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Author Topic: CNC porting cylinder heads - options?  (Read 4591 times)

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Offline KdF

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CNC porting cylinder heads - options?
« on: June 30, 2008, 09:25:18 AM »
Here is a tidbit for those that always wanted to CNC port cylinder heads but did not know how to probe the ports for a reference.

If you start out with a probe tip the same shape as the cutter tool you will use (WE use a 3/8" ball tipped tool on a long taper shank) then you can take the polar array and probe a slice section of the port one step at a time.

For example position the head in a fixture you would use to hold it during machining. Pick a center point location that allows you to get as far down the port as possible. Next start the array running and allow it to step down a certain amount each cycle. When you have reached the bottom you will have series of slices that can be used to create a cutting program to replicate what you have probed.

Next rotate the head in the fixture to setup the other end of the port and repeat the process.

As long as you use the same shape tip as the cutter then the probe shape will translate directly to the cut shape accurately.  You can then cut it like you probed it.

Just A Thought (;-) TP

First post. Let me know if I'm in wrong place  ;)
Got some complete beginner questions regarding the possibilities of CNC head porting. The above post was the only mention I could find regarding cylinder heads....

I'm carrying out a CNC retrofit to my Bridgeport vertical mill and have been looking for a method that might allow me to achieve at least a basic level of CNC porting and digitizing ports. Ideally full 5 axis is the way to go but trying to build a dual rotary table (A & C) looks like it'd be a nightmare with respect to backlash (adding servos or stepper motors to a couple of rotary tables). I'm not aiming for mass production level, more a case of saving my semi arthritic wrist from countless hours of grief manipulating an air-powered die grinder  ;) This is more a hobby project than mega buck investment.

Not sure I've interpreted the post above correctly but am I right in thinking the probing is done on a 3 axis machine to the point where the head must be rotated to allow better port access? If the exact rotation co-ordinates are noted, CNC porting of the head can then be carried out in various stages on a 3 axis machine, as long as each batch of probing/cutting is carried out at the same exact position?  It would obviously be more time consuming than using a dedicated 5 axis machining centre (such as the Centroid) but for me, time is more readily available than cash  ;)

I'm a total newbie when it comes to digitizing....  say for instance I wanted to digitize an existing, painstakingly developed, hand shaped port, can the Mach3 automatically send the probe on a journey around the port, recording the co-ordinates as it goes, therefore saving an awful lot of time having to manipulate the probe's position manually? Does some kind of preliminary plot have to be entered to tel the probe how close together the contact points should be? Will it do this in XY&Z?

Can those co-ordinates then be imported into a separate CAD program (eg Rhino) for further manipulation/smoothing before directing the mill to carry out the work?

If anyone can point me towards any tech articles/books etc that might help, please do. I've found very little info with regard to CNC porting of cylinder heads, apart from the very high end, dedicated machining centres by the likes of Rottler and Centroid etc. I'm sure much of what I'm trying to achieve has similarities with work carried out by many people on this forum, even if theirs isn't automotive in nature.
« Last Edit: June 30, 2008, 09:30:02 AM by KdF »
Re: CNC porting cylinder heads - options?
« Reply #1 on: June 30, 2008, 12:31:02 PM »
Hi Kdf and welcome to Mach

I think you are approaching this from the wrong end, there must be an optimum profile for the ports ?

You use a cad program to draw that profile then a cam program to produce a toolpath to produce that profile. "Cad/Cam"

You will want to centre the port at the top and centre as far down as you can go with your cutter and the cad program can be used to centre the two then just hand finish into the curved part of the chamber.

HTH
Phil_H
The Good Thing About Mach3, Is It's very Configurable

The Bad Thing About Mach3, Is It's Too Configurable

Offline KdF

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Re: CNC porting cylinder heads - options?
« Reply #2 on: June 30, 2008, 01:38:24 PM »
Hi Phil,

Due to the tortuous route many ports take, you'd need to set up the head in several configurations to complete a full CNC rework of the port on a 3 axis machine. Hence the appearance of various high dollar 5 axis machining centres designed specifically for the purpose. Setting up on a 3 axis machine for a single CNC pass down the port wouldn't offer much advanatage as it would leave a significant portion of the port untouched. Check this link for a video of the Centroid 5-axis CNC head porting machine:
http://www.centroidcnc.com/cncvideo/cnc_video_porting.html
Amazing stuff. If only I had the cash  :(

Usual approach is to work on one port the old-fashioned hard way - reshaping by hand and carrying out frequent flow tests on a cylinder head flow bench to monitor progress. The ultimate aim being to make the port as efficient as possible i.e. achieve good flow numbers whilst maintaining good port velocity. Once the optimum port shape has been found, digitizing the port accurately allows that hard earned port shape to be replicated on the remaining ports, achieving identical performance on each and every port. Very tricky and time consuming to do all that by hand but that's how I've been doing it for years!

Once a library of port shapes has been created and digitized, it's then a simple matter of selecting the design best suited to the current application and all ports can be CNC cut. Having to carry out only minor blending by hand would be an absolute luxury  8)

I'm sure top level race teams subject computerized port designs to CFD analysis and maximize flow potential before a head casting gets anywhere near a milling machine. But I think it'll be a while before that filters down to the rest of us - unless someone knows of a relatively expensive CFD program offering that capability?  :)
« Last Edit: June 30, 2008, 01:52:26 PM by KdF »

vmax549

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Re: CNC porting cylinder heads - options?
« Reply #3 on: June 30, 2008, 04:25:56 PM »
KDF it will depend on the shape of the port as to how well you can replicate it on a 3 axis. Basically can you reach 100 % of the surfaces with just fixed fixture rotations. AS to the five axis it just allows them to do an entire head without intervention.
AND provides paths that are a  better in finish quality with little to NO overlapping lines.

IF your fixturing is accurate in both planes you can actually do a good job replicating a port.

Probing is the same deal. You can probe the port in two planes each plane then can be optimized for shape in a good 3d cad program and produce code based on the fixture plane to produce a decent port replication. The easy way is to use a probe tip the same shape as the cutter tip. Then it will probe what it sees and then cut what it probed.

You may have to optimise your probing routine to gather the best set of data points. THer are two basic probing routines that can work. One is a probing array. the other is a creepy crawler method both can do what you need.

Just make sure your fixture plate is properly indexed to the mill so it will replicate the base position every time. OR you will see cross over lines when you change planes.

Let me know if you need help, (;-) TP

« Last Edit: June 30, 2008, 04:47:51 PM by vmax549 »

Offline KdF

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Re: CNC porting cylinder heads - options?
« Reply #4 on: June 30, 2008, 05:55:49 PM »
 Hi vmax549,

Glad you chipped in because it was your post on the guitar thread (quoted above) that prompted my enquiry.

The port design of the heads I work with means there's no way you can gain access to the entire port from a single fixed position - it'd probably take at least four static positions to probe the entire port. I've been looking at ways of implementing 5 axis but suspect it may involve a lot of time and expense to achieve something less than satisfactory. I'd happily sacrifice a little time having to reposition the head a few times than have inaccurate machining due to backlash in a rotary table (or two) giving inconsistent co-ordinates. I understand the need for absolute repeatability in respect of positioning the heads on the mill table.

I've yet to carry out the CNC upgrade to my mill. Still trying to decide which route to take eg servos or stepper. Also need to either fabricate or buy a ballscrew Z axis, in addition to the ballscrew upgrade for X&Y. I'm sure this is essential as I'll be relying on accurate control of the Z axis. Pretty much ruled out motorizing the knee.

I've been trying to work out if a rotary 4th axis might be a step in the right direction. I'd still have the issue of dealing with backlash but if I can sort out a decent rotary table with minimal backlash, maybe it'd make additional probing and machining setups easier. Maybe not? From what you say, I suspect 3 axis with some accurately machined fixtures might be the way forward.

I guess I then need to find a reliable and accurate way to digitize the port and use some software that's capable of combining arrays or the creepy crawly method you mention. This is an area of which I have zero knowledge (so far!). Can Mach3 handle either or both of these digitizing methods?  I've got a copy of the program but I know I won't start to fully understand and learn its capabilities until I actually start using it for real.

Thanks again for taking the time to share your knowledge. It's much appreciated.