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Finish Sizes & Bar Clean-Up Part 2

The following is a continuation from our last blog on ROUND bar clean up.

It is important to share both your finish diameter AND length. They would like to know if you have a short stubby shaft, where straightness is not often an issue. Or, if it is a small diameter shaft that is 18ft. long. Big difference in how you approach the raw material for the job.

We are not suggesting that you ask the supplier what size they recommend, since a savvy supplier will avoid answering that. Yes, they are there to help you, but they can’t possibly know how talented you are in making a difficult shaft. It always comes down to; When things go South, who’s wallet comes out? Sufficient stock allowance for one, might be trouble for another.

You might also share information on what grinding, or machining method you intend to use; are you grinding or turning on centers, or will it be a centerless operation; which is by far the most common. Clean-up stock and straightness requirements will be different.

What surface finished are they able to supply? Do they have pre-machined, or, rough turned stock, is it fine-turned, cold drawn, hot rolled, or as-forged? Each may have a different allowance for removal that has been left on the bar. There are of course standards that govern each type of finish, BUT, there are variances from one mill to another.

Share “end-use” information with the supplier. What is the application it is being used in. If you are making Hydraulic Rods for instance, that would be an important bit of information you might want to share.

Subsequent Operations that will be incorporated, is also important. Will you be changing the hardness, adding surface hardness or surface-coating? If they don’t seem to be interested, no harm done. They just assume you’re verbose. A good supplier, however, will recognize that your contributions are important to a successful transaction.

In the next post, we will finish up elaborating on the last of the 10 points relative to “Stock Allowance on Round Steel Bar”.

 

-Howard Thomas, March 6th 2019

 

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