Author Topic: My parting was sucky sweet sorrow - how to fix this?  (Read 4733 times)

Offline PStechPaul

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My parting was sucky sweet sorrow - how to fix this?
« on: February 20, 2015, 09:16:42 AM »
I'm copying this from my thread in HSM about my second semester of Machine Tool classes. This is a piston for an air/steam wobbler, and apparently it wobbled loose from the chuck:  :( :o
I thought it would be good to cut off the ends of the piston in the school shop, where they have some good parting tools and more powerful lathes. Maybe not the best idea:



 Up close and ugly  :'( :



 The first toolholder with parting blade needed some adjustment to reduce the overhang, but the socket head clamp screw was buggered up, and jammed by the threads having been forced against their limit. The assistant instructor tried a number of things to fix it, but after probably more than a half hour later, I had to get another set out of the drawer. Looking closely at the cutting tip, I saw that it was gouged on the top edge, so I cleaned it up with a diamond hone until it looked and felt OK. I left the shaft end inside the chuck and set the tool perpendicular and on center, although I gauged it at the long end of the 5/8" stock that was extended probably 5" out of the chuck. The actual cut was about 1/2" from the chuck jaws, which was about as close as I could position the tool without hitting the jaws with the toolholder.

 I started parting using manual feed, and it seemed to be cutting nice curls, so I switched to power feed at about 0.003" per revolution, and I was running about 200 RPM. All was going well, but suddenly the part twisted loose from the chuck and caused the damage shown. The instructor noticed that the tool was not really properly ground, and it was also slightly below center. So he resharpened it and showed me how he could part the material using manual feed (where he could feel how it was cutting), and at 600 RPM. But he also said that the three-jaw chuck I was using was old and worn, and I should have used a better one. So this is the piece he cut off:



 I asked if I could cut the damaged piston off of the shaft, and use this piston by using a press fit, and he said it would be OK. So I may bore a hole slightly undersize to fit the shaft, which reads 0.374". I also thought about turning a slight taper on the end of the shaft and a matching tapered hole in the piston, and another idea is to thread the mating parts. In any of these cases, it may be best to use some Loc-tite.

 Not trying to make excuses, but I think a significant reason for this problem was the fact that I was (and still am) in considerable pain due to my multi-level spinal stenosis, and my left knee (the one with no cartilage) has been hurting a lot. Standing at the machines is hard on me, although I often get involved in the work and don't think much about the pain until I need to move around and walk.  :old:
Here is the drawing for this part:

I think I have a thread here already on this project, but I thought this screw-up belonged here. I'd like some ideas on how to fix this, especially if someone can suggest a better way than I have proposed. If I did it over again using one piece, I think I'd just use the bandsaw to get it close, and then face to size.  8)

Offline philjoe5

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #1 on: February 20, 2015, 02:14:57 PM »
Quote
I think I'd just use the bandsaw to get it close

That's a perfectly good approach to avoid parting.  However, despite all the bad press, parting is not all that difficult, but it is unforgiving of a few essential techniques.  From your photos it appears that you're tearing the metal.  The large "pip" on the work indicates to me that the parting blade was not centered on the spindle or the feed rate was too fast or both of these.  Lastly, there should be as little overhang as is possible since you don't want to work against a lever.  That will cause chatter or tearing instead of cutting.  I'm assuming you have a rigid setup, using cutting oil and your rpm is as low as the machine permits, definitely less than 100 rpm.  If I read your plans correctly, you're parting a 5/8" rod which should take a few minutes to complete especially on a light machine.

Keep at it and you'll eventually succeed

Cheers,
Phil
If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man.  - Mark Twain

Offline ShopShoe

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #2 on: February 20, 2015, 02:36:37 PM »
PSTechPaul,

I think you are learning well and are finding out what works and what doesn't work with parting just like many of the rest of us have done. It will get better.

I am only a hobbiest and I have a small lathe, but here are some things I think apply to your attempt.

Five Inches of a 5/8-inch rod being parted is probably too long: Gravity will get you as you get near the end of the cut. Don't even think about using the tailstock to support the piece while parting: A worse crash will result.

Good idea to cut as near the chuck as possible and you did. Also it's good to have as little of the tool sticking out of the toolholder as possible.

I agree with your instructor that you need the manual feed and need to feel the cut. In my experience, steady feed is best, taking into account that things change as cut nears the center. If you get chatter, feed less, but don't stop feeding. (See below about work hardening.)

Properly sharp cutter and proper setup of tool is more important in parting than in some other lathe operations. If you don't have it right you will force the cut into chatter or bend the piece or put too much of a load on the cutting edge.

You didn't say what material you were cutting: Different ones take different approaches and you will learn in time. Some have a harder surface and need an aggressive start to the cut. Some "grab" more than others. Some will work harden, particularly if your tool is dull or not set right. Some will harden if you take off too little.

Many have suggested that you part down to a certain point, then finish the cut with a hacksaw. I would certainly agree if you are cutting off a long piece. Then face the part.

You didn't say if you used a cutting fluid. That might help.

These are starting observations. I'm sure others can weigh in.

Good luck to you. Thank you for posting and for asking here.

--ShopShoe

PS,

philjoe5 just posted while I was writing.

I have some of the same things, but my whole post is above

Offline Roger B

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #3 on: February 20, 2015, 06:32:25 PM »
I would say much the same as the others. Rigidity, minimum overhang, slow speed, sharp tool on centre height, lubrication. I have used the tailstock centre to support long work when parting off. As long as you don't use too much pressure and release it just before the end it works ok. When parting large diameters you either have to start off too fast or stop and increase the speed as you go deeper.

I recently parted off a piece of 16mm silver steel (drill rod) at 250 rpm with a bit of chatter at the start which went away as the cutting diameter decreased.
Best regards

Roger

Online Jasonb

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #4 on: February 20, 2015, 06:48:16 PM »
To me it was being held by the wrong end, rather than hold the short bit of 5/8" that you wanted you should have held it by the remaining 5/8" stock and just parted off the bit you wanted. That way you have the full length of the chuck jaws in contact with the work not less than 1/4"

5/8" I would be parting at 5-600rpm and using power feed and a little coolant, but a syou are starting out handfeed will give you teh feel of what can and can't be done but use two hands on te hrim of the handwheel so you can get a constant feed.

Offline PStechPaul

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #5 on: February 20, 2015, 10:22:56 PM »
To answer some questions, the piece was 1018 steel, and I was using the shop's 12" Clausing lathe, which should be pretty stiff, although it is old and it has been used in a school environment. I had adjusted the blade as close to the chuck as possible, with minimum overhang, and it was a very robust P-type (T-shaped) parting tool, about 1/8" wide and 1" tall. I was using lubricant, and I had honed the cutting edge to what I think was a good shape and sharpness. It was cutting nice curls when I was using manual feed, which is what I'm used to on my home shop 9x20. Here is the piece as it was when I chucked it in the lathe:

 
I had the machined shaft in the chuck and spindle, and the rough end was extended toward the tailstock. I actually had the end touching a live center that I had used to align the part in the chuck. It may have been a red flag when I noticed the piece wobbling a bit and I tapped it to a pretty good (but not perfect) concentricity. I thought about drilling a center and using the live center for support during the start of the cut, but I was in a hurry (another red flag) as it was getting near the end of the class, and I wanted to "git 'er done". Also, (another red flag), the chuck was set for a much larger piece (probably 1.25" or 2"), and as I turned the wrench it became very stiff before loosening slightly as I got to the 5/8" of the part.  :facepalm:
 
I am almost certain that the end portion of the chuck jaws were not in good shape (bell-mouthed), and thus did not grip the part very well, so the radial pressure of the parting tool caused the piece to "cock" which resulted in the grab on the side of the cut. When that happened, the piece started spinning in the chuck, and the amount of wobble was limited by the machined shaft which shows some scarring on the end.  :thinking:
 
The instructor parted the small piece shown above, that has the nub. It appears to have a taper, and when I looked closely at the tool as I was putting it away, it also seemed to be not quite square. That accounts for the taper and the somewhat premature separation of the part from the stock. It also most likely turned out better because the bulk of the stock was in the chuck and was gripped along a much larger area, plus the short extension of the part beyond the chuck reduced the possibility of movement from vibration. The instructor did not use any lubricant.  :naughty:
 
I think it would have been better for him to have let me sharpen the tool and perform the parting operation, but it seems that his style is to just show how he does it. When he was showing me how to use a spiral tap on the mill, he just did it himself. So now I have seen how it's done, but I didn't get the experience of actually doing it.  :shrug:
 
Now I'd like to discuss how I might best be able to fix this.  :help:  Since it will be a piston assembly, the force will mostly be compression of the piston against the shaft (perhaps properly termed a crankshaft), and that will be the case also on the return stroke, where the inertia of the flywheel pushes the shaft into the piston. I have some machine taper reamers, so I could get a pretty tight fit if I cut a matching taper on the shaft, but then the hole would have to extend through to the face of the piston. This may not be a problem for this wobbler, as it would be more airtight than the blowby around the piston in the cylinder, but it would be prominently visible. This is also not really a problem, unless someone takes the engine apart, and if I face the piston along with the shaft, the joint would be difficult to see.  ;)
 
Thanks for the ideas on parting.  :ThumbsUp:

Online b.lindsey

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #6 on: February 21, 2015, 12:42:00 AM »
Paul, it seems that by the time you cut a taper on the existing piece, then cut/drill and taper ream a sleeve to fit over it, and then face things off to be smooth...you could just as easily make a whole new piece and never have to worry about it. Just my thought on it.

 Bill

Offline philjoe5

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #7 on: February 21, 2015, 03:25:46 AM »
I agree with Bill.  You haven't invested that much time or material in making this part so I'd say chalk it up to experience and make a new part.

Good luck

Cheers,
Phil
If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man.  - Mark Twain

Offline PStechPaul

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #8 on: February 21, 2015, 05:22:16 AM »
I thought it would be a good exercise to try the repair. I faced off the new piston and drilled it with a size U drill (0.368") to a depth of 0.375" in the 0.531" part. Then I turned down about 0.37" of the end of the shaft to 0.370", but after I smoothed it with a file and abrasive cloth, it became about 0.367" which is a smooth clearance fit. It's pretty close, and it "pops" when the shaft is removed, but it's not the press fit I wanted. I can probably use Loc-Tite, but it may be better to turn the shaft longer into the damaged section, cut off the undersize part of the shaft, and try again to get a good fit. It may be better to leave it with some grooves as turned, as they can deform and make a tight interference or press fit. I couldn't spend much time machining these parts because my shop is unheated and it's about 10 degrees F. Supposed to be warmer over the weekend. So here it is at this point:
 




Online Jasonb

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #9 on: February 21, 2015, 07:55:12 AM »

Pressing or sticking the shaft into a drilled hole is not ideal as a drill seldom cuts perfectly true to the axis of the lathe so you may loose some concentricity with the piston rod, this will only get worse with a slightly sloppy fit. If you are just using a 3 Jaw chuck to hold the bar then even turning it as you originally did may mean teh rod is not concentric to teh 5/8" bar. either set it up in a 4-jaw or use 3/4" stock and turn both diameters in one setting to ensure they are true to each other.

J
« Last Edit: February 21, 2015, 08:10:47 AM by Jasonb »

Offline steamer

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #10 on: February 21, 2015, 11:57:05 AM »
To me it was being held by the wrong end, rather than hold the short bit of 5/8" that you wanted you should have held it by the remaining 5/8" stock and just parted off the bit you wanted. That way you have the full length of the chuck jaws in contact with the work not less than 1/4"

5/8" I would be parting at 5-600rpm and using power feed and a little coolant, but a syou are starting out handfeed will give you teh feel of what can and can't be done but use two hands on te hrim of the handwheel so you can get a constant feed.

What Jason said.   It's a sad thing 3 jaw chucks   They are so very convenient!...but very limiting.    They are not very ridged, and when they get old, they get less ridged!
Personally, I would grab the long part of the part where ever possible, and use a 4 jaw, or a robust collet chuck ( 5C) for parting.  If you must use a 3 jaw, make sure you have stock in the chuck that engages with the full width of the chuck jaws.   Using just the ends springs the worn jaws out, and the part will start to move around in the chuck, when that happens during a parting operation, you're going to have a "dig in" and the part will get pulled out of the chuck...

My 12" Logan was a trade school machine, the 3 jaw was totally worn out!     Completely useless!    The 4 jaw still had the cosmoline on it and was perfect!  I don't think it ever got mounted!   It's my go to chuck for big parts or ridged setups.
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Offline ShopShoe

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #11 on: February 21, 2015, 02:32:48 PM »
Paul,

Thank you for sharing your thought processes and work procedures with us. You are doing a good job of analyzing your successes and failures and coming up with changes for the next step.

You have recognized the "Git-R-Done" pitfalls. In the end that usually results in more time spent than doing it carefully.

I would agree with others who have posted that sometimes it is better to remake a part rather than start making modifications to solve the problem. I know I have been guilty of that myself.

--ShopShoe

Offline PStechPaul

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #12 on: February 22, 2015, 01:35:49 AM »
Well, I did learn something from my efforts to repair this muck-up. And I may continue along this line to obtain a usable assembly, as it is very close to being there. I may drill and tap the piston and the connecting rod (not a crankshaft as I said earlier), and use a piece of 1/4-28 threaded rod to hold the pieces together. Then I can face off the piston and the extended threaded rod and it may be only barely detectable. If I used Loc-Tite or JB Weld on the mating parts, it should look and function the same as a part made in one piece. It actually took about two hours for me to make the part, but I probably could have increased the speed, DOC, and feed. I want to get as much of this done as soon possible, so that it might be ready for the Cabin Fever expo, and a few days after that I will be having cervical spine surgery on my neck, so I'll be on "light duty" until mid-May.  :old:
 
At least I learned how much difference a thousandth makes, and how much material can be removed from a rough turned shaft using a file and abrasive cloth. What I have now is a very nice sliding or bearing fit, and it feels very good with a bit of oil. The drill produced a pretty good hole, at least good enough for a force fit (some roughness might even be an advantage). I would go into my workshop now, but instead of the 15 degree cold, it is now about 28F and there's about a foot of snow on the ground. I just don't want to deal with that now. I'd rather  :atcomputer:  and  :DrinkPint:   :)

Offline philjoe5

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #13 on: February 22, 2015, 03:31:32 AM »
Quote
At least I learned how much difference a thousandth makes

I've seen grown men (and women) cry over 0.001"  :'(  Personally, I fell into that but then I discovered "over and under reamers".  Not cheap, but I haven't  :'( since I bought a set.

You're giving a good picture of your project. :ThumbsUp:  I hope to see it finished and you at Cabin Fever.

Cheers,
Phil
If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man.  - Mark Twain

Offline PStechPaul

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Re: My parting was sucky sweet sorrow - how to fix this?
« Reply #14 on: February 23, 2015, 05:49:46 AM »
I tried the repair option. I faced off the smaller end of the connecting rod, and drilled and tapped it and the piston for 1/4"-28. Then I cut a piece of threaded rod and screwed it tightly into the rod, then screwed the piston onto it until it seated solidly. Finally, I cut the extended portion of the threaded rod, and faced it flush with the piston. But the first part of the facing operation seemed to cut more deeply than the inner portion. After trying a couple of times to fix it, the piston measured about 0.510 rather than the 0.531 that was the specification. It doesn't take much to shave 20 thousandths! Note that the threaded portion of the repair on the face of the piston is barely visible, and probably would have been OK.
 
The problem was a poorly ground cutting bit, so I used a different one. I turned the rod through the mucked-up portion down to 0.375" and long enough to cut off the too-short piston and part of the rod. Then I tried making a HSS parting tool and got down about 3/16", but it grabbed and stalled the lathe (actually, activating the low speed torque limiter). I think it's because the cross-slide has significant play and the slight positive rake on the top of the cutting edge probably caused it to pull in and take too much bite.
 
The parting groove made a nice start for the hacksaw, so that's what I used to finish the parting. Then it was just a matter of facing the new piston to size and facing the rod, then using a file and abrasive cloth to remove the tooling marks and make it nice and smooth. I left a couple thousandths over before filing, which worked out well. Here is the result:
 


« Last Edit: February 23, 2015, 05:53:22 AM by PStechPaul »