How to Drill Cast Iron

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Cast iron can be challenging to work with. It is not a hard metal and it can splinter or crack if you do not drill it properly. However, its more than just picking the correct drill bit for cast iron. Drilling through cast iron is more about technique than it is about the proper tools.

First we’ll look at how to drill your cast iron, and then we’ll address picking the right bit, along with some common questions.

Drilling Through Cast Iron

The drilling process is pretty simple, but it’s easy to mess up the cast iron if you do this wrong. Again, this is a soft metal, so you have to be careful!

1. Pick the Right Drill Speed

This is a softer metal, so you should never drill any faster than 150 surface feet per minute. This value is connected with your drill revolutions per minute through the simple formula:

Drill

RPM=SFM x 3.82/ drill bit diameter

This means that if you are drilling a ½ in hole, the max drill speed you should aim for is 1,146 rpm. This is the midrange for most cordless and corded drills.

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Cut this speed in half for hard cast iron or if you are drilling outside in the winter. In winter conditions, which can influence the brittleness of the metal, reduce your drill speed to 300 rpm.

2.  Mark Where You’ll Cut

Start out by making a dimple, or an indent where you intend to drill your hold. It is always a good idea to mark this spot clearly so that you don’t:

  • wander off the intended spot for the hold
  • don’t drill to the side of an area already drilled into if you have to lift your drill during the process.

A permanent marker is a big help for this part of the process.

Before drilling your hole, you might want to drill a pilot hole. Done with a smaller bit, this will allow you to practice on the cast iron a bit before making the permanent hole.

3. Verify Your Drill Bit

You will want to double-check that the drill bit that you have selected is actually the proper size. Compare it to the area that you have marked out, or you can even do a couple of test revolutions to see what the size of the hole will be.

We have a section below on how to pick the best drill bit for cast iron.

Brace the cast iron if it is a loose piece, or one that will not sit flat on your table. If it is connected to something else, you will need to brace this item as well in some cases.

Be sure that your work surface and the cast iron itself will not move as you drill.

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4. Start Drilling

Once you have marked the hole and stabilized your work surface, you will want to carefully start drilling.

  • Hold the trigger firmly and slowly move the drill in an out of the cast iron. This deburrs the hole and makes sure to slough off any excess metal shavings or roughness around the hole.
  • Wipe down the surface when you are done drilling with a dry cloth, then follow up with a wet one.
  • Dry it off again and see what the hole looks like. If the inside is still rough, you can make another couple of passes to deburr it further.

Always make sure to feel out each drill pass that you make to be certain that the drill bit is not snagging or catching in the hold. This can be a sign that you are causing damage to the cast iron that will lead to breakage.

Cast Iron Drilling Tips

Here are a few simple but important cast iron drilling tips to pay attention to:

  • Always remember to use proper drilling safety which includes eye protection and keeping your work space free of debris and other items that might hinder your work or cause harm to you or others near you.
  • Use extra cutting fluid if you feel that your drill bit is getting too warm. You can always wipe off excess debris as needed and reapply if there is dust and grime sticking to the cutting fluid.
  • Start at slow drilling speeds and work up to a comfortable speed. More speed causes more friction which can lead to breakage of the drill bit or the cast iron itself. Don’t be ashamed of easing into your drilling process.
  • Step up through bit sizes as needed. You can always start with a smaller bit and work your way up as needed. You cannot however, put metal back into a drilled hole if you have carved out too much.
  • Mark the surface that you are drilling with a permanent marker if needed before you start to drill. There is no shame in knowing exactly where you intend to place the hole as you work.
  • Verify the width of the material that you are drilling into before you damage it. Keep a ruler or a tape measure handy to check the width before you select the right drill bit. There is no shame in knowing what thickness the metal you are working with actually is so that you can achieve a nice finished product.
  • NEVER drill into loose cast iron. You must always use a vice if you cannot hold down the piece of iron readily. This is a major safety consideration and one that you should not skip.

Is Cast Iron Easy to Drill?

Cast iron is very soft and brittle, so it is very easy to drill into compared to other metals. However, this also means that it is easy to break and chip, which means that you have to be careful.

Cast iron pot

Drilling cast iron can be a necessary skill to learn if you fabricate a lot of materials in your job or when you are making items at home for your own use.

Cast iron has its own unique drilling process and you will need to learn about the right way to drill with this metal before you get started trying to make items in cast iron.

Should You Use Lubrication?

This is one of the biggest points of debate that you will encounter when you ask people to teach you to drill cast iron. Many machinists will say that you should not lubricate when you drill cast iron, while others swear by lubrication to prevent cracking or other imperfections in the finished result.

Cast iron has a high carbon content and carbon acts as a natural lubricant. This means that you will often not need a lubricant to prevent issues with drilling into cast iron. This is a nice benefit to working with cast iron since lubricants can be messy and they can affect your ability to control your drill bit as you work.

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If you are going to use a lubricant, make sure to use a water-based or an oil-based lubricant to eliminate the risk of wear and tear to the drill as well as cracking of your drill bit. Most expert cast iron metal workers will tell you that if you are going to drill more than two holes, you will need to lubricate to prevent the built-up heat in the drill bit from damaging the cast iron.

Drill Bits for Cast Iron Metal

You will not need a specialized drill bit to handle drilling into cast iron. Regular drill bits should do the trick so long as you pay attention to the speed that you are running them at and the heat in the drill bit.

  • You should never use a bit that is made for wood or masonry due to their shape and you will want to avoid hammer drill bits as well.
  • Dull tips are never going to go through the cast iron and are much more likely to damage it. Sharpen your drill bits if needed, and invest in a drill bit sharpener for future projects.
  • Forstner and auger bits will also not be suitable for any kind of metal drilling, including cast iron drilling.

The best drill bit that you can use is a convention bit with a 135-degree point angle. The angle in these bits will make the tip quite sharp.

You should also consider a cobalt bit because cobalt is more accurate and stronger when heated. If you have a titanium nitride bit, that will do the trick as well.

When using a drill press, you will probably find that the right bit is a 118-degree point angle drill bit. These bits work more smoothly and create less shards.

Find the best inexpensive drill press in our review!

Summary

Drilling cast iron is a simple task that just requires some finesse and some planning. You will just need the right drill bit and the right technique to create properly drilled out cast iron that can be used for a variety of project.

With some planning and patience, you will be able to drill any piece of cast iron that you want and create beautiful, tidy holes that can be used for a variety of different purposes.

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An expert at home repair, remodel, and DIY projects for nearly 40 years. His first experience came in completely restoring an antique home. Completely redone from the inside out, and restored to its original form, the home is a featured design by renowned Southern California Architect Cliff May, considered to be the father of the California Ranch Home. Now Dennis spends his time on fine woodworking projects and tool comparisons.