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50BMGBOB
10-01-2011, 06:46 AM
I am trying to teach myself to machine and recently bought a Grizzly G0619 mill.

My question is when using a slitting saw, how do you figure the rpm's and cutting speeds? Is it the same as an endmill? When I do, they are turning so slow that I have vary little torque. I looked at a cold saw (metal cut off saw) with a 14" blade and it was spinning much faster than what I was spinning a 4" blade (both are carbide tipped blades). What am I missing? I can't find this covered in any of the books I have.

SGW
10-01-2011, 07:53 AM
Yes, a slitting saw of any significant diameter does need to run fairly slowly.

A 4" dia. saw has a circumference of approximately a foot -- actually a bit over, but call it a foot. To get a cutting speed of 100 fpm, which might be appropriate to cut mild steel with a HSS sawblade, the saw has to turn 100 rpm.

Now, you're talking carbide, and you don't say what you're cutting. The carbide ought to let you boost the fpm, and if you're sawing aluminum instead of steel you can crank it up even more.

But basically yes, you figure it like an end mill. At least I do.

J Tiers
10-01-2011, 08:41 AM
Going slow you SHOULD have tons of torque. but that is if you get slow speed by belts and/or gears.

That vertical milling machine has a brushless DC motor, and while it has a stated range from 100 to 1750 RPM on spec sheet (0-2000 in catalog), it seems to do that by electronic speed control, and not by belts, other than a 2:1 speed changer.

Because of the speed and torque issue, it may just not be suitable for a large slitting saw, which is mare a cutter intended for a horizontal mill. Horizontal mills typically can be run below 100 RPM, with full torque.

Even though it will go at 100 rpm, and despite being set in the low speed range, you are getting about 1/10 HP at 100 RPM. If you turn up (because of carbide) to 200 or 250, you can get 1/5 or 1/4 HP applied to the saw.

You still will be torque limited to the basic motor torque, (or 2x in slow speed), so that may be an unacceptable (but unavoidable) limitation. With belt or gear drive, at 100 RPM you would have about 17 x the basic motor torque. Instead, you may have no more than 2x.

By contrast, my small horizontal mill has a 1/4 HP motor, but is all belt drive plus 6:1 back gear. When set to run at a slow rpm for larger cutters, the machine happily chews through steel with a 2" or 4" wide high helix "slab mill" cutter. It has all sorts of torque, far in excess of the motor basic torque, because all the speed reduction is mechanical only, multiplying torque.

knedvecki
10-01-2011, 12:06 PM
Surface feet per Minute X 12 divided by PI X Cutter Diameter (inches) = RPM ???

Deus Machina
10-01-2011, 12:09 PM
Yes, you figure it like an end mill. Or any other cutting tool, for that matter.

I currently have a mini-mill (such as the G8689), that I have and can work with tools far larger than what the spec sheet says I can. The trick is to push to the upper edge of the suggested speed and take excessively light cuts. Yes, it sucks. But it can be done.

lynnl
10-01-2011, 02:05 PM
Surface feet per Minute X 12 divided by PI X Cutter Diameter (inches) = RPM ???

I think you mean: divided by cutter diameter, don't you?

Scottike
10-01-2011, 03:17 PM
Surface feet per Minute X 12 divided by PI X Cutter Diameter (inches) = RPM ???

I think it's supposed to be SFM / 12 (= in./min.) divided by Pi x Cutter Dia.(circumference (in.)) = RPM.

edit: Nope! he was right the first time!

Black_Moons
10-01-2011, 03:43 PM
Correct, you calculate it like an endmill
And yes, your mill just may not be up to the task of running any kind of large diamiter cutter like a flymill or sliting saw, or even a larger boring head.

This is the big problem in wide speed 'VFD'/variable speed electric drives, And why most good VFD conversions of belt/gear driven mills retain the pullys/gear box, even if its going to be a CNC and the speed all controled automaticly, its still a HUGE benifit to be able to alter the gearing on the motor manualy via actualy pully/gear ratios.

MichaelP
10-01-2011, 05:08 PM
I think you mean: divided by cutter diameter, don't you? Not really. :) As he said, divided by (Pi times cutter dia. in in.).

If simplified, the formula will be:

RPM=SFM x 4 / Cutter diameter (in inches)

50BMGBOB
10-01-2011, 08:19 PM
OK, I was figuring it right then and it is just the limitation of my small mill. The thing that confused me was the metal cut off saw at work (I am a mechanic, not a machinist by trade) spins a larger 14" blade at a much higher rpm. I try to cut most things close to size at work but needed to cut a 1.5" thick piece of steel at home. Even taking light passes, it was bogging down to much and I ended up waiting until I could take it to work to cut it.

Space is the big premium at my house/shop. I only have a 8'x12' shed for my hobbies. I have a my mill, an Atlas 10" lathe, two reloading presses and my ammo and shooting supplies all in that space. I guess I will have to try and make room for a saw now too.

Thanks for all the responses! I am learning a lot here and try to only ask questions after searching as much as I can first.

dp
10-01-2011, 11:14 PM
Some of the guys here have adapted a second tool holder head to their mills for extremely high speed milling. The same idea can be used to reduce the speed to a geared down crawl. The existing spindle drives a cog belt and a clamping arm attaches to the quill (assumes your quill doesn't spin) and has a simple bearing pair and a tool holder such as a good drill chuck or collet chuck.

The add-on spindle can be mounted to the power head if needed. This is an example if a simple add-on spindle:

http://www.cncathome.com/spindles.html

J Tiers
10-02-2011, 12:33 AM
I think you need a horizontal bandsaw...... Get a HF or similar Asian one if you must, although I have and like an old Atlas saw with hydraulic feed.....

Every metal shop needs a horizontal bandsaw for cutting stock. Some of them (like my Atlas) flip up to be a vertical bandsaw as well, although the table is typically tiny.

The problem with your mill is also that the saw is on a vertical spindle. That makes cutting a piece off a longer "stick" exceptionally hassle-prone....... the head interferes, you have to hold a stick of metal vertical, but there is no good vise for it, generally the whole process is a mess.... if it is even possible.

50BMGBOB
10-02-2011, 05:43 PM
I work on large container cranes at the Port on Oakland. We have saws and welders there for repairs, just no machine tools. I am lucky in that I can do most of my cutting there. I cut this piece there but made it 3/8" to long. That is a lot to mill off with my little mill. Like I said, space is the premium at home. I am debating between a cold cut off saw or a hand held band saw for home. Both have good and bad points. I would love a larger saw, but that will have to wait until I move to a bigger shop, and that isn't even in the picture right now.

lynnl
10-02-2011, 07:43 PM
Not really. :) As he said, divided by (Pi times cutter dia. in in.).

If simplified, the formula will be:

RPM=SFM x 4 / Cutter diameter (in inches)

Yes, it does become right when you put parentheses around the "pi X cutter dia." :) I wasn't reading it that way.

Oh well, whatever.
I just think of it in terms of your reduced approximation: 4XSFM divided by dia. (in inches).

KEJR
10-02-2011, 08:17 PM
Have you tried just cranking upt he speed and taking the light cuts as suggested? I'd go by ear and you should hear a low buzzing sound, not a high pitched squeal. With the carbide you don't have to worry about wearing the teeth out as much.

Davo J
10-03-2011, 01:09 AM
I work on large container cranes at the Port on Oakland. We have saws and welders there for repairs, just no machine tools. I am lucky in that I can do most of my cutting there. I cut this piece there but made it 3/8" to long. That is a lot to mill off with my little mill. Like I said, space is the premium at home. I am debating between a cold cut off saw or a hand held band saw for home. Both have good and bad points. I would love a larger saw, but that will have to wait until I move to a bigger shop, and that isn't even in the picture right now.

Another vote for a cheap bandsaw, you could mount it onto a bench instead of it's crummy stand, or even keep it outside under a small awning depending on where you live.

Dave

fixerdave
10-03-2011, 03:47 AM
...Space is the big premium at my house/shop. I only have a 8'x12' shed for my hobbies...

Oh, that's huge! I'm in a 9'x20' with a 3in1 (mill/drill/lathe), sheet metal 3in1 (slip, sheer, brake), wood bench, metal bench, mig welder, oxy/act kit, plasma cutter (broken - sniff), all my automotive tools, compressor and air tools, wood planer, router table, bench saw, supplies, storage, more hand tools than I know how to use, a wood bandsaw, a metal bandsaw, wood drillpress, metal drillpress, wood bench beltsander, metal bench beltsander, 5" bench shear (oh so handy), 2 mounted bench grinders, a miter saw, a jointer, a big whack of handtools for metal and wood, lots of wood clamps (but never enough), oh, and my motorcycle - and my riding gear (5 helmets last count). I still have loads of space to work in.

Just buy the metal bandsaw... you'll need it. It's just one of those tools that makes everything else so much easier. With a little though, you can make it fit just fine. Right now, mine is mounted on a handtruck so I can wheel it outside if I need to. My plan is to make a swing-out rack for it so I can stuff it under a particular bench. I've seen one where a guy just mounted it between two pieces of equipment with a board to keep the chips off. When he needed a cut, he just rolled it out and flipped it up. Worked just fine. There's always a way.

dp
10-03-2011, 10:26 AM
A cheap band saw solves a problem but doesn't solve all problems any more than a slitting saw arbor solves all problems. Both tools have purposes and sooner or later you will need both tools to work effectively and accurately as required.

A slitting saw arbor was one of the first tools I made. I'm fortunate in that my small bench mill has a two-speed gear box and will drive my slitting saw without any hint of bogging down. I've never had a job that needed a slitting saw that could also be done with a band saw.

SGW
10-03-2011, 11:48 AM
I think the generic 4"x6" horizontal/vertical bandsaw I bought for my shop has given me more bang for the buck than just about anything else. Get the Horror Fright one or similar. Mine (not a HF brand) needed some tweaking to get best performance out of it, but nothing serious.

They aren't really all that big. One will easily go under a workbench.