Proper technique to enlarge a hole

[manoweb]

I had to repair my tractor and needed to enlarge holes to make new threads using a hand help cordless drill. With a brand new, high quality drill of the correct size, it was a nightmare: it was catching on all the time. At the end of the fourth and last hole, it finally broke in half. This is not so uncommon apparently, a drill needs more material in order to be able to work correctly. So I wonder, what is the *proper* way to enlarge a hole slightly when drilling into thick steel?

 

[mike93lx]

How thick?

Step drill is my go to, just need one with steps larger than the material is thick

 

[BrandonV]

What lubricant/cutting oil were you using?

 

[Oldsnapper]

Is it a through whole or a blind hole? If it is a through hole, a reamer would be the way to go. A blind hole you are more limited.

 

[Pinemarten]

Reamers work way better than drill bits for small increment hole enlargement.
How many thousandths are you going up in hole size?

 

[rbgearz]

Take a stone to the cutting edge of the drill bit This definitely helps to keep it from grabbing.

 

[manoweb]

The depth of the hole (blind, into the (cast) transmission case) is about 1-1/2inch. As I was working with stripped threads I don't have an ultra precise measurement but it was more or less from 1/2" to 37/64"
The idea to "dull" the edges a bit might be actually pretty good, as counter-intuitive as it sounds...

 

[manoweb]

What lubricant/cutting oil were you using?

Some CRC cutting oil in a spray can. As the holes are vertical and I was going in from below, I sprayed in and the cut would stay nicely lubricated with the oil dripping down, but this did not prevent grabbing. Also, being this material a cast transmission case, I am not even 100% I should have used lubricant at all

 

[Steve_P]

As you said, this is about the worst thing for a drill bit as only a small portion of the bit, at the OD, is doing all the work. But sometimes you just have to do it this way. A few months ago, my neighbor brought me a steel part with a 7/16 hole that he wanted enlarged to 1/2, or similar, and I was like, "oh man...."

 

[isb cornbinder]

I had to repair my tractor and needed to enlarge holes to make new threads using a hand help cordless drill. With a brand new, high quality drill of the correct size, it was a nightmare: it was catching on all the time. At the end of the fourth and last hole, it finally broke in half. This is not so uncommon apparently, a drill needs more material in order to be able to work correctly. So I wonder, what is the *proper* way to enlarge a hole slightly when drilling into thick steel?

The reason your drill was catching and kicking back was because you were not strong enough. There must be enough resistance/traction to hold the drill-motor firm. A twist drill will likely hook and stop cutting if there is too little metal to cut. Too much drill speed is another problem.
I bought a HOUGAN mag drill and annular cutters for a job much like you describe.

 

[larry_g]

The depth of the hole (blind, into the (cast) transmission case) is about 1-1/2inch. As I was working with stripped threads I don't have an ultra precise measurement but it was more or less from 1/2" to 37/64"
The idea to "dull" the edges a bit might be actually pretty good, as counter-intuitive as it sounds...

The idea is not to dull the cutting edge, lip, but to change the angle behind the lip that the spiral flute forms. Change this angle so that the bit does not tend to self feed and screw into the part. Change this angle to something like 7-12 degrees.

lg
no neat sig line

 

[manoweb]

The reason your drill was catching and kicking back was because you were not strong enough. There must be enough resistance/traction to hold the drill-motor firm. A twist drill will likely hook and stop cutting if there is too little metal to cut. Too much drill speed is another problem.
I bought a HOUGAN mag drill and annular cutters for a job much like you describe.

Maybe, but the drill (bit) broke in half at the last hole, so the problem is only in part due to lack of rigidity from my part. It is well documented that this kind of operation leads to breakages or chipping of the cutting edge, even in rigid setups. I would have loved to put this part on the Bridgeport, but it's the bottom of a tractor's transmission. Which by the way does not have machined surfaces and it's all curved, so it would be pretty difficult to fit a mag drill anyway to fit existing holes.

 

[manoweb]

The idea is not to dull the cutting edge, lip, but to change the angle behind the lip that the spiral flute forms. Change this angle so that the bit does not tend to self feed and screw into the part. Change this angle to something like 7-12 degrees.

Ohh this sounds so interesting but I am not 100% sure I understand. I will lookup drill geometries to try to understand

 

[LopezBart]

I assume this was a cast-iron transmission casing? You can stone a small flat on the drill cutting lip to reducing the rake angle to as little as zero, as one does for drilling brass. Those machining a lot of brass often maintain a set of drills modified like this.

 

[larry_g]

Ohh this sounds so interesting but I am not 100% sure I understand. I will lookup drill geometries to try to understand

Think of it as the top relief on a lathe bit.

on edit, Bart shows the modification.

lg

 

[manoweb]

I assume this was a cast-iron transmission casing? You can stone a small flat on the drill cutting lip to reducing the rake angle to as little as zero, as one does for drilling brass.

It's a ferrous cast part, it's an old Deere tractor, I cannot imagine they used any advanced process, but at the same time I don't have the elements to be more precise as I am not really an expert of this brand or tractors in general. It started as "I need to tighten those bolts as the front loader seems to wobble" and quickly becoame "I need to extract two broken bolts, rethread two more because they are completely stripped"

 

[larry_g]

It's a ferrous cast part, it's an old Deere tractor, I cannot imagine they used any advanced process, but at the same time I don't have the elements to be more precise as I am not really an expert of this brand or tractors in general. It started as "I need to tighten those bolts as the front loader seems to wobble" and quickly becoame "I need to extract two broken bolts, rethread two more because they are completely stripped"

I've learned over the years that sometimes it is better to 'leave well enough alone'. I've got an old backhoe that has enough slop in the swing that you let off the swing handle 12" before the spot you want to hit....

 

[manoweb]

Well in this case it got so bad that I couldn't realistically continue using this tractor. Something would have broke as the bottom mount was essentially completely loose.

 

[manoweb]

I believe that I should do what is presented in this video, correct?

 

[Cane]

Make a guide block out of 1 1/2 steel and bolt or clamp it down then use a bottoming bit.

The bit broke because it was in a bind.

 

[LopezBart]

Front end loader bolts are well-known for coming loose on a wide variety of tractors. Retorquing them should be part of regular maintenance, but it's not called out in manuals. Grade 8 bolts and washers will help, but these bolts are under a lot of stress in some tractor designs.

 

[manoweb]

Make a guide block out of 1 1/2 steel and bolt or clamp it down then use a bottoming bit.

The bit broke because it was in a bind.

I had planned to do that but there is no machined surface and the holes go into a cast, curved part. I desisted from using a block.
There is no doubt that it broke because the setup is less than ideal, but the fact remains that it was grabbing so much that it made the whole experience really miserable. I might still bind and break a bit next time, but if the zero rake mod makes it less painful to drill it's still a win.

 

[dchawk81]

@PelicanPines

 

[manoweb]

Front end loader bolts are well-known for coming loose on a wide variety of tractors. Retorquing them should be part of regular maintenance, but it's not called out in manuals. Grade 8 bolts and washers will help, but these bolts are under a lot of stress in some tractor designs.

The follow up question is, once I cut new threads, if I buy grade 8 bolts will I be able to apply all the nominal torque, that for this size is 180ft*lbs (that should result in a clamp load of around 23k lbs) without stripping the threads in the cast part?

 

[rustyzman]

https://www.mscdirect.com/product/details/01585322

Core drills are made for enlarging an existing hole. Best used in a machine, but you can do it in a stout hand drill as well.
Reamers work too, but they require smaller steps between sizes.

 

[manoweb]

Core drills are made for enlarging an existing hole. Best used in a machine, but you can do it in a stout hand drill as well.
Reamers work too, but they require smaller steps between sizes.

I see, yeah, those look like the right tool for the job if one is willing to pay up

 

[JradM]

Is it a through whole or a blind hole? If it is a through hole, a reamer would be the way to go. A blind hole you are more limited.

This is the way.

 

[PCustoms]

@PelicanPines

I was thinking, oh wow, this made it to 26 posts without any innuendo...then I opened the thread.

 

[dchawk81]

I was thinking, oh wow, this made it to 26 posts without any innuendo...then I opened the thread.

Well that's your fault.

 

[LopezBart]

The follow up question is, once I cut new threads, if I buy grade 8 bolts will I be able to apply all the nominal torque, that for this size is 180ft*lbs (that should result in a clamp load of around 23k lbs) without stripping the threads in the cast part?

Your new bolts are bigger, of course, so you can up the torque considerably in any case; I'd guess that something along 75% more than the original values. You'll get a qualitative idea of the casting strength when you're tapping the new larger holes; the fact that your drill bit broke makes me think it is pretty tough stuff. It looks like you're tapping a 5/8-18 thread 1.x" deep... [a coarse thread is usually recommend for cast iron; if the original ones were fine thread I wouldn't worry about it]. A bottoming tap will definitely increase the depth of threads you can use. For good quality cast iron, I see torque values of 190 ft-lbs for grade 8 5/8-11 bolts in cast iron, and 215 ft-lbs for fine thread; these numbers are substantially the same as grade 5, which makes me think the limits here are the casting. If this is an older tractor, I'd reduce these.

https://www.melling.com/wp-content/uploads/2017/03/Common-Bolt-Markings-Torques-Tech-Bulletin-AERA-Melling-Engine-Parts.pdf

 

[mreisner]

Drilling to put in helicoils? 4020?

 

[larry_g]

If this application used bolts that were a bit short then you should have some good threads at the bottom of the hole, with luck. Consider putting in a stud and in as deep as you can and then torque things with the nut. The stud can be installed with the loader in place if necessary if necessary.

lg
no neat sig line

 

[BlitzcrankJapan]

Almost related to above. You can may be able to loctite and insert a soft bolt or stud. Cut it off and then redrill the filled hole (with the larger diameter). This is similar to welding a hole and redrilling it. Except you don't need to deal with the hardened weld material. You have a soft bolt to drill into. This should substantially decrease grabbing.

 

[theoldwizard1]

Depending on the thickness of the metal, buy a step drill.

I have not tried this, but it should work. Get a cheap 45° carbide router bit. It will cut metal, just don't lean on it too hard. Now you larger drill won't have an "edge" to catch on.

 

[manoweb]

Drilling to put in helicoils? 4020?

I figured that the hole I'd drill for the helicoil is exactly the same I could drill for a new bigger bolt. As I am not restricted by the size of the bolt I thought I'd save the helicoil process and go directly with a bigger bolt.

 

[manoweb]

If this application used bolts that were a bit short then you should have some good threads at the bottom of the hole, with luck. Consider putting in a stud and in as deep as you can and then torque things with the nut. The stud can be installed with the loader in place if necessary if necessary.

I did not think about using a stud, I am not sure what would be the advantage over bolts. These four bolts are just used to fix a plate in place, that it not very unwieldy; the bottom of the front loader attachment then bolts to this plate.

 

[manoweb]

Your new bolts are bigger, of course, so you can up the torque considerably in any case; I'd guess that something along 75% more than the original values. You'll get a qualitative idea of the casting strength when you're tapping the new larger holes; the fact that your drill bit broke makes me think it is pretty tough stuff. It looks like you're tapping a 5/8-18 thread 1.x" deep... [a coarse thread is usually recommend for cast iron; if the original ones were fine thread I wouldn't worry about it]. A bottoming tap will definitely increase the depth of threads you can use. For good quality cast iron, I see torque values of 190 ft-lbs for grade 8 5/8-11 bolts in cast iron, and 215 ft-lbs for fine thread; these numbers are substantially the same as grade 5, which makes me think the limits here are the casting. If this is an older tractor, I'd reduce these.

https://www.melling.com/wp-content/uploads/2017/03/Common-Bolt-Markings-Torques-Tech-Bulletin-AERA-Melling-Engine-Parts.pdf

The originals were coarse. I decided to go fine because the next size up of the stripped holes matched a fine thread. Fine threads are typically easier to tap (as I will have to do it by hand from under the tractor, I was trying to maximize my chances of success) and should have some advantages, but I did not consider they'd not be recommended in cast iron. There is no danger for contamination to foul these threads, but I hope I'm doing the right thing. The 5/8-18 (fine thread) tap set is supposed to be delivered tomorrow, If the consensus is that fine threads are awful in cast material I am still in time to send everything back, place a new order and re-drill and re-tap in a coarse size... An extra week added to this repair won't make a huge difference... But how bad really are fine threads in cast material?

 

[Diabolical D]

Delicately sir.

 

[LopezBart]

I'd suggest tapping 5/8-11; these threads will be far more resistant to pulling out and loosening due to vibration, esp. if the cast iron is of less than stellar quality. You'll get a lot more shear area in the cast iron with the coarse threads.

 

[Wamsutta]

I've gotten away with spinning up the drill bit real fast and then sending it through slowly to enlarge a hole. Something about spinning slow causes the drill bit to get caught. Now if I'm drilling a new hole, I prefer to go in slow with lots of cutting oil.