270 Offy

[mayhugh1]

Given that a cam lobe is convex, a flat tool tangent to any point can't touch any other point. So I don't understand why gouging could come into play as long as the path keeps the tool tangent to the intended surface. The A axis should be able to move continuously.

Good point ...

 

[dsage]

Perhaps when you get down to a small finish cut this is possibly true. But if the cutter diameter is small compared to the size of the cam, the "corner" of a flat cutter can be cutting before the center if you rotate the cam blank into the cutter. If the cutter is very large then the corner is not an issue but there is still some point other than the center of the cutter touching the work before the center (where your calculations are for). The exception would be if you use a ball nose cutter.
Of course if the software is calculating possible collisions of the edge and adjusting for it (as Terry suggests his software does) then it probably a non-issue. But without some allowance for the diameter / corner of the cutter you cannot just rotate a cam into a flat cutter without some un-intended pre-cutting ahead of the center where you intended the cutter to be cutting.

 

[Peter Twissell]

Dsage - it is true that the 'corner' of the cutter will, as you say, cut material from the blank before the centre of the cutter arrives at the required location, but the material removed by the 'corner' is always material which will be removed by the cutter centre as the blank continues to rotate.
My approach with such operations is to try to make the machining operation replicate the final application as closely as possible.
The cam ring for my radial has only two cam profiles (although each has three lobes) so it was possible to machine it with the side of an endmill. By using an endmill of the same diameter as the roller followers, it was relatively straightforward to machine the cam to the desired lift/angle profile.
My mill (Sieg X3) has the facility to rotate the head through 90°, allowing horizontal axis milling. With a cutter larhe enough to allow the mill head to clear the fixturing, I would probably use this configuration to cut cams on a long shaft.
Incidentally, the large diameter of a radial engine cam ring (relative to the roller followers) means that the contact point between cam and follower is always very close to the theoretical contact point of a zero diameter follower, when measured as an angle about the cam centre. This also means that lateral loads on the follower are relatively small.

 

[dsage]

Dsage - it is true that the 'corner' of the cutter will, as you say, cut material from the blank before the centre of the cutter arrives at the required location, but the material removed by the 'corner' is always material which will be removed by the cutter centre as the blank continues to rotate.

I don't agree. Consider the geometry in the first picture below. Also consider that this is a finish pass so all of the rough material is removed already so the cutter is supposed to just barely touch the surface. The center of the cutter is at the desired / calculated cutting point. If you simply rotate the cam clockwise you can see that without any intervention by the software that the left corner of the cutter will contact the cam BEFORE the center does. Hence the corner of the cutter is going to gouge the cam. Without proper software it is better to do the geometry using the left edge of the cutter (not the center). If you had made the cam from the start using the center as the cutting point then you'd have a cam shape with the gouge already machined and you be oblivious to the error in the profile. It would "look" ok but it wouldn't be exactly as intended if measured.
The situation gets worse if you have a larger cutter - in the second picture. The top of the cam would be truncated if you rotate the cam.
The only marginally better situation is if you use a ball nose cutter - third picture. BUt then you're left with grooves around the cam face unless you do very very small step overs (requiring multiple passes).
BTW if you are side milling a contour shape around the cam (as you say you do) without using the A-axis then the geometry is like the third picture. All cam programs are fully aware of the point where the mill is contacting the surface so it should work out fine. As does any contour milling operation.
The key to using a flat cutter is having some software that knows the tool diameter and can calculate where the actualy cutting is going to take place. As apparently Terry's software does.

 

[tornitore45]

The point where a flat tool touches the horizontal tangent is not always aligned with the cam's center of rotation. To avoid cutting metal that should not be cut one must be willing to move the spindle axis back and forth from the rotary table axis. Not a very practical approach on a manual mill. This back and fort movement is what Terry noticed on his CNC machine. His software knows that the tool tip is going to cut ahead so it backs out to keep the porward periphery of the tool on the cam axis and the other side of the tool is riding far high over previously cut area.

 

[byawor]

I cut this one on wire EDM from your dimensions straight flanks. wanted to do the one with radius but could not decifer the center
and radius for the big arc.
bob

 

[mayhugh1]

Bob.
I didn't notice I'd omitted the centers of the flank radii. SolidWorks was happy with the flanks being tangent with the heel and nose circles and didn't warn me that I'd not specified their centers. Here are updated sketches with those centers:

 

[awake]

I don't agree. Consider the geometry in the first picture below. Also consider that this is a finish pass so all of the rough material is removed already so the cutter is supposed to just barely touch the surface. The center of the cutter is at the desired / calculated cutting point. If you simply rotate the cam clockwise you can see that without any intervention by the software that the left corner of the cutter will contact the cam BEFORE the center does. Hence the corner of the cutter is going to gouge the cam. <snip>

Dave, it looks like, from your pictures, that you are depicting a straight-flank cam. If the flanks are radiused, wouldn't that change things?

 

[byawor]

so would it be x.5869 y.2317? Nice thing about this way its only a few lines of code, a good guy with a good machine could get it exact with a surface finish that would need no touch up. Also if so desired could harden the material prior to cutting. Bad thing not so easy to attach to shaft with the right timing. Just did it out of interest would never tackle this engine.
Bob

 

[MikeG]

Hi all, I was just remembering a dear friend....Ron Bement............google him and his offy.

Regards
MikeG

 

[Peter Twissell]

Dsage, you're absolutely right about the cutter corner digging in.
I had in mind a cam with convex flanks, but even then, if the cutter axis remains aligned with the cam base radius axis, it will dig in.
My radial cam has straight flanks and they were machined by moving the mill table to create a surface tangent to the base radius.

 

[kvom]

A program following a flat flank wouldn't rotate the table until the start of the next non-flat section. The tool would move along the Y axis as well as Z to maintain tangency.

 

[mayhugh1]

With four valves per cylinder, the Offy's head isn't to be taken lightly when it comes to its seats and seals. Fortunately, cages were used to improve the odds of getting concentricity between the valves and their sealing surfaces.

My starting material was 3/8" diameter 544 phosphor bronze. Each blank began with polishing its diameter down to .373" using with 400g paper. I used 1/8" rather than 3/32" for the diameter of the valve stem hole to improve my chances of drilling/reaming it straight through the center of the blank. The mouth of the cage which was bored in the same setup. The side entry port was drilled using a v-block setup in the mill before returning to the lathe where the perch for the valve spring was turned.

The final step will be to manually cut the seat using a piloted seat cutter. With the cages located so deep in the head, however, my favorite seat cutter won't be able to reach them after they're installed. If I don't make a new one, the seats will have to be cut and tested the before the cages are installed. In any event, since they're .002" undersize, the cages will slide into the head without distortion where they'll be sealed and secured with high temperature Loctite.

At the beginning of this build, Ron warned me that he had run into problems with his exhaust cages slipping out of position. The shear strength of a Loctite 620 (hi-temp, slip-fit) bond between two properly prepared steel parts at room temperature is about 2.5 kpsi. Accounting for usage with aluminum and a 400F operating temperature, I estimated that 400 lbs would be required to dislodge one of the Offy's cages from its head.

I prepared a test with four .002" undersize bronze blanks installed in a 1/2" thick aluminum block with Loctite 620. With the adhesive fully cured and the block's temperature somewhere between 300F and 400F, all four cages slipped under only a 200 pound load. This 2X discrepancy with the data sheet may have been a result of using bronze in aluminum since Loctite's parameters appear to be dependent upon the actual metals being bonded. Two hundred pounds force on a cage would be roughly the result of a 2500 psi combustion pressure.

I repeated the test using Seal Lock's Fluid Weld which is a similar but new (to me) product mentioned in another thread on this forum. The results were essentially the same. Although I wouldn't expect such a high combustion pressure inside a model engine, Ron's warning has convinced me to also pin the cages inside the head.

The exact lengths of the stock cage subassemblies and their installed heights in the heads are critical to setting the ends of the valve stems a sufficient distance under the cam lobes for operation with the followers. The travel of the valve inside its cage is limited by contact of the loaded spring collar with the top of the cage. The stock parts provide just enough travel for the required .10" lift so long as the valves are sunk into their cages with the .030" wide seats that Ron used. The valves can't be made any longer and still operate with reasonably thick followers. Since I plan to use much narrower seats, I shortened the top ends of the cages by .050". This provides a comfortable margin for the lift requirement without robbing material from the followers.

While working on the valve cages, I also machined the spring collars to break up the monotony of working on the cages. I (hopefully) made way too many spares but thought it wise to do so while I was set up and my workflow seemingly working. The spring collars were designed for use with E-9 commercial external retaining rings. I also made lots of extra collars since those are the parts that typically go flying across the shop during assembly. - Terry

 

[dsage]

Terry:
There has been a series of live seminars recently by Henkel - the makers of Loctite. They stress that Loctite does not work well with "non-active metals". I think that means metals that do not contain copper (but don't quote me on that). In any case there is a primer that you need to add to the joint that adds the necessary ions to the joint before assembly. There is also a new product that replaces the common blue 242 that has the activator in it. It is 243. Blue may not be strong enough for your application but I believe there is an equivalent in red.
I think they posted those seminars on their Youtube channel after they went live.
They stress throughout the seminars that they are available any time for consultation. You should give them a call. I'm sure they can fix you up with the proper product to use.
The info is also available in their catalog but I've found the catalog difficult to read.
Ask the experts.

 

[awake]

Very nice! And glad the cut wasn't any worse. I did a number on my hand when trying to drill some wood angle blocks with a forstner bit, holding by hand. I found out that wasn't such a good idea ...

 

[tornitore45]

Ouch!

 

[kvom]

Love the Sunday updates.

 

[Richard Hed]

I won't tell you what I did, but the same results.

 

[mayhugh1]

Since I'll likely cut the valve seats before installing the cages, I'll need a way to test them independently of the head. I designed a simple leak tester around the Offy's cages, but the concept could be easily applied to others. It works by allowing me to pull a vacuum behind a seated valve so the leak-down time of its seal can be determined. My goal for similar measurements in a head has been at least ten seconds for a leak-down from 25 inHg to 15 inHg.

The body of the tester is a short piece of bored-through one inch diameter Delrin. The front of the bore was internally grooved for an o-ring that seals against the front o.d. of the cage under test. The cage's side entry port is open to a second internal groove that's connected through four longitudinally drilled holes to a plenum in the rear of the tester. In use, the plenum is evacuated using a Mity-Vac hand vacuum pump. With a perfect valve (or a thumb) on the seat, the tester will pull down and maintain a maximum vacuum of some 25 inHg after after a few pulls on its trigger.

In the past I've leak-tested my valves after installing the cages, and this required adapting the pump to the ports on the head. The valve guide ends of the cages also had to be sealed to prevent leakage around the valve stems from affecting the measurement. A silicone cap slipped over the rear of the cage usually took care of this. With the entire cage inside the tester, this seal isn't necessary, and the testing cycle is actually quicker and easier.

I had a spare valve left over from my Howell V-4 build that happened to be a perfect fit to my Offy cages with their .125" valve guides. I cut a .007" wide seat on one of the cages, and the combination held a 25 inHg for a few minutes. To be certain I was actually measuring the seal, I put a tiny scratch across the seat and retested. This time, I could only reach a few inHg, and it leaked down almost immediately. - Terry

 

[gadabout]

Terry, I am interested to know if testing the cage and valve with the Spring ,retainer and keeper would change the result? It shouldn't I think but was wondering if you had tested that way!
Regards
Mark