I have been developing the capability to cut the gears for the Wallaby. I will be using the tools and techniques I picked up from Chris over at ClickSpring. If you are not familiar with his work, look him up on YouTube, he is very talented and makes beautiful videos. I will not repeat his instructions here as I could not do them justice. To make a gear you need to be able to cut the spaces between the teeth with a properly shaped cutter, and to make the cutter you need special lathe cutters, mandrels, cutter blanks, and sharpening tools. This is where I started. Below is a picture of the print for one of the spur gears and an aluminum test gear, the final spur gears will be made of brass and the pinions of steel.
This is the print for one of the two pinions
Sheet two has the information necessary to create the cutter to cut the space between the gear teeth.
Below I am practicing making gear tooth cutter blanks from 01 tool steel.
Below is the beginnings of one of the button cutters that will cut the profile into the gear tooth cutter. these are precision diameter made from 01 tool steel and hardened. In the back ground is a tool that allows the proper relief angle to be cut into the button cutters and is also used to precicley sharpen them.
As seen below two of these button cutters are mounted in a holder and cut the proper profile into the cutter on the lathe.
Then the mill is used to create the face of the cutter.
Below is the gear tooth cutter ready for heat treatment.
This is a rotary table that I picked up from Amazon and uses the same chucks as my lathe. I use some Arduino code and a PC to advance the gear blank the correct amount so the mill can cut each tooth space.
I cut the gear blank to diameter on the lathe, then move the entire chuck and work piece to the rotary table on the mill. then I cut each leaf (the space between the gear teeth).
Finally I move the chuck back to the lathe and perform the other operations to finish off the gear.
The following picture shows the formed cutter fit into the forming tool.
Here is a closeup of the cutter tool for the gear cutter, you can see the 15 degree relief marked in red.
Below is the mandrel used to cut the gear cutter. the gear cutter is mounted off center in order to create the 15 degree relief angle when actually cutting the gear. The pin holds the cutter in the proper position as the cutter profile is turned on the lathe, then the cutter is rotated to the next position and the lathe operation is repeated. This mandrel is also used in the mill to cut the notches which reveal the cutter faces.
Below is the setup for sharpening the gear cutter. It presents the face of the cutter perfectly to the grinding stone. This same set up is used to sharpen the button cutters. I have highlighted in red the holes that the button cutters are mounted in while they are sharpened.
You asked the question: How did you determine how much to off center the cutter? Below is a drawing of the gear cutter blank, I have circled in red the desired 15 degree relief angle that is presented to the gear as it is cut. Then I have also circled in red a small point that represents the rotational axis the cutter blank needs to be turned on in the lathe to create this relief angle.
Below is a test fit of the practice aluminum gears on the engine. How do you like my precision carbide 1/4″ gear shafts? The top right gear is connected to the cam shaft and the bottom gear drives the oil pump.
Below is a mega close up of the two small gears meshed. The leaf cut depth is .07″ for scale.
Now that I have made my gear cutters and a set of practice gears in aluminum it is time to make the real deal. I have two brass gears to make and two steel; I’ll start with the brass. I have to say that I love working in brass, it machines so nice.
I mount a mandrel in the four jaw chuck, I like using a centering four jaw for cutting gears as they are a little more rigid. I face the mandrel end and cut a few grooves for the super glue to flow into.
I then super glue the gear blank to the mandrel. I use a rag so I don’t drip super glue on the ways. The tail post is used to press the work piece onto the mandrel. You can see I have punched a center point and scribed the rough gear blank outline. I have found that the thin super glue works better for me than the thick glue shown in the picture.
This particular brass gear will drive the oil pump and is mounted on a 1/4″ shaft, so the first operation is to face the brass, drill and ream to .251″. I then follow up with a #20 drill into the aluminum and tap for a 10-32 screw. This provides extra holding power to the mandrel while turning down the outside diameter and cutting the gear leafs.
I use a stepper driven 4th axis on the mill table to cut the gear. It uses the same chucks as my lathe so I transfer the entire chuck and am able to maintain concentricity. It is very important to precisely establish the correct Z position for the mill spindle, it needs to align with the gear blank center or the teeth will be misshapen. If you look at the bottom of the gear you can see some scratches in the dykem. I get the cutter as close to center as I can, then I scribe a line on the blank with the cutter. the gear is then turned 180 degrees in the 4th axis and the cutter Z position is compared with the scribed mark on the opposite side. If it is off a little, the cutter Z position is moved to split the difference and the test is repeated. I have had very good results with this centering method.
Brass cuts so nicely that I cut the leaf full depth at one go, I run the spindle at about 2000 RPM and use the X table power feed to make the cut. I find that if I cut both directions, I end up with a better finish on the gear tooth. I use the sound of the cut and the size of the swarf to set my cut speed.
I then transfer the chuck back to the lathe to finish the machining.
My first attempt at making a steel gear was a fail, the cutter wore out before I made it half way around the gear. In hindsight I was being too aggressive in my cuts and I was running the spindle too fast. I may have gone to the other extreme, I successfully used a spindle speed of 500 RPM and I removed .007″ of material in a pass. My cutter cut two pinions of 20 teeth each with no noticeable wear. I also constantly bathed the cut with oil. You can see my cardboard splash guard in the right side of the above photo. Another change I made was the tempering I put on the cutter. The first cutter I made, the one that wore out, after heat treating the 01 tool steel cutter I tempered it to a pretty dark straw color, almost into the blues. This trades off some hardness for less brittleness. Since the first cutter did not fracture, but wore down, I tempered the second cutter a light straw. This can be seen in these photos.
An aside: I was wondering what the difference is between a “pinion” and a “gear”. A pinion and gear mesh, the pinion is smaller, is the driver and the gear is driven. I also have a secondary “pinion” between the crankshaft pinion and the camshaft gear that is really an “idler”. A spur gear is a straight cut gear like mine, these are the simplest kind. A bull gear is the larger of two meshed spur gears, … I digress.
The procedure I used to cut the steel pinions differed from the way I manufactured the brass gears.
First I made the steel gear blank turned to proper diameter, all machining on both sides completed, the center hole drilled and reamed; basically complete except the teeth. Then I mounted an oversized steel rod in the lathe chuck to form the mandrel. I turned an area smaller than the internal leaf diameter so the cutter would not cut the mandrel, then I turned a spigot on the end matching the diameter of the pinion blank center hole and not quite as long as the width of the pinion. Finally I faced and spot drilled the end of the mandrel to interface with a dead center.
I then super glued the pinion blank to this mandrel and moved it over to the 4th axis on the mill. If you look at the above picture you can see I am using a dead center in the tail stock that has been relieved on one side to clear the cutter. This provides a ridgid setup.
The total depth of cut for the leaf was .070″, for each I made multiple passes of .007″ depth increments until I hit .063″ then moved on to the next leaf, when all leafs were cut to this depth I locked the mill Y axis at .070″ and made another complete turn of the pinion cutting each leaf to this final depth.
The above picture shows the cutter after cutting my two pinions and it can be seen that there is no noticeable wear. I consider this a win.
I used excel to create the GCode for the 4th axis. I know how many steps it takes to turn the spindle 360 degrees so I simply divided this by the number of teeth on the gear. this is an example:
G1 F500 <——– this sets the feed rate, or in my case how fast the spindle moves from tooth to tooth.
M18 S5000 <———– this sets the stepper motor timeout, if it is too short the motor will
de-energize before the cut is done and you will lose your place.
G92 X0 <———– this command sets the current position to zero on the axis
G1 X0.00 <— this tests to see if we are really at zero
G1 X-5.93 <— then each of the following commands move to the next tooth position
G1 X-11.85
G1 X-17.78
G1 X-23.70
G1 X-29.63
G1 X-35.55
I copy and paste one line at a time to my motor controller when the time comes to move to the next cut position on the gear. I am doing this on a laptop with a usb cable to my motor controller.
In my next post I will show the gears mounted on the engine.