Tool Diameter and Speed
You are probably already familiar with how the speed changes by the work piece diameter on lathe operations. The bigger the diameter the less of the cutting speed. The same principles apply on milling operations too with the only difference that we use the tool’s diameter to determine the speed. For the previews operations we have used a 12mm HSS End mill on a piece of aluminum at 750RPM. What I’m using as a rule is to increase or decrease the speed by 7.5% approximately for every 2mm of change in the tool diameter. For example with a 16mm End mill my speed will be 640RPM, approximately 15% less than 750RPM. Let’s take a look at the chart below.
Diameter – Speed
16mm ~ 640RPM
12mm ~ 750RPM
10mm ~ 800RPM
8mm ~ 870RPM
6mm ~ 930RPM
This rule it may have been proved effective but in the reality the optimum cutting speed is increasing dramatically as the cutters diameter decreases. For example the recommended speed for a 3mm End Mill will be some thousands more than the maximum speed of the D180. So, for cutters smaller than 4mm I sagest you to use the highest speed possible, always use some cutting fluid and reduce the feed end the depth to minimum. Otherwise you will break the cutter when you barely touch the work.
Materials and Speed
A different material will affect our cutting speeds in any way? Yes, again as on lathe operations the same principles apply on milling too. For example for steel we will decrease the speed approximately by 15% but for brass we must increase it by 15-20%. For brass always use good quality tools and razor sharp.
Carbide vs HSS
Can we use carbide tool? Generally I prefer HSS End Mills since the carbide tools demand much higher speed and more aggressive cuts to perform well. The only exception is when I need to cut a very hard material, stainless steel for example, but I’m taking very light cuts with small cutters and only in very small pieces. I should mention also that for steel I’m using HSS tools with an additional 8% of cobalt on it and can be identified by the letters HSS-Co8. They are much harder than plane HSS tools.
Those are a few tips to help you with your first milling operations on the lathe. Of course we have more to talk about, like how to square the work, how square is actually a square and how to find out, plunge milling, fly cutters and many more but first I want to talk for my milling attachment and to give you some more details because I’m getting a lot of questions about it.
My milling attachment