Slitting saw problem

[Roadracer_Al]

Hey there gang...

I had a repeating job in the shop yesterday. I belong to the South End Rowing Club in San Francisco, and I am one of the volunteers who maintain our antique wooden boat fleet. The job which came up was to make "nut plates" out of SAE 954 Aluminum-Bronze. The parts are 5.75" long, .5" wide, .25" thick and have 5 tapped holes, and 3 countersunk mounting holes. Very straight forward. These are for a 6-person "Barge" (quite similar to a Cornish racing gig), with 3 required for every position, plus a couple of spares, for 21 pieces total.

Late last year, I did the same job with slightly different dimensions for 3 different boats.

The mill finish on 954 is very ripple-y and is oversized by roughly .08" so it can be cut down to it's nominal size. The smallest stick I could find is 1" x .375", so there's a LOT of roughing.

As a nerdy aside, while the blanks were cutting off in the bandsaw, I used my Rhodes shaper to square up one side. This worked out great!

Last time, I used a slitting saw quite successfully to split the piece lengthwise, if memory serves... taking the .375" DOC in a single pass.

This time, it wasn't working well at all. I set the RPM to 300, which was a somewhat slower than last time. I was using a modest feed - it's a manual mill, no power feed. I'd estimate about 2" per minute. The first cut was wildly crooked: it sloped up by almost .1" over 5" ! When I went back to try to trim it flush with a second pass, the slitting saw blade snapped.

On the second piece, I found another, similar saw, got the height of the cut set right, and tried to take roughly half the depth ~.18" in one pass. The first pass went OK, but the second pass showed some crookedness as a slope up toward the end of the cut, much like the first one, then promptly snapped when it touched the third workpiece.

Down to my last saw, I fell back and punted: taking 5 passes, using heavy sulfured cutting oil.

I'm at a loss as to why the first two blades broke - I've rarely had any trouble with slitting saws. They always seem to work great, and will take lots of feed. Any thoughts? The arbor is a nice made-in-USA quality piece.

Thanks!

[DaveH]

300rpm? a lot will depend on the slitting saw dia. and the pitch of the teeth. Normally they need to be run quite slowly.
ie a 5" slitting saw about 50rpm
 
DaveH

[PixMan]

Agreed, the rpm doesn't tell us much without the saw diameter.

There can be vast differences in the SAE 954/AISI 660 cast bronze material too. If it cut good with the previous tool and cutting parameters, that material issue is the likely culprit.

Can you describe the saw you were using?

Me? I know that stuff can be ungodly expensive, but not having any reason to believe I'd ever have a use for small little pieces, I just plow though it with a lead angle carbide insert face mill and it's butter.

[TomG]

A wandering saw is almost always due to a dull cutter, so either the ones you chose were already dull or your rpm was too high for the material. For the record, if I have a long cut to make, I'll plunge the saw into the side of the part rather than let it feed the entire length of cut. That way if the is a wandering issue, it won't get out of hand and scrap the part.

Tom

[Mayhem]

Slitting saws still confuse me. I guess I had best buy one, don a suit of armor and attack a piece of unsuspecting metal. I still think they are the reason why CNC milling centres have doors on them!

[Roadracer_Al]

Yes, diameter is important, isn't it. My bad. Both broken ones were 3", 36 tooth. One was 3/32", the other 1/16" thick.

Maybe I've been lucky - I've always run them fairly fast. They don't seem to require much HP to cut. Wouldn't hurt me to dig out Machinery's Handbook and look it up.

I had some suspicion that wandering was due to a dull cutter (woodworking circular saws do the same thing), I could dig the pieces out of the trash and have a look.

I like the plunge technique. I'll remember that and use it.

The material was all from the same order, and it ran nice last time. Yeah, it's expensive -- 4 6' sticks was $375!

Maybe I'll dig out the balance of my cutters and take them to the local tool grinder and get them sharpened.

Thanks for the thoughts, everyone.

[PixMan]

300 rom with a 3" cutter is 235 surface feet per minute, or 72 meters per minute. While that bronze is easy cutting, it can be fairly abrasive. Tool wear would be my bet.

I'd be at half to one third that speed with HSS.

[DaveH]

(06-30-2015, 06:15 AM)PixMan Wrote: I'd be at half to one third that speed with HSS.

 I agree.

I use 2" dia saws at a speed of 125rpm. 125rpm is the slowest speed on my mill so there is no point in using a larger dia saw. The high surface speed will just blunt the HSS saw very quickly.

DaveH

[Roadracer_Al]

OK, then I think we have found the culprit.

Thanks!

The downside of being self-taught is that I'm not in the habit of checking for tool wear b/c when I was learning, they broke loooooong before they got dull.