Slitting saw speeds

[Will DParticipant @willd]

[Will DParticipant @willd]

Hi all, another question to help get me on my way with my LBSC Bat loco.

I wish to cut slots in brass with a 63mm x 1.5mm slitting saw . Having never used one before what speed should I be cutting at?

[vintage engineerParticipant @vintageengineer]

Start off slow and increase speed until it cuts without building up any heat.

[Anonymous]

Depending upon the brass with a HSS saw you can be cutting at 200-300 feet per minute. So for a 63mm (2.5 inch) diameter saw that's about 300-460 rpm.

Andrew

[HopperParticipant @hopper]

For your future reference, the formula to calculate RPM of a cutter is 4CS/D.

Where CS is Cutting Speed in feet/min and D is Diameter of the cutter in inches.

So with a cutting speed of 200f/m for brass

4 x 200 = 800

Divided by the cutter Diameter of 2.5 = 320rpm

Same formula works for jobs in the lathe, only D refers to Diameter of the workpiece in inches.

That's the theoretical cutting speed of course. In practice with a thin slitting blade you might want to start slower and work up to it just to be sure, as already suggested.

[not done it yetParticipant @notdoneityet]

Posted by Hopper on 22/02/2019 23:28:45:

For your future reference, the formula to calculate RPM of a cutter is 4CS/D.

Where CS is Cutting Speed in feet/min and D is Diameter of the cutter in inches.

So with a cutting speed of 200f/m for brass

4 x 200 = 800

Divided by the cutter Diameter of 2.5 = 320rpm

Same formula works for jobs in the lathe, only D refers to Diameter of the workpiece in inches.

That's the theoretical cutting speed of course. In practice with a thin slitting blade you might want to start slower and work up to it just to be sure, as already suggested.

Unless you work in metric!. Still simple enough. pi.D is cutter circumference. Surface speed, in length per minute, from tables. Choose your own units (mm, cm or m) and divide former into latter.

[John HaineParticipant @johnhaine32865]

Starting too slow can be a mistake if you don't have a feel for the cut. Easy to feed too quickly, the saw jams, may break, arbor gets scored etc.

[EmgeeParticipant @emgee]

Will, following on from what John says also make sure when cutting the direction of cut is pushing the work away, not pulling it in to the saw, if not any backlash in the feedscrew will allow the work to be dragged in to the saw, either stalling the saw or damaging the job, perhaps both.

Emgee

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Haine on 23/02/2019 07:06:07:

Starting too slow can be a mistake if you don't have a feel for the cut. Easy to feed too quickly, the saw jams, may break, arbor gets scored etc.

That's the disadvantage of what I'm about to recommend! An easy to remember and calculate approximation for cutting speed in metric is simply: 10000 ÷ tool diameter (in mm)

The approximation is about right for mild steel, and it's a good starting point for finding a reasonable speed for the material being cut in your workshop on your equipment by you. Brass cuts better at a higher speed than mild steel, and it pays to increase the speed experimentally for best cut. About double. Industrial cutting speeds will tell you if a particular metal should be cut slower or faster than mild-steel.

The problem using recommended cutting speeds is they are industrial. Speeds are calculated to optimise metal removal vs tool life in a production environment. This is unlikely to be appropriate in a home workshop, not least because small machines rarely have the power and rigidity necessary to remove metal that fast. Nor is working at that rate likely to suit the operator, unless you are working to feed a starving family.

Generally then, the cut rate in a home workshop will be markedly lower than theory recommends. BUT! I am eternally in the debt of Mr Andrew Johnston of this forum, for the advice to NOT take light cuts. In a way this is worse than the gorilla approach to metalworking! Pussyfooting causes tools to rub rather than cut and soon blunts them. So you need to strike a balance, most likely spinning the saw somewhat less than theoretical rpm while pushing it into the work as fast as it will go just short of stalling. Takes a certain amount of practice, but its not rocket science.

Cutting speeds are guidelines rather than the law, the range of speeds over which most metals will cut is relatively wide, provided you position the cut between the sin of being too brutal and the sin of being too weak. 10000 ÷ tool diameter (in mm) works for me as a reasonable starting point.

Dave

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