PM-1640TL problems getting reasonable surface finish

[macmccaskie]

At home (as opposed to school), I have only ever used cheap import HSS tool bits. It does the job for a while, & then requires resharpening sooner than I would like.

At the one school I went to, they used the good stuff. I never saw a tool bit require a trip to the grinder.

I should change to quality HSS tool bits.

Agreed, I pitched all my Chineseium. The T-15 is Vasco Supreme - VERY hard to grind but I have found a very good routine: 36 grit ceramic belts, then CBN wheels. Plus my shop made tool rest for the grinder helps big time.

 

[Lo-Fi]

Is anybody else doing the roughing with carbide & then switching to HSS for the last .005” or so?

Not unless I've managed to stuff up my balanced cuts with the carbide and haven't landed on my target dimension. I use HSS if I have a not so rigid setup that requires very positive cutting action and low cutting force or where a form tool or odd shaped tool is needed. If you haven't watched Stefan's video I posted above, I'd heartily recommend it as he's talking about exactly this topic. For me, it turned frustrating fussing with finishing cuts into lovely finishes right on dimension straight off the carbide. For tough materials it's absolutely essential.

 

[erikmannie]

Not unless I've managed to stuff up my balanced cuts with the carbide and haven't landed on my target dimension. I use HSS if I have a not so rigid setup that requires very positive cutting action and low cutting force or where a form tool or odd shaped tool is needed. If you haven't watched Stefan's video I posted above, I'd heartily recommend it as he's talking about exactly this topic. For me, it turned frustrating fussing with finishing cuts into lovely finishes right on dimension straight off the carbide. For tough materials it's absolutely essential.

I will check out the video.

I have yet to be successful in any balanced (“managed”) carbide cut.

I have always been so scared to overshoot it (and thus scrap the part) that I purposely undershoot it with the plan of doing the last few thou with HSS (and files and emery paper). And then a green Scotch-Brite makes it pretty! Yes, I know that this is not the optimal strategy.

One day I will have more time to practice on *scrap* in order to learn how to be successful with a balanced cut.

Most of my lathe work is making pipe welding coupons so the finish is unimportant.

 

[jwmelvin]

Most of my lathe work is making pipe welding coupons so the finish is unimportant.

Then so is hitting a precise dimension; seems like a good opportunity for practice with balanced cuts.

 

[ddickey]

Ray C has a nice write up about hitting a dimension.
Starts on post #43

Bull Nose Live Center

Ray I one of your many posts you mentioned the inset that you use to achieve the fine finishes. I know that I saved it somewhere , but can't find it in my smarter than me filing system Tks for your patience ed

www.hobby-machinist.com

 

[Lo-Fi]

Don't forget that you can balanced cut your entire way through any hogging operation, checking as you go. By the time you get to the final cut you will have built confidence that dialing up, say, 15 thou is going to remove exactly 15 thou because it has the last seven times. Applies to carbide or HSS just the same. I really like it as a methodology, it just makes so much sense to me.

 

[mattthemuppet2]

just for fun, here's the result of a 50 thou roughing cut with a name brand (Mitsubishi I think) CCMT insert in mysterium steel. OD is ~1"

Not a super shiny finish, but good enough for what I needed. I'd probably reduce the DOC to 20 thou or so and up the speed a bit for a finishing cut.

 

[davidpbest]

I use a balanced cut method all the time, somewhat similar to what Ray C is trying to convey but without all the math. It's fully described in much simpler terms in this video, and I use the DRO instead of the dials. Using this technique on my PM1340, I can routinely hit target dimensions to within 2/10ths, and there is no way this is possible creeping up on the final OD with shallow finishing passes. Granted, I have a solid tool post which helps, but even with the compound, this method works. Watch Stefan's video to the end, and realize that the technique is even more accurate using a DRO. He is also demonstrating the technique on 4140 with a similar sized lathe, using the style of insert (CPMT) that I would pick for your material and machining condition.

 

[ddickey]

just for fun, here's the result of a 50 thou roughing cut with a name brand (Mitsubishi I think) CCMT insert in mysterium steel. OD is ~1"

Not a super shiny finish, but good enough for what I needed. I'd probably reduce the DOC to 20 thou or so and up the speed a bit for a finishing cut.

What was you feed rate?

 

[mattthemuppet2]

about as fast as I could go without the motor belt slipping You can see one of the chips out of focus on top of the toolholder at the bottom of the pic.