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fixerdave
04-30-2018, 01:46 AM
I decided to cut some aluminium dies for a pipe bender. If it works, and I wear out the aluminium, then I'll cut them from steel. I want to bend 5/8" OD pipe and have a rough idea of how I'd like the bender to work. Nothing fancy, just some circular dies and leverage.

That's the plan. Problem one: now that I've got the blanks turned (mostly), how do I cut that 5/8" semicircle in the circumference? If I made a 5/8" forming tool, I expect it would just chatter like crazy on my little lathe.

Do I make forming tools, left and right with side rake, that are 1/2 the diameter of the semicircle I want to cut, then work them in to a roughed out slot?

Do I CAD out the depths and just plunge in a parting tool, leaving steps I can file out?

Do I give up on the lathe and mill it with a rotary table?

Do I get better advice on what to do from the people here?

David...

J Tiers
04-30-2018, 02:08 AM
Small single point cutter, and a radius attachment, somewhat the same as a "ball turner", except made so you can extend the cutter past the pivot point and cut a concave form. Some ball turners allow that easily.

Boostinjdm
04-30-2018, 03:05 AM
A ball Turner would be best, but I've roughed out and then filed a groove up to 1/2".

Richard P Wilson
04-30-2018, 03:48 AM
Your suggestion of 2 half form tools is probably best, just work them in gradually, in a little, sideways a little, a bit at a time. You don't need a super precise groove, just keep checking with a piece of pipe, as you form the groove till you get something that the pipe seats nicely into.

dian
04-30-2018, 04:22 AM
sometimes you can clamp a bar to the toolpost and use that for radius turning.

elf
04-30-2018, 04:25 AM
Use a ball end mill held in a boring tool holder.

deltap
04-30-2018, 07:20 AM
For one off I plunge to depth in center with a cut off tool going a bit side to side. Then a series of steps. I used a cad
program to calculate the steps. Finish with radiused tool bit. This for a small not rigid lathe.

Toolguy
04-30-2018, 09:15 AM
You could set the rotary table up in vertical mode, fix the blank to the rotab, then cut the groove with a 5/8" ball end mill. You may want to rough it out first with a 1/2" ball end mill, then finish with the 5/8. Or, just make several passes with the 5/8, going from shallow to finish depth.

Bob La Londe
04-30-2018, 09:48 AM
Having slotted with a 5/8 ball mill I can tell you it screams in aluminum and chatters unless you have a really rigid machine. It just so happens I have to make 5/8 slots every day (I actually go .630 now). Its the standard spru size for hand injection soft plastic bait molds. Now I use a 3D CNC operation to make them with a 1/4 inch ball end mill. Its about as fast and I can even do it on my lighter faster machines.

For the guys who suggested taking a step wise approach on the lathe they are dead on. Cut it a little at a time. Then finish it up. You might even be able to use some hand turning methods (wood lathe techniques) to get close. I've seen a guy make aluminum crank handles that way.

A ball turner would also work.

The tool post swing method would only work if your tool post is single bolt to the compound, and not on a T-slot. I think you might have some trouble turning a small enough radius that way though.

A ball mill would not work very well on the lathe unless you have live tooling. Fixed in any kind of holder you would be cutting on one side and rubbing in reverse on the other side. In the interest of being fair I suppose you could use one if you made the bending die in two pieces, and machined half a round in each piece then bolted them together. It would probably still chatter and scream at the end of the cut.

Nobody seems to mention them anymore, but a simple set of radius gages will help you get really close even if you are free hand turning. I have a very old set that came from my grandfather. Other than that, nothing special about them, but I do use them from time to time.

754
04-30-2018, 10:27 AM
I used a homemade radius cutter like a ball turner, 1/2 shaft, relieved to around .43 in the middle. Round hole for toolbit setscrew from side to hold it. Works good but needs a bit of polishing.
Of course you use a radius guage to check.
I did a die In steel, I think 2 inch radius..

tom_d
04-30-2018, 12:53 PM
Do as you have suggested and use your CAD to determine tool position and plunge out the material with a parting tool. If you plot enough points then there might not even be a need for a file.

BCRider
04-30-2018, 01:17 PM
On a smaller lathe that is chatter prone I'd suggest a 3/8 round ended form tool which you use to hog out the material and work the tool back and forth until it's "close enough". There's no need for half thou accuracy with a half cutout of this sort. Get it close with small forming cuts then blend out the high ridges with file and later by hand out of the lathe, with files or half inch stock wrapped with emery cloth.

You WILL get some distortion of the tube with this setup so don't go full half depth on the groove. Or if you want to go half or slightly more than make it a slightly wider groove with a slight oval shape so the tube won't lock into place after bending. with a templ

gellfex
04-30-2018, 01:21 PM
If you have more than one to do you might consider making a rig like this, I did a small one for bending dies also, but I made the dies of UHMW.

http://www.practicalmachinist.com/vb/attachments/f38/72569d1363870576-need-some-help-cutting-concave-radius-2013-03-20-12.46.24.jpg

fixerdave
04-30-2018, 01:30 PM
Thanks for all the ideas :)

I'm really having a hard time visualising a pivoting, ball-turning, kind of rig that would do the inside of a 5/8" diam. I suppose a 1/2" tool holder, rounded nose, and a hole drilled right near the tip. I could then make a some kind of stud to fit in that hole and somehow hold a length of 1/8" HSS. Something on the other end of the HSS to control the pivot. It too sounds like quite the project.

But, it never occurred to me to use the actual pipe I want to bend as a form tool.

Along that line, what if I cut an inch section off the pipe and welded it vertically to something I could clamp in the toolpost? That wouldn't take much effort. Then, a die-grinder on the top-center open end, sort of like making a simple ball-turning cutter, would make for a nice relief all around, leave me a nice cutting angle. It wouldn't have any relief on the outside... might rub some unless I kept it below center. Might have to carefully grind a bit of relief, though that would be a pain. Lathe it before welding? Also, if necessary, I could grind one side away so I was only cutting half at a time. If I did it smart, I could then flip the tool upside down and it would do the other side. Think it would work?

But, yeah, removing most of the waste with a parting tool or something makes sense. And, after that, maybe I'll try a bit of hand turning to see how I do. I hadn't thought of that either. Sounds fun. Might not need any custom tool at all.

Oh, and radius gage... just going to use the pipe :)

Thanks for all the replies,

David...

Mcgyver
04-30-2018, 01:32 PM
AFAIK radius turning attachments don't work for small internal radii. You need a set up that uses some sort of tangential tool. I'd set up a tool bit vertically and use the compound for rotation.

The other way, perhaps more exact and solid is to use a boring head. You need to make something to hold it on the cross slid and be able to rotate it - such that its axis is parallel to the X. Something mounted in the tool post works so you adjust height. boring head adjustment becomes radius adjustment. Again, it uses a tangential tool

fixerdave
04-30-2018, 01:55 PM
... You WILL get some distortion of the tube with this setup so don't go full half depth on the groove. Or if you want to go half or slightly more than make it a slightly wider groove with a slight oval shape so the tube won't lock into place after bending. with a templ

That's good to know... I was wondering how to get a full half-circle with a ball-turning attachment. Yup... that would be the kind of mistake I'd make, or more likely NOT make, by mistake, overshooting the mark or something.



If you have more than one to do you might consider making a rig like this, I did a small one for bending dies also, but I made the dies of UHMW.

http://www.practicalmachinist.com/vb/attachments/f38/72569d1363870576-need-some-help-cutting-concave-radius-2013-03-20-12.46.24.jpg

Looks nice... but scaling it down to do 5/8"diameter, 5/16" radius... I don't think I'm that good.

I did think of using a boring bar in the mill. With some trial and error, I could set that up to do the 5/8". That and a rotary table would do it... eventually. Sounds like a awful lot of winding though.

Actually... why not a boring bar arrangement with a center hole and a matching pin in a holder? Pin way below center, could be a decent size. Bar going up, with a cutter set in a cross-drilled hole Set screw holding it up from the bottom, access through the pin hole. Handle on top of the bar for control. Still seems like it would bite and bend in though. Maybe some kind of U-shaped bar holder, above and below the work, and then a handled as above mounted to the bar at the top? Sigh... another project.

David...

Barefoot
04-30-2018, 02:03 PM
I made this exact die out of 6061 years ago.

For roughing in I'd definitely nibble out with a normal groove tool as others suggested. For the finish tool, I ended up making one using a cut off section from an old junk 5/8 shank end mill TIG welded onto the end of some 1/2 inch square stock. Left the face a bit proud for a final grind and hone after welding. Used low amps to retain as much hardness as possible.

Don't know how rigid your set up is but I had to go extremely slow rpm to avoid chatter. Think I finished it under hand power, followed up some 220/320 grit under power to polish up any discrepancies.

A light coat of grease and a smooth OD surface on your stock will prolong die life. G/L.

old mart
04-30-2018, 02:06 PM
What material is the pipe? Could the mandrel be made from two pieces?

754
04-30-2018, 02:43 PM
I will explain further. Take a ring about 4 in od. About 1 1/8 wide.
Drill a 3/4 hole thru both sides on center.
Then make bushings with 1/2 I'd to fit these holes, weld them in, cut off the part of the ring past the bushings.
Now you have a semi circle with bushings on the ends, add a square bar to the outside , so you can clamp on toolpost.
Now make a 1/2 in shaft, reduced to .43 in the middle....add a 3 /16 hole and setscrew in the middle reduced section.
The 1/2 shaft should be .500 longer than the bushing top and bottom.
Add a set collar on bottom, collar with handle on top.
Grind a toolbit out of a drill or center drill... done.

Rich Carlstedt
04-30-2018, 03:16 PM
There are maybe three factors involving chatter and perhaps the third can help
1. Tooling ...you have read various approaches...I prefer the parting tool making stepped penetration first as the poster pointed out
2. Lathe rigidity ...you are stuck more or less with the design of the lathe, but can make sure your gibs are NOT loose.
3. Mass of the Part...here you can help yourself by using or adding mass to the part. In industry, we added lead weights to spokes for example so that the vibrations were stopped . Think ..Lead, ....sand...plaster. ( Even Cast Iron is wonderful at killing vibration !)

All reduce vibration by adding mass and are non vibratory in their nature. ( Lead tuning Forks anyone !)
One comment on plaster...it will rust your parts like mad, so bare metal must be protected !.
We turned many thin Aluminum rings for example , so corrosion on Aluminum was a not a factor , but our chucks were.
So we greased the heck out of the chuck and jaws and wrapped/laid Saran Wrap on the chuck face and jaws. ( Not the aluminum part)
Then poured the Plaster of Paris on the chuck . Be sure that the plaster can bite into the part and be stable and ATTACHED to the work piece.
One technique was to wind wire tightly around the jaws /and between ,where the part did not touch, so the plaster has bite in the wire (support)
Watch your RPM !
You may want to screw a coffee can to the face of the part and fill the can with sand.
We turned all drums with a sand fill also .

Rich

boslab
04-30-2018, 04:43 PM
A way to finish a full radiused groove, use a milling cutter at a tangent to the disk, my last one was vertically fitted to a plate, bolted to the tool post stud, I just ran it into the roughed groove with the cross slide at low rpm, kind of reamed the radius but without the full load of a form tool radius, the flutes sheared off metal, in fact it was a chipped out cutter but it didn’t matter as the side was doing the cutting, it ras a wire rope pulley but exactly the same as a bend form ( it was the only way at the time, wonders of the night shift.
A long series cutter might be better, mine was held in a holder, the 40 taper Allen screw kind, welded to the block.
Mark

fixerdave
04-30-2018, 07:13 PM
A way to finish a full radiused groove, use a milling cutter at a tangent to the disk...

You want to know something really stupid? Me.

I have a mill, and a rotary table, but somehow I got fixated on using the end of the endmill, and I don't have a 5/8" ball endmill, so I was thinking about using a carbide bull-nose router bit... which would have been a chattering mess so I opted for the lathe instead. But, you know, it never occurred to me to use the side of a 5/8" endmill. Really, it didn't. How dumb is that? Didn't think to use the side of an endmill. Like, you know, I've --never-- done that before :confused: Sheeze... Even thought of using a boring bar on the mill, but not an endmill. Doh!

Might have some clearance issues on the big 6" disk, but the rest should be easy.

I think I'll go for a walk now,

David...

boslab
04-30-2018, 07:24 PM
It’s not stupid, it’s quite normal to miss the obvious, if the obvious is so obvious why the hell do we all miss the damn thing, the mrs usually points it out to me, wouldn’t it have been easier to drill the hole before you cut the 1/2” bit off the bar, easier to hold, aw **** (she used to be an Ali fabricator!)
Always amuses me, she reckoned I’d got more tools than brain cells lol, might have been right before I got the cleaners in my shop!
Hope it works out
Mark

gellfex
04-30-2018, 08:27 PM
Looks nice... but scaling it down to do 5/8"diameter, 5/16" radius... I don't think I'm that good.


Cmon, it's not tough! This is mine, that's 3/8" shaft in a piece of 1/2" 6061. Set screw in the bottom to hold the tool is probably not the greatest design, but it works fine for soft materials. The milling plan would fail with a large enough radius of your bender that your tool is too overhung.

https://i.imgur.com/Oy4Uo9m.jpg

strokersix
04-30-2018, 08:44 PM
i've done this exact thing to make forming dies for 2 inch tubing. End result dies didn't work too well but the machining was fine.

I set up a rotary table and a fly cutter set to 1 inch radius in right angle head on a turret mill.

plunger
05-01-2018, 03:38 AM
Cmon, it's not tough! This is mine, that's 3/8" shaft in a piece of 1/2" 6061. Set screw in the bottom to hold the tool is probably not the greatest design, but it works fine for soft materials. The milling plan would fail with a large enough radius of your bender that your tool is too overhung.

https://i.imgur.com/Oy4Uo9m.jpg

I made some 18mm dies with a setup like this. Biggest problem is swarf buildup around the tool as it has no where to go. I would just use a parting tool and plot a simple diagram to rough most away and then try a form tool and file to get a good finish. Or rough as much as possible with a parting tool and finish off with this tool you made.

ahidley
05-01-2018, 01:36 PM
Make a BALL TURNING ATTACHMENT . You'll thank yourself each time you use it.
There are many designs that a Google search will reveal

dian
05-02-2018, 12:29 PM
https://s19.postimg.cc/8miza6cg3/ball_cutter_026.jpg (https://postimg.cc/image/uygs3kbjz/)

rohart
05-02-2018, 05:33 PM
To follow up the idea of milling the side of the disk with the end of an endmill, and clearance issues. What profile would result if you cut the groove with the endmill not reaching all the way down to the horizontal ? I can't quite get my head around this one.

Cutting on the horizontal diameter gets a semicircular profile - cutting on the very top gets a a rectangular profile. Is it an ellipse ? A parabola ?

No, can't visualise it right now, and I'm quite sober. And usually good at this stuff.

elf
05-02-2018, 05:42 PM
An ellipse and a parabola are both sections of a cone, so either would be possible as well as a hyperbola.

754
05-02-2018, 08:41 PM
I think with all the info given here, if you can't make up some basic radius cutter to do this or similar jobs..
You will have trouble being proficient on a lathe.

fixerdave
05-02-2018, 09:41 PM
I think with all the info given here, if you can't make up some basic radius cutter to do this or similar jobs..
You will have trouble being proficient on a lathe.

My goals in lathe work are, in this order:
1) have fun
2) make stuff I want
3) be proficient

I don't pretend to be the best nor have the best equipment. I do, however, have a blast. Don't exactly know why, I'm just standing there spinning knobs, but it's fun. Been fun for a decade now... pretty cool hobby actually. On this go, the point is to make a pipe bender, quickly, so I can make a rack to trailer my kid's pedal bike behind mine. Not entirely sure it will work at all, I know it won't work well (angles are wrong using the steering head bearing as a pivot point), but it just might work well enough to solve a particular problem... getting my kid to a place where he can learn to ride safely, before winter.

I don't get a lot of shop-time these days (shop is across town from where I live), maybe 4 or 5 hours a week, and lots of things compete for that time. Always something to fix. So, this time, I'm focused on getting that bender so I can bend some pipe so I can make that rack so I can go for a ride on the other 6 days a week with my kid.

So, yeah, I'll probably make a radius cutter at some point but right now I'll skip it. If there's not enough clearance to use the side of a mill on the big one, I'll probably rough it out and free-hand. The little ones should be no problem on the rotary table. And, really, I could probably skip the big one for now anyways. Just need to bend some pipe.

David...

754
05-02-2018, 10:44 PM
The last one shown is extremely simple, looks like 2.5 hrs or less.
And then you can cut die blocks or rollers for up to 1 in or more tubing.
It's nice to be able to change bend radius, without major expense.

Mine gives a finish that still needs sanding, but it's far easier to sand smooth or a bit of clearance, than to file off lumps or norgs.

fixerdave
05-30-2018, 11:12 PM
Finally got some machine-time... had to take a vacation day (and not tell anyone) to do it. But, results:

https://upload.wikimedia.org/wikipedia/commons/thumb/d/df/Concave_milling_with_rotary_table_1.jpg/1024px-Concave_milling_with_rotary_table_1.jpg

Didn't bother taking the latch chuck off my little 3:1 as there was clearance (just).

https://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Concave_milling_with_rotary_table_2.jpg/1024px-Concave_milling_with_rotary_table_2.jpg

And, the results (so far):

https://upload.wikimedia.org/wikipedia/commons/thumb/4/45/Concave_milling_with_rotary_table_results.jpg/1024px-Concave_milling_with_rotary_table_results.jpg

Not exactly a full set, and just made out of low-grade home-poured "billet" aluminium, but the idea of using an endmill on edge worked the treat. I just put the bottom of the endmill on center, advanced in about a mm at a time, and started winding. Not hard to do and not a bad finish either.

Thanks for all the advice :)

David...

754
05-30-2018, 11:48 PM
That small die looks tight like 2xR or so .
Are you fuelling the work around the die, it's pretty tight.

fixerdave
05-31-2018, 12:22 AM
That small die looks tight like 2xR or so .
Are you fuelling the work around the die, it's pretty tight.

The idea behind the smallest one was that it would be doing the pulling, mounted on the lever arm, when bending around the biggest die. It's near 2" dia to bend 5/8" OD pipe. My original idea was to have a fixed distance between dies, and then just having matched sets. 7" and 2", 6" and 3", 5" an 4"... but, my scrap pile of aluminium didn't support my plans... ended up being 6.5" and things went downhill from there ;) We'll see how the rest of the week goes.

If I manage to get some pipe bent, and the idea of using aluminium isn't a complete waste of time, then I'll probably make some specific sized blanks the next time I get around to doing some casting. Right now, aside from the one need, it's mostly experimenting.

David...