill chuck in it currently. Tighten the gibs and it will single point a rough hole with delicate feed. Not limited to annular cutters. Wouldn’t have bought it if it was.

greg_n said:

How could you drill undersize with an annular cutter if they are blind holes ?

Click to expand...

He is not "drilling" a hole, he is enlarging a hole that is already there, so it would have no center slug.
Otherwise do as john.k said and use a wedge to break the cast iron core off.

Is this a job for a vera-mil?

Click to expand...

It will work quite well if the rest of the casting in the area of the work is wide enough to shim & bolt the base down so the feed is perpendicular.

Is the area that a mag drill would hold on a machined, perpendicular flat?
It's not an uncommon failure mode for studs if the install is cocked to put bending loads on the threads where they exit the threaded hole, instead of pure tension loads.

There's a fixture/jig here made decades ago, with inter-changeable drill & tap bushings for putting inserts into aircraft engine cases....

A relatively simple bored piece of flat, heavy scrap that can be bored for slip fit bushing(s), bolted & shimmed to the machine, with the mag drill on that could be a solution. 1.) step pin through bushing bore aligns platform with existing hole while it is shimmed perpendicular and bolted. 2.) insert bushing reacts much of the drilling load through the bushing while drilling. 3.) Tap bushing aligns tap vertical. (if desired)

smt

It is easy to get cutters ground to size.

I have a mag drill I paid $300 for (used) and I use it horizontally from time to time.



Might be cheaper to get a 1.375" tap drill ground than a rotabroach cutter, but that's up to you.

metalmagpie

NFS_87 said:

Have a repair to make in cast iron (fortunately) 6 threaded holes need repaired, drilled out and tapped to accept thread repair inserts. Original thread size is 1"-8, the inserts will require a 1-3/8-12 thread. Holes are in a horizontal orientation and due to the size, the part is not moving. I.E. The machining will have to be done in place.

It started out as one of those, just drill out the holes, helicoil them and be done. But then the reality hits.

My first thought is a mag-drill with an annular cutter, but I'm having a hard time coming up with one in the tap drill size for 1-3/8-12. Next thought was drilling undersize with an annular cutter and then single point boring to final tap drill size, but I'm not sure how well an unsupported mag-drill would do in that scenario. Holes are blind, can't line bore to size.

Only other thought is to rig a Bridgeport head on some kind of rigid stand and use it as a 'horizontal drill press'.

Tapping will be done by hand, I'm not concerned about that part.

Would prefer not to drill by hand just due to the hole size and enlarging an existing hole, I'll need my wrists for the tapping.

Also, I'm not looking to break the bank on this, couple hundred dollars (I can come up with a mag-drill or Bridgeport head) for tooling max.

Thoughts?

Click to expand...

When I read of hand tapping anything bigger than 3/4" I start to get concerned

I had to drill and tap some 1 1/2 Whit holes in a big cast iron jig plate....The cast iron drilled and hand tapped with incredible ease....I already had the 1 1/2 W tap ,which is why I chose that size......I needed about 20 holes,could have done every square in the plate in a day,I reckon.

Manual Horizontal Drilling over 1" Dia.

Plus all above for a mag drill. If there’s an MT3 option available I agree that’s the most versatile solution. Beg, borrow, steal the mag base, a Weldon-shank holder and an MT3 center if you don’t already have them. You’re enlarging and existing hole for a threaded insert. With the time involved to get to that point I would recommend something better than a helicoil, personally, but you do you.

Since there’s already a hole, it doesn’t matter whether it’s blind or not. If theres anything at all left over sizing with an annular cutter it should break off with sideways glance in cast iron. I can’t see how the bottom of the hole comes much into play in this application.

I do think you should make a rudimentary plug to support the centering pin on any annular cutter you use. I’ve never made a blind hole with an annular cutter, but I have opened up holes in heavy steel plate using a mag drill and a bushing to support the centering pin. Blind or through-hole this doesn’t strike me as a particularly difficult job.

Use the MT3 center and another set of hands to get the tap started. It’s a big tap you’ll be driving, thankfully it’s a biggish hole in cast-iron and you only have to tap deep enough for whatever insert you settle on. I wouldn’t recommend actual helicoil inserts. I would probably make my own insert and glue it in. Otherwise I’d be looking at something cast-iron specific.



Be safe



Jeremy