MT 6 1/2 One Piece Dead Center

[erikmannie]

I'm not sure if you're asking for help/suggestions, but if you are, here are some ideas.

Make your setup as rigid as possible. That would mean taking as much slop out of the cross slide and compound ways as possible. If you haven't already, I would recommend removing the gibs on both control surfaces, hone them on a stone to remove any burrs, and check them for flatness. Assuming they are flat and free of burrs, I would then fit them and adjust them using a dial indicator to remove as much sponginess as possible and still have the surfaces free moving. Here is a link to another thread where one technique is documented to check the rigidity of the compound after adjusting the gibs. Go here.

Looking at some of your surface finishes, I suspect you have some rigidity issues, which is why I mention the first point about the gibs. But you can improve rigidity by pulling in the compound so that the QCTP is sitting directly over the swivel-mount for the compound, and 90-degrees to the spindle axis.

Further along in that same thread, the problem he was having was lack of rigidity because the compound was not sitting flat on the cross slide. You can see that here. Rigidity is king, so check out your setup with some dial indicators - who knows, you may have the same warped compound base that TX Cowdoc did.

I am not a fan of the type triangular insert you are using. It has a relatively weak cutting point. I would favor an 80* rhombus style insert and tool holder - CPMT style. But, if you want to use that triangle-shaped insert, I would pick one graded for use with tougher materials. Assuming the I.C. of your insert is 3/8", something like this would be a better choice. I would also tweak the lead angle of the tool by rotating the QCTP 10° or so CCW from where you show it in your videos, so that the radiused tip is doing most of the work and the cutting forces are toward the chuck rather than across the spindle axis. With decent rigidity, shifting the lead angle to balance the tool forces toward the chuck, and a proper grade insert you should be able to peel of 0.100" per pass no problem and get close to mirror finish.

ddickey suggested a good post by Ray C on how to hit target diameters without trying to creep up on final size with tiny skim cuts. Creeping up is definitely asking to overshoot your target OD and leave questionable surface finish. I use a balanced cut method, 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.

Hope this helps.

The main purpose of this thread & most of my forum activity is to provide & receive help with machining. Your quoted response is tremendously helpful. I can only hope to pay it forward someday.

 

[erikmannie]

Well, I can’t think of a better example of forum people helping to solve a problem than what I saw today.

Here is how I ran it today:
(1) set compound to 90°,
(2) used a chuck,
(3) adjusted my cross side backlash in such a way that there is now more resistance in turning the cross slide hand wheel (it was really loose before),
(4) fresh insert tip to start the day,
(5) checked that the tool was on center,
(6) set my toolpost for about a 10° angle of attack,
(7) no lubrication because of what was going on in my house (not a good time to make the house smell like sulfur),
(8) changed the feed to .017”/rev,
(9) set RPM to 250,
(10) DoCs between .030” to .045” (and keeping track of how much it actually cut!),
(11) minimal tailstock quill and tool shank stickout,
(12) locked the compound, and
(13) I was using the power feed and the taper attachment the whole time.

Here are some pictures of the set up:

 

[erikmannie]

The final pass was .0277” (radial). The situation in the house changed, so I was able to turn on the CF. Here are 4 action photos:

 

[erikmannie]

Here is the finish that I have to work with.

 

[davidpbest]

Looking a lot better. A different grade of insert can help further. As a roughing operation that surface finish is fine. Getting to higher surface finish will probably require an inset graded for finishing operations. If you have the original package for your insert, and if it has any printed nomenclature on it, I‘d love to see a photo of it. A finishing insert will have a smaller nose radius, and the feed rate will need to be lowered as well. This is the kind of information I’m talking about - all that gibberish includes the grade info as well as the feed/speed formula. The P05-P30 is the ISO grade specifying this insert is ideal for medium to fine finishing operations on steel. This one would likely perform better in your application If it’s the right size for your tool holder (it’s ⅜” I.C.), and available here as a single insert for $9.

Your chips look good considering the insert’s chipbreaker style, but I’m curious if most of the heat is ending up in the chip and not in the part being machined. So if you have some idea how hot the part gets after a pass that would give some hint as to your feed/speeds settings.

I didn’t realize you had a taper attachment, but that too can contribute to lower rigidity - introducing additional backlash slop in the carriage if it isn’t dialed in properly. With the carriage locked, which is great, I believe you are essentially eliminating any of the taper attachment sloppiness from the equation. Once you start using the taper attachment, it could be a source of additional issues. Hard to know at this point, but something to watch out for.

 

[erikmannie]

I used the compound to taper a 30° angle. I got tuckered out before I could put on a fresh tip for the final pass.

There is a witness mark at the base of the taper. The length between this witness mark and the chuck will be turned down so that it fits well inside the 2.55” spindle pass through.

 

[erikmannie]

Looking a lot better. A different grade of insert can help further. As a roughing operation that surface finish is fine. Getting to higher surface finish will probably require an inset graded for finishing operations. If you have the original package for your insert, and if it has any printed nomenclature on it, I‘d love to see a photo of it. A finishing insert will have a smaller nose radius, and the feed rate will need to be lowered as well. This is the kind of information I’m talking about - all that gibberish includes the grade info as well as the feed/speed formula. The P05-P30 is the ISO grade specifying this insert is ideal for medium to fine finishing operations on steel. This one would likely perform better in your application If it’s the right size for your tool holder (it’s ⅜” I.C.), and available here as a single insert for $9.

View attachment 354217

Your chips look good considering the insert’s chipbreaker style, but I’m curious if most of the heat is ending up in the chip and not in the part being machined. So if you have some idea how hot the part gets after a pass that would give some hint as to your feed/speeds settings.

I didn’t realize you had a taper attachment, but that too can contribute to lower rigidity - introducing additional backlash slop in the carriage if it isn’t dialed in properly. With the carriage locked, which is great, I believe you are essentially eliminating any of the taper attachment sloppiness from the equation. Once you start using the taper attachment, it could be a source of additional issues. Hard to know at this point, but something to watch out for.

Here ya go. I will also add a link here, as well. Not much information on the packaging, right?

I’m actually so broke for the next 8 weeks that I can’t afford $9! This machining hobby can be a little expensive.

Carbide Insert TNMP-332-A6

www.aloris.com

My LH, RH & chamfer shanks all use the same inserts in the green box pictured above, which I like. My tool shanks are these type:

Aloris Double-Lock Tool Holder ADS6-3

 

[erikmannie]

So I used 8” and 10” smooth files and green Scotch Brite to get the surface finish to my liking.

The OD at the base of the taper was spot on at 2.634”.

The resulting taper is .090” over 3.5206”, so I ended up with .0256”/inch taper. The first time I did this, I aimed for .0256”/inch.

However, additional measurements made from the spindle nose and by remeasuring the factory reducing sleeve called for .0240”/inch and .0236”/inch respectively. Later today I will be able to find out how well this fits.

 

[erikmannie]

Here is the parting operation.

I just had to make the base smaller than the spindle through-hole.

I will fine tune the 60° tip when it is in the spindle nose.

 

[erikmannie]

Parting the 4140 prehard took longer than parting low or medium carbon steel. I broke one insert being overly aggressive.

I was so happy when it fit nicely.

Now the only problem I would have left is if it gets jammed in there while I’m cutting the 60° tip.