Disc grinder or mill fly cutter for surface grinding?

M

Mancipia Mariae

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Oct 24, 2020
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If these were your only options for surface grinding, which would be more accurate if I was going to hand sand on a granite surface plate after?
 
That very much depends on the rigidity of the mill you'll be fly cutting with. Sloppy spindle bearings and gibs/ways will make for a lot of sanding. And don't try to hard machine with a fly cutter...

You can get fairly accurate flatness, parallelism on a disc grinder with practice and patience. If you need real accuracy a surface grinder is the correct option
 
Keith Nix said:
That very much depends on the rigidity of the mill you'll be fly cutting with. Sloppy spindle bearings and gibs/ways will make for a lot of sanding. And don't try to hard machine with a fly cutter...

You can get fairly accurate flatness, parallelism on a disc grinder with practice and patience. If you need real accuracy a surface grinder is the correct option
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Got it, so a disc grinder would be better for heat-treated steel?
 
I use my glass platen on the 2x72 to start with and do light touch up on the disc and always finish on a granite block.

I just could not get the accuracy I wanted solely using the disc grinder.
 
I tried a fly cutter on my tormach on blade steel and it did not work well. It was a rough cut, I think lots of flex and chatter. If you spent time refining speed/feed and depth maybe you could find something that works. A 1/4” end mill works far better, leaving a finish that you can easily sand out, but it’s slow. You can take off material faster than a disc I’d imagine.
 
Nathan the Machinist said:
Actually, fly cutters are pretty lame. I would recommend a decent face mill and just use a single insert. It works way better, trust me. But I still wouldn't recommend that on hardened steel.
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been planning on get one one of these days likely 2 or 2.5 inch im thinking depending on the cutterr style 3 inch woudl be too much for my bridgeport
 
Nathan the Machinist said:
Actually, fly cutters are pretty lame. I would recommend a decent face mill and just use a single insert. It works way better, trust me. But I still wouldn't recommend that on hardened steel.
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Is there a reason not to use all the inserts? You have me curious now!
 
REK Knives said:
Is there a reason not to use all the inserts? You have me curious now!
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The inserts will have slightly different depths and diameters and will also have higher horsepower and rigidity needs. It is slow, but you will get your best finish with a single insert and you won't overwhelm a small machine with just one.

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.005" death of cut and feed per revolution is a pretty good starting place for a finish cut
 

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Nathan the Machinist said:
The inserts will have slightly different depths and diameters and will also have higher horsepower and rigidity needs. It is slow, but you will get your best finish with a single insert and you won't overwhelm a small machine with just one.

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.005" death of cut and feed per revolution is a pretty good starting place for a finish cut
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Nice, thanks for the explanation! I never thought about only using one.
 
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