This is a follow-up post to some earlier threads about using carbide on homeshop lathes and is oriented toward beginning users. -Nothing to ooooh & aaaaah about but the topic comes-up often and somewhere along the line, I promised to show the differences when using different bits and speeds -so here is a start. BTW: It's a PM 1236 lathe with a cheapo brand AXA QCTP. The AXA is my preferred holder. Seems to be the right size and have never found it lacking. The BXA is fine too and it gets used a lot but, it will probably get retired for now.
This first one is a cheapo harbor freight 3/8 shank RH bit. I use those because they're fairly durable and cheap. Here I was knocking off the corners of square with cuts about 20 thou DOC -any deeper and the carbide will chip because it's a nasty interrupted cut. That stock is A36 material, 1.25" square and is just big enough to get enough SFM at high RPMs for the carbide to work properly. The finish is bright & shiny. There are some lines in it from small chips in the carbide. This was spinning around 1200 RPM and was the 1st medium cut of about 30 thou dialed in once the corners were removed. You can see in the second picture that the surface is not as shiny... Why? -Because as diameter decreases, SFM decreases and the carbide can generate enough heat to weld the surface smooth.
In the next pictures is the 3/8" shank diamond point. It's the set from Shars (commonly sold at other places too). I love those diamond points and they're my personal favorite because they're versatile. At this point, the diameter won't allow enough SFM to get a gloss finish but, these diamond points can still go deep. Anyhow, DOC was about 50 or 60 thou. I dont bother with coolant or oil when trying to remove material but in the second pic, cutting oil was applied just so you could see with your own eyes, it makes no difference when cutting like this. That finish is beautifully even albeit matte looking. I actually prefer that kind of finish because it's much easier to black oxide treat. BTW, If I took the time to switch belt ratios on the machine, the RPMs could have been kicked-up to 1800 RPMs and at that diameter, the surface would probably be glossy. Once you get below 3/4", there's not enough RPMs in the machine to really make carbide work. Keep in mind, this is A36 metal -very low grade. This is a situation where you can't expect to put lipstick on a pig.
Next photo is pretty stupid but here goes... This is the kind of swarf you really need to be mindful of and you don't typically get it with HSS. I't a coil, probably 50 or 60 thou thick that comes reeling off the insert fast and it's sharp as a razor (and obviously hot too -but that's the least of your worries). Its coming out at about your SFM and if you're one of those guys who pokes their face in the machine to watch the cut, you won't be able to get out of it's way. Not trying to sound like mother hen here but, seriously... this can come flying at your face PDQ and you need to wear goggles and should watch out that your hands are kept low. It's not going to cut a finger off but it will make you bleed and it can "poke your eye out" (think Christmas Story).
Some time in the future, I'll show some different feed rates. Pretty soon, I need to cut some SS and carbide loves SS... Stay tuned.
Ray
EDIT: The only reason a tailstock is not being used is because the piece is extremely stumpy. Keep that in mind when you experiment.
This first one is a cheapo harbor freight 3/8 shank RH bit. I use those because they're fairly durable and cheap. Here I was knocking off the corners of square with cuts about 20 thou DOC -any deeper and the carbide will chip because it's a nasty interrupted cut. That stock is A36 material, 1.25" square and is just big enough to get enough SFM at high RPMs for the carbide to work properly. The finish is bright & shiny. There are some lines in it from small chips in the carbide. This was spinning around 1200 RPM and was the 1st medium cut of about 30 thou dialed in once the corners were removed. You can see in the second picture that the surface is not as shiny... Why? -Because as diameter decreases, SFM decreases and the carbide can generate enough heat to weld the surface smooth.
In the next pictures is the 3/8" shank diamond point. It's the set from Shars (commonly sold at other places too). I love those diamond points and they're my personal favorite because they're versatile. At this point, the diameter won't allow enough SFM to get a gloss finish but, these diamond points can still go deep. Anyhow, DOC was about 50 or 60 thou. I dont bother with coolant or oil when trying to remove material but in the second pic, cutting oil was applied just so you could see with your own eyes, it makes no difference when cutting like this. That finish is beautifully even albeit matte looking. I actually prefer that kind of finish because it's much easier to black oxide treat. BTW, If I took the time to switch belt ratios on the machine, the RPMs could have been kicked-up to 1800 RPMs and at that diameter, the surface would probably be glossy. Once you get below 3/4", there's not enough RPMs in the machine to really make carbide work. Keep in mind, this is A36 metal -very low grade. This is a situation where you can't expect to put lipstick on a pig.
Next photo is pretty stupid but here goes... This is the kind of swarf you really need to be mindful of and you don't typically get it with HSS. I't a coil, probably 50 or 60 thou thick that comes reeling off the insert fast and it's sharp as a razor (and obviously hot too -but that's the least of your worries). Its coming out at about your SFM and if you're one of those guys who pokes their face in the machine to watch the cut, you won't be able to get out of it's way. Not trying to sound like mother hen here but, seriously... this can come flying at your face PDQ and you need to wear goggles and should watch out that your hands are kept low. It's not going to cut a finger off but it will make you bleed and it can "poke your eye out" (think Christmas Story).
Some time in the future, I'll show some different feed rates. Pretty soon, I need to cut some SS and carbide loves SS... Stay tuned.
Ray
EDIT: The only reason a tailstock is not being used is because the piece is extremely stumpy. Keep that in mind when you experiment.
