Best U Drill for Steel, Stainless Steel and Cast Iron: Material-Wise Selection Guide

The best U drill for a job depends first on the material, then on hole diameter, depth, and coolant. In steel, use a balanced insert geometry with a tough coated grade. In stainless steel, use a sharper edge and strong coolant so chips do not pack in the flute. In cast iron, choose a wear-resistant grade, because abrasion is the main problem. The drill body can often stay the same across all three materials; the insert, the cutting parameters, and the coolant strategy are what change. This guide covers each material, then depth, holders, insert codes, troubleshooting and a shop-floor checklist.

What Is a U Drill, and When Is It the Right Choice?

A U drill is an indexable insert drill: a steel body with helical flutes that carries two replaceable carbide inserts, one near the centre and one at the outer edge. When the edges wear, you index or replace the inserts instead of the whole tool. In most workshops, "U drill" and "indexable drill" mean the same thing.

Because the body lasts through many insert changes, the cost per hole is usually lower than with solid carbide once holes get larger. As a general guide, U drills become the preferred choice from around 12 mm diameter upward. Below that, solid carbide drills are usually the better option.

Best U Drill for Steel, Stainless Steel and Cast Iron

Manufacturers group workpiece materials into ISO categories, and insert grades are developed for each. The table shows how the three materials differ in a U drill.

U drill for steel

Steel is the easiest starting point. Choose a general-purpose insert with a balanced chipbreaker that forms manageable chips without overloading the edge. Low-carbon steels tend to produce long, stringy chips, so chip control matters more than raw speed. Through-coolant helps flush those chips out of the flute.

For alloy steels such as EN24, or for pre-hardened material, run more conservative parameters and pick a tougher grade. Always check the workpiece hardness against the insert maker's chart instead of assuming a mild-steel setting.

Example: a batch of 22 mm holes in EN8 flanges on a vertical machining centre suits a 3D U drill with through-coolant and a general-purpose steel grade.

U drill for stainless steel

Stainless steel work-hardens, forms tough, sticky chips and holds heat in the cut. A U drill for stainless steel needs a sharp edge, a tough grade and strong through-coolant. Two habits ruin inserts in grades like 304 and 316:

• Dwelling or rubbing. If the feed is too low or the drill pauses in the cut, the edge rubs instead of cutting and hardens the surface ahead of it. Keep the feed steady and positive.
• Chip packing. If chips cannot escape, they jam in the flute and can chip the insert or break the drill. If the coolant holes are partly blocked or the flow is weak, fix that before changing anything else.

Through-coolant is not optional for most production work in stainless, particularly at 3D depth and beyond.

U drill for cast iron

Cast iron is abrasive and forms short chips or powder, so the priority shifts from chip control to wear resistance. Choose a wear-resistant insert grade and expect the outer corner to wear first. Coolant is less critical than in stainless: for shallow holes external coolant, an air blast or minimum quantity lubrication can work. For deeper holes through-coolant still helps clear chips. Extract or contain the dust, since it is abrasive to slides and guards.

Cast iron can also chip at the exit side of the hole. Consider reducing the feed as the drill breaks through.

For aluminium, titanium and heat-resistant alloys, different insert grades and geometries apply, so check the manufacturer's chart first.

How to Choose a U Drill: Five Checks Before You Order

1. Diameter and depth

Depth is quoted as a multiple of diameter: 2D, 3D, 4D and so on. Choose the shortest depth that clears your hole. A longer body is less rigid, and a 4D drill on a 2D job gives up stability for nothing. Most indexable drill systems run from 2D to 5D. Deeper holes generally call for gun drilling or dedicated deep-hole tooling. Jaibros's Chuanrun WC series U drills are listed in 0.5 mm diameter steps with 2D, 3D and 4D options.

2. Shank and holder

The shank size must match your holder or sleeve. The Chuanrun drills listed for 16 to 23.5 mm use a C25 shank, and the 25.5 to 27.5 mm drill uses a C32 shank. A worn or dirty holder adds runout, which shortens insert life. See our guide to tool holders and collets for more.

3. Coolant

Check whether your machine has through-spindle coolant and at what pressure. If it does not, you can still use U drills on shallower holes in cast iron or steel with external coolant, but stainless and deeper holes get much harder.

4. Insert code and compatibility

The insert must match the drill body. The insert code appears in the drill's specification. The listing for the 16 to 17.5 mm Chuanrun drill names WCMX 030203 inserts with a T8 screw, and the 25.5 to 27.5 mm drill names WCMX 050308 with a T10 screw. SP-style inserts such as SPMG 050204, 060204 and 090408 fit different bodies and are not interchangeable with WC inserts. Confirm the code before ordering.

A multi-purpose grade such as TB40G is convenient because one grade covers several materials. For high-volume work in one material, a dedicated grade will often last longer.

5. Tolerance and finish

A U drill is built for productivity, not for the finest hole quality. If a hole must hold a close tolerance or a fine finish, plan a follow-up boring or reaming operation. Our boring bars cover that step.

Setup Details That Protect Tool Life

A stable setup matters as much as the insert. Keep the overhang as short as the part allows, use a clean holder, and keep the spindle runout low. On a lathe, set the drill exactly on centre height, because misalignment leads to uneven insert wear and oversized holes. Entry is best on a flat surface. If the surface is angled or curved, reduce the feed until the drill is fully engaged. Before starting, check that every coolant hole is open.

Always begin from the insert maker's recommended cutting speed and feed for your grade and material. As an illustration only: if a maker recommends 120 m/min for a 20 mm drill, spindle speed is 1,000 x 120 / (3.14 x 20), roughly 1,910 rpm. A feed of 0.10 mm per revolution at that speed gives a feed rate of about 190 mm/min. Then adjust based on chip shape and insert wear.

Troubleshooting U Drill Problems

Quick Checklist for U Drill Users

• Identify the material (P, M or K) before choosing the insert.
• Check the hole diameter and choose the shortest depth (2D, 3D or 4D) that works.
• Match shank size (C25 or C32) to the holder.
• Confirm the insert code for your specific drill body.
• Check that through-coolant is on and the coolant holes are clear.
• Keep overhang short and the holder clean.
• On a lathe, set the drill on centre height.
• Start from the maker's recommended speed and feed.
• Inspect both inserts for wear at each change, and replace worn or chipped ones.
• Plan a boring or reaming pass for tight-tolerance holes.

Frequently Asked Questions

Which U drill is best for stainless steel?

Choose a U drill with a sharp-edged insert geometry and a tough grade, and run it with strong through-coolant. Keep the feed steady to avoid rubbing, and make sure chips can leave the flute.

Can one U drill be used for steel, stainless steel and cast iron?

Usually yes. The drill body can stay the same and you change the insert. Multi-purpose grades cover several materials, but for repeated work in one material a dedicated grade will usually last longer.

Do I need through-coolant for a U drill?

For stainless steel, deeper holes (about 3D and beyond) and high-feed production, through-coolant is strongly recommended. For shallow holes, cast iron and low-volume work, external coolant, air or minimum quantity lubrication can be acceptable.

Should I choose a U drill or a solid carbide drill?

Use a U drill for larger holes, generally from about 12 mm upward, where cost per hole matters. Use solid carbide for smaller diameters or when hole quality is the priority.

When should I replace U drill inserts?

There is no fixed interval. Index or replace the inserts when you see flank wear, chipping, a rougher hole finish, oversize holes or rising load. Keep a count of holes per edge so you can change them on a schedule instead of after a failure.

Conclusion

Picking the best U drill is mostly a matter of matching three things: the material, the insert and the coolant. Use a balanced geometry for steel, a sharp and tough setup with strong coolant for stainless steel, and a wear-resistant grade for cast iron. Then confirm the insert code, keep the setup rigid, and start from the maker's parameters. To build your setup, browse Jaibros's Chuanrun U drills and drilling inserts.