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Any clever ideas to fix this problem?
2

Any clever ideas to fix this problem?

Any clever ideas to fix this problem?

(OP)
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

RE: Any clever ideas to fix this problem?

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

Ted

RE: Any clever ideas to fix this problem?

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.

RE: Any clever ideas to fix this problem?

(OP)
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.

RE: Any clever ideas to fix this problem?

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

RE: Any clever ideas to fix this problem?

(OP)
I'm guessing you're referring to these (http://www.apexpowertools.com/catalog_pdf/pdf_page...). 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.

RE: Any clever ideas to fix this problem?

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

RE: Any clever ideas to fix this problem?

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

RE: Any clever ideas to fix this problem?

(OP)
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.

RE: Any clever ideas to fix this problem?

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/index.cfm/27025071...

Ted

RE: Any clever ideas to fix this problem?

(OP)
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.

RE: Any clever ideas to fix this problem?

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.

RE: Any clever ideas to fix this problem?

Link

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

RE: Any clever ideas to fix this problem?

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

RE: Any clever ideas to fix this problem?

(OP)
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?

RE: Any clever ideas to fix this problem?

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

Morse taper size 0 ?

Ted

RE: Any clever ideas to fix this problem?

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

RE: Any clever ideas to fix this problem?

(OP)
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.

RE: Any clever ideas to fix this problem?

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.

RE: Any clever ideas to fix this problem?

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

RE: Any clever ideas to fix this problem?

RE: Any clever ideas to fix this problem?

Basically you would just slot the mandrel to align with the current circlip location to commiserate with bit engagement. Then make a U shaped spring steel clip slightly wider than the slot. With the clip in place you will have pinned the bit on two side using existing locking features. Removal of the clip is performed with a flat blade screwdriver by tapering the lower edge of the slot to allow acces to the clip.

Clear as mud right.

RE: Any clever ideas to fix this problem?

B

Quote:

basically you would just slot the mandrel to align with the current circlip location to commiserate with bit engagement. Then make a U shaped spring steel clip slightly wider than the slot. With the clip in place you will have pinned the bit on two side using existing locking features. Removal of the clip is performed with a flat blade screwdriver by tapering the lower edge of the slot to allow acces to the clip.

Clear as mud right.

i like this idea

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

RE: Any clever ideas to fix this problem?

There are a few things you can do, along the lines of the attachement. A guy could come up with similar mechanisms based on this model, I had a telescopic pin concept rather than the Locking Segments, but not sure of your loads. You mentioned robustness, this would definitely meet that criteria.

I think the challenge would be to get chip flow out of the broached hole of the female piece. Most likely you would machine this recess after broaching, I think a long continuous path for the shaper would be best in terms of an interrupted cut.

I also prefer liquid nitration as a case hardening process in order to reduce the metal-to-metal transfer between mating components. It also removes the free iron from the surface of the pieces, hence no rusting.

I've used similar ideas like this in the past, works okay, nothing special though.

Regards,
Cockroach

RE: Any clever ideas to fix this problem?

(OP)
Cockroach,

This seems like a perfect idea, and should work for my application. The bit pullout needs to withstand 70 lbs, do you think this is possible? Also, can you explain your design in more detail, I'm still a little confused on how the locking mechanism works. Thanks so much for the help, you're a life saver!

-M

RE: Any clever ideas to fix this problem?

Basically, you have a hex nut (Locking Segment) that is activated by a Compression Spring. The male is inserted into the female and by hand, those segments are compressed flush to the male shaft. This allows passage into the female, aligned with the internal hex. Continuing to push the male to the female, they shoulder out at which time the Lock Segments encounter the recess and "snap" outwards due to the Spring force below. The system is now locked into place longitudinally (axially) with the hex shaft taking the torque.

To release, I have three 1/16 holes circumferentially around the female. Simply insert an o-ring pick or equivalent into the holes to push the Lock Segments down. Pulling back on the male shaft will disengage the components and allow for dis-assembly. Spring force is light, no much is required.

A guy could do the same with a telescopic pin, spring activated, running orthogonal to the male hex shaft. In this case, the pin would snap into a hole or groove in the female, so close to what we have here. That would have a big cost advantage compared to what I did, but the prints you have take much more load, 70 lbf is relatively easy to obtain. But as mentioned, I would do the stress mechanics on the system in order to validate the design for your input statements and requirements.

There are other examples as well, this is not a particularly challenging design problem.

Good luck with it. Chip flow from the internal bore of the female while cutting the recess, I think that would be the issue. Hopefully it is a bore thru design, I've shown it closed ended only to mirror what your initial requirements where.

Regards,
Cockroach

RE: Any clever ideas to fix this problem?

Just a quick and dirty, the male part will take 2,556 lbf of axial load, but is subject to failure in shear due to torque at 5.284 ft lbf. That's because the relief machined in for the hex is only 0.172 inches in diameter. The quick fix here would be to counter bore the opening of the female and push the hex of the male past that opening to the right. You could then remove the counter bore and simply mill the hex to the male shaft and have a flank angle run-out at 30 degrees to the OD of that shaft. Simple.

The female part will take 5,011 lbf, the wall of the recess being the weak point. Note that it sees no torque because I extend the hex above and below that recess, loading IS NOT thru the wall of the recess. I should of seen that for the male, but following the above would eliminate the limitations in torque.

So I think this would meet your requirements once you make the above noted quick fix.

Regards,
Cockroach

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