General question; milling slots

[pepi]

I've noticed when machining a slot or step, the side surface is ruff. To clarify further, the surface the top of the step has a clean machined surface, the side not as nice...

Is the end mill the proper bit to be using, or this is a rooky mistake....... I know who the hell is going to look at the inside of a slot ........ me LoL

Greg

[Mayhem]

I usually make it 0.010" undersize and then take that off in a final pass.

[awemawson]

When using an end mill to cut a slot the same width as the cutter, one side of the slot is being cut 'conventionally' and the other is 'climb milling'

There is also a tendency to re-cut the chips that have already been removed, which can put the cutter off line if it is slender.

As has been said, best to cut a central undersized slot, then shave the edges to finished size. (Preferably using climb milling if your machine is rigid enough)

[TomG]

As the others have pointed out, the "rookie mistake" is using an end mill the same diameter as the slot you intend to mill. Use one slightly smaller and climb mill for the finish pass. Here's a video I made on climb milling vs. conventional milling and how it affects the surface finish.

Tom

[Dr Stan]

[pepi]

(11-19-2017, 09:45 AM)NTomG Wrote: As the others have pointed out, the "rookie mistake" is using an end mill the same diameter as the slot you intend to mill. Use one slightly smaller and climb mill for the finish pass. Here's a video I made on climb milling vs. conventional milling and how it affects the surface finish.

Tom

Ok, two conclusions for me, the slot machining, did use a tool equal to the width. The step technique demonstrated, the step depth was constant, the milling was to form the width....

I was doing it backwards rooky, had the width constant and milled to the depth.

Thank you

While I'm thinking about it, the piece you are milling, is alum. You mention that if not making the demo, you would mill @ 400 rpms, tool 7/8.

 I have a chart, that basically says the same for steel, then is small print it states, alum, cast iron & bronze dived the speed 1/2. ?

Not saying this is correct, cause I always side on the side of experience, so I'm questioning the chart.

Greg

[f350ca]

Also, when cutting a slot a 3 or 2 flute cutter is better than a 4. With a 4 flute cutter, the edge at the front of the cut is flexing the tool toward you if your cutting to the left, this causes the cutting edge closest to you to cut deeper while the edge away from you is pulled from the surface.
The speed sounds right for steel but should be twice for aluminum. Bronze Im not sure.

[TomG]

(11-19-2017, 11:19 AM)pepi Wrote: While I'm thinking about it, the piece you are milling, is alum. You mention that if not making the demo, you would mill @ 400 rpms, tool 7/8.

 I have a chart, that basically says the same for steel, then is small print it states, alum, cast iron & bronze dived the speed 1/2. ?

Not saying this is correct, cause I always side on the side of experience, so I'm questioning the chart.

Greg

The general rule for rpm is to multiply the cutting speed of the metal being milled by four and then divide by the diameter of the end mill. In the case of mild steel, the cutting speed is 100 surface feet per minute, so 4 X 100 / .875 = 457 rpm. The cutting speed of aluminum is 400-500, so the same formula would result in 1828 to 2288 rpm. There really is no upper limit on aluminum, as long as you can get the chips out of the way fast enough so as not to clog the end mill.

I also have a video on cutting speeds and if you look in the reference section of my website, there is a nifty rpm chart for various metals.

Tom

[pepi]

(11-19-2017, 02:50 PM). TomG Wrote:

(11-19-2017, 11:19 AM)pepi Wrote: While I'm thinking about it, the piece you are milling, is alum. You mention that if not making the demo, you would mill @ 400 rpms, tool 7/8.

 I have a chart, that basically says the same for steel, then is small print it states, alum, cast iron & bronze dived the speed 1/2. ?

Not saying this is correct, cause I always side on the side of experience, so I'm questioning the chart.

Greg

The general rule for rpm is to multiply the cutting speed of the metal being milled by four and then divide by the diameter of the end mill. In the case of mild steel, the cutting speed is 100 surface feet per minute, so 4 X 100 / .875 = 457 rpm. The cutting speed of aluminum is 400-500, so the same formula would result in 1828 to 2288 rpm. There really is no upper limit on aluminum, as long as you can get the chips out of the way fast enough so as not to clog the end mill.

I also have a video on cutting speeds and if you look in the reference section of my website, there is a nifty rpm chart for various metals.

Tom

You read my mind in the very last paragraph LOL. Think it is time to buy The Machinist Hand book. I do remember your video, showing the cutting speed and formula. I like to use large mills to cover large areas to keep the tool marks to a min.

Thanks again Tom

   

   

Finished the part ....... probably guessed it is the steady bed clamp, to much torque on hold down bolt, broke it in half. My guess is I'm not the first one to do that .

Greg

[randyc]

This is sort of gilding the lily but for my $.02 I'll add these:

If it's at all possible, get a shop vac hose close to the slot to pick up the chips as they are cut.  This will help cool the cutter, too !

Also, I seem to recall that Machinery's Handbook (somewhere) recommends slower speeds for slotting than for edge milling.  That seems reasonable since both sides of the slot are being cut simultaneously and more heat will be generated (the chip re-cutting adds to that load, of course).