Pull bore?

[Walt]

Trying to make a compact bike light, I'm having trouble fitting the pieces together.

The photo shows the pocket I've milled into a small block of aluminum. At the upper left side of the pocket is the piece I've left ("boss"?) for a gland nut that will screw in from the outside. This gland nut will allow a power cable to enter the light. The gland nut is close to the smallest feasible size for the cable I'm using and the O-ring seal I hope to use.

Above the pocket on the right is the switch I plan to use. It's a very nice switch with a rubber O-ring at the base of the threaded shaft. I'm holding it in approximately the orientation it will have in the finished light head, with the exception that it will be mounted in a hole to be drilled in the top of the light head, next to the cable boss.

The problem I have is that the O-ring needs to pull up against a surface that is flat to a first approximation. The pocket wall next to the gland boss is flat, but the flat portion is too narrow for the O-ring to seat properly. I've used a 0.250" end mill to cut the walls of the pocket and the curvature at the inside corners makes the flat area about 0.271" wide. The O-ring diameter is ~ 0.380" OD.

There is plenty of clearance for the switch between the cable gland boss and the inside wall of the light head, ~ 0.6" or so, if I could cut a flat section into the curvature of the inside corners for the O-ring to seat.

I'm wondering if a boring bar could be adapted to cut by inserting it into the switch mounting hole, 0.250" and pulling backward? Is this even possible? The bar would have to have a removable cutter that will extend out to cut a radius of 0.190" (could be bigger) and have a shaft diameter 0.250" or smaller.

http://www.use-enco.com/CGI/INSRIT?PARTPG=INLMKD&PMPXNO=948646&PMAKA=376-6010

This is the closest I've been able to come, but the shank is too big at 3/8", and the minimum cut is larger than I'd like at 0.437" (although this would probably work.) Not sure the cutting bit can be reground to cut backward either.

Walt

 

[Rbeckett]

Walt,
That is a pretty piece of aluminum. I think if you explain your question differently you might get some replies. I am confused and dont understand so if you reword the question others may be able to help too. But I do like the clean well cut look of what you have done so far and it is an outstanding start. Do you plan to anodize it with color when done or leave the finish natural? Either way I think will look really unique.

Bob

 

[Walt]

Walt,
I think if you explain your question differently you might get some replies. I am confused and dont understand so if you reword the question others may be able to help too.
Bob

Thanks for your reply Bob.

Picture my problem this way: I'm trying to drop a washer flat ways into a U-shaped trough. The diameter of the washer is smaller than the trough width, so it can fit down inside the trough. But I want the washer to rest flat against the bottom of the trough, and it won't because of the curve radius at the bottom of the trough.

The solution is obvious! Just use a flat end mill to cut the bottom of the trough flat, and we're done. Oops! There's a hitch. The trough is actually the side of a pocket hole. There is no "top" access to cut the bottom of the U. Because it's really not a trough, it's the wall of a 4-sided pocket.

If I had access to a shaper, I could cut the trough, the inside of the pocket square and the problem would be solved. But I don't have access to a shaper, so that solution is out.

I could make the light head itself larger resulting in longer walls and more space to mount the switch. But I want to keep the unit as small as possible because I plan to wear it on a bike helmet and smaller is better. Call it stubbornness on my part.

I guess the easiest way to think of this is it's sort of a square peg in a round hole sort of problem, but twisted around sideways with no good access to the hole.

Walt

 

[Tony Wells]

Can you modify the washer to slide in and be narrow enough to clear the radii left by the end mill? Or perhaps compromise and use a smaller end mill to reduce the radius in the corners? That may gain enough to clear the washer.

 

[stupoty]

I've seen write ups of people useing a lathe compound slide a a shapper, maybe lock the lathe spindle any try that?

Maybe the same could be done with a mill quill.

stuart

 

[OldMachinist]

Sounds like you're looking to do a backside counterbore. The tool to do that looks like this.



To use it the mandrel goes thru the hole and then you attach the cutter. Start the spindle and pull up on the spindle until you've cut the required depth. Then stop the spindle, remove the cutter and pull the mandrel back out of the hole.

 

[Tony Wells]

That's known as a back spot-facing tool. I have a few of them in select sizes. The do work miracles if you need a flat surface on the back side of an inaccessible spot. Not cheap, but very good.

The poor man's way to do that is to grind a sort of boring bar with a small enough shank to give the cutting edge enough throw, and an upper edge square to the bore. Then with the mill centered on the hole (obviously the tool won't fit while held in the collet) tilt/drop the tool through the hole, and then put it in the collet. Pull up on the tool and you can cut a reasonable back spot face. Low RPM recommended.

 

[Walt]

Thanks Tony,

Smaller washer doesn't work. (Sorry to be confusing with this, the "washer" is actually the O-ring seal around the mounting shaft of the switch. The switch has to be the size it is because that's the smallest I could find after many hours of searching.)

Smaller end mill? I could just barely do the job if a 1/8" end mill was capable of side cutting to a depth of ~0.800". I searched Enco and did not find any with a LOC (length of cut) that long. When I tried using the 1/8" end mill I have with a shorter LOC, as I advanced the end mill past its LOC, the shaft damaged the pocket wall above the cut (It looked like galling to me). The damaged area will prevent the O-ring seal around the switch from sealing. I'm not sure that I could get a good quality cut with a longer 1/8" mill, even if I could find one, there seems to be a lot of flex and chatter with the relatively short one I have.

Thank you for your time and thoughts.

Walt

- - - Updated - - -

(Meant to start a new reply to Mr. Old Machinist)

- - - Updated - - -

Sounds like you're looking to do a backside counterbore. The tool to do that looks like this.

View attachment 50874View attachment 50875

To use it the mandrel goes thru the hole and then you attach the cutter. Start the spindle and pull up on the spindle until you've cut the required depth. Then stop the spindle, remove the cutter and pull the mandrel back out of the hole.

Yes, that is exactly what I was looking for. I bet it is very expensive! Now I know the correct name to search for. Thank you!

Walt

 

[Walt]

That's known as a back spot-facing tool. I have a few of them in select sizes. The do work miracles if you need a flat surface on the back side of an inaccessible spot. Not cheap, but very good.

The poor man's way to do that is to grind a sort of boring bar with a small enough shank to give the cutting edge enough throw, and an upper edge square to the bore. Then with the mill centered on the hole (obviously the tool won't fit while held in the collet) tilt/drop the tool through the hole, and then put it in the collet. Pull up on the tool and you can cut a reasonable back spot face. Low RPM recommended.

Very sharp Tony!

I'll try this first, but it will be *very* close. The shaft mounting hole is only 0.250" and the back-side facing cut needs to be 0.380". That's something greater than a 0.065" throw on shaft smaller than 0.250" diameter. There's a chance an undersize shaft will allow the cutting edge to tip in because the wall the shaft hole goes through is only 0.075". Lacking a rotary table or a lathe to cut my own tool, I'm going to be limited probably to adapting a miniature boring bar.

Lots to think about. This project is fun, in a crazy-making sort of way. However, my friends are beginning to change the subject when I tell them what I'm doing.

Walt

 

[Walt]

Quick update:

Searching on-line under "backside counterbore tool", I found a discussion thread on another website that gave a link to Brown Aviation Tool Supply Co.

http://www.browntool.com/Default.aspx?tabid=254

I ordered this mandrel/cutter combination for ~$40 with shipping. Not cheap, but it will do exactly what I need. It would have been interesting to try to make a tool myself, but I am not sure of my ability to do the job. Throwing money at the problem will allow me to move on to the next part of the build.


Walt