Cam Grinder questions / suggestions

[Charles Lamont]

Mine grinder uses a white aluminium oxide wheel mounted on a home made spindle. I would not take anything like so agressive a cut with that wheel. In my case the cams are machined to about 0.005" over size (on radius), hardened, and then ground, taking cuts of 0.0005" and allowing it to run almost to spark-out before adding more cut. Even with some cooling, trying to go any faster would overheat and colour the cam surface.

My grinder does not really have a bed as such, but is instead based on two pieces of 5/8 birch multi-ply glued together. This may make the construction clearer:

 

[werowance]

I went back and looked at your website Charles, now that ive had time to study and learn a little more i am starting to understand the mechanics of it all a little better (which isnt saying much 1+.00001 for example is very little but getting there) one part im still researching is the pivot points or ratio. so if i take a master cam that 2x the desired outcome how would i calculate the pivoting arm? example you start out at the hinge, move out 6 inches and that is the cam blank rotating assembly and them move out 6 more inches and that is the master cam bump or rotating asembly. the 6 inches i am using is just an example, how would you calculate the distance to move out for each point on the arm in order to use a 2x master cam? i feel like this should be simple math but it eludes me. or does it matter as long as i keep the distance between the shafts the same?


here is an edited version of the pdf you just sent. and im just trying to figure out if i design one of my own how would i go about calculating the distance between the points below? not asking for your measurements that you used in your design, just wondering how i would do the math is all

 

[bluejets]

The grinder I designed and built mostly from recycled materials and parts from commercial printers (cogs, belts etc.) has served me well for over 15 years now.
Had to rebuilt it when the flood came and damaged most things in my workshop but apart from that, it's the same as built originally.

I did have a stepper drive on the feed-in mechanism which I replaced with a dc motor/gearbox due to water damage.
Otherwise same same.........
I usually grind one pair of cams from a turned blank at a time and then pin that cam pair to a silver steel shaft after hardening and polishing the cam.

The reference cam (at top) is 4 times finished size and there are colour coded reference points to locate the reference cam. This feature also will allow a complete one piece cam to be ground but in the past this has proven to be a problem with bending during the hardening process, therefore a one pair cam method now the preferred.

The grind motor (Sunbeam mixer ) did have a speed control at the back but this used to "hunt" too much at the required grind speed.(centrifugal governor) so this was bridged out and a standard panel mount thyristor control was added right from day one. (suck it and see I guess one could say)
Black tin can is simply a cooling duct for the motor and contains a filter to keep grinding waste out.

 

[peterl95124]

also, just ran across another photo and it looks like the cam stock is hand rotated wtih a crank. any issues you all know of doing it that way? would eliminate some cost and initial work but might end up with a sore arm after a while of using it.

since I usually advance the grinding wheel by .001 to .002 per revolution of the cam it takes quite a few revolutions to grind a cam with .078 or .094 lift, which need to go slowly and evenly, so yes you'd much rather have a simple motor driving the revolutions, after all the time and effort for everything else in the machine its a small thing to add.

 

[peterl95124]

I went back and looked at your website Charles, now that ive had time to study and learn a little more i am starting to understand the mechanics of it all a little better (which isnt saying much 1+.00001 for example is very little but getting there) one part im still researching is the pivot points or ratio. so if i take a master cam that 2x the desired outcome how would i calculate the pivoting arm? example you start out at the hinge, move out 6 inches and that is the cam blank rotating assembly and them move out 6 more inches and that is the master cam bump or rotating asembly. the 6 inches i am using is just an example, how would you calculate the distance to move out for each point on the arm in order to use a 2x master cam? i feel like this should be simple math but it eludes me. or does it matter as long as i keep the distance between the shafts the same?


here is an edited version of the pdf you just sent. and im just trying to figure out if i design one of my own how would i go about calculating the distance between the points below? not asking for your measurements that you used in your design, just wondering how i would do the math is all

View attachment 156722

you've mis-identified the cam rotation point, see the "pointy" ended rods, those are "dead centers" for the cam blank which has to have been "center drilled".

hopefully you remember "similar triangles" from elementary school geometry, if the master cam center is 4 times as far from the pivot as the cam blank (being ground) is from the pivot, then the master cam (for this design of grinding machine) needs to be 4 times as large as the cam being ground. this design of grinding machine also requires a timing belt to keep the cam blank and the master cam (which are on separate parallel shafts) rotating exactly in phase at all times.

the other design, that I used, has the cam blank and the master cam on the exact same shaft, so there's no extra cog belt, and the master cam is an "additive" shape (EG. 2" diameter with 0.094" lift) instead of being a perfectly scaled image of the desired cam (EG 2" diameter with 1/2" lift)

hope that makes sense...

 

[peterl95124]

here is the Don Bell "Master Cam Design" article, I find it hard to read because it tells you what to do but you then have to reverse-engineer what's going on, which is this, you start with the center points for the 3 arcs at exact scale, then ADD the same amount to each radius and you end up with 3 arcs that still meet each other tangentially (so the cam is "smooth"), and has the same lift as the original, but has a larger base circle.

FYI the timing (beginning and ending angles of the lift) appears to be off, but when you take into account where the grinding wheel actually contacts the cam blank it works out that your ground cam does have the correct duration angle.

when you make this type (additive) of master cam, be sure to grind on some test bar stock first, it will probably end up with some faceting, imperfections in the master that don't seem to be visible to the naked eye can show up (sometimes glaringly) in the ground cam, so some fine filing on the master may be necessary to get a perfectly smooth ground cam. for me, if this happens, it is at the meet points of the 3 arcs, especially where the flank and nose arcs meet.

 

[petertha]

Tuning in now because a while back I was struggling with this same geometry. I read the same SIC Don Bell article but it confused me. Oh well, I often confuse myself. I'll throw out some screen grabs using his specific cam example. Maybe it will clarify matters or maybe will add to confusion. I suspect the 'answer' may lay in semantics and/or what occurs behind the scenes in CAD programs.

Start out with example cam. My CAD program shows all black when the geometry is fully defined, so far so good.

If I simply use an 'offset curve' function to the cam perimeter in my CAD program using his 1.375" value, I get his same resultant dimensions & nose radius (R=1.406). The 1.375 comes about because he defined base circle = 3.00 diameter on the no-lift zone. So if true, this would be appealing because generating the enlarged template offset is just a single mouse click. I could further generate intermediate Angular or X,Y coordinates to this curve.

Now here is a different perspective which is where I thought this was going. If you take the same 1.375 offset dimension & radiate them out from center but the segments, the construction geometry looks like this.

Now connect those construction segment end points with a curve. I also additionally constrain the curve end points so curve is perpendicular to the adjoining X & Y axis respectively. From the back of the room, that looks reasonable?

But when you overlay the 2 methods they are visibly different. Orange is aforementioned radial offset segment method. Grey is 1-click built-in CAD offset command. I'm not 100% here but I think the explanation is we get a different resultant curve if the construction segments are projected to COINCIDE with the center point as opposed to only projecting normal to the cam curve (which will not intercept the center). Now which is 'right' for this particular cam grinding geometry mechanism purpose?

BTW, the smaller you make the base diameter, the more discrepancy can be seen

 

[petertha]

I think what the CAD offset function is doing behind the scenes might be visualized like this. I'm using same cam but different offset values to the Bell example just to make the screen grabs more compact. Here I define offset amount = 0.500" represented by circle at 12:00 position. Then I make an rotational array of circles with identical diameters. The enlarged grinder template would be represented by red curve which is a smooth curve connecting all the circles tangentially at their OD's. The red would just continue on around the bottom which is just a circle anyways. Where this gets tricky manually is the offset circle must be tangent to the correct corresponding curve segments depending on its angular position. It can occur on the cam nose, flank or the base circle depending on the angle.

 

[Charles Lamont]

you've mis-identified the cam rotation point, see the "pointy" ended rods, those are "dead centers" for the cam blank which has to have been "center drilled".

hopefully you remember "similar triangles" from elementary school geometry, if the master cam center is 4 times as far from the pivot as the cam blank (being ground) is from the pivot, then the master cam (for this design of grinding machine) needs to be 4 times as large as the cam being ground.

You are right on both points. I would add that 'similarity' as in similar triangles, is the key to the design. This has further implications. Firstly, and continuing throughout with the example scaling factor of 4, then the shoe or follower that the master cam rides on needs to be radiused at four times the radius of the wheel. A 5" diameter wheel would need a 10" radius follower. Dressing the wheel will make a difference, but not enough to worry about.

You can use a flat follower, but the master cam profile should be adjusted to compensate, and I have never bothered to get my head round exactly how to do that.

While other geometrically correct designs are possible, an 'all square' layout is easiest. If the hinge to master cam centre distance is 6" then the hinge to cam radius is 1.5". The horizontal, plan view, distance from hinge to wheel centre will also be 1.5" and the shoe will be also be at 6". Finally, the hinge is adjustable for height, and the centre line must be set to the same height as the top of the wheel. This will mean that when grinding is complete, the shoe will also be the same height and similarity assured.

The fact that the hinged carrier is at an angle above the horizontal means that the centre of the master cam will not be vertically above the centre of the follower, but neither will the cam over the wheel, and the similarity built into the design ensures accurate reproduction of the master.

My grinder actually uses a scale factor of 5. It seemed to make it easy to set out the table of offsets for maching the master cams. A good reason for choosing this scaled-up-master-cam type of design is that making the master cam profile to an accuracy of say ±0.001" means that the finished cam will be within a couple of tenths. One would not be going to the trouble of making a cam grinder if one did not obsess care about precision.

 

[peterl95124]

I think what the CAD offset function is doing behind the scenes might be visualized like this. I'm using same cam but different offset values to the Bell example just to make the screen grabs more compact. Here I define offset amount = 0.500" represented by circle at 12:00 position. Then I make an rotational array of circles with identical diameters. The enlarged grinder template would be represented by red curve which is a smooth curve connecting all the circles tangentially at their OD's. The red would just continue on around the bottom which is just a circle anyways. Where this gets tricky manually is the offset circle must be tangent to the correct corresponding curve segments depending on its angular position. It can occur on the cam nose, flank or the base circle depending on the angle.

TL;DR, the CAD program does not know anything about the "center" of your cam design and does NOT simply extend radially from that "center" (which would only work for the base circle and not for the flanks or nose).

CAD programs find the "normal" (perpendicular to the "tangent") at each point on the curve, and extend along that "normal" line which in general does NOT go through what we're arbitrarily calling the center of the cam.

hopefully a picture is worth a thousand words...,
here the circles are the cutter/grinder, and "to the center of the cam" is straight down,
the middle shows incorrect cutting/grinding if simply adding an offset from your chosen "center" of the cam, which over-cuts into the cam, that must not be allowed to happen,
the right shows how CAD software actually works, the offset is along the "normal" line (perpendicular to the "tangent"), not along the line to the "center" of the cam

 

[petertha]

the CAD program does not know anything about the "center" of your cam design and does NOT simply extend radially from that "center" (which would only work for the base circle and not for the flanks or nose). CAD programs find the "normal" (perpendicular to the "tangent") at each point on the curve, and extend along that "normal" line which in general does NOT go through what we're arbitrarily calling the center of the cam.

I completely agree, my description was not clear. If the fixed offset amount is perpendicular to the tangent of the cam (which is what the cad program does), then the extending the normal line inward demonstrates it will only be coincident to the cam rotation center at particular points as you say. But what I was not clear on is if a particular cam replicating mechanism ASSUMES the follower mechanism is acting through the center & therefore had to cook a different template for that reason - in which case the cad offset shape would not be compatible. I have not delved into cam grinding machines in any detail but one in particular I was contemplating had a ball ended stylus on a lever. But I wasn't quite clear how the template was generated which is what got me to drawing. Hopefully I haven't confused matters more.

 

[werowance]

you've mis-identified the cam rotation point, see the "pointy" ended rods, those are "dead centers" for the cam blank which has to have been "center drilled".

hopefully you remember "similar triangles" from elementary school geometry, if the master cam center is 4 times as far from the pivot as the cam blank (being ground) is from the pivot, then the master cam (for this design of grinding machine) needs to be 4 times as large as the cam being ground. this design of grinding machine also requires a timing belt to keep the cam blank and the master cam (which are on separate parallel shafts) rotating exactly in phase at all times.

the other design, that I used, has the cam blank and the master cam on the exact same shaft, so there's no extra cog belt, and the master cam is an "additive" shape (EG. 2" diameter with 0.094" lift) instead of being a perfectly scaled image of the desired cam (EG 2" diameter with 1/2" lift)

hope that makes sense...

yes it does now, but for the life of me i couldnt figure out how to calculate that. i appreciate the help from everyone.

 

[peterl95124]

yes it does now, but for the life of me i couldnt figure out how to calculate that. i appreciate the help from everyone.

in a perfect world (?) the master cam's cam follower and the grinding wheel would have the same diameter/curvature, in that way the ground cam will be "exact", in the real world my master cam is about 2" diameter and my grinding wheel is about 4" diameter but I'm not able to detect any anomaly in my ground cams as a result of this. Also in a perfect world (?) the actual cam's cam follower's diameter and the afore mentioned diameters would be the same, but in the real world that follower's diameter is always pretty small because there's just no room for a large follower in the rocker arm, but they all seem to work well anyway. I have a hunch this all works out well because my cam and follower diameters have a similar ratio to the master cam and its follower diameters. I'm sure there's a mathematical way to compensate for the small diameter of the rocker arm's cam follower, but I haven't bothered to figure it out as the result could be that the master cam is not a simple 3-arc cam and that would make it impractical to make, and the cams I've been making have turned out entirely suitable for model engine'ing (they appear, under high magnification, to have the correct nose radius, flank radius, and overall duration).

note that the follower for the master cam in the drawing you posted of Charles' grinder is flat (infinite diameter) does not match the diameter of the grinding wheel, but his ground cams I'm sure work fine. what doesn't work is a master cam's follower whose diameter is too small, that case is easy to draw and easy to see that it results in a ground cam lobe that is too small.

IMHO, YMMV, VWPBL, yada, yada, yada...

 

[GreenTwin]

I am tempted to mock up some cam grinder shapes in Solidworks, and try and get it to do a trace in simulation.
Then various shapes/sizes could be tried, and the results would be drawn out, and used for comparison.

This is how they use to check the old steam engine valvegear, but manually with cranks and arms, drawing on paper with pencil, to trace valve movement in forward, reverse, and in various link positions.

I have read everyone's comments, but if asked to summarize it all, I could not effectively do that.
Solidworks simulation does determine movement exactly though.
If I were designing a multi-cylinder engine, I guess I would deep dive into this topic.
For one cam on a hit and miss engine, I would probably hand-grind that cam.

Quite an interesting topic for sure, and more complex than I imagined.
I really thought a cam profile scaled up on a copy machine would work.
.

 

[Roger Taylor]

I am tempted to mock up some cam grinder shapes in Solidworks, and try and get it to do a trace in simulation.
Then various shapes/sizes could be tried, and the results would be drawn out, and used for comparison.

This is how they use to check the old steam engine switchgear, but manually with cranks and arms, drawing on paper with pencil, to trace valve movement in forward, reverse, and in various link positions.

I have read everyone's comments, but if asked to summarize it all, I could not effectively do that.
Solidworks simulation does spell out movement exactly though.
If I were designing a multi-cylinder engine, I guess I would deep dive into this topic.
For one cam on a hit and miss engine, I would probably hand-grind that cam.

Quite an interesting topic for sure, and more complex than I imagined.
I really thought a cam profile scaled up on a copy machine would work.
.

 

[Roger Taylor]

Just an article to mention on a cam shaft grinder build in the Nov/Dec 2004 issue of Home Shop Machinist magazine. by Jerry Keiffer. It is made from Sherline lathe parts, and a very similar one is exhibited at the model engineering Craftsmanship Museum.

 

[Charles Lamont]

note that the follower for the master cam in the drawing you posted of Charles' grinder is flat (infinite diameter) does not match the diameter of the grinding wheel, but his ground cams I'm sure work fine. what doesn't work is a master cam's follower whose diameter is too small, that case is easy to draw and easy to see that it results in a ground cam lobe that is too small.

No. As explained in post #29, the follower is radiused (though it is not really discernible in the view). The master cam scaling factor is five, and the radius of the follower is therefore five times the radius of the grinding wheel. Anything else would introduce errors.

 

[GreenTwin]

There are so many variables being mentioned that it becomes difficult to see the forest, because all those trees are in the way.

I would have to start with something simple, such as the master cam being exactly the same size as the finished cam, the wheel that rolls on the master cam being the same diameter as the grinding wheel, etc., and then vary one parameter at a time to see what effect that has.

Unfortunately this topic at this point is as clear as mud to me, but again, I am not making a cam or cam grinder, so just making casual observations.

The only way I would be able to begin comprehending this is if some very simple set of starting point parameters was defined; ie: 1:1 ratios, etc.

Edit:
It would seem that if you placed a finished cam lobe in the machine (lobe that has been machined to final size), and then placed a 5x sized round wheel on the far end of the shaft, and secured a sharpie (or something more accurate) on the circumference of the 5X wheel, then rotated the cam by hand while its surface was pressing against the grinding wheel, the sharpie should trace out the shape (on paper) needed for the master cam ? Is this a safe statement ?
.

 

[Vietti]

I think I mentioned it before but the old time auto machine shops bought pistons in rough sizes and then cam ground them. These machines have not been used in years but can make a pretty cool cam grinder. The late Jim Riggle and a few others, my self included, made cam grinder from these machines, The most common brand seems to be Van Norman. They also make a decent radial grinder.

 

[petertha]

I would have to start with something simple, such as the master cam being exactly the same size as the finished cam, the wheel that rolls on the master cam being the same diameter as the grinding wheel, etc., and then vary one parameter at a time to see what effect that has.

I suspect the simple 1:1 example works because certain variables have been nulled by definition. A 1:1 replicator might be a perfectly good grinder for the intended purposes. But when you try to scale it up to a 2X or 3X lever mechanism because template enlargement offers some desirable advantage, then new variables come into play & now you have to pay attention to what is more fixed, for example grinding wheel diameter VS. what must then vary for example follower radius or lever pivot lengths or master template shape or.... Its like a generic formula X=f(A,B). By specifying A & B, X becomes known. Alternately specifying X & A, B becomes known.

Since you mentioned SW you could use sketch blocks to your advantage. Of course you know that when all elements are black, it means the dimensions are fully defined; you have a correct solution. If you have blue lines it means something still needs to be defined. This is just a generic sketch so don't pay attention to dimensions. But like the above formula analogy,
- if you define grinder OD + lever length + template is 2X cam, then follower radius is calculated to satisfy result
- if you define grinder OD + follower radius + template is 2X cam, then lever length is calculated to satisfy result
- if you define grinder OD + lever length + follower radius then the master template shape must be calculated to satisfy result. This is a lot more work but could be approximated by rotating the cam some increment result & make a reference point of what the master would have to be. Rinse & repeat & connect all the dots with a spline I suppose. Ugh.

Now in real life I suspect most of these mechanisms just locking down a certain collection of parameters out of practicality & if the resultant cam is out a couple thou, oh well. And because there are different means to accomplish the same end result, it may not be readily apparent what is going on just looking at the mechanism even though it was clear in the designers mind. If you build a model, please share results.