Bandsaw blade tip off

Dec. 1, 1999
Simply installing a carbide-tipped bandsaw blade doesn't guarantee that it will outperform a bimetal blade.

Simply installing a carbide-tipped bandsaw blade doesn't guarantee that it will outperform a bimetal blade.

Carbide-tipped bandsaw blades like this Sandvik 3868 Triple-Set Xtra cut hard or abrasive alloys and large workpieces.

Carbide-tipped bandsaw blades can effectively saw thick-walled tube stock.

Carbide-tipped blades can handle tough alloys, such as this abrasive aluminum-bronze alloy.

This carbide-tipped Sandvik 3868 Triple-Set Xtra blade generates more chips and more heat than a bimetal blade. It requires richer coolant mixtures for high cutting speeds.

Carbide-tipped bandsaw blades cut difficult-to-machine alloys fast and economically. Although they generally cost about four times as much as bimetal blades, they justify their higher initial cost by cutting up to six times faster and lasting more than three times longer. In some cases, they pay for themselves simply by cutting metals that are too tough to be cut using bimetal blades without special equipment. However, to maximize cutting speeds and blade life, blade users must pay careful attention to blade selection, bandsaw maintenance, and machine settings.

When cutting titanium or Inconel, for instance, carbide-tipped blades run dramatically faster than bimetal blades. A bimetal blade cuts an 8-in. solid round billet of 6A14B titanium at less than 1 in. 2 /min. Carbide blades, on the other hand, withstand much higher speeds and cut the same workpiece at 6 in. 2 /min.

For some stainless steels, carbide-tipped blades may cut a little faster but will last much longer. A bimetal blade cutting 10-in.-diameter pieces of 316 stainless typically lasts around 4,000 in. 2 , while a carbide-tipped blade in the same application cuts 12,000 to 15,000 in. 2 — three to three-and-a-half times longer.

When cutting extremely abrasive alloys at high speeds, a finer pitch blade with carbide, such as Sandvik's 3869, which has two teeth set left and right to provide swarf clearance followed by a third higher tooth chamfered on both sides, withstands vibration better.

High strength or hard-surface alloys and larger workpieces are best cut with a slightly coarser pitch blade using a grade of carbide better suited to machining more difficult-to-cut materials. One such grade is on Sandvik's 3868 blade for large workpieces made of stainless steel, titanium, and other tough-to-cut alloys.

Bandsaw maintenance
Bandsaw machines require few modifications to run today's carbide blades. In fact, using blades with Sandvik's triple-chip set of carbide teeth lets shops use carbide blades in place of bimetal on small, low-horsepower bandsaws without heavy, rigid frames. However, the higher chip production rates of carbide blades as compared to bimetal blades require effective chip removal systems. Vertical bandsaws must have chip brushes, and large machines with chip brushes may need one or two more.

Higher chip rates and cutting temperatures associated with carbide blades also require increased coolant flow. Blade manufacturers often recommend auxiliary flood lines for machines using carbide blades for the first time. Carbide-tipped blades are compatible with bimetal blade coolants, but mixtures should be richer. While a bimetal mixture may be diluted 10:1, the ratio for carbide blades is 7: 1 or less. Faster, hottercutting carbide blades also generate more contaminants and quickly evaporate coolants, so machine fluids should be monitored and changed frequently.

A poorly maintained bandsaw increases blade wear and wastes the performance advantages of carbide. Given the higher speeds of carbide blades, machine wheels must be kept true and round, and tracking must be checked regularly. Guides should be in good condition. Also, tension mechanisms must be working properly, and chip brushes well maintained.

Additionally, carbide tips are relatively brittle compared to high speed steel, and users should exercise care when opening, loading, and refolding these blades. Protective edge covers should be kept in place until the blade is loaded on the bandsaw.

Machine settings
Blade tension settings are the same for both bimetal and carbide-tipped blades. The range is usually 30,000 to 38,000 psi, depending on the width of the band.

Carbide blades run at higher speeds and feed pressures, and running them at bimetal speeds and feeds actually increases wear. Speeds and feeds naturally depend on the material. They are determined by experimentation, usually in conjunction with the blade vendor. Because carbide tips are more readily damaged by operator error than bimetal blades, feeds and speeds should be established by the blade maker or experienced operators to protect the investment.

Carbide blades in a new application are generally started at bimetal speeds and feeds. Speed is increased gradually, followed by greater feed pressure until the acceptable chip load peaks. Minute increases in blade speed always precede increases in feed pressure. Heavy vibration or evidence of chatter on chips and workpieces is a cue to reduce speed 5 to 10%.

Like bimetal blades, new carbide blades require a run-in period to give their sharp, pointed teeth a slight radius. However, run-in speeds for carbide blades, in most cases, are equal to full running speeds of bimetal blades, and the run-in period is longer. New carbide blades start cutting with heavy vibration upon entering the material, but the vibration nearly disappears as the teeth hone themselves during run-in. Run-in periods typically last from 20 to 35 min, depending on the material being cut. More abrasive alloys shorten run-in time.

Cost calculations

Even when bimetal blades can cut extremely hard or abrasive alloys, machine cycle times are often so long and blade lives so short that the cost-per-cut is high. The choice of a carbide or bimetal bandsaw blade should be made on the basis of total cost-per-cut, not the cost of the blade. Cost-per-cut calculations should include the total value of machine time, including local labor costs and idle time for blade changes.

Machine/labor cost

  1. Cost of machine excluding blade cost ($/hr)
  2. Cost of machine excluding blade ($/min)
  3. Actual cutting time
  4. Idle time between cuts
  5. Total cycle time (add figures from 3 and 4 in decimal min)
  6. Time per piece (single or bundled)
  7. Cost per piece from machine ($/piece)

Blade cost

  • Price of blade ($)
  • Time of blade change (min)
  • cCost of machine excluding blade ($/min)
  • Cost of blade change
  • Total cost of blade
  • Number of pieces cut by blade
  • Cost per piece from blade ($/piece)

Total cost per cut = cost per piece from machine + cost per piece from blade

Ten Tips

  1. Choose carbide or bimetal blades based on cost-per-cut, not cost-per-blade.
  2. Choose the right carbide blade based on workpiece size and material.
  3. Whatever the machine, proper maintenance is essential to carbide blade life.
  4. Chip removal systems must keep pace with carbide blades.
  5. Coolants for carbide blades should be rich and changed frequently.
  6. Tooth protection should be kept in place while carbide blades are loaded.
  7. Speeds for carbide blades start at typical running speeds for bimetal blades.
  8. Increases in blade speed and feed pressure should be made in minute steps.
  9. Increase blade speed first; then increase feed pressure.
  10. Vibration and chatter after carbide blade run-in are problem warning signs.