The flute configuration of a reamer is important.  Selecting the correct configuration helps optimize tool life and hole finish.

Left hand spiral reamers should be selected for through holes.  A through hole has an opening on the other end.  If you put your eye up to the hole, you would be able to see out of the other side of a through hole.  A left hand spiral pushes the chips ahead of the reamer and out the other side of the hole.  This reduces the chances of scarring the work-piece and reduces the chance of damaging the reamer by previously cut chips.  Additionally a left hand spiral helps bridge interruptions such as cross-holes or keyways.  Left hand spirals are excellent for reaming hard materials and typically provide the best size and finish as compared to right hand spiral or straight flutes.

Right hand spiral reamers should be selected for blind holes.  A blind hole is a hole that does not have an opening on the other end.  If you were to put your eye up to the hole, all you would see is darkness.  In a sense, you would be “blind” while looking in the hole.  A right hand spiral pulls the chips towards the back of the reamer.  This helps prevent the chips from getting packed into the bottom of the hole reducing the chance of damaging the reamer and the work-piece by previously cut chips.  Additionally a right hand spiral also helps bridge interruptions such as cross-holes or keyways.  A right hand spiral reamer may cut slightly oversized holes due to the aggressive flute geometry.   They perform very well in highly ductile materials.

Straight fluted reamers are very good general purpose reamers.  They are typically cheaper than left hand or right hand spiral reamers and work best in non-chip forming materials such as cast iron, bronze and free cutting brass.  The preferred hole condition for a straight fluted reamer is a through hole all things being equal but they perform well in blind holes as well due to their non-aggressive geometry.  If all the application parameters are known typically a right hand or left hand spiral reamer will provide superior results in finish and performance then a straight fluted reamer.

For additional information on left hand and right hand spiral reamers while reaming with coolant please see our previous post Reaming with Coolant.

As always if you have any questions about left hand spiral, right hand spiral, or straight fluted reaming (or any other cutting tool questions) feel free to leave us a comment here on our blog or fill out the form on Super Tool’s Contact Us Page.

Bryan Enander
Super Tool, Inc.

  1. claudia says:

    Hello there,

    I’m wondering if you could help me what does
    aggressive flute geometry mean?
    why do left hand fluted reamers cut harder materials?

    Many thanks 🙂


    • brednane1981 says:

      Hi Claudia,
      Aggressive flute geometry refers to a high radial rake angle on the flute of the reamer. The higher the radial rake angle, the more “aggressive” it is. For more on rake angles here’s a link to my blog on Milling Cutter Rake Angles. Even though the blog is about milling cutter rake angles the rake angle on a reamer is the same and the same concepts apply. The radial rake angle of a reamer is the angle between the flute face and a radial line passing through the cutting edge in a plane perpendicular to the cutter axis that the flute sits at relative to the center of the reamer. The rake angle can be positive or negative. This angle is important because different materials machine better at different rake angles all other things being equal.

      Your other question was “Why do left hand fluted reamers cut harder materials?” The majority of cutting tools are right hand cut meaning they spin to the right when they are cutting. A left hand spiral fluted reamer that is right hand cut is better for cutting harder materials because the opposing forces of the left hand spiral and the right hand cut balance each other out resulting in rounder and straighter holes. You would get the same result with a right hand spiral reamer that is left hand cut but left hand cut tools are not very common in the industry although they do exist.

      I hope this answers your questions. Let me know if need further clarification.


      • claudia says:

        Hi Bryan,

        Thanks for your extensive reply – that is really helpful!

        I was looking at your milling page and at the materials listed. Presumably, when reaming there are only certain types of metal reamers that will work on other harder metals – is there a strict format / chart that I could follow for advice on which materials can be reamed by which types of reamer e.g. cobalt reamers work on … as opposed to a high speed steel reamers that only work on …?

        Thanks once again for all your detailed information!

        Claudia 🙂

        • brednane1981 says:

          Hi Claudia,
          There is not a chart or formula that will tell you which type of reamer to use but here are some things to consider when trying to determine what material the reamer should be made out of. Cobalt, HSS, and carbide reamers each have different hardnesses with carbide being the hardest and HSS being the softest . As the hardness of the material you are reaming approaches the hardness of the reamer you are using you will get a reduction in tool life because the tool wears down faster. If you are reaming harder material you would want to use a reamer that is significantly harder than the material to get longer tool life. That is one advantage to using carbide tipped; it’s wear resistance at the cutting edge is far superior to HSS or Cobalt. Another advantage of carbide tipped over the other two is you can use higher speeds and feeds which results in shorter cycle times for the job you are running. If the hardness is low, either of the three will “work” but there will be a reduction in tool life for HSS and Cobalt and the cycle times will be longer because you can’t run the tools as fast as the carbide tool. As a result typically HSS and Cobalt tools are cheaper than equivalent carbide tools. If you are just reaming one hole it may be more economical to use a HSS tool. If you are doing a long production run than carbide is more economical because, you can run it faster, you will get more holes, and you will have less machine downtime for tool changes.
          I hope this helps answer your question.

  2. claudia says:

    Hi Bryan,

    Thanks for your answer – it’s super and clarifies a lot of the information I have come across!

    Once again – thank-you very much for sharing all your knowledge!

    Claudia 🙂

  3. Peter Newton says:


    I need to clean up a 10mm dia. blind hole in an aluminium casting. The hole is (was) a close-tolerance housing for a hardened steel spindle carrying a valve rocker – this is in a petrol engine. The hole has worn by a few thou and is no longer circular and cannot hold the original spindle. I need to bring the hole true such that it can take a light interference fit sleeve (steel or perhaps bronze) in which I can fit a smaller diameter spindle for the rocker (I can rebush the rocker accordingly). I am hoping that a reaming operation might be all that is needed – grateful for your comments.


    • brednane1981 says:

      What size are you going to take it to before sleeving?

      • Peter Newton says:

        Hi again,

        The hole was 10mm dia and I simply need it to be circular, so an increase in diameter need only be small, and possibly up to an imperial size, say 13/32″ which will be an increase in diameter of about 0.32mm. I guess this might be a little heavy duty for a reamer, even accepting that the material being cut is cast aluminium, but perhaps I should drill first? The holes are in a cylinder head and would be very difficult to bore to a size suitable for reaming, hence my thought about reaming straight to size.

        Any thoughts welcomed.


        • brednane1981 says:

          A good rule of thumb is that a reamer should remove 2-3% of it’s diameter. So a 13/32″ (.4062″) reamer should work well if you are starting with a 10mm (.3937″) hole. We would not recommend drilling the hole first.


  4. saket says:

    How the geometry of left hand spiral flute reamer helps it in pushing the chip out ward

  5. abhi says:

    hey, though the answer is correct, but my ques is why the left hand spiral flute reamer will push the chips out of hole?

    • brednane1981 says:

      Most reamers are Right Hand Cut meaning the direction of the rotation of cut is to the right. The Left Hand Spiral is the opposite direction of the rotation of cut. The opposing forces cause the chips to be pushed forward in front of the reamer out of the hole. Conversely, a Right hand Spiral would have the same direction of the rotation of cut. The forces are in the same direction and pull the chips out towards the back of the reamer. It is possible to have a Left Hand Cut reamer although it is not typical. A Left Hand Cutting reamer with Right Hand Spiral Flutes would also push the chips out in front of the reamer because of the opposing forces. Also a Left Hand Cut reamer with Left Hand Spiral Flutes would pull the chips out towards the back of the reamer because the forces are in the same direction. So to answer your question, it is not so much the direction of the spiral on the reamer that determines where the chips go but whether or not the forces involved are opposing or in the same direction. I hope this is helpful.

  6. Ben says:


    Could you clarify what you mean when you say straight fluted reamers work best in non-chip forming materials such as cast iron, bronze and free cutting brass? I thought all material was chip forming of one sort or another, continuous chip, discontinuous chip or build up chip. Are non-chip forming materials more brittle?

    Thanks in advance,


    • brednane1981 says:

      Hi Ben,
      Thanks for your comment. You are correct in that all material forms some sort of chip. When machining cast iron, bronze and free cutting brass the chips are really small. Machining grey iron ends up forming more of what I would call a sludge but there are definitely chips in there just really fine chips. Perhaps it would have been better worded if I said “Straight fluted reamers are best suited for short chip-forming materials such as cast iron, bronze and free cutting brass.”

      Thanks for keeping me honest.

  7. Wadee Kajiwara says:

    I’m testing left hand side spiral reamer to enlarge blind hole.
    and set the counter clockwise operating to cut burr.
    Material stainless SUS404.I’m facing with problem
    gouging at the entrance of hole (chamfer 45 degree).
    In the past i use straight reamer and clockwise operating.
    burr occured at the exit way but have no gouging problem appear.

    I want to use left side spiral reamer to cut the burr.
    I’m to solve problem gouging.

    Please help me teacher…^^

    • brednane1981 says:

      Can you clarify a few things for me?
      Is the reamer you are testing a right hand cut (spins clockwise) or left hand cut (spins counter-clockwise)?
      Does the reamer you are testing have right spiral flutes or left spiral flutes?
      You mentioned that you were enlarging a blind hole. A blind hole only has one entrance/exit. Please confirm you are reaming a blind hole and not a through hole (a through hole has an entrance on one side and an exit on the other side).