Nic, you may be cutting only aluminum with small end mills right now but someday that may change. You need to know how to calculate the required rpm for the cutter you're using and the material you're cutting. One common formula most of us use is:

**RPM = cutting speed** (for the material for the type of cutter used, in surface feet per minute or SFM)

**X 3.82 / diameter** ( of the cutter).

Cutting speed is obtained from a table like this one:

https://littlemachineshop.com/reference/cuttingspeeds.php#Milling. Note that there are separate tables for turning, milling, etc. Find the cutting speed for the material you are working with under the column for the type of tooling you're using. That speed will be in SFM. Plug that into the formula.

The cutter diameter is whatever you're using - 1/4" for a 1/4" end mill, etc. If you're using a flycutter set at 3" OD then use 3", got it?

Now solve for RPM and you'll have a good starting point for speed. Keep in mind that this is not written in stone. If the speed seems to high for your mill or operation then slow it down. Note that speed will often be higher than your machine can run if you're cutting aluminum with small diameter end mills unless you have a high speed spindle. Also remember that speed is only one factor. You have to sort out the proper speed and depths of cut for the end mill you're using.

To sort out the depth of cut, it depends on whether you're making a slotting or profiling cut. The commonest newbie mistake is trying to cut everything with the tip of the end mill; this just wears it out sooner. To guide you, I'm attaching some guides from Niagara Cutter - see below.

If you look at the "T6 feeds and speeds" chart you will see that you can do either slotting or profiling cuts. The specific cutting speed depends on which operation you're doing and the type of cutter you're using; take your pick. Note also that they refer to axial and radial depths of cut and this is based on a fraction of the cutter diameter. For example, if you choose to use 1/2 the cutter diameter of an uncoated HSS end mill as a depth of cut for a profiling cut, the cutting speed would be 400 sfm. You take this and plug it into that RPM formula above and that will give you the speed to use for that cut. For your 1/4" end mill, this would be a 1/8" radial depth of cut and you can go as deep as 3/8" (

**EDIT: you can use up to 3/8" of the length of your end mill's cutting flute length**) while taking that 1/8" deep cut.

Note also that you can calculate feeds but this doesn't help unless you have CNC or NC or power feed. If feeding manually, feed so that you feel a slight resistance to the cut and you'll do fine. Don't forget coolant.

Anyway, have a look and see if you can sort this out. If you have questions, post them up. Hope this helps.