Possible to Cut O-ring Slot in Inside Wall?

[geraldvowles]

Hi, Can anyone advise if it's possible to cut O-ring slots in the inside wall of a small ID tube or pipe and what tooling would be needed?

In my case the material is Plastic (PVC and/or UHMWPE). The OD is 1"+ but I want to keep the bore ID to 1/4" if possible. One or more O-ring slots would be cut into the inside wall.

The application is a slow cycle (6-15 cpm) reciprocating shaft water pump. The max pressure of 125 psi is well below O-ring max. capabilities from what I'm reading. - Thanks

[Harold_V]

Very simple, assuming you have a lathe. One grinds an inside grooving tool from HSS stock. Do you need a photo to help you understand proper design?

Harold

[Victor_R]

How ironic is this. I just did this very thing to an adaptor for a microphone calibrator made from black delrin or ABS. (They're very similar) This is a ½ inch O ring with a .093 thickness. I set the position up with a micrometer stop on the lathe and plunge in to depth. The tool is a little less then .093, so I widen the groove to size horizontally. I had the tool from a previous job. Real simple.

Victor

[geraldvowles]

Thanks much Victor, it's nearly exactly what I'm wanting to accomplish! Also, thanks for taking the time to take the photo's, it's very helpful.
- Gerry

[Harold_V]

Internal grooves can be challenging in small bores, the problem being shank size. In order to accommodate the depth of cut, the grooving portion of the tool must extend beyond the shank diameter, which must be deducted from the shank for clearance in the bore when the tool is plunged to depth. A good design on your tool will be in order. You will have to grind it specifically for the intended groove, with clearance on the shank relative to the bore when the groove is to full depth, plus enough clearance to get the tool inside the bore without touching the ID. Fortunately, plastics machine will very little resistance, so you'll be able to get away with a relatively small shank and still not have trouble with the cut.

Grinding a tool such as this is best done with a narrow grinding wheel, which will keep the relieved area as short as possible. That's good for rigidity. That, of course, depends on where you intend to machine the gland as it relates to the end of the material. I strongly advise it be near the end if there are no reasons to place it elsewhere.

Keep these things in mind when you grind your tool. The cutting edge should be the widest point (naturally!), meaning that if you look at the tool from the perspective of the cut, the tool should narrow slightly under the cutting edge towards the bottom, and also slightly towards the back. If you don't provide relief in these areas, you risk dragging of the tool. That will destroy the finish and possibly lead to a broken tool. All depends on how bad things go when the plastic starts streaking off on the tool.

The tool need not be the full width of the groove. As long as you know the width, you can plunge cut then move over and plunge again. You can achieve a perfect blend in the groove by staying shallow a couple thou until you are to width, then with the tool at full depth, move the carriage from one side of the groove to the other. Using a DRO or long travel dial indicator makes this very simple.

Don't make a random cut. Glands that are successful should be machined to specifications. The type of action and how the O ring is applied determines sizes, so be certain you have the proper gland design when you determine width and diameter of the groove. In this case, the smaller cross section that will work, the better your chances of success. Cross section will have a profound effect on the shank size of your tool.

It's important to keep your tool as near center as possible, especially when working in tiny bores. Otherwise your cut won't register according to the dial, plus changing center has a profound effect on rake angle when you work that close to the center of material. You can generate positive rake (very good for the material in question) by grinding an angle from the cutting edge towards the rear of the tool. You can see a grinding line on the top of the tool I've posted that would be similar to the grind you'd put on your tool. The rake angle would be ground using the periphery of the wheel, so you get a pronounced hollow grind.

The tool pictured should not to be construed as one that would cut your O ring groove, but it resembles one that would work satisfactorily. In the case of cutting inside a ¼" bore, the front relief of the tool will be extreme, so your tool will have a strange appearance, with a much smaller radius than the one you see.

A special tip for working in small holes:

Instead of grinding the cutting edge on the top of the tool, it's not a bad idea to grind the top portion into a round bar, leaving a hump on the end that will become the grooving tool. The round portion then becomes the centerline of the cutting edge (the top would have been), so a corresponding amount of material is removed from the hump on the end, to the centerline of the round shank you've generated. That provides for the maximum shank strength possible. If this doesn't make sense, lets talk about it until it does. If you had a job in stainless, this would be critical to success.

Hope some of this helps-----let us know how it turns out.

Harold

[geraldvowles]

Harold,

Thanks much for the detailed advise. It will serve me well. I'm on a Mac so the pic's often don't come through so if there was one included, and if your time permits you could send it to my e-mail at gvowles@mac.com. Thanks again for the detail and clarity.

- Gerry

[geraldvowles]

Harold,
How about that! The pic's popped up as soon as I posted my reply so no need to send again.

Thanks, Gerry

[awake]

Gerry, I have a theory about the "Mystery of the Appearing Pictures" -- it has nothing to do with being on a Mac, but rather with whether or not you are logged in to the forum. If you are not logged in, you can't see the pictures.

[RET]

While things should work the way you've been told (all good advice by the way) sometimes it can be easier if you rethink the problem. With the OD. you have, it would be possible to bore out the rod to 3/8" and then machine a few plastic spacers to fit into the bore between the "O" rings. If you then counterbore & thread the end to a bigger diameter, either 7/16" or 1/2", you can make a threaded end that will hold the whole assembly together.

Since it is plastic, you can arrange the interferences so that the assembly either won't leak at all or will leak very little. Just a different way of "looking at the elephant."

Richard Trounce.

[geraldvowles]

Hi Harold, looking at your two tooling pic's you posted above, how the heck did you do that! I'm just starting to realize the importance and value of good tool grinding and design but can't imagine how you managed to grind such an intricate small tool? I'm looking at what I'm assuming may be a front view of the cutting end? Thanks much, Gerry (the Newbie)

[Harold_V]

Hi Harold, looking at your two tooling pic's you posted above, how the heck did you do that! I'm just starting to realize the importance and value of good tool grinding and design but can't imagine how you managed to grind such an intricate small tool? I'm looking at what I'm assuming may be a front view of the cutting end? Thanks much, Gerry (the Newbie)

Well, for starters, I've done this all my life, or so it seems. I worked in the trade for many years-----long before "machinists" were afforded the luxury of buying insert tooling, avoiding learning how to grind proper tools.

I've harped, endlessly, on learning how to grind HSS toolbits. I can't think of any one thing that will liberate a guy more than understanding how tools cut, and how to grind them for their lathe.

Recently I ran a series of posts that covered grinding HSS----which can serve to guide you fairly well in the learning process. I'm going to include the links here for your perusal. One of them even details the kind of system I use-----a far cry from the typical pedestal or bench grinder with a tool rest. As long as you restrict yourself to one of those, between the wheels that are too wide and hard, a tool rest that is almost always in the way, plus the typically low height at which such grinders are usually found, I can't imagine EVER learning how to grind useful tools. Anyway, check the links and see if there's anything there that might serve you.

http://www.chaski.org/homemachinist/vie ... hp?t=75985

http://www.chaski.org/homemachinist/vie ... hp?t=75969

http://www.chaski.org/homemachinist/vie ... hp?t=76065

Harold