Lift Table: The Basics

The lift table is the cam profile. I say that a lot. Kind of like the rise and run on stairs, the lift table shows the tappet lift at each degree of lobe rotation. If a dial indicator was placed on the tappet and the camshaft lobe rotated one degree at a time, the dial indicator readings would be the the lift table. Some lift tables may show increments of less than 1-degree. I have previous posts about lift tables so be sure to go back and also read them.

"The lift table shows the tappet lift at each degree of lobe rotation." It is very important to understand that statement. "Tappet lift", is any tappet, not specific. This is the rise (lift) of the tappet. It doesn't apply to any type of tappet, only the lift of the tappet. The same lift table can be for a flat tappet profile or a roller profile. It can be for a roller profile with any wheel diameter. The "tappet lift" is just that, the lift of the tappet. Remember that.

The actual tappet used will determine the final shape of the lobe. That is because of the contact point between the tappet and the lobe. The contact point does not stay on the centerline of the tappet. The contact point moves away from the centerline on a flat tappet and a roller tappet. The shape of the lobe and the contact point will determine the tappet lift at each degree of lobe rotation. Obviously if you put a flat tappet on a roller profile or vice versa you will get a different set of specifications. That is because the contact point is different on the flat tappet and the roller, make sense.

When someone orders a cam profile design from me they will get a .p file, an .s96 file, and a data sheet with the lift table. All of these are the profile lift table just in different formats for different applications. The .p file is used for the Landis CNC cam grinding machine, the .s96 file is used for most other CNC cam grinding machines. The lift table is part of the data sheet for the cam profile. Other important information is also on the data sheet. Most CNC cam grinding machines that I know of use the lift table to produce the cam profile. Apparently the CNC machines have built-in software to calculate the contact point for the actual tappet being used to produce the cam profile.

Sometimes customers will want to convert the lift table to cartesian coordinates (x and y). Using a direct conversion equation from the polar coordinates of the lift table to cartesian coordinates will not work. The contact point for the actual tappet being used must be considered in the calculations. The base circle diameter of the lobe also plays a part. Using a direct conversion will produce a shape that looks more like an inverted roller profile and is of no use. Physically drawing the profile using the lift table and the correct tappet will create the correct lobe shape. I am not a CAD/CAM person but that would probably be the best way to draw it.

Back in the day before CNC machines I would create a model lobe by using the lift table and a rotary chuck on a manual milling machine. This was a visually effective way of seeing and understanding the contact point between the lobe and the tappet. In this case the cutter is the tappet and it creates the lobe shape. I would use a flat face end mill for a flat tappet profile and a ball end mill of the correct diameter for a roller profile. Many original roller profile lift tables were used to produce a mushroom flat tappet profile. I just used a flat face end mill cutter instead of a ball end mill. Pretty cool. Today with CNC milling machines the model lobe can be produced using cartesian coordinates. This website may be of help for converting the lift table to the proper x and y coordinates. https://saltire.com/