HELP! Stroker crank

[Rich Fife]

Are crank weights typically just mild steel?

If I had to guess, 8620. Some of the top companies may use 4140 or 4340. Buuuut, being that most are mass produced, I'm leaning towards 8620... most auto-manufacturers etc use 8620 for their gears, shafts, etc...

 

[srcarr52]

If I had to guess, 8620. Some of the top companies may use 4140 or 4340. Buuuut, being that most are mass produced, I'm leaning towards 8620... most auto-manufacturers etc use 8620 for their gears, shafts, etc...

Regardless of the material, they are forged and don't cut very easy.

 

[Rich Fife]

Regardless of the material, they are forged and don't cut very easy.

Just gotta use Good carbide, proper speeds and feeds and they cut nicely resulting in mirror/rainbow finishes. Lol

Edit: Providing the setup and machine is rigid.

 

[srcarr52]

Just gotta use Good carbide, proper speeds and feeds and they cut nicely resulting in mirror/rainbow finishes. Lol

Edit: Providing the setup and machine is rigid.

Sure, but if you use carbide meant for cast/mild steel it will only shine up the surface.

 

[Rich Fife]

Sure, but if you use carbide meant for cast/mild steel it will only shine up the surface.

Well, you wouldn't try to cut a tree down with a butter knife, right? So yeah, it's important to select the proper tools for the job...

 

[Dolkitafreak]

Well, you wouldn't try to cut a tree down with a butter knife, right? So yeah, it's important to select the proper tools for the job...

Even so, takes a pretty sizable machine to cut cranks very well. Been done without, but I don’t recommend

 

[Rich Fife]

Even so, takes a pretty sizable machine to cut cranks very well. Been done without, but I don’t recommend

Tit for tat... small lathes that are rigid do just fine...
You can cut them on a Bridgeport if one was feeling ambitious.
Or in a lathe with a angle grinder attached... whatever works for the individual.

 

[Firewood Hoarder]

Tungsten inserts are often used to balance engine cranks; the density is more than the steel and they weigh more. This could be an option to offset the additional weight of a stroked crank

You could drill and ream a hole through the counterweight, press in an old carbide endmill shank, and then TIG a little bit around the edges of the hole to hold it in place.

 

[Ketchup]

Just thinking about how you bore the hole, what you use for a shim and how it all welds together.

 

[Firewood Hoarder]

If I were offsetting the hole, I would fixture the crank half vertically my mill table (held by 3 jaw chuck clamped to my table, probably), pick up the center, align the existing hole in-line with my y-axis, and then move to the center of where the new pin will go. I would use an offset boring head to bore open the hole, to about 1/64" smaller than finish size. I would use a reamer to finish the hole, for a press fit on the crank pin.

Repeat for the second half.

I've never welded a crank, though, but I've straightened a couple on dirtbikes back in the day before I had money.

 

[srcarr52]

If I were offsetting the hole, I would fixture the crank half vertically my mill table (held by 3 jaw chuck clamped to my table, probably), pick up the center, align the existing hole in-line with my y-axis, and then move to the center of where the new pin will go. I would use an offset boring head to bore open the hole, to about 1/64" smaller than finish size. I would use a reamer to finish the hole, for a press fit on the crank pin.

Repeat for the second half.

I've never welded a crank, though, but I've straightened a couple on dirtbikes back in the day before I had money.

Better have a carbide reamer, it's hard enough to roll the edges on a HSS reamer.

 

[Firewood Hoarder]

Better have a carbide reamer, it's hard enough to roll the edges on a HSS reamer.

M42 cobalt won't get through it?

 

[Rich Fife]

M42 cobalt won't get through it?

The cranks are around the low to mid 40s on a Rockwell hardness tester... "C" scale... I checked a YZ250F crank with low hrs and it was 38 in a couple spots... the cranks may work harden.

 

[Firewood Hoarder]

The cranks are around the low to mid 40s on a Rockwell hardness tester... "C" scale... I checked a YZ250F crank with low hrs and it was 38 in a couple spots... the cranks may work harden.

Carbide would be best with harder material, there is no denying that. I regularly ream out pin holes in 400 series stainless where I work, which have been hardened to 450-500 brinell, which is mid to upper 40's on the rockwell C scale. 8% cobalt isn't obsolete, by any stretch of the imagination.

 

[Rich Fife]

Carbide would be best with harder material, there is no denying that. I regularly ream out pin holes in 400 series stainless where I work, which have been hardened to 450-500 brinell, which is mid to upper 40's on the rockwell C scale. 8% cobalt isn't obsolete, by any stretch of the imagination.

It should work. I try and use what makes the job easier and faster... providing I have what I need... if not, I'll interpolate it on the HAAS. Lol

 

[Firewood Hoarder]

It should work. I try and use what makes the job easier and faster... providing I have what I need... if not, I'll interpolate it on the HAAS. Lol

And I'd interpolate in the prototrak after-hours as well, given the chance!

 

[Rich Fife]

And I'd interpolate in the prototrak after-hours as well, given the chance!

We got a new white prototrak, late 2023... very nice machines!

 

[Rich Fife]

We also just received our New VF3 HAAS... I'm not a fan of the controls/touch screen. The 2013 VF2 is Much more friendlier to navigate... more direct.

 

[Firewood Hoarder]

We got a new white prototrak, late 2023... very nice machines!

The last Haas I ran regularly was a little TM-1 that had been ridden hard and put away wet for years.... but it still did alright. 22 years making chips for money this year; you run a bit of it all eventually.

I'm a fan of the conversational programming aspect on prototrak machines, and really want to add one to my home shop. Great for one-off parts and short run production.

 

[Rich Fife]

I'm a fan of the conversational programming aspect on prototrak machines, and really want to add one to my home shop. Great for one-off parts and short run prproduction.

We have a Trak K4 that's been used... I wouldn't mind having it in my garage... I'd gladly get rid of my Bridgeport