Machining an offset taper

maynah · May 24, 2018

I would have posted this in general forum but I can't hide the fact that I'm not a machinist so easily over there.
Oh and by the way, I'm not a machinist.
I'm making a part for a friend and am not sure about how to make the taper.
The part is for a Taylor Fur Getter Set-gun. It wasn't legal very long and is fairly scarce. I'm making the brass piece in the white circle, the pink circle is the taper. I've machined a brass bar down to .508 square. The only way I can figure to make the taper, notice the taper is only on two opposite sides, is to bore two center drill holes off of true center and cut one taper, switch center holes and cut the other. If this is the best way, what is the math needed to determine the distance from true center, to the taper center, to get a 9 degree taper on each side?
I hope I'm making myself clear. Or is there a better way to do the job?
The first picture is the example I made my drawings from.
Second picture is the part I'm making.
Third picture is where I am in the job.
Any help much appreciated.

athack · May 24, 2018

This is a turning job.

johnoder · May 24, 2018

Put in lathe

Support end with tail stock as needed

Set compound rest to 9 degrees off spindle axis

Machine taper

M.B. Naegle · May 24, 2018

Figure the radius off of one side and off-set the part in a 4 jaw chuck to hit just the one side as it's being turned. Measure the amount of taper you need and set it with the compound. Keep your carriage locked in place and use the compounds travel to cut. You can step it in with the cross slide. Then without moving the jaw position, flip part over for the other side. Probably how it was made to begin with.

Renardiere · May 24, 2018

Centre up bar in 4 jaw chuck in lathe. Offset one pair of jaws so that the adjacent two faces are clear of the path of the cutting tool. Cut taper using compound set to desired angle. Add more offset if required but ensuring that adjacent faces will not be cut. When happy with first side remove by releasing only 2 jaws , rotate part 180 degrees and repeat.

thermite · May 24, 2018

maynah said:
I would have posted this in general forum but I can't hide the fact that I'm not a machinist so easily over there.
Oh and by the way, I'm not a machinist.
I'm making a part for a friend and am not sure about how to make the taper.
The part is for a Taylor Fur Getter Set-gun. It wasn't legal very long and is fairly scarce. I'm making the brass piece in the white circle, the pink circle is the taper. I've machined a brass bar down to .508 square. The only way I can figure to make the taper, notice the taper is only on two opposite sides, is to bore two center drill holes off of true center and cut one taper, switch center holes and cut the other. If this is the best way, what is the math needed to determine the distance from true center, to the taper center, to get a 9 degree taper on each side?
I hope I'm making myself clear. Or is there a better way to do the job?
The first picture is the example I made my drawings from.
Second picture is the part I'm making.
Third picture is where I am in the job.
Any help much appreciated.

Jeff?

That taper you circled in pink is "cosmetic", rather than functional. It only has to "look nice" and not be rude to the hands nor snag on a trapper's carry bag.

In brass and for a "one-off", I would probably not even have it on a machine-tool of any kind. Not worth the set-up nuisance. The originals were cast, not machined. Pattern probably machined. Concave router on wood perhaps, not lathe work.

Vise, rasps, files, stiffener-backed abrasive papers, maybe a powered wheel is all. Small part, done with it in short order.

WHERE. you need partial taper "sections" that ARE functional - for fastening, alignment, latch and release?

Simplify the trig and angles to the core that gave rise to them. Work with ratios of sides.

They are easier to measure accurately with the tools we have, and "SOHCAHTOA" is the only "trig table" needed.

Two centre divots is a valid approach. If/As/when too close together to work well, consider two separate attachable "false ends'. Or just one. One-eighty reversible, so its single offset divot delivers symmetry as a byproduct, no real fuss required..

Grigg · May 24, 2018

Another approach is use the belt sander, rest the back end on the table and lean it onto the belt to get 9 degrees, and sand to an arc marked on the end.
A little easier and faster than file work.

If you had to make a bunch, or if that was a critical dimension/feature, you've already received some great answers.

thermite · May 24, 2018

Grigg said:
Another approach is use the belt sander

Thanks for that!

Reminds me I should cease waiting for a decent used / rebuildable Burr-King to fall out of the sky and go get at least a cheap one for piddly's hit whilst awaiting such a blessing!

What was that old saw? "Wish in one hand and spit in the other....?"

Standard "cargo cult" bait of Old Iron isn't working, so...

maynah · May 25, 2018

Just thought I'd follow up. The 4 jaw chuck off-set worked well. I only had round stock to work with so just left the work in the collet block so I could turn 180 degrees easily. I dialed it in then moved the work .100" offset. The other thing that worked well for me was my 12" Skinner 4 jaw independent/universal chuck. After the first cut I used the scroll feature to release all 4 jaws at once, turn the block 180 degrees, then tighten all 4 at once again. I think that's the first time I really used the chuck as it was meant to be used. I usually just use it as a independent 4 jaw.
Thanks for all the replies. I was trying to make the job harder than it needed to be.

thermite · May 25, 2018

maynah said:
I was trying to make the job harder than it needed to be.

No fear. One of the ways we self-identify AS craftsmen, that can be!

JohnMartin · May 26, 2018

Guessing that your friend was at the Oxford gun show last weekend...

maynah · May 26, 2018

Absolutely. My table had the 2X Garand trainer, hard to miss.

JohnMartin · May 27, 2018

maynah said:
Absolutely. My table had the 2X Garand trainer, hard to miss.

Hard to miss indeed. Last table in the back room, IIRC.

Interesting how the interests of different people often parallel. Or maybe it's just a man thing. I'll say hi if I see that big M1 again.

L Vanice · May 27, 2018

Decades ago, I had a very similar device that was supposed to be 18th Century. The wood frame had plenty of worm holes, so maybe it was. It had a flintlock and blunderbuss barrel. Legend in the 20th Century was that it was to fatally discourage poachers wandering aristocratic woods. No worry about making perforations in the fur.

They are pretty rare, but the Internet has pictures: flintlock poacher trap gun - Yahoo Image Search Results Flintlock trap gun 832162 | National Trust Collections

The Fur Getter patent: us patent 1,098,742 Patent Images
Not shown in the post #1 pictures is the bait hook that was supposed to assure the hungry critter shot itself in the face, maybe avoiding a hole in the wrong place.

Larry

thermite · May 27, 2018

L Vanice said:
to assure the hungry critter shot itself in the face, maybe avoiding a hole in the wrong place.

Larry

Memories! Could be both! "Mountain William" Dad teaches "City Girl" mum to shoot. Really well. Seems she was a "natural".

He's off building B-36 runways in Saudi, she gets aggravated at a whistle-pig predating her kitchen garden and takes him off the radar and onto to the dinner menu with a "Cadet" Model pre-WWII Mauser .22.

G'dad - called to advise - compliments her not a wit about the entry hole less than a sixteenth-inch off centre of a line between the eyes.

He focusses instead on the .22 solid's exit hole about 1/4" away from the anus.

His patient explanation that she had not done herself any favours as to food preparation by shooting it walking and scrambling its innards, full body length, bone shards included.

One is meant to head-shoot a whistle-pig when erect, checking out its zone as it exits its burrow. Doesn't mess up the meat that way.

maynah · Jan 28, 2019

I forgot to put the finished picture in this post.
The owner "aged" the brass after my work and it matched pretty good.
I did use a big brass wood screw I had rather than the original steel.

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