Centerless vs. On-Center Grinding

Bar Grinding Centerless Vs. On-Centers – Second Part of Four Part Set

As we mentioned in our last blog; in the maintenance industry, if someone refers to grinding a steel shaft, they are most likely talking about “Centerless Grinding”. There is another method, however, and that method is called “On-Center Grinding”. A misunderstanding on which method is actually required usually results in expensive errors, and general unhappiness for all parties. Of the two types, centerless is by far the most common. So much so, that if you mention grinding a shaft, the mill or service center will assume you are discussing centerless grinding.

Centerless grinding tends to follow the outside diameter of the bar; think apple peeler. When the skin is off, you still have a recognizable apple; naked, but still looks like an apple. Grind an egg-shaped hot rolled bar, and you will have a precision finished egg. In the hands of an experienced grinding operator, many troubling issues may be corrected. Taking it to an art form, the right operator can minimize irregularities and even affect straightness; to a point. The standard in industry is centerless. So, unless specified, tolerances being discussed are taken to be based on centerless.

On-Center grinding, on the other hand, indexes on the center of both ends of the bar. The grinding head then machines the O.D. of the bar to be concentric with the I.D. (chucked up centering holes). If your bar is egg shaped, now, your ground bar will be concentric. If the bar is bent, the finished ground bar will be straight, depending on how bent it was and how much stock removal you are able to take. The roundness (concentricity) and the straightness come from the “On-Center” grinding. On center grinding requires more stock allowance “to-clean up” than centerless grinding. Where there are low spots, no stock will be removed. The on-center grinding operation will not only true up the diameter size, but, it will “machine” the bar into a true round and straight part. How do you avoid these potential problems if you are not aware of the intended grinding method? Qualify, Qualify, Qualify. If “finish size” is mentioned, ask about the grinding method. And remember; “If it doesn’t clean-up, whos wallet comes out?”

-Howard Thomas, September 5th 2018

Who’s Wallet Comes Out

ALLOWANCE TO FINISH

Between the end-user, machine shop, and/or service center, when discussing round steel shafts, there are issues with “allowance to finish”, and even with the method of grinding that will be utilized to produce the finished polished shafts. If subsequent bar finishing or grinding will be done, always let your vendor know what method of grinding will be utilized; are you centerless grinding or grinding on centers. Remember this: “When the bar doesn’t clean up, who’s wallet comes out?”

Each method will have a unique set of requirements; we will discuss those in a future note. In a perfect world there would be one semi-finished condition for all rounds. Call it Hot Rolled, Drawn, Peeled, Rough or Fine Turned. All sizes would have a standard “stock allowance for clean-up”, no matter the mill of origin, or size of bar. All lengths would also have the same perfect straightness.

Regrettably, that is just not the case. At any given time, a service center may have stock from a half dozen various bar mills. Each one has their own description of what constitutes a “Hot Rolled” finish allowance. Some mills will only give a “peeled” or rough turned finish. Another may have hot rolled, or even forged to size with allowance, not machined.

If you are selling steel, how do you come up with a textbook answer that explains which size will make the finished size? When your customer asks what size they should order to make a given part; assume they are asking: “What is the price of a car?” As a seller, can you control the machining or grinding process? Can you insure the capabilities of the operator, or even potential “movement” of the steel? Obviously, you cannot. To even attempt to help the customer, you need much more information. Qualify, qualify, qualify.

Whether you are buying or selling, make sure both parties understand each other’s needs and abilities… When the bar does not “clean-up”, who’s wallet comes out?

-Howard Thomas, August 6th 2018

Does Heat Affect Steel?

This is directed to: steel novices, steel challenged, and people who might otherwise cause harm to themselves, those around them, or pieces of steel.

So, will steel be affected by temperature? That depends. What Temperature? That depends.

Let’s define temperature as “Service Temperature”. That is, the temperature the steel will encounter where it is being used. It is worth mentioning that service temperature may be “Intermittent”, or “Constant”. If the steel is exposed to intermittent temperatures, it is not exposed long enough to thoroughly take upon itself the service temperature. (It just passes in and out of a furnace but not long enough to get as hot as the furnace.) If the steel is exposed to constant temperature, it takes-on the service temperature.

When the steel mill hardens steel to obtain specific properties, it involves heating the steel and cooling it to a very specific formula. If you are now going to expose it to temperatures that approach those used in the original recipe, you increase the chances of changing the original properties (hardness, brittleness, ductility, etc.). That is reason for caution if you are intending to do anything other than drop it or throw it.

The temperature to which the steel was originally heated were specific to the elements that were in the steel. The temperature it was cooled to, as well as the rate of cooling and even the time required to move the steel from one process to another affected the properties obtained.

Heat will affect steel based on the composition of that steel and relative to the past thermal processing that steel has undergone. 

Give or take a country mile; steels will melt around 3000°F. Whereas aluminum will melt around 1200°F. Short of those temperatures, you should not have to worry about your steel leaking off the shelf. Steels will begin to soften, however, at a wide range of temperatures based on their chemical composition and the thermal processing that got them to the current hardness.

Temperatures need not be extremely high to begin to lower the properties of the steel. Some of the very hard wear plates found in industrial applications (near diamond hard) will begin to soften at 280° to 350°F. You can cook a pork butt at 280°F.

In very general terms, if you have a very hard piece of steel that will be exposed to elevated temperatures, there is a good chance it may soften. Conversely, if you have a soft steel and expose it to elevated temperatures, you may cause hardening.

In all cases, with known grades or unknown grades of steel; when heat is involved and the steel you are using may be hardened or may be hardenable, exercise caution. (safety glasses, hard hat, gloves, etc.)

-Howard Thomas, July 5th 2018

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Howard Thomas

Howard Thomas

Experience

Sr. Acct. Mgr. (US Southwest) / Metallurgical Consultant
Associated Steel Corporation
Jan 2017 – Present

Past Vice President / General Manager
Associated Steel Corporation
Apr 1998 – Jan 2017

Past Vice President / General Manager
Baldwin International
Apr 1974 – Mar 1997

Education

Cleveland State University
Kent State University
University of Denver