Precise and repeatable profiles are best cut on my tiny lathe with custom form cutting tools. The Lola T70 restoration has big tires - relative to my regular sizes. To properly interpret the 1:1 tires I've made this first tool using techniques new to me but ancient in origin. Annealing, quench hardening and tempering are tough to learn on the quick, so I've invested a fair amount of time to learn enough to meet my needs.
My other metalworking machines are quite limited, so I've hand ground and shaped the profile. The tire's OD, ID and other major features can be done simply, but the radiused portion took considerably more time. The radius must be precisely sized and positioned.
Testing on a wooden tire blank throughout the process has shown good results. With a few more tweaks it'll be ready for final burr removal, quench hardening and tempering.
The nearly completed tool is shown in position on the tool post. Its primary cutting action is done in the -Y axis, moving toward the chuck as it cuts. The end points of the radius where they meet the flat surfaces of the blank are the tricky bits. The radius must be smooth and symmetrical, but the distance from starting point to the finish - the chord of the arc - must be exactly right. Patience is required.
Because it is a small lathe, with normally only about 1.75" (4.5cm) swing over the bed, 1.25" (3.2cm) risers are used to allow cutting the larger blank down to a 3.35" (8.5cm) OD. The length of the part, including the tire jig, is close to 4.5" (11.4cm).
The extra height and length mean that the cutting forces are at the outer limits of the lathe. Careful attention to the speed and feed rates keeps the tool chatter to a minimum.
The HSS (high speed steel) tool blank is only .25" x .25" x 3.5" (6.4mm x 6.4mm 89mm). With a proper relief angle ground onto the tool’s front surface and grinding away the radius with its relief angle, there is barely enough steel remaining to provide enough tool rigidity. Adequate relief angles were ground on the top and leading surfaces as well. Only the edge of the tool does the cutting.
Two bench grinders, a handheld grinder and files were used to cut away the radius. Shaping was done freehand with a great deal of care. Of course, there are machine tools to make the job easier, but I rarely need them, so they're not a good investment.
A Sharpie provided quick marking bluing so the profile could be laid out. Simple hand tools were used for the layout. My existing CAD for the tires was used to provide exact layout measurements.
Full disclosure: I didn't end up using any of the specially bought grinding tools in the photo. They'll come in handy later, I'm sure.
Annealing the HSS cutter blank to soften it before shaping was done on fire bricks in a BBQ. I don't have a proper metalworking oven, so the blank didn't receive a completely controlled heat soaking to reach full softness before shaping began.
After final adjustments and the burrs are stoned off the tool will be quench hardened and tempered. A final careful honing will finish the job.
Next up will be making a second tool to cut the front tire profiles.
Links to previous Lola T70 posts are below.