# A simple pen mill for metal lathes



## skiprat (Dec 2, 2008)

I've been messing around for ages with old gears and cogs to make another pen mill that can be used on my metal lathe and cut metal as well. I only wanted a hexagon shape with a slight twist - about 60deg.

I couldn't get the ratio high enough with the bits and bobs I had and eventually came up with this idea.
It is nothing more than an indexable jacobs chuck. The long chrome arm slides up ( or down ) the ramp, turning the mandrel while the cross slide of the lathe is moved. 
The aluminium collar with the threaded studs is changeable for different designs ( pentagon, octogon etc )
I didn't realise it at the time but with the ramp parralel to the mandrel, the rotation of the blank is not linear. No biggie as I can spin at an angle till I get it correct.
The aluminium is from an office wall partition.
I've only tried it on this piece of red PR so far. On the weekend I'll try ali, then some stainless. I'm pretty pleased with the result and it was real easy to make:biggrin:


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## Daniel (Dec 2, 2008)

nice work. It does not appear that there woudl be a problem with the rotation as it is. except maybe roughness in the ramp or something. I guess that is why it is called learning. any more info as to just why the twist is not linear?


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## bobindayton (Dec 2, 2008)

The twist is not linear probably because the distance between the centerline of the mandrel and the ramp changes as the arm goes down the ramp. Mark the arm in the starting position and compare the mark at the end of the cut.
Bob


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## skiprat (Dec 2, 2008)

Daniel, geometry is not my cup of tea, and I could not explain it correctly. But the big chrome arm rotates from approx 1 o'clock to just past 3 o'clock positions.  When the arm is travelling near the top of the ramp it rotates less than it does when at the bottom of the ramp.
Look at the far end of the blank, it is almost straight.
My theory is that if I move the top of the ramp away to the right hand side but keep the bottom where it is, then it will work.
Essentially I want the same point on the arm to be in contact with the ramp all the time. Clear as mud??:biggrin:


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## skiprat (Dec 2, 2008)

Just checked and I was wrong:redface: ( Nothing new there!!!! )
The top of the ramp needs to be closer to the *left*. ( as in these pics )Although still not perfectly linear, at least the start and finish points are in the same place. I think that to make it spot on, then the ramp would have to be curved ( to the left at the bottom ) but I think it will be close enough for me.:wink:


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## Daniel (Dec 2, 2008)

"Clear as Mud"?
Actually yes it does make since. At least the picture I have going on my head does.
no time to elaborate right now but will come back later.


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## Paul in OKC (Dec 2, 2008)

Kind of have to think of the fulcrum point, and keep it the same through out the stroke. Very clever idea!


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## btboone (Dec 2, 2008)

I love the idea of changing pitch. You might try different shapes such as an arc to accentuate the effect. I think it would be cooler than cool.


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## SuperDave (Dec 3, 2008)

skiprat said:


> ...and it was real easy to make:biggrin:




...yeah, whatever...


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## ldb2000 (Dec 3, 2008)

Skippy , you are my hero and when I grow up I want to be just like you :biggrin:
That is the coolest simple machine I've ever seen . With a wavy ramp you could make some wild twists .


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## akbar24601 (Dec 3, 2008)

That is fantastic Skip!!! Smart thinking! U Da Man!!!


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## Chuck Key (Dec 3, 2008)

Very cool and you could do tapers with additional hack saw blades.  Also, it looks like the facets will be coves rather than flats witch are also nice.  A little adjustment to the angle the stock is presented to the end mill would fix that if flats were desired.  Sand paper will do it too .  Three facets would cut the grinding, sanding and polishing in half.


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## BRobbins629 (Dec 3, 2008)

Skip-geneous at work.


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## skiprat (Dec 3, 2008)

Hi Chuck, long time no hear.

The facets are flat if the mandrel stays perpendicular to the cutter. If I move the tail stock end of the mandrel a bit to the right, then I think they will be concave. Move it to the left and they will be convex. I'm not 100% sure on that, but I think thats how it will work. Raising either end will of course produce tapers. I used two cheap pairs of feeler gauges to do the height shimming. The way the gadget is assembled in the pics is to use the side of a cutter. The extra holes that can be seen in the aluminium are the original and make the mandrel the same height as the lathe centre. I'll use that for end milling. 

It's a pity that metal lathes run so slow really, ( max 2500 rpm )as I would like to try different router bits just for the hell of it.:biggrin:

Later, I'll stick a pic or two in my album of the tool on its own ( sans lathe and distracting background ) and then it can be seen just how basic it is.

All I have to do now is find some time to play:biggrin:


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## VisExp (Dec 3, 2008)

Interesting how the comments on this thread have been predominantly from folks with a machinist background. 

I've read and studied this thread four times and finally am able to say:

"Very cool Skip, I understand how it works" :biggrin:


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## jhs494 (Dec 3, 2008)

Another great idea Skiprat! It looks simple and makes a nice looking part when completed. Job well done I'd say!
Thanks for sharing it with us!


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## Randy_ (Dec 4, 2008)

Hey Skippy:
 
I haven't had a chance to fully think through the geometry and the trigonometry yet; but my initial reaction is that the rotation of the pen blank in this configuration is sinusoidal rather than linear.  That is why you see less rotation at the 1 o'clock position (30°) than at the 3 o'clock position (90°).
 
If I am correct about this and forgetting about the rotation of the ramp for a minute, you won't ever get a perfectly linear rotation; but you will be able to come very close if you can change the geometry so that the arm movement is from 2 o'clock (60°) to 4 o'clock (120°)..........symmetrical around the 3 o'clock (90°) position.  From the pictures, that might be a little tricky to do; but you would be the better judge of that with the machine in front of you.   

I haven't quite wrapped my brain around the effect of rotating the ramp and don't quite see how it could help unless it gets the center of the arm movement closer to the 3 o'clock position?  Any possibility you could take another picture or two?  For some reason, I am not quite picturing exactly how the ramp works.  Ideally, one with the ramp parallel to the mandrel and the arm in its lowest position and then one with the ramp rotated to the position that gives you the best result and with the arm in its lowest position.  I don't know if the extra pictures will turn on a light for me; but it will give me a better understanding of exactly how the ramp is configured.

Thanks.


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## PenPal (Dec 4, 2008)

*Hi Skiprat*

I made a small Clisby Lathe function to make straight divisions of a 72 tooth gear I had fixed to the head stock using a small mating gear to hold the shaft in position. I sleeved the tail stock and lock the blank using a knurled nut spacers are Corian. The blank is fitted by slacking the tail stock.I include a pic of my modified lathe.

The reason for my reply you mentioned the slow speed problem, I bolt the unit to my mill drill XY table, fitted an outrigger on the quill to hold a finishing router and use a dovetail bit that has a handy flat on it at 32000 revs.

I am not very computer savvy but I did find the router very useful. It will be beyond my thinking to provide a means of twisting and I admire your concept.

Regards. Peter


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## PenPal (Dec 4, 2008)

*Hi Again Skiprat*

Just remembered I had taken pics of the mill set up. In the pics I was experimenting with a straight bit and by reversing the lathe on the bed I can leave the Router on the outrigger or remove the mill chuck.

Peter.


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## NewLondon88 (Dec 4, 2008)

VisExp said:


> I've read and studied this thread four times and finally am able to say:
> 
> "Very cool Skip, I understand how it works" :biggrin:



I'll be able to say that too. 
Someday.

(how old am I now?) :biggrin:


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## skiprat (Dec 4, 2008)

Hi Randy, LOL, I knew I could rely on you to ask difficult questions.:biggrin:

The ramp is fixed and does not move. As the mill is moved across the lathe bed by the cross slide, the weight of the arm rotates the mandrel as it (the arm ) slides down the ramp. That is the reason for the big heavy arm.
I was going to use a lighter, shorter one assisted by a spring.
In the first three pics, the ramp is parralel to the mandrel / mill.
In the second two, I have adjusted the position of the ramp but not the angle. With the ramp turned to this angle, the mark on the arm is resting on the ramp by the same radius from the mandrel whether it is at the top or bottom of the ramp. Half way down the ramp, the mark is off somewhat. This is why I figured that if the ramp was curved then it would be spot on. 
But hey, if I wanted perfection I'd buy a machine to do it for me:biggrin:
Below is another pic of the thing on it's own.

PS, pwhay, thats a cool machine you have there. Have you rigged it to doo spirals yet?


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## skiprat (Dec 4, 2008)

Ooops!!! Forgot the pic:redface:


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## Paul in OKC (Dec 4, 2008)

I got it! To make the thing rotate the same all the way thru you just need a curved slide! The distance from where the 'arm' touches the slide at the top and bottom of the stroke is longer than the distance at the middle of the stroke. Clear as mud? So, just calculate the radius needed, and walaa!


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## JLL (May 10, 2017)

Great


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