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dlsinak
10-30-2009, 02:06 AM
As I am using my Benchmaster mill for the first time, I am getting strange results. I am using a 4 flute end mill on the long side of a T-nut that I am fitting to my lathe. The surface looks like chicken scratch, and feels rough.

I am sure there is a loose nut running the mill, but besides that, I have no idea what is going on. I am using the lowest speed belt pulley combination and taking a .025 cut. The machine seems to do fine making the cut, but it looks like crap. I wonder if the cutter speed is too low or my feed rate is off, or could I be using the wrong cutter?

What cutter should be used when? 2-flute, 4-flute, fly cutter or ??????

I read a lot of material about machining, but either I am retarded and unable to retain the information or I haven't found it yet.

Any suggestions?

Thanks, Dennis

Black_Moons
10-30-2009, 02:13 AM
What diamiter is your endmill? How fast where you feeding it? How many rpms is 'slowest speed'
How deep of a cut where you taking?

Fine work is done in two steps, roughing, usally multiple passes (if required) untill the work is close to final size (within 0.01" or so) and then finishing passes done often at higher RPM and VERY slow feed rate, like one revolution of the handle per 2 seconds. (assumeing 0.1" per rev)

You don't need a good finish on a T nut unless its going to be slideing around.

'endmilling' often results in visable if not feelable scratchs in the surface unless great care is taken, try cutting with the side of the mill as much as possable for the best finish.

4 flutes will give the same finish as a 2 flute but at twice the feed rate.

Retaining information is 10x easyer once you are using the information :)

darryl
10-30-2009, 03:35 AM
Are you using cutting fluid- and are you trying the feed in both directions? Sometimes taking a spring pass in the other direction helps. Generally known as climb milling. You're pretty much ok with any machine doing this as long as the depth of cut is very little. It can leave a better finish. You don't want to hog material out by climb cutting unless you know your machine is up to it.

One other variable is the steel you're using.

andy_b
10-30-2009, 07:52 AM
i have the same problems as Dennis.



Fine work is done in two steps, roughing, usally multiple passes (if required) untill the work is close to final size (within 0.01" or so) and then finishing passes done often at higher RPM and VERY slow feed rate, like one revolution of the handle per 2 seconds. (assumeing 0.1" per rev)


for this roughing versus finish pass, do you use the same endmill, or do you pop in the crappy Chinese endmill for roughing and a fine American steel for the finish pass? :) i guess my question is, will a good endmill produce a poor finish when roughing, or if you get a poor finish with the endmill is it many times due to a low quality endmill?

andy b.

J Tiers
10-30-2009, 08:52 AM
Soft steel does that.

Also, which surface is the end and which surface is the side of the cutter? is tehre a difference in finish for you? What size is the cutter?

When you mill soft material, and don't 'climb cut" (you can't, not on that mill, so don't try it), you end up re-cutting chips that the cutter leaves on the work and carries around, plus they can get re-welded onto the surface, which messes up the surface. between teh two effects it can look like 'chicken scratches in the grass", or worse.

Cutting oil may prevent some of the re-welding, but it will make the re-cutting worse, by sticking the chips to the cutter.

The only real cure is full flood coolant, and/or climb milling. Coolant washes away chips, and cools to prevent re-welding. Climb milling tends to reduce re-cutting of chips and carry-around of chips.

That small machine has no provisions for either of those.

Your best bet is to first know what you are cutting, and use some cutting oil at least brushed on. Soft steel and soft aluminum often does finish like that. harder material tends not to be as bad, and material that forms good chips, like many brasses, also cuts well.

For soft low carbon steel, 100 FPM is a good cutter speed, which would be about 1500 rpm with a cutter 0.25 inch diameter. if you are going a lot slower, you have to feed crazy-slow to keep the chip load per tooth down to a reasonable amount.

For 1500 rpm, and 4 flutes, with 0.001 feed per tooth..... you get 1500 x 4 teeth per minute, or 6000 teeth per minute. 6000 x 0.001 is about 6 inches per minute feed.

You'd cut the rpm to maybe 40% for a tool steel, or closer to 600 rpm, and reduce the feed proportionately, to 2.4 inches per minute.

For aluminum, you could probably double it or triple it easily, although your mill may not spin that fast.

You do want the feed to be enough to keep all the teeth cutting, otherwise some just rub, and that dulls them.

I find that a 2 flute cutter works better in a lot of cases. Not all. it usually doesn't carry around as many chips

Black_Moons
10-30-2009, 12:42 PM
andy: I often use the same cheap chinese endmill for both :P but thats mainly because my endmills are newish and still very sharp. (As they remind me every day when I use them and cut myself by just grazeing them in the mill, or in the toolbox.. etc)

Idealy you should use a diffrent endmill, ie new endmills be for finishing and older ones for roughing. but as a beginner all your endmills are finishing soo :)

Alguy
10-30-2009, 12:52 PM
My benchmaster which is i belive as delivered, has lowest speed of 450 rpm
then 900 rpm close to 1500 rpm then about 1900 rpm in the 4 steps of the pully. Most of the time i run at the second step 900 rpm ( approximate) using a 1/2 endmill , the slowest speed 450 is used for fly cutting. I usually cut dry sometime wd 40 on aluminuim and some time will hand apply cutting fluid . My finishes are good. Feeds are kinda by feel , i am an amature dont know what is supposed be most of the time.. often after cut that feels good ill mic. the cuttings and find they are usually in the .0015 range. So far this is working for me , What a pro would do i am not sure.
I have noticed when i go to small endmill 1/8 inch i up the speed . I suspect you are going too slow in spindle rpm for the endmill cutting diameter. Some times i wish i had a higher speed for smaller endmills. also slower speed for slitting saws.

Black_Moons
10-30-2009, 01:30 PM
alguy: A second pully (like a tripple pully 15~ speed drill press) could be a wise investment for you. Don't overspeed your mill too much for long periods of course (Could overheat/ruin bearings), lower speed is fine.

Or even better (but more expensive) a VFD+3phase motor.

You'll find RPM's (And hence, SFM, surface feet per min) depends a lot on material your cutting, and what material your cutting it with too.

900 rpms at 1/2" is slow for aluminum, especialy if your using carbide.
900rpms at 1/2" is about right for mild steel with HSS, a little slow for mild steel cut with carbide.
900rpms at 1/2" is too fast for harder steels with HSS, and maybe about right for harder steels cut with carbide.

dlsinak
10-31-2009, 11:41 PM
Well, I finally got back to the mill. I increased the spindle speed to the second pulley, about 900 rpm. The surface looked a little better, but still not satisfactory. I increased the feed rate and the surface was a little better yet. I figured what the heck, and increased the depth of cut and got a pretty good finish. The chips were coming off a brown color, and life was good.......until I discovered why the vise I bought off EBAY was for sale there. Not a disaster, but the piece moved in the vise and gave me the "opportunity" to try again. I was also getting some sparks at the end of the cut.

I believe my problem is not one, but several that are acting together. My cutter is a 4 flute 1/2" with a 3/8" shank. I think I would do better using a larger shank cutter. I am also thinking that I should get a 2 flute cutter. I also think that the steel I am working on is pretty soft.

The machine makes a lot of noise from the headstock / pulley area, similar to a loose pulley, even though I know the set screw is tight, I think I will drill and tap the pulley for a second set screw eliminating any chance of pulley wobble.

I wonder if I installed a treadmill motor for variable speed adjustment might work? Then I could put a quality pulley on the spindle.

Thanks for your help, Dennis

J Tiers
10-31-2009, 11:50 PM
Getting sparks suggests that the steel may be harder stuff to cut than you think..... and that you need to slow, not necessarily speed up.

900 rpm would be about 120 FPM with a 1/2" cutter. still Ok for CRS, but maybe rather fast for mystery metal that could be tool steel, or alloy material of any sort.

I had a piece of hot rolled 1018, or so I thought.... I rubbed off the teeth on a slitting saw before I saw the error of my ways and slowed way down. Dunno what it was, but 1018 it was not.

It cut like soft steel, UNTIL it work hardened, or whatever hardened, at which point it was really hard, and yes, I did see sparks. That wised me up in a hurry, and I had a look at the saw.

Davek0974
11-01-2009, 03:39 AM
I'm still learning and one of the best things i have learnt so far is that unknown or junkbox steel is not good for practice, my results improved massively after i started buying known stock. Either freecutting, leaded freecutting or just plain mild if i want to weld it.

A couple of years later, i find i can grab just about any lump of crap and make something useful, although i do occasionally get bad results, usually tearing and poor finish.

Dave

BobWarfield
11-01-2009, 10:17 AM
Well, I finally got back to the mill. I increased the spindle speed to the second pulley, about 900 rpm. The surface looked a little better, but still not satisfactory. I increased the feed rate and the surface was a little better yet. I figured what the heck, and increased the depth of cut and got a pretty good finish. The chips were coming off a brown color, and life was good.......until I discovered why the vise I bought off EBAY was for sale there. Not a disaster, but the piece moved in the vise and gave me the "opportunity" to try again. I was also getting some sparks at the end of the cut.

I believe my problem is not one, but several that are acting together. My cutter is a 4 flute 1/2" with a 3/8" shank. I think I would do better using a larger shank cutter. I am also thinking that I should get a 2 flute cutter. I also think that the steel I am working on is pretty soft.

The machine makes a lot of noise from the headstock / pulley area, similar to a loose pulley, even though I know the set screw is tight, I think I will drill and tap the pulley for a second set screw eliminating any chance of pulley wobble.

I wonder if I installed a treadmill motor for variable speed adjustment might work? Then I could put a quality pulley on the spindle.

Thanks for your help, Dennis

dlsinak, my machinist's calculator says 1200 rpm @ 14.5 IPM feedrate for mild steel and a 4 flute HSS endmill. The sparks are definitely indicating there is some hard stuff here and there in your material. Perhaps some hard scale or something?

I haven't seen how much depth of cut and width of cut you're trying to get yet either?

Cheers,

BW

J Tiers
11-01-2009, 10:35 AM
That's 160 FPM and 0.003 feed per tooth.

Probably at the high end of the range for CRS on most home type machinery, probably OK for a heavier industrial mill. I don't think it applies well to a tiny Benchmaster.

General rule, if you see sparks, cut speed in half, order new cutter.

lazlo
11-01-2009, 10:39 AM
General rule, if you see sparks, cut speed in half, order new cutter.

LOL! That's a binary search -- an expensive way to learn speeds and feeds. :)

Looking up the right speed and feed in any machine shop text would be a lot cheaper...

TECHSHOP
11-01-2009, 11:01 AM
lazlo:

Do you have a PDF of the recently updated Mystery Metal Manual?

dlsinak
11-01-2009, 12:37 PM
lazlo:

Do you have a PDF of the recently updated Mystery Metal Manual?

Where can I get one?:rolleyes:

I know the cutter is trash. The piece I was working on was the (mystery steel) blank that came with a qc toolpost. After I increased the speed and feed rate, .025 cut at half the cutter width, the little mill was groaning a bit, but the chips were a brown color, shortly thereafter sparks started flying. Being a manual mill, I suppose I could have unknowingly changed the feed rate.

Part of my problem is that I don't know what a good chip is supposed to look like or even how a small mill is supposed to behave or sound, how close a tolerance it is capable of, etc. I feel like I am reinventing the wheel, so to speak.

In any case, the resulting finish is not very good, but the piece fits where it is supposed to go and looks like it will be functional.

Now, when I am ordering different cutters, how do I know whether I am getting Chinese cutters or USA cutters? Are all Chinese cutters crap? If I can get some known good quality cutters, from wherever, then I have at least one known part of the equation.

In the mean time I am still looking for a good full size mill.

Thanks, Dennis

Davek0974
11-01-2009, 02:21 PM
Where can I get one?:rolleyes:

I know the cutter is trash. The piece I was working on was the (mystery steel) blank that came with a qc toolpost. After I increased the speed and feed rate, .025 cut at half the cutter width, the little mill was groaning a bit, but the chips were a brown color, shortly thereafter sparks started flying. Being a manual mill, I suppose I could have unknowingly changed the feed rate.

Part of my problem is that I don't know what a good chip is supposed to look like or even how a small mill is supposed to behave or sound, how close a tolerance it is capable of, etc. I feel like I am reinventing the wheel, so to speak.

In any case, the resulting finish is not very good, but the piece fits where it is supposed to go and looks like it will be functional.

Now, when I am ordering different cutters, how do I know whether I am getting Chinese cutters or USA cutters? Are all Chinese cutters crap? If I can get some known good quality cutters, from wherever, then I have at least one known part of the equation.

In the mean time I am still looking for a good full size mill.

Thanks, Dennis

I try to limit the feed/speed to keep my chips a light straw colour at most, brown / blue is a bit too hot for me. If i'm using carbide tips, i just go for the top end of the listed limits and the chips come off very hot, set fire to my waste bin which was sitting next to the mill once:o

J Tiers
11-01-2009, 05:43 PM
LOL! That's a binary search -- an expensive way to learn speeds and feeds. :)

Looking up the right speed and feed in any machine shop text would be a lot cheaper...

Ah, but it's MYSTERY metal..... I won't object if you suggested cutting speed to 0.25 x................

After all, the correct speed is a mystery!

but if you see sparks, the condition of the cutter is probably NOT a mystery.................:eek: