Swiss Screw Machine GuideBush scratching material??

CrashBandicoot · Dec 12, 2019

Hey all, I'm a fairly green machinist working with a Swiss Style Tornos Nano Lathe. Last week, the bar stock (.0625 Sandvik 20ap Steel)got bound up behind the stationary Guide bushing and caused the machine to alarm out and stop (also breaking my parting insert). I cleaned out all the bent material, and verified that my guide bushing and collet were not damaged. Now, my guide bushing is scraping off material when being fed through. The guide bushing isn't too loose or tight (verified by those who know better) and it scrapes only when the spindle is in rotation. Also, my tools are getting chipped out in basically the first usage trying to cut metal. The runout in the stock is .0007 before the guide bushing and .0003 after so I doesn't seem like the material is bad or the collet is a problem. We've tried replacing the guide bushing, collet, different stock diameters (and materials) and it all gets scraped by the guide bushing. We've tried every reasonable variable at this point and it's becoming quite a head-scratcher. Any suggestions are welcome, Thanks!

Andrew

rcs60 · Dec 15, 2019

I would try lapping the guide bushing
use your material and some lapping compound this will make the guide bushing exact to the material and should remove anything that could be scratching the material

DouglasJRizzo · Dec 15, 2019

Very common. Chips/swarf get stuck in the guide bushings and wreck the bar stock. Can also cause Z axis feed problems and runout. I ran Tornos for years as well as Citizens. Remove the guide bushing and put a rubberized abrasive in a air die grinder and clean out the embedded dirt. The guide bushing is lined with carbide so this really won't hurt the bushing.

Nerdlinger · Dec 15, 2019

You said you tried different diameter material, collets, and guide bushings....is this one .0625 guide bushing the only one that scrapes the material and the others are fine? Are you sure the collet isn’t scraping the material?...I’m not familiar with Tornos but you should be able to see a few inches of material between the collet and GB to double check.

Rick Finsta · Dec 15, 2019

I used a standard round file or some emory paper to knock loose crap out of carbide lined bushings and a diamond file if I found a sharp edge somewhere on the carbide liner. It happened to me after getting some thin material wound up like you describe.

ptuttle78 · Dec 15, 2019

I have certain jobs I run that I have to clean guide bushing out about every 5 hours. Shaving will build up and cause problems

Sent from my iPhone using Tapatalk

Delw · Dec 15, 2019

What might be happening is that you have to long a gap between your guide bushing and your collet. so when your pushing the bar the .062 dia stock is bending and binding up a tad or scraping on the back side of the .062 dia in the collet.
I generally take pc of delrin/teflon (what ever I have) and punch a hole in it a tad bigger than the bar dia and make it as long as you safely can then press it into the back of the bushing with a light press. this will help guide the material straight with less bending at the sharp edges on the back of the bushing dia.

Also what I do on most small materials is instead of using the bar feeder , I use bushing to pull the stock. it doesn't scratch the material at all if you have a slight snug fit. as the material is very light in weight.

Rick Finsta · Dec 16, 2019

Yeah and I don't run my G300 lines at high feed for thin stuff, either. Anything under 0.187" I slow that thing way down and usually throw some G04s in between ungrip/move/grip steps for good measure.

Also, Pro Tip: Take the first piece of swiss spaghetti you made and tape it to the controller like an antenna. Tell people it is to help with signal reception.

CrashBandicoot · Dec 17, 2019

We tried a .078 Collet + Guidebush + Brass stock and there was still scratches on that material, but not as deep. Also tried brand new .0625 Collet and Bushing from a brand new machine.

TeachMePlease · Dec 17, 2019

Show us a picture of the scraping?

If you've changed out bushings and collets and the problem stayed, the issue isn't the bushing or collets...

Have you re-touched all your tools since your pretzel?

TeachMePlease · Dec 17, 2019

Are the stripes longitudinal, or radial?

If radial, could they be created from tool pressure during your cutoff?

Delw · Dec 17, 2019

it could also be your clamping collet dragging when feeding, have you checked that to make sure?

Nerdlinger · Dec 17, 2019

I always stop the spindle before regripping material for the next part. One time parts were coming out with the OD all scraped up and it’s because I wasn’t M5ing at the end of the program. I figure the material was slipping a bit like a clutch plate trying to match speeds either when unclamping or clamping. The first clue was looking at the length of bar between the collet and guide bushing and seeing the scrapes on the bar before it even got to the guide bushing. Soooooo maybe try throwing in an M5 if it isn’t already there just to see what happens?

Milland · Dec 17, 2019

I'm not a Swiss Miss(ter), so forgive if this is a dumb idea. But what about making a temporary bushing from a plastic like PEEK? Pretty hard, very wear resistant, perhaps a tiny bit undersize to maintain some drag but still allow movement of the stock. Could be a press fit rod in a larger hard bushing that's then drilled for the stock diameter.

The other thing I'm wondering about is the uniformity of the stock. Has it been measured for consistent diameter and cylindricity?

Delw · Dec 17, 2019

Milland said:
I'm not a Swiss Miss(ter), so forgive if this is a dumb idea. But what about making a temporary bushing from a plastic like PEEK? Pretty hard, very wear resistant, perhaps a tiny bit undersize to maintain some drag but still allow movement of the stock. Could be a press fit rod in a larger hard bushing that's then drilled for the stock diameter.

I think that would be pretty cool,and can see use fo it.
Dont see why it wouldnt work other than it might induce vibration or flex while cutting. but on the other hand its small material. you would have to capture the bushing pads somehow to keep it from sliding out of the collet. they would have to be pads because the bushings have 3 slots in them like a collet. maybe if you split them most of the way through you could still tighten?

I am trying to envision it on my citizens I think its possible

TeachMePlease · Dec 18, 2019

Delw said:
I think that would be pretty cool,and can see use fo it.
Dont see why it wouldnt work other than it might induce vibration or flex while cutting. but on the other hand its small material. you would have to capture the bushing pads somehow to keep it from sliding out of the collet. they would have to be pads because the bushings have 3 slots in them like a collet. maybe if you split them most of the way through you could still tighten?

I am trying to envision it on my citizens I think its possible

You make your bushing out of delrin (PEEK would work, if you wanted to shell out for rod that large), slightly undersize it, and groove it for a couple of o-rings that do the actual retention. Or you press fit it in the guide bushing and leave it short of the lands, so the bushing can still compress as you adjust it with the drawbar nut, and consider it non-removable.

Also, to the OP, not a fix for your current problem, which sounds like it transfers from bushing to bushing, and is in fact a problem with the machine, but if you ever need guide bushings that won't mark a part, and are even better than meehanite, these polymer guide bushings are the bee's knees. NEUKOMM

They also make main and pickoff collets as well, made to order, reasonable lead times, and not a whole lot more expensive than a comparable meehanite guide bushing.

Ox · Dec 18, 2019

TeachMePlease said:
Are the stripes longitudinal, or radial?

If radial, could they be created from tool pressure during your cutoff?

Per the wording in his previous post - I got that same feeling.
We're not on the same page.....

The M5 post was good.
Could use a comment on that....

Can you run this machine w/o the bushing?
If so - I would pull the bushing and doo a test cut on the spindle only - to verify that it is straight.
You say that this happened right after a wad up, and is now seen on every collet/bar combination, so that seems to scream spindle (bushing?) alignment to me.

That's the only thing that changed eh?

-------------------------

Think Snow Eh!
Ox

CrashBandicoot · Dec 18, 2019

TeachMePlease said:
Show us a picture of the scraping?

If you've changed out bushings and collets and the problem stayed, the issue isn't the bushing or collets...

Have you re-touched all your tools since your pretzel?

Hi, I would include a picture but my Iphone 5 camera might as well be drawing it with crayons. It's coated Sandvik steel so it's just the coating getting scratched off in small stripes, each stripe being about .3 inches apart, which is about the length of the part we're making. After the initial "spaghetti" of the stock, my parting tool was broken. Since then I've replaced and re-centered all my tools.

CrashBandicoot · Dec 18, 2019

TeachMePlease said:
Are the stripes longitudinal, or radial?

If radial, could they be created from tool pressure during your cutoff?

I've suspected my cutoff tool is pushing on the stock, since it gets material built up on the cutting edge after minimal usage. Also, the chips getting scraped off my bar stock is in the corner of the bushing opposite the cutting tools. Our rpm's and feed haven't changed, any ideas of why the tool would be pushing the stock? Thanks

CrashBandicoot · Dec 18, 2019

Milland said:
I'm not a Swiss Miss(ter), so forgive if this is a dumb idea. But what about making a temporary bushing from a plastic like PEEK? Pretty hard, very wear resistant, perhaps a tiny bit undersize to maintain some drag but still allow movement of the stock. Could be a press fit rod in a larger hard bushing that's then drilled for the stock diameter.

The other thing I'm wondering about is the uniformity of the stock. Has it been measured for consistent diameter and cylindricity?

That's not a bad idea at all, ill have to talk to my foreman about that possibility. As far as the bar stock, it has less that .001 runout between the collet and bushing and literally .0003 after the bushing. This material sucks to turn and is easily bent but this was one of the first things we ruled out. Thanks!

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