Any clever ideas to fix this problem? 2

[metalman8357]

Hi all,

I have a cylindrical mandrel (0.34" OD) that has a 0.188" flat to flat hex recess broached into one end of the mandrel. A hexagonal bit fits into this 1" broached recess, and the bit has an oversized ring on the end that expands into an undercut once it is inserted far enough into the mandrel. This is to ensure that the bit cannot be pulled out of the mandrel while it is in use. To remove the bit, we have a hook tool that fits into a pilot hole on the mandrel and acts as a cantilever on the back of the bit that allows the snap ring to compress and the bit is ejected. The problem is, we are lengthening the bit and the recess, and once the snap ring is cleared of the undercut by the ejection tool, it is still too hard to remove by hand. I have been asked to redesign the mandrel so that the bit cannot be pulled out during service, but then can be easily removed when needed. At first i thought of a ball detent, but the assembly required is outside the budget for this part. I'm just looking for a clever mechanism that can achieve the same function that I described. Any suggestions would be greatly appreciated.

Thanks,
M

 

[hydtools]

Try an o-ring instead of the circlip. Although sharp edges may cut the o-ring.


Ted

 

[Screwman1]

We used to use the rubber bit retainers from Apex on some railroad tooling. it is a rubber o-ring with a cross pin attached to one side. You slide it one over the nose and feel the cross pin portion into the cross hole and then work the o-ring into place to retain everything.
We also used light duty spring pins that you could push out by hand with a pin punch. We also used spring wire retainers that would expand into a groove in the chuck. You would grab the bit with a vise-grip to pull them out.
It doesn't take a lot of force to retain a bit so you don't have to be very heavy duty.

 

[metalman8357]

An o-ring is definitely interesting. Besides damaging the o-ring, my only concern is that when the bit is ejected, the o-ring will just roll right off the grove in the bit and stay in the mandrel undercut.

 

[metalman8357]

Screwman, can you point me in the direction of these apex rubber bit retainers?

 

[metalman8357]

I'm guessing you're referring to these (

http://www.apexpowertools.com/catalog_pdf/pdf_pages/TC100_84.pdf).

Looks like these are way too large for the application that I need. But definitely on the right track. If i could have them made to the size of the bit it could work.

 

[metalman8357]

This idea most likely wont work anyway since the mandrel spins in a housing that doesnt have much clearance.

 

[Screwman1]

You found 'em. Would the metal clips on that same page work?

 

[metalman8357]

Unfortunately no. It's going to have to be something that operates either on the outside of the bit, or the inside of the mandrel. We have no room on the OD, and we can't groove it since the wall thickness is already extremely thin.

 

[hydtools]

Install ball plunger(s) in the mandrel and engage the groove in the tool shank. You could still use the same release tool, but the sprung retainer would stay in the mandrel and not need to be dragged the full length of engagement with the tool shank.

http://www.carrlane.com/catalog/ind...20609090C0015482013180B041D1E173C3B2853524458

Ted

 

[metalman8357]

The smallest total length for spring ball plungers that I could find is 3/16". This is already more than the radius of the mandrel.

 

[mfgenggear]

Metal

I purchased a knife recently that uses I think a very ingenious locking method.
a small straight spring when the blade is pulled open it locks in place.
to unlock it push on the spring to disconnect it. marveles.

if it would be possible have a small spring in the mandrel that locks the bit
when it is inserted to the mandrel.


Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[ajack1]

Link

If I am understading correctly would something like a ball lock not work?

 

[hydtools]

If you are making the tooling, put the retaining groove near the entrance of the mandrel to shorten the distance the circlip must be pulled.

Ted

 

[metalman8357]

hydtools, the problem with that is that the circlip puts a substantial groove into the bit. This is because it needs to resist up to 70 pounds of pull out power upon receiving inspection. As a result, any hex material behind the circlip can be considered 'useless' for bearing torsional load since the section area is so thin for the circlip groove. Ideally we want the most hex engagement because these mandrels have a tendancy to break as well. The thought of a simple set screw into the side of the mandrel came to mind, but I'm worried that the vibrational motion might unscrew this during service?

 

[hydtools]

Install and retain a circlip in a groove in the female hex in the mandrel.

Morse taper size 0 ?

Ted

 

[Cockroach]

You can machine an internal groove to accept 1/4 inch diameter spherical ball bearings, fed in from the OD and then retained with a set screw. I have put two drill and tap holes extended radially from the surface to centre line at 180 degrees, so one for entrance of the balls and the other for exit. I use a wire snake to chase the balls around such an internal groove. Sucha system has been used before in downhole equipment for oilfield usage, SHE Ltd, sounds like you're in the drilling industry.

Basically the centre line for the spherical balls is such that an exact integral number will lie on the groove. The midpoint of the groove lies at the interface of the drill bit to mandrel bore. Was hoping you had a picture, I could be much more exact.

Analysis of fracture mechanics, resistance force is a function of shear through the diameter of balls times the population housed in the groove common to the Mandrel and Bit. The joint takes a considerable amount of bit bounce and drill string vibration, so it is fairly robust for your application.

Good luck with it.

Regards,
Cockroach

 

[metalman8357]

Without getting too technical with the design, i'm thinking that a simple conical set screw into the side of the mandrel will suffice. It can then make contact with a groove on the outside of the bit to lock it in place. I've heard that we tried to use a set screw before, but that it got unscrewed during application (These mandrels see 4k RPM, and get a lot of usage). Would spring loading the set screw prevent it from unscrewing during use? I've attached a rough sketch to show the dimensions I'm working with.

 

[LewisWHilliard]

Take a look at the way a 2010 Corvette shifter knob is secured. It is basically a U shaped clip like a staple for electrical cable. It would be a simple mandrel modification and would be very effective for your application.

 

[metalman8357]

Lewis I'm not familiar with this design and couldn't find anything useful from a google search. Can you provide additional information?