By Alan (Tiny) Coffin, Starrett Saw Division Sales Manager 

When is Material Too Hard for Conventional Sawing?
It is commonly accepted among metal sawing technologists, including those in the Saw Division of Starrett, that cutting steel hardened to Rockwell numbers in the 60s is a job for a diamond grit-edge blade, not a regular band saw blade.  Starrett admits such hard metals are exceedingly difficult to cut, even with the best carbide tooth blade.  However, this Athol, Massachusetts company has done just that, and shares their story.

The sawing problem began with a company that operates a four-high rolling mill. Torsional shear had caused one of their 9-in. diameter by 7 ½ -ft. long mill rolls to fail.  To determine what had caused the shear, they needed to sample sections of the roll.

They called on Sawtech, a metal cutting specialist in Lawrence, Massachusetts to cut the damaged roll.  No stranger to cutting difficult metals, Sawtech took on the job, but they still had questions. “We knew we could use a carbide-tipped blade to get into the metal,” said Carmen Grasso, Vice President and co-owner of Sawtech.

“But we didn't know two things – how long the blade would last and how long it would take to saw the samples".

So Grasso called in more expertise by contacting Ray Jack, sales coordinator at Starrett. Jack had Sawtech send the roll directly to their Athol plant so they could test–cut the roll as an R & D project.

Analyze First, Saw Second
Several factors had to be analyzed and properly balanced for the cutting to be a success.  Jack's first thought was that a grit-edge blade might be the best option, but he needed to examine which blade would be the best choice for the particular saw.

Jack explains, “Grit-edge blades generally run at higher band speeds than toothed blades.  But, because of the dimensions of the work piece and the size limitations for band saws that accept grit-edge blades, we decided the most effective method would be a blade with teeth.”

Even though higher speeds are more easy to reach on vertical saws, there were other problems as well.  “For example, it would be difficult to mount the roll onto a vertical saw table.”  according to Jack.  Mr. Jack also considered a high-capacity horizontal saw because it could hold the work piece; however, it too would not accommodate a grit-edge blade, either in blade width or speed.

So, Starrett's Behringer HBP 420A Band Saw, which has a 16 ½-in. diameter capacity, was selected. 

Mr. Gene Ramsdell, Starrett Metallurgist, also arrived at a similar conclusion when factoring blade type.   “The blade would have to be carbide tipped to work,” explained Ramsdell.  “Normally, we use 55-57 as the changeover point from carbide blades to diamond grit.  For diamond blades, the saw must have a precise downfeed.  Diamond grit bands are only 0.020-in. thick, and it's easy to wear them down.  Also, we always consider economics for our customers.  Diamond is a last option due to its cost.”

Starrett Saw technicians also had to consider the hardness of the roll surface.  At first they were told that the surface of the case-hardened Chrome Molly roll was Rockwell 64 C, but testing revealed a 60-62 reading.  In addition, the hardness was 50 Rockwell C at the core.  They figured that pushing a carbide tipped blade past accepted limits could be accomplished by properly balancing feed rates, band speed and coolant flow. Finally, they chose a triple-chip grind, one pitch carbide tipped blade to do the job.

“Triple-chip grind is done only for our carbide line,” said Ramsdell.  “Experience has shown us that the triple-chip performs better than a set-tooth blade in the overall cutting, particularly in solid sections.”

Interestingly, the triple-chip blade has two different tooth grinds which are positioned alternately along the blade.  One called the chisel tooth is a triangular surface that cuts both sides of the kerf and produces two chips.  The other is the triple-chip tooth, forming the third chip, and is thinner and several thousandths of an inch longer.  This cuts the bottom of the kerf.

The Proof is in the Cut
After all the careful analysis and reasoning, the Starrett lab went to work. Technicians made a total of five cuts.  Noteably, two cuts, which required 67 minutes each, included 1 ½-in. thick samples from the full 9-in. diameter of the roll.   Band velocity ranged from 65 to 100 SFM.

“You would think a blade might be shot after a workout like this,” said Jack, “but the blade was in great shape, even after the fifth cut.”

“By dialing in the optimum feed rate and band velocity, we were able to make one cut in only 25 minutes.” explained Jack.

Building A Knowledge Base
The sawing project succeeded on two fronts.  First and obviously, it provided Sawtech's customer with the samples they needed, and secondly, by sawing these rolls, Starrett gained solid knowledge on a difficult application.

Jack cautioned that the results of the project might not work in similar situations.  All variables must be considered before attempting this type of cut.  “We made five cuts on this high-hardness material without damaging the blade.  If we were in production, results could vary.”

“This was a job like no other, even though our technical department receives exceptional requests on a daily basis.” says Gene Ramsdell.  However, the knowledge gained from each job has the potential to assist another.

“For instance, data gathered from a past experience with sawing rods and shafting helped contribute to the success of this job.  In that project, we ensured the saw was solid and had an accurate feed system (never force brittle carbide tips into a hard surface).  We used either grit edge or carbide-tipped blades to cut Thompson case-hardened or hard chrome plated shafting and hydraulic cylinder rods.  These were nitrided or carbon nitrided with a case up to 1/8-in. thick.”

Overall, the best results when sawing super-hard materials are produced through careful analysis and testing.

	Email Page			Printer Friendly