Can't hold tolerance with finish cuts

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SteveHGraham
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Re: Can't hold tolerance with finish cuts

Post by SteveHGraham »

I wonder if anyone has thought about making sure the bed clamps are snug.
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liveaboard
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Re: Can't hold tolerance with finish cuts

Post by liveaboard »

I changed the crossfeed screw and nut on my lathe; the wear on the old ones were incredible.
I googled "trapezoidal screw" and found a supplier easily, and not expensive, about $20 + postage from the UK.
Since your machine is in inches, I believe what you want is an "acme" screw + nut. Trapezoidal is the metric equivalent.

I bought a suitable length and machined mine by locking the cross feed and using the compound to do the job while the original screw was removed. I cut the handle end off the old screw, bored it, and cut a step in one end of the new screw to press in.
The difference is huge; the lathe is MUCH better now, I'm really happy I did the job.

Some time ago I also had mysterious jumping at the tool bit, but in my case it was the whole apron that was unstable; there was a large chip between it and the ways [shudder, feeling of horror and pain...]. In this case, it's been determined that the apron is stable but I thought I should toss the experience into the pot.
Glenn Brooks
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Re: Can't hold tolerance with finish cuts

Post by Glenn Brooks »

Liveaboard,

May I ask what lathe do have and What did you use for a cutting tool when you cut your thread? Did you grind one with HSS or buy something from a supplier. Also,did you make up a nut also- if so again, what did your thread cutting tool look like? I would be interested in how you went about the process- does it differ from standard screw threading, etc.

Yes, we do use the acme thread here in the US. I thought about attempting to cut my own screw, but my other lathe cuts a taper due to bed wear, and didn't think about setting up the compound to do the work on the South Bend. Just goes to show you, creativity is half the battle in machining.

While I am having a new screw and nut made up by Miller Machine and Fabrication, I do want to take a shot at making acme thread. So will grind a tool bit and maybe make one for the other more tired machine.

Thanks much for relating your experience.

Glenn
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Re: Can't hold tolerance with finish cuts

Post by liveaboard »

I bought the threaded rod and nut, I only machined it on one end to fit it to the spindle of the original screw.
I was surprised at the low price; the website lists their accuracy standard level as 7E, and it's all pretty impressive.
I'd bought an 8' length of trapezoidal screw from them before, that wasn't for a tool and the accuracy was unimportant, but I was really impressed with their service and prices, and their reasonable postage charges.
http://www.worldofcnc.com/
They mainly sell cnc kits and such, and seem to be only metric.
The nut I ordered was cube shaped, I had to weld it to a small plate to raise and attach it, and I had to cut the other side down quite close for clearance.
There is very little slack; maybe 1/10mm compared to 1mm on the old screw/nut [1/3 of a rotation!].

My lathe is Polish made, "Zalenda". They were produced in metric and inch versions .
Mine is a little one but the company made fairly large machines too. I'm pretty sure they no longer exist. Obviously, there are no parts to be had. It's very old, but I don't know how old.
I knew absolutely nothing when I bought it [compared to nearly nothing today], so I was lucky; the ways are excellent, reground I think because everything else is clearly worn, even the section of.bed where the tailstock slides and the leadscrew.

[attachment=0]zalenda lathe 600mm.jpg[/attachment]
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zalenda lathe 600mm.jpg
Glenn Brooks
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Re: Can't hold tolerance with finish cuts

Post by Glenn Brooks »

Hi guys, well got back from a mini vacation in the Sonoran desert Monday night and installed the new screw and nut. Mixed results. The screw and nut are well made and took a lot of play out of the compound. Unfortunately the tool holder STILL jumps .004" into the work every time I turn the machine on, and/or take out slack and dial in a new setting for a cut. So disappointed the new screw didn't completely fix the problem.

Also noticed the I can pull and push the cross feed assembly .014" back and forth, I guess due either to gib wear, or some play in the coward park of hand wheel shaft somehow. If I tightened the compression bearing nut on the cross feed screw to take out this play, the compound wheel becomes hard to turn, as the bearings are to tight. So, loosen up the bearings so that the screw turns properly and the play shows back up. Hence looking at the gibs -although I don't know if this is an appropriate approach. I made up and added a 015" brass shim alongside the gib. but it was to thick, and the gib adjusting screw doesn't engage properly. So will try a thinner shim tomorrow and see what happens.

After that I may try to swap out the QCTP for a different model- but I've got the tool holder cranked down so tight now I can't see how it would contribute to them.004" movement. Frankly still stumped... :| :( :?:
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spro
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Re: Can't hold tolerance with finish cuts

Post by spro »

Well I deleted a boat full of stuff which came to mind. You are already close and perhaps today go back to why it "jumps". There is some torsional movement or stress to cause that. Aside from the "under" gibs and shims, the lead screw should be in perfect alignment with the carriage travel.
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Re: Can't hold tolerance with finish cuts

Post by John Evans »

I can't see how any adjustment on the cross slide has ANY effect on the compound ! My 18" lathe tends to move on start up but that is a lot due to the inertia of starting a 150lb chuck to rotate without a clutch !. Try setting the cutting edge of your tool past 90* to the work to the tailstock,that way cutting forces will tend to move the tool away from the work.\
There is 2 places for backlash in the cross slide ,the thrust bearing and the thread clearance between the nut and lead screw. thrust bearing should free turning with no noticeable end play,most lathes have a adjustment on the nut to reduce any play there. Saying that my 12" Chinese does and my SB heavy 10 does NOT ! Excessive gib clearance on both the cross and compound will allow both to move in a slight arc do to cutting pressure . And a tool that leads into the cut will rotate clockwise causing a deeper cut. Again try moving the cutting edge of the tool to a trailing position ie toward the TS. I learned that lesson on my 12" Atlas with a rocker tool post!
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Glenn Brooks
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Re: Can't hold tolerance with finish cuts

Post by Glenn Brooks »

Hi John, thanks I will try to set the tool post a bit more toward the rear and see what happens. Although the whole problem seems to be somehow the compound jumps into the work before starting the cut. With the new lead screw and nut, once the cut commences, its rock steady -with a dial indicator set up to watch the tool post during each cut.

Glenn
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John Evans
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Re: Can't hold tolerance with finish cuts

Post by John Evans »

Well Glenn I tend to keep my compounds either locked or snugged up fairly tight depending if same has a locking screw or depends on the gib. Only time I want it to be fairly free moving is when cutting a short taper. Threading snug it is. Like i say tip of tool towards TS so cutting pressure moves it away from the cut,think of a clock face looking down in the tool ,tip between 12 and 1. And lock the compound gib down tight. Then see what the results are.
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Glenn Brooks
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Re: Can't hold tolerance with finish cuts

Post by Glenn Brooks »

More progress. I followed Spro's advice and went back to the beginning and dialed in all the separate parts of the cross slide and saddle. Finally isolated most of the .004" unwanted movement at the end of the cross slide screw at the worn thrust bearings in the taper attachment, and some in related movement in the worn cross feed Gib. After reversing the end bearings and adding a 1/2" shim to distribute the load better at the end of the taper attachment, I was able to get run out down to a consistent .001" overcut :!: :D

This also in conjunction with adding a new brass shim to the cross slide gib and much fiddling with gib tension.

So am ordering new thrust bearings and an NOS gib from Latheman and will see what happens.

Definitely the new screw and nut significantly stabilized my depth of cut problem, and improved finish and x axis movement of the tool along the work. So very happy with that.

Clearly at some point, I will need to scrape the cross slide ways to level and true the compound et al as it slides on the cross slide. But I think if I can consistently anticipate the .001" error, I can carry on with the actual Building and Repair of Railroad stuff. So maybe the end is in sight!

Also, FYI, Last night I discovered a bunch of forum comments from SB lathers and other lathe owners talking about their cross slide/toolpost also 'jumping' a thou or so upon turning on the machine. Seems a fairly common occurrence, for which no one had an answer. I am thinking that worn thrust bearings and possibly related worn/ oblong hole in the thrust bearing housing, through which the end of the screw resides, are possible trouble spots- causing the shaft to torque to the rear when spinning up the chuck, AFTER dialing in depth of cut. On my lathe at least, the chuck and drive system, and jarring action of the motor spinning up, definitely Impart enuf torque to move the entire cross slide assembly if not properly tightened down. So, If the thrust bearing seat and bearing surfaces that support the compound screw are worn, this could easily add an extra thou or two to the depth of cut as the cross slide screw moves rearwards towards the tailstock, rotating the tool into the work, as John Evans describes above.

- Glenn
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GlennW
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Re: Can't hold tolerance with finish cuts

Post by GlennW »

Glenn Brooks wrote:Also, FYI, Last night I discovered a bunch of forum comments from SB lathers and other lathe owners talking about their cross slide/toolpost also 'jumping' a thou or so upon turning on the machine. Seems a fairly common occurrence, for which no one had an answer.
I never make a handwheel adjustment on my lathe with the spindle stopped and then start it to make the cut for that reason. I always make the adjustment with the spindle running and then make the cut.
Glenn

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Re: Can't hold tolerance with finish cuts

Post by NP317 »

GlennW wrote:
Glenn Brooks wrote:Also, FYI, Last night I discovered a bunch of forum comments from SB lathers and other lathe owners talking about their cross slide/toolpost also 'jumping' a thou or so upon turning on the machine. Seems a fairly common occurrence, for which no one had an answer.
I never make a handwheel adjustment on my lathe with the spindle stopped and then start it to make the cut for that reason. I always make the adjustment with the spindle running and then make the cut.
Same practice for me: Adjust the tool position after the lathe is running at selected speed.
~RN
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