# Thread: Calculating feeds for milling a helix

1. ## Calculating feeds for milling a helix

Hi,
I have to basically mill a twist drill type form on the front end of a part I have turned on a CNC Swiss machine. I just guessed a feedrate and it worked OK. But I would like to know how to accurately calculate the feedrate
when using C axis and any other axis. Anybody out there help with this?

plarkin@acumed.

2. If you are making a simultaneous ZC motion, I would guess that the linear axis feedrate is going to be 'boss' and will determine how long the motion requires to complete. The rotary axis is internally synchronized via interpolation, to end the feed motion at exactly the same time as the Z motion completes.

For a sole rotary axis feedrate, you need to calculate the circumference of the part, to determine how many degrees per minute it takes to travel an equivalent linear distance. There may even be a setting in your control parameters to enter the diameter that the C axis is working on.

To determine the combined vector feedrate of both the linear and rotary combined motion as you mill the helix, you need to find the length of the hypotenuse of a right triangle:
The base of the triangle is the lead of the helix, or how far along Z it takes to complete a full revolution of the feature.
The height of the triangle is the circumference of the part, generally assumed to be at the largest diameter.

From those two sides, you can compute the length of the hypotenuse, which is the path the tool is on. Subsequently, you can arbitrarily choose how fast you want the tool to travel along this hypotenuse.

Now, you reverse engineer the Z component of the 'hypotenusal feedrate' using what....COS of the helix angle (between hypotenuse and the Z base). So the linear feedrate of the Z should always be less than the actual desired feedrate along the helix.

Since a helix like a twist drill is quite a gentle slope, there is not going to be a huge difference between the Z component and the hypotenusal net feedrate. Its barely worth the trouble to calculate the difference since its like only a few percent on the feedrate override.

3. I have to linear mill around a flat part (manual program), with a Z-.125. The cut is .15. If I start with a X-.5 Y.5 part zero offset,with a .375 end mill (top L corner of work) I am having a hard time calculating the Y move needed to
move to for my X move. I have X0 Y.0375, but this does'nt seem right.

4. Originally Posted by HuFlungDung
If you are making a simultaneous ZC motion, I would guess that the linear axis feedrate is going to be 'boss' and will determine how long the motion requires to complete. The rotary axis is internally synchronized via interpolation, to end the feed motion at exactly the same time as the Z motion completes.

For a sole rotary axis feedrate, you need to calculate the circumference of the part, to determine how many degrees per minute it takes to travel an equivalent linear distance. There may even be a setting in your control parameters to enter the diameter that the C axis is working on.

To determine the combined vector feedrate of both the linear and rotary combined motion as you mill the helix, you need to find the length of the hypotenuse of a right triangle:
The base of the triangle is the lead of the helix, or how far along Z it takes to complete a full revolution of the feature.
The height of the triangle is the circumference of the part, generally assumed to be at the largest diameter.

From those two sides, you can compute the length of the hypotenuse, which is the path the tool is on. Subsequently, you can arbitrarily choose how fast you want the tool to travel along this hypotenuse.

Now, you reverse engineer the Z component of the 'hypotenusal feedrate' using what....COS of the helix angle (between hypotenuse and the Z base). So the linear feedrate of the Z should always be less than the actual desired feedrate along the helix.

Since a helix like a twist drill is quite a gentle slope, there is not going to be a huge difference between the Z component and the hypotenusal net feedrate. Its barely worth the trouble to calculate the difference since its like only a few percent on the feedrate override.
all sound complicated to me could you not just use IPM and convert to IPR
just askin HU you know more than i do i just look for simple

• Originally Posted by jackson
all sound complicated to me could you not just use IPM and convert to IPR
just askin HU you know more than i do i just look for simple
This is a Live tool that may not read IPR (G99) with this application. I'd say that using IPM (G98) would be more practical. Then again I've never done anything like this Part with the exception of Common Threading. Remember to that this is a Swiss. So you can't Pull the material all the way back throught the guide bushing. It has to be Machined in One Pass given that the length of the part is under 3/8 to 5/8.

Difficult to say without being in front of the Machine.

• Originally Posted by tobyaxis
This is a Live tool that may not read IPR (G99) with this application. I'd say that using IPM (G98) would be more practical. Then again I've never done anything like this Part with the exception of Common Threading. Remember to that this is a Swiss. So you can't Pull the material all the way back throught the guide bushing. It has to be Machined in One Pass given that the length of the part is under 3/8 to 5/8.

Difficult to say without being in front of the Machine.
My firt thought was just to use IPM but being a swiss"wich im not real familiar with" i didnt know if it needed IPR that was why i was thinking you may need to convert

• Originally Posted by jackson
My firt thought was just to use IPM but being a swiss"wich im not real familiar with" i didnt know if it needed IPR that was why i was thinking you may need to convert
Oops!! my bad, sorry A Swiss is very simular to a regular Lathe that will handle both IPR (G99) and IPM (G98) with the exception of the Live tools. It really depends on how old the Control is too. If it is like the Fanuc 1LE that I worked with it won't take IPR (G99) for a live tool because it doesn't know or have an input area for the number of Flutes on the End Mill. A Mazak with a Fusion Control will though. That is a different machine all to gether. (JUNK!!! IMHO).

This Swiss Program puts a CAM on the front of a part. The machine wasn't designed for this but a SMART Co-Worker made it happen. You will notice the G98 in the begining of the Milling Operation.