Lead screw shear pin size and material?

[machinist60]

Further on the boat anchor Chinese lathe mill/drill I am going to put in a new pin to connect the drive socket to the lead screw. The original fit was very bad causing eccentricity in the lead screw producing drunken threads. The pin (spring) hole was drilled by someone after a couple of beers, not square to the axis nor through the center with much damage to the holes in the socket. I have repaired the holes with threaded plugs silver soldered in and turned everything true. There is a .007" shim locktited into the socket which gives a nice sliding fit on the end of the lead screw.

The lead screw is LH M20x4 acme and the socket/lead screw drive is 15mm dia. The lathe length is ~20" and swing ~16".

Any ideas what shear pin size would be suitable? Material choices I am considering are brass, aluminum and possibly copper.

Thanks, John.

[doc-zeus]

My first comment is a question. Did you make any necessary alterations to make replacing the shear pin easy to replace?

If it's easy to get to/replace, you might start with the softest (aluminum) and give it a try. If not satisfactory, move to the next harder mtl.

The principle behind shear pins is to provide a cheap and easily replaced part to take the damage in the event of trouble. (Yea, that's basic! But you'd be surprised how many repair jobs come into the machine shop where some knothead got tired of replacing the shear pin and put in a grade 8 bolt!)

Sometimes these shear pins have easy access, sometimes they are a real pain. I'm sure that promotes some lazies to go to a harder materiel!

Good luck, and keep smiling (It drives your nosey neighbors nuts!)

[bob ward]

FWIW the lead screw shear pin on my generic 14 x 40 is 4mm 5/32" brass rod peened over at each end.

[lazlo]

The shear pins on my Clausing 12x36 (for relative size comparison) are made of aluminum. I think any brass/aluminum rod that will fit in the shear pin hole will be fine.

[lakeside53]

My Emco V10P has a 3mm aluminum pin; my Polamco 14x40 has a steel pin (factory...) about 6mm!

[machinist60]

Thanks guys, I have to drill the hole because I repaired the old ragged one as described. The pin location is readily accessible and can be peened at both ends. I'll try 3mm aluminum and if it shears too easily I'll move up to brass or enlarge the hole.

John.

[Duffy]

When I got my 11 x 24 Standard Modern, the shear pins were finishing nails! The holes LOOKED like they tapered, so I used a #3 taper pin reamer and turned some matching brass taper pins, (one spare!) They work a treat. I have also heard that 1/8" brazing rod is a good choice, but it is pretty brittle when you attack it with a pair of side cutters. Duffy

[Bruce Griffing]

My 13x34 Standard Modern has brass taper pins used as shear pins. They are 1/4" to the foot taper and the large diameter is 0.145"

[Rich Carlstedt]

Its a trial and error thing.
You are correct in starting with Aluminum and going to brass.
You could calculate the shearing torque/stress, but that becomes a
wild axx guess too, when the pin shears, and the hole deforms.
you want sharp edges on the screw to get consistant shear points.
So many folks do not understand that shear pins should never be harder than the parent material.
If you find that you have to use steel for a pin, where Aluminum /brass was earlier, turn a thin groove in the pin with a parting tool, and place that groove
at the interface of the leadscrew and the coupling. A 1/4" pin could have a 1/8" diameter shear point.

To check the proper shear pin setting
set your quick change to a very small thread, like 4 or 6 or very high feed rate. (max)
Put some round scrap in the chuck ( between centers !)
and then engage the feed and feed in the tool aggresivly until the motor start to stall. If the pin shears at that point , its perfect ( if you never want to damage your Lathe)
Anything heavier will damage the lathe potentionally, and anything lighter will damage the workpiece if threading (only !)
Aluminum pins should be checked periodically, especially if the fit is loose
Unfortunitely, most machine tools have shear pins far in excess of needs and that means damage.

To give you an idea, my buddy has a 15" Nardini ( 7.5 HP) , that stands in front of a solid wall.
he had the power crossfeed on one day and the tool bit went past the center on a facing cut.
The far end of the cross-slide hit the metal chip guard, and proceeded to push against the wall, tilting the lathe towards him ! ( This happens fast fellows !) When the load became too great (Lathe weights about 5K I think) the cross feed gear bearing in the carriage expanded, and that locked in the Longtitudenal feed AT THE SAME TIME . Now the carriage was trying to move towards the tailstock (Pushing the whole lathe to the left, while pushing the Lathe away from the wall.
This HUGE force bent the rack gear shaft like a pretzel,busted the rack gear and tore up 2 inches of rack.
In 5 seconds, it did over 2,000 dollars worth of damage..
The shear pin ? never worked....over-engineered !
Where was my buddy? he was trying to disengage the crossfeed handle on the carriage, but when the load was transferred to the gears, the handle froze, and he couldn't reach the E stop till it was too late.

Your choice
Rich

[machinist60]

Quite a story, glad I was not involved. Thanks for the tip on a testing method. After I first posted I did consider the maximum torque required and concluded it would be a heavy cut at the most coarse thread pitch, 4 tpi in this case.

Thanks, John.

[Don Young]

As a frame of reference, the slip clutch on a 12" Atlas leadscrew is supposed to be set to 5 foot pounds of torque.

Some manufacturers use hardened steel shear pins with grooves around them so they will snap cleanly and not jam like softer pins can.

[firbikrhd1]

The shear pins are obviously there to protect parts that would be more expensive and difficult to replace should something bind. Going with that theory, the pin only need be as strong as is required to withstand the normal loading or the twisting force. Aluminum or brass may work but different alloys may not shear cleanly and with varied force. Increases in diameter will probably increase shear forces too. Experimentation is probably in order and including some grooves, as has already been mentioned, is probably a good idea as well. Have you considered using a synthetic like a hard plastic?

[wooleybooger]

when i got my rockwell 14 x 40 the pin was gone. the guys on the rockwell/yahoo group said to use a piece of wire coathanger. i had to beg a couple from a buddy who has rental uniforms provided. i no longer had a single one of those most handy welding rod,fishing tool,shop light hanger,hose clamp,tool support things anywhere.

[Don Young]

when i got my rockwell 14 x 40 the pin was gone. the guys on the rockwell/yahoo group said to use a piece of wire coathanger. i had to beg a couple from a buddy who has rental uniforms provided. i no longer had a single one of those most handy welding rod,fishing tool,shop light hanger,hose clamp,tool support things anywhere.

I never went for that new-fangled stuff, I still got a good roll of baling wire!