Small mill - reasonable expectations in steel?

sru_tx

sru_tx

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I have a small mill, G0704, and am wondering what this machine can reasonably do when milling steel. The picture below shows an endcap for 2" square tubing with 0.25" wall thickness. The stock is 3/4" x 2" flat bar steel cut into a 2" square piece. I needed to remove approximately 0.25" on the 4 sides and 0.5" deep. I was going to mill out that perimeter area and quickly found that it was going to take a lot of passes to achieve that. I ended up roughing out the recess area with a bandsaw and then finishing out to final dimensions using the mill. Even with the roughing process it took quite a bit of time.

So the point to my post. What are reasonable expectations when milling steel on this machine?
I was using a 2 flute, 1/2" diameter HSS end mill running at 1200 RPM. I find that i'm only able to take off a few thousandths (0.003") per pass. Conventional milling is hard on the machine, lots of noise and chatter, even with the lightest of cuts and climbing cuts seem to be easier and less jolting but well aware of what could happen with too deep of a cut.

Is that DOC all that I can reasonably expect from this machine?

IMG_20200305_145253.jpg
 
I would expect you to be able to get 10x that, assuming a sharp cutter. Check the tram and gibs. Loose gibs on my Bridgeport causes chatter. Can't imagine it's better on a smaller machine.
 
I don't have a G0704 but I do have a smaller horizontal miller which can struggle with heavy cuts. One thing you may want to consider is going to a roughing end mill. They're more expensive than comparable quality 2-flute cutters, but I got about 3X the cut from a rougher than what I was getting from a plain cutter and with way less chattering and jumping around. They leave a kind of corrugated finish but lots of times that doesn't matter, and if it does you can take a finish pass with a regular cutter after.

You may never be able to take monster cuts with a small machine, but the roughing cutters sure help.

-frank
 
ditto above and I'll extend that to running smaller endmills. 1/4 and 3/8" are a good size for this size mill and you can run them faster. I also tend to avoid 2fl endmills unless I'm slotting. 3fl for alu and 4fl for steel will give you an easier time.

Plus you might have been running things a bit fast - my (very rough) rule of thumb is: speed = 200 to 400/HSS cutter diameter for steel and 400-800/cutter diameter for alu. Start at the low range and work up. So between 400 and 800rpm would be a good starting point for a 1/2" endmill in steel. Those aren't hard or fast numbers, but starting on the low end won't hurt anything other than some time.
 
1200 RPM, about 160 FPM, is maybe a bit fast for a HSS end mill in steel, especially when running in less than ideal conditions. I would most likely run that at about 100 FPM (760 RPM) for a 1/2 dia end mill.

Your picture also looks like the cutter is a bit dull. This could be caused by running too fast with a too light chip load, those conditions would dull a cutter very quickly due to rubbing rather than cutting.

In your machine I would probably not run anything larger than a 3/8 end mill in steel to reduce the cutting loads. Unless I have a real need to do so, I don't normally run larger than a 1/2 inch end mill in my 3500 lb machine.
 
I have a little Sherline mill that could take that 1/2" axial depth of cut with a 1/4" radial depth of cut in a single pass in mild steel so something is off. SFM for the above cut would be around 120 SFM so RPM should be around 1800 rpm with a 1/2" finishing end mill and about 20% faster with a roughing end mill. It seems to me that there is a dull cutter involved or something is moving. So, I have a few questions:
  • Are you using a sharp cutter?
  • Are your gibs adjusted well? Did you lock them down before taking your cuts by any chance?
  • What kind of tool holder are you using? Is it just an R8 collet? If so, was the drawbar pretty snug? Any chance the tool was slipping?
 
I assumed he was taking a 1/4" radial doc. Am I mistaken?
 
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