unbreakable shear pin on Vectrax Lathe

Here is a question for a machinery design engineer or bright lathe
operator.
Hopefully you are familiar with a typical lathe. I believe my Vectrax
15 x 50 lathe is typical. I have been told it is one of the better
import brands. Either way, I have had it for 8-10 years with no
problems - till now.
It broke when I failed to disengage the half nuts and the tool post
base ran into the work. I was threading a 1-3/4-4 ACME "nut". I had
successfully done about 29 pieces and was about to finish my last one
when I got lazy and didnt get the threading half nuts disengaged in
time and BAM. My lathe has a shear pin located in the coupler that
drives the acme threaded "threading" rod. The rod I am talking about
runs the full length of the lathe. The half nuts engage this rod and
drive the carriage along to do threading operations.
Enough background. I am questioning the selection of the shear pin
material the mfg used. The threading rod has 4 TPI. I built an exact
replica of the coupler to test various materials to use in the
replacement shear pins. The coupler on the machine is basically two
collars that are arranged such that they are against one another.
There is a single shear pin running parallel to the shaft and about
.794" axis to axis, that ties the two collars together. By axis to
axis I mean the center of the shear pin is located .794" out from the
center of the threading rod. They used a single 5MM (.197") roll pin
as a shear pin. It is supposed to break when an unexpected load is
felt. Well, it didnt break. In fact, the threading rod pushed so hard
that it broke the casting that holds the threading rod to the lathe
frame. This casting is located on the far right end (away from the
chuck) of the bed frame.
I built a shear pin testing tool that has the same dimensions used in
their coupler, only in mine I am able to use a torque wrench to
measure the torque at which the pin will break. I also used a slightly
smaller hole for the pin. I used a 3/16 diamter pin in my design
instead of the 5MM (.197") pin.
Based on my tests using various pin materials heres what I got:
Alum sheared at 70 ft/lbs
360 brass sheared at 90 ft/lbs
1018 CR sheared at 120 ft/lbs
Roll Pin sheared at 160 ft/lbs
Using an alum pin and 70 ft lbs, I calculate that the threading rod
would push on the casting (and the carriage) with 21112 lbs of force
before the pin would fail. Using a Roll Pin, there would be a whopping
48255 lbs of force. Seems to me they have screwed up in their design
of the shear pin. This casting is only held in place with two 6MM
bolts and two 3/8 dowel pins. The only reason the carriage stopped
advancing is because the casting broke and allowed the threading rod
to move about half an inch to the right, which allowed the coupler to
seperate enough to pull the "shear pin" out of the hole.
I am thinking of at least going with an alum shear pin instead of
spring steel Roll PIn. What do you all think? Is there anything else I
could do to design a more crash proof design.
Any comments would be greatly apperciated here and to my email
address.
thanks
Steve Bales
Reply to
steve bales
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Steve- This is not the whole answer ,but you could use a 3/16 solid brittle steel pin and turn a groove centered at the joint between mating surfaces. Vary the depth until you find a depth you feel comfortable with. Seems you want it to snap and not bend. I think some outboard lead screw and threading rod bearings are purposely designed on the weak side to act as a shear pin. Jim.
Reply to
Jim L.
Hi Steve,
I admire all the research you've gone to in coming up with the shear strength of the pin. I'm not convinced all of us would be so dedicated.
My only comment would be that in using the spring pin, it was not intended to be a shear pin, although I agree that it should have been. Spring pins are used for their relaxed tolerance in most instances, in other words, it permits a sloppy drilling job to still be an acceptable piece of work. Most any other type of pin would have required a tighter tolerance, meaning more time spent, to accomplish an acceptable procedure.
If the project were mine and I figured aluminum would work satisfactorily, I'd sure as hell use it. You could even vary the strength by choosing a particular alloy, say 7055-T6, for example. That would give you a pin nearly as strong as one made from mild steel. The real advantage, in my opinion, is if it ever shears, it's highly unlikely to do any damage to the mating components, yet will serve very well if it isn't called upon to shear.
Sorry to hear of your accident. A buddy of mine allowed a fellow on his Mori Seike 17" lathe and he did the same thing to it. Cost him his lead screw. sigh!
Harold
Reply to
Harold & Susan Vordos
I admit I'm, not the most experienced, but I use a lot of shear pins and never heard of one used that way. My shear pins are always a hard bolt going radially from one side thru the center out the other side. ALWAYS use a hard bolt, grade 8 - the soft ones distort instead of breaking clean and can be a dog to remove.
Reply to
Nick Hull
Steve. it should have an IRON taper pin in there for sheer. Least most lathes do. They are readily available from MSC, and most bigger hardware or bolt places.
worst comes to worst, use a nail. It will shear long before that spring steel pin will.
Gunner
"Gun Control, the theory that a 110lb grandmother should fist fight a 250lb 19yr old criminal"
Reply to
Gunner
This might lead one to the conclusion that the pin in question wasn't designed to be a 'fuse' element.
Unless it is called out in his parts diagram as 'shear pin' it may just be the strongest part, near the real weak link.
Jim
================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================
Reply to
jim rozen
This is the way snowblower shear pins are designed. When they do snap, it's easier to get the pieces out.
Reply to
Jim Kovar
snipped-for-privacy@earthlink.net (steve bales) wrote in news: snipped-for-privacy@posting.google.com:
A bit more of an expensive fix, but you can get torque-settable slip clutches. These look similar to a flex coupling, and act as one, but they have an adjustable collar that you can preset the allowable slip torque. When the torque is exceeded, they ratchet. Nothing breaks, and nothing gets damaged. Don't have any manufacturers names handy here at home. These are used on quite a few of our CNC machines.
Reply to
Anthony
The JET 1340GHB lathe is like this, and its known to break the casting as well before the shear pin give. Soits just not a problem with Vectrax lathes. A new casting for the JET is about $135.00.
Where I used to work the had a 11 x 30 (36?) Standard-Modern lathe, which also uses this same arrangement. They managed to shear the pin on theirs, and the machinist (thats what they call him anyhow) did not know Standard-Modern included extra shear pins along with this lathe when it was purchased. He used a long #30 or #40 aluminum AN type rivet and drove it into the taper pins holes and proceeded with his machining work. That was over 8 years ago and it has yet to give up the ghost. The lathe is used a lot and abused a lot by lots of folks, and that aluminum rivet is still holding just fine inplace of the original shearpin, which IIRC was brass to begin with. The original in my JET is steel but the replacement pins I got with the lathe when new are made of brass. I can't see how a roll type pin would be acceptable for a shear pin, as they are usually tempered and used mainly for a connection that can be removed but not as a connection that is designed to shear. They sell brass tapered pins, but since yours is a straight hole and not tapered perhaps a piece of brass welding rod woud be sufficient, but then again that old aluminum rivet is holding fine after many years Visit my website:
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expressed are those of my wifes, I had no input whatsoever. Remove "nospam" from email addy.
Reply to
Roy
Clausing supplies an aluminum shear pin for the lead screw on their 5914 type lathes. Its actually a little thinner than the size you're talking about. I'd go with either aluminum or brass but not steel. It sounds like someone at Vectrax either got lazy or wasn't thnking when they put a steel roll pin in yours.
Good luck-
Paul T.
Reply to
Paul T.
I have a Bunch of these btw..but maybe a bit big for his application.
Gunner
"Gun Control, the theory that a 110lb grandmother should fist fight a 250lb 19yr old criminal"
Reply to
Gunner
I'd use brass. If it fails when it shouldn't, probably no harm done and you could go to a stronger alloy of aluminum as Harold suggested. My 1956 Johnson 35hp outboard uses a 1/4" brass shear pin. This has failed a number of times when someone hit a rock with the prop. The pieces were easy to remove and no damage was done to shaft or prop other than minor nicks on the prop blade.
Ted
Reply to
Ted Edwards
I agree with this opinion. Hardened steel roll pins are not designed as sheerpins they are used by some cheep manufacturers after design because they are cheaper to deal with, there flexibility and hardness makes them totally unsuitable as Sheerpins. Brass sheerpins are excellent they have a fairly constant breaking point and break cleanly without marring the matting surfaces. Cooper may be a next best replacement they are not as good and smear a little when sheering of. Aluminum may work but it will smear a lot which could result in a incomplete separation and additional cleanup work when replacing. Steel often breaks in multiple smaller pieces that can result in damaging the mating surfaces sharp edges will bite into the surface and may not separate consistently. Good Luck
Reply to
Torsten
Hey Steve,
Sorry for your problem. Next project going to be an auto-release??
Anyway, I did the same thing once (so far!!) myself on a 10" Atlas. It too sheared off the tail mount. It was a white metal casting. I thought that was a stupid way for them to have made it, but someone at the time said to me that was a design feature, and how happy would I have been if the max force was instead only applied against the quick-change!! And you have to admit that fixing or buying a new bracket is reasonably economical, versus a lead screw or gear-box. The tail-bracket and the thrust bearing(s) incorporated into it is what sets the "back-lash" of the lead-screw, so there can't be any play allowed.
Anyway, have you looked at the design placement of this "shear"? It does seem to me that what you described will have a real tough time shearing as you expect it to, because the instantaneous force will be to bind/jam the two surfaces of the shear plates (got a better word??) together, and not to shear the pin. If the pin was perpendicular to the shaft/lead-screw, then the force would shear it instead, although shear points this way are double. That is, it has to shear the pin on two "sides", whereas the design you have now only has to shear one thickness of the pin.
Take care.
Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXXXXXX Anyway, >Here is a question for a machinery design engineer or bright lathe >operator.
Reply to
Brian Lawson
I'm wondering if that would be such a good idea. With a clutch, you may not know if it has slipped, even if it does make some kind of clicking noise. Some machining operations are quite noisy.
With a leadscrew, I'd want to make sure the operator knew the torque limiting device had been used such that he would not go on trying to use the screw (quite possibly resulting in a very expensive, but broken, threading tool.)
May or may not be an issue.
Regards,
Robin
Reply to
Robin S.
And Clausing will sell you a pack or replacement shear pins for a few $. It didn't take long to buy those once the cotter pin was discovered in the used 5914 I bought a while back.
Reply to
Mike Henry
I added a quick change (bought new from Clausing in the late 80's) to my Atlas 12. Installing that also converted the lead screw to having a fibre disc clutch up near the QC box and the lead screw support at the tailstock end was replaced with a heavier piece made of brass.
The clutch was adjusted at assembly by tightening it until it slipped at a given number of foot pounds on the screw. I never tested it by overloading it in use, accidently or otherwise.
Reply to
Jack Erbes
"Robin S." wrote in news:lNmKb.4420$k snipped-for-privacy@news20.bellglobal.com:
On our CNC machines, there is a proximity sensor at the clutch disengagement point. If the clutch slips, the machine faults. Something similar could be made for a manual machine, with just an indicator light being made or something to let the operator know. It would be a one-time fix for a possibly recurring and somewhat expensive crash.
Reply to
Anthony
> > >Hey Steve, > > > >Sorry for your problem. Next project going to be an auto-release?? > > > >Anyway, I did the same thing once (so far!!) myself on a 10" Atlas. > >It too sheared off the tail mount. It was a white metal casting. I > >thought that was a stupid way for them to have made it, but someone at > >the time said to me that was a design feature... > > I added a quick change (bought new from Clausing in the late 80's) to > my Atlas 12. Installing that also converted the lead screw to having > a fibre disc clutch up near the QC box and the lead screw support at > the tailstock end was replaced with a heavier piece made of brass. > > The clutch was adjusted at assembly by tightening it until it slipped > at a given number of foot pounds on the screw. I never tested it by > overloading it in use, accidently or otherwise. > > > >
Reply to
steve bales
I want to thank all the wonderful responses I had to my broken Lathe. I have finally made the repair. If you'd like to see a photo of the new aluminum shear pin, go to:
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Pic 4 is a close up photo of the new 3/16 diameter aluminum pin. In this photo, you can see the other hole that the manufacturer used for the 5MM roll pin. Because I wanted to use an American size, I drilled and reamed a new 3/16 hole to use. Besides, the roll pin hole was mangled quite a bit.
To see the new casting in place go to:
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Some of you may notice in these photos a rather odd looking attachment I have added to solve another problem Vectrax had in the design of their lathes. I'll give a prize to anyone that can tell me what this is for. Well, perhaps there isn't a prize, but you could guess anyway.
Happy chip making
Steve Bales
Reply to
steve bales

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