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dpb

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#1
1340gt & 1440gt owners: put an indicator on the side of your spindle chuck, apply some pressure, and tell me how much movement you see. By pressure, I just mean to lean on it a bit, no shoving, no levers. Imprecise, I know, but I'm curious about the results. Thank you.
 

mksj

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#2
On a 1340GT at 4" out from spindle, pushing and pulling on my 5C chuck, I get +/-0.0001" deflection with about 20 lbs of force.
 
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#3
I don't like where this is going. No one asks a question like that unless there is a noticable amount of movement.
 

jbolt

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1340gt & 1440gt owners: put an indicator on the side of your spindle chuck, apply some pressure, and tell me how much movement you see. By pressure, I just mean to lean on it a bit, no shoving, no levers. Imprecise, I know, but I'm curious about the results. Thank you.
Indicated from where?

Sent from my SM-G955U using Tapatalk
 

dpb

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#5
I put a magnetic base on the carriage, with a dial indicator on the right end of the side of my 3 jaw.
 

dpb

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#6
I don't like where this is going. No one asks a question like that unless there is a noticable amount of movement.
There's not that much, about what MKSJ is experiencing. I'm trying to track down a minor taper issue, and I didn't have anything to compare to.
 

mksj

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#7
Where/how are you experiencing a taper. Have you done the leveling and headstock alignment. Many chucks have some degree of skew in clamping stock, it can be quite significant and also can vary based on the clamping diameter of the stock. If you provide further information, we may be able to give more specifics.
 

dpb

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#8
The lathe is level, and Rollie's Dad's method agrees with this assessment. I have not adjusted the headstock.
1.25" 4140, protruding not more than 1.5" from the chuck, will show .0002-.0003" of taper over 1". This seems to occur regardless of depth of cut, feed, etc.
2" aluminum shows no taper, over 1". I don't have a longer piece of aluminum to test.
The difference between the two materials suggests that perhaps I am just dealing with material flex, I don't have enough experience to tell.
I realize this taper is pretty insignificant, for most purposes, I'm just curious about it.
 

Ray C

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#9
The lathe is level, and Rollie's Dad's method agrees with this assessment. I have not adjusted the headstock.
1.25" 4140, protruding not more than 1.5" from the chuck, will show .0002-.0003" of taper over 1". This seems to occur regardless of depth of cut, feed, etc.
2" aluminum shows no taper, over 1". I don't have a longer piece of aluminum to test.
The difference between the two materials suggests that perhaps I am just dealing with material flex, I don't have enough experience to tell.
I realize this taper is pretty insignificant, for most purposes, I'm just curious about it.
A one inch test cut is a little short -especially if you're using carbide cutters. Depending on the cutter and several other factors, it could take about 3/8" of travel just for the cut to stabilize.

Ray
 

dpb

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#10
I am using carbide, CCMT / CCGT in this instance. The diameter is larger, away from the headstock. I did not know about the need for a cut to stabilize.
 

Ray C

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#11
I am using carbide, CCMT / CCGT in this instance. The diameter is larger, away from the headstock. I did not know about the need for a cut to stabilize.
Yes, depending on the insert type, angle setting, speeds/feeds etc, when the cutter hits the metal, it can either push the cutter away or cause it to dig in at first. I think you're in safe territory and need to learn and explore the machine. Are you measuring the piece with a micrometer or calipers?

What are you speeds/feeds and DoC? Is it 4140 annealed, pre-hard or rusty HR? Are you getting a shiny finish or a matte/fuzzy finish? All these things are factors and/or tell-tale signs of what is happening with the cut. But anyhow, my lathe is known and trusted and I would not lose sleep over 3 tenths on a 1" cut. If it was a 3" long cut on a 3" dia piece of clean/annealed bar-stock with a light DoC at proper F/S and it was getting 3 tenths per inch on a steady taper then, I'd dig-in and fix it.

Ray
 

dpb

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#12
It is HR, not rusty, and not hardened. Other than that, I'm not certain.
I'm using an Iscar IC907 insert with a 1/64th radius. Cutting speed of about 250 sfm. With DOC above .025 (radius), and feeds above .005, I get a shiny finish. Shallower or slower feed, and it is matte.
This creates another question.. how would I get the shiny finish at a shallower DOC?
 

Ray C

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#13
It is HR, not rusty, and not hardened. Other than that, I'm not certain.
I'm using an Iscar IC907 insert with a 1/64th radius. Cutting speed of about 250 sfm. With DOC above .025 (radius), and feeds above .005, I get a shiny finish. Shallower or slower feed, and it is matte.
This creates another question.. how would I get the shiny finish at a shallower DOC?
IC907 specifies a carbide grade type so I can't tell what your profile or insert size is. What is the CCMT number like 2151, or 321 or 431 etc?

You should be spinning at/around 600-675 RPM for a 1.25 dia piece. Feed at 0.006 ipr (ballpark). Assuming you have a 2151 or a 321 and a typical profile, DoC could be around 0.025 to 0.040".

It's fairly hard to get a shiny finish on cuts under 0.015 with carbide. Need to spin faster.


Ray
 

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#14
Is the taper larger away from the chuck , or close to it. How far from the chuck , what diameter are you cutting . Lots of things to consider before worrying the headstocks out. Even the chuck isn't out of being the problem . Only real test is between centers after setting the tailstock to center.
I surely wouldn't worry over that slight amount of taper , it's not a 5,000 lb cnc machine purpose built for tight tolerances . Too many times I read these kinds of worries the only problem I see many times is we expect far more then we should has a hobby . Not putting down any machine or person but ,0003 - .0002 , 1/10th of a thou. Even the micrometer and the person measuring can get that much just measuring.
 

vtcnc

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#15
Is the taper larger away from the chuck , or close to it. How far from the chuck , what diameter are you cutting . Lots of things to consider before worrying the headstocks out. Even the chuck isn't out of being the problem . Only real test is between centers after setting the tailstock to center.
I surely wouldn't worry over that slight amount of taper , it's not a 5,000 lb cnc machine purpose built for tight tolerances . Too many times I read these kinds of worries the only problem I see many times is we expect far more then we should has a hobby . Not putting down any machine or person but ,0003 - .0002 , 1/10th of a thou. Even the micrometer and the person measuring can get that much just measuring.
Agree, in many circumstances that amount of error could be attributed to measurement error. I don't think we still know if he is using mics or calipers. On the other hand, the only clue that it isn't measurement error is that he reports no taper on the aluminum test bar.
 

dpb

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#16
I'm using Starrett & Mitutoyo micrometers. Measurements are repeatable.
The taper is larger away from the chuck.
Turning between centers does not produce any measurable taper, with my micrometers.
Taper occurs when turning items using only the chuck, which is why I refer to such short lengths, as I am trying to eliminate material flex as a cause.
I do realize this is not a serious problem. It's more an issue of wanting to understand the problem. I realize that things are not going to be perfect, but I like to know why.
Thank you for the help.
 

navav2002

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#17
Out of curosity, what size are your tool bit holders?? What tool post??
 
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dpb

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#18
They are 3/8" IC CCMT, held in a 5/8 SCLCR, on an Aloris BXA QCTP.
 

jbolt

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#20
It has been my experience on my PM-1440GT with tougher materials like steel what you are seeing is tool deflection. Material deflection becomes more of an issue when you get past two diameters of stick out.

The movement can come from the spindle bearings (more so when not at operating temps), carriage to bed, cross slide to carriage, tool post and sometimes the backlash of the cross slide lead screw depending on which direction the cross slide approached the cut dimension. The material, cutter geometry, DOC, feed, cutting speed and tool height all affect how the tool and stock are loaded when cutting.

For tight tolerance work on our lighter duty machines I prefer a well ground and honed HSS tool for cutting steel where I can take a lighter finish pass than with carbide.

Another thing to take into consideration when dealing with sub thousand dimensions is heat generated during the cut.
 

navav2002

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#21
Another thing that I have read over the internet (gotta be true) is that we are using carbide bits wrong...Carbide cutters/drills are apparently "suppose" to be run @ 3 to 4 times the speed of HSS..Most of our home based machines are not capable of reaching the speeds to allow the carbide tool to achieve optimal performance...

I have mostly carbide stuff myself...When I'm having "trouble" with something I will usually speed up rather than slow down...
 

Ray C

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#22
Another thing that I have read over the internet (gotta be true) is that we are using carbide bits wrong...Carbide cutters/drills are apparently "suppose" to be run @ 3 to 4 times the speed of HSS..Most of our home based machines are not capable of reaching the speeds to allow the carbide tool to achieve optimal performance...

I have mostly carbide stuff myself...When I'm having "trouble" with something I will usually speed up rather than slow down...
Just about any modern lathe can hit 1800 RPM. Most of the work I do is 3/8" diameter and greater and I've never had issues with not enough speed. Here's my favorite SFM chart; copies of which are plastered near the lathe and mill.

A good formula for RPM using HSS is [4 x SFM / Diameter] where the diameter is measured in inches. For carbide multiply the HSS value by 2 or 3. For positive rake inserts, the 2x value should be your starting point. For neutral rake or negative rake inserts, start with 3x. For soft materials, increasing speeds is usually the right thing to do. If you go beyond 4x, double-check the material type and condition. For VERY hard materials that resist scratching with a file (or if you know its above RC 40), use the 2x speed as the starting point and drop down 10% steps to find the final speed.

1523280763745.png

As far as feed rates are concerned, for typical steel like 1045 or 4140 at 32 Rc, machines in decent condition with 1 HP motors are comfortable up to about 0.008 IPR. 2 HP an go up to 0.012 IPR. 3HP is around 0.016 IPR. Depending on insert style, best finishes are obtained at/around 0.005 IPR. Any slower and the edge drags/rubs too much. Lines start to show-up when going faster. All of this is really "ballpark" information that assumes diameters of at least 1/2" and the workpiece is properly supported with the tailstock and/or center support.

Some folks are looking for that clean bright shimmer. On some metals, it's not going to happen (like 1117 or anything with L (lead)). For shiny finish, your best bet is medium carbon like 1045 or 4140 prehard (about 28-32 Rc). Harder gives a much nicer finish. As you turn a piece of metal, hardness decreases even if the material is claimed to be "thru-hardened". Only certain tool steels can be fully thru-hardened -and you can't turn HSS that's been hardened. A 1.5" diameter piece of 1045 that is Rc 42 at the surface will be about (ballpark) 28 down at the 1/2" diameter range -and that depends on how the foundry did the hardening. The only way to control this, is to harden it yourself -which is what I do when making high-horsepower shafts etc...

Over the years, countless people have asked for advice on speeds/feeds and countless times people have responded with "hey, that worked pretty good". This is the secret sauce that I go by.

Have at it.

Ray

PS: Here's the start of a miter gear made about 20 minutes ago. 4143 Rc 38 (self heat treated). The blank was formed about 50 thou oversize in all dimensions then heat treated. The shoulder and shaft are now taken down to final size but, the 45 degree angle is still in rough dimension. The shoulder is 1.3" diameter, shaft is 1" diameter. I chose an SFM of 80, used negative rake CCMT (431) insert. DoC' were roughly 35 thou. Feed was about 0.007 IPR. Machine is a PM1236.

It will be a 30 tooth Mod-1 gear.

IMG_20180409_100535[1].jpg
 
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