I want to do something right, so I'm doing it myself.
I learned how to program an RPM ("Speeds") calculator in Excel 2010 so I can derive my own speeds chart. It's handy because I can add any materials I like at any time as long as I can provide that material's SFM*.
(For those of you new to the game of rotary cutting tools, the "Speed" measures how fast the spinning element spins; the "Feed" measures the speed of pushing the cutting tool into the material. Necessary for clean, safe, repeatable cuts!)
I made this at Techshop Detroit (www.techshop.ws) and it's proven to be USEFUL!
*Going to explain that in a bit.
Step 1: The Formula for Speeds
You can find the SFM in a chart produced by your tool manufacturer; I used one from the Machinery's Handbook. I found that while this SFM number is "scientific", it DOES depend on the material your cutter is made of, too, and it DOES depend on what the chart-makers thought was "efficient." I thought the Handbook represented a solid authority.
So we can pull material SFM and cutter diameter together to find the RPM for spinning that cutter to produce a good cut.
The formula is:
RPM = (SFM *12in/ft) / (pi * Cutter Diameter)
Cutter diameter is going to be in inches, and SFM is Surface FEET per minute, so we multiply the SFM by 12 to get the units the same.
For this situation we're using mild steel as our material, and we're cutting with a pretty standard .5in bit made of high speed steel. The SFM for this scenario is 80. So:
RPM = (80ft/min *12in/ft) / (pi * .5in)
IF YOU'RE GLAZING OVER, just think of this as a recipe. You need the SFM for your material and you need to know the size of your bit. I'm setting you up so you just have to know the basics, then plug in values and have Excel do the calculations.
Step 2: Making It Happen in Excel 2010
First, open Excel 2010.
Start in with a spreadsheet that lists your material (Mild steel), the SFM you found for that material (80) and the cutter diameter you're going to use (.5in)
To program in Visual Basic for Applications, you first have to activate it! (The photos illustrate the following steps)
Go: File -> Options
Go: Customize RIbbon
Then check the box for Developer.
You'll see that the Developer tools are now active in your spreadsheet, above the tool bar.
Step 3: Opening Visual Basic for Applications
Go: Insert -> Module
This opens a window for programming your formula.
In the next image, you see the code that creates the RPM calculator. You can copy and paste this into the window on your own machine:
Function RPM(SFM, Diam)
'SFM = surface feet per minute
'we multiply this by 12 to convert from feet to inches per minute
'Diam must also be in inches
Pi = 3.141592
RPM = (SFM * 12) / (Pi * Diam)
That first line is important to understand. "Function" tells Excel that your program is going to behave like a function. Then "RPM(SFM, Diam)" tells the computer that when you go to use the funtion, you're going to type this in the cell:
Where X is a value for SFM and Y is a value for your cutter diameter. More on this in a bit.
If you're familiar with the Basic programming language, you can use VBA to make any custom functions you'd like.
You don't have to save or click "done" or anything. Once the code is there, you can go back to your spreadsheet and do some calculations.
Step 4: Using the RPM Function
Click to edit the cell in the RPM column. Type:
Excel is waiting for your SFM value. Click on the SFM cell to use that as your value - easy! The cell will be highlighted, and you'll see the cell coordinates have been added to your formula, something like:
Type a comma. Highlight your cutter diameter cell. Then close the parentheses:
Press enter and your RPM formula gives you a calculation! Set your drill press to that RPM - around 611 - to make a proper hole in mild steel with a .5in drill bit.
Here's the fun part - populate your chart with whatever materials and drill bits you've got in mind!
Step 5: Save Your Chart
Go: File -> Save As
Choose: Excel Macro-Enabled Workbook in the file extensions drop-down
Step 6: What About That "Feeds" Part?
We'll manage this with Rules of Thumb:
- Make your first contact with a light touch, then apply force
- Press as hard as your bit "wants" to cut - your motor shouldn't slow; your bit shouldn't spin; squealing should be RARE
- Look at the chips:
+ Your aim is small, distinct C-shaped chips. Long curls means lighten your cut.
+ You want to keep the cut cool. In steel, chips can be coffee brown at the hottest. Blue or purple means lighten your cut.
- Use a pecking technique - pressure followed by a light release - presssssss...lighten. presssssss lighten. This helps equalize heat and stress on the tool
Step 7: Go Forth and Cut
- To make a speeds chart specific to the materials and bits of your project
- To calculate speeds for lathe and mill work too, using the appropriate SFM values
- To compare/troubleshoot tool performance vis a vis the RPM charts you find in books and on internets.
- To work in with the steel corner brackets instructable I made - or any other "just drill it" kind of instruction.