A Milling Problem

Split cane (bamboo) rod makers use a planing form to make the tapered triangular rod sections for gluing together to form a hexagon. The form consists of two 1" square steel bars about 5 feet long whose separation can be adjusted by screws. The top of the bars has a tapered 60degree groove in it, the groove gets deeper by 1 thou in every inch length.

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How would you go about milling this groove using a milling machine (vertical or horizontal) with a table travel of only 18".

cheers,

Rod

Hi Rod,
I think what I am about to suggest is a bit mad. First make two guides that will hold the bar down to the table but allow it to slide. Place one either side of the cutter. The guides could be ball races positioned either side and above the bar. Arrange support rollers either side of the mill to support the bar. Now here is the mad bit. make some way to fix some steel cable to the end of the bar. Work out what diameter circle would be required such that an inch of its circumference corresponds to 1 thou on the handle of the Z feed. Fasten a drum of this diameter to the Z feed and arrange pulleys etc so the steel cable can be wrapped round the drum. winding the drum will pull the bar across the table and the same time put on 1 thou of feed for every inch you pull the bar. Another way would be to arrange the movement of the bar to drive an optical encoder. Feed the pulses from this through a divider circuit to drive a stepper motor attached to the Z feed.

Les.

It's not much help, Rod ... but I have to say that the ground ones, on the site you linked, look magnificent.

MichaelG.

Rod, I think your problem would best be solved with a small jig that you could run down the bars by hand. I would arrange some method of varying the depth as it went - such as rollers on a screwed shaft so that they separated as it moved along (you may need a gear train to drive the thread at that taper?) or a jack screw linked in the same way? You could then sell beautiful looking bits of bar for a huge sum and retire the week after.

Mark

I'd space the bars apart by 120 thou at one end, perhaps with some intermediate spacers, and screw them to an equally long or longer base bar of accurate section using location dowels and countersunk screws into the underside of the bars (won't affect their use).

Now make some sort of guide jig on the mill table to allow you to align the base bar centrally under a 60-degree cutter and truly parallel with the long axis of the mill table. Fitting each bar in turn you can machine them in lengths of 18" at a time, unclamping and moving the base bar along to a new section.

Done well, any step between sections should be well under a thou and could be removed by hand finishing a short section.

Neil

Hi Neil,
A very nice bit of lateral thinking. I would have thought the spacing should be 69.3 thou. 60 thou x Tan 30 ( 0.5774 ) x 2

Les.

Thanks guys. I think Neil is pointing me in the right direction. The taper in depth is taken care of by separating the bars with a wedge shaped gap. This is how the planing form works anyway, there are pairs of screws every 5 inches, one of which acts to pull the bars together and the other as a stop screw to keep them apart. The trick will be to keep the milling cutter in the centre of that wedge. If I mount a pair of toolmaker like buttons on the milling table, the form can be pressed against them so that its centre line is on the X axis and clamp it down, all I then need to do is to move the Y axis so that the cutter centre is over the form centre line. After taking a cut I unclamp, slide it along, clamp and then re-centre the Y. I think that might work. I'll need to do some sums .

The alternative is to use a triangular file. I've done one side and a) I'm not particularly happy with the result and b) It's very hard work and causes blisters Having the tools, I feel there ought to be better way.

Cheers,

Rod

You're probably right Les.

Neil

Hi Neil.
The main thing is your novel approach to the problem. That way of doing it had not occurred to me. I'm sure someone else will comment on the trigonometry. One thing I would add is to also have some space at the end that was in contact and have the difference in the gaps whichever value turns out to be correct. This is to avoid cutting on the tip of the cutter.

Les.

Good point Les, thanks.

I think I need to look out for a 60degree included angle (double sided) cutter for my Sharp mill in horizontal mode.

Rod

Hi Rod & Neil,
Here is a suggestion that may help in positioning the bars. Clamp a length of bar to the milling table and align it so one end is offset by 10.4 thou from the true X axis over a length of 18" (This is if my trigonometry is correct ) Clamp the pair of bars against this bar such that it position the centre line between the two bars in alignment with the X axis. This bit I have not come up with a good solution unless the space between the bars allows an electric edge finder to fit between the bars to centre them. The idea now is to mill 18" as Neil suggests. Move them along 18" again clamping them against the guide bar and move the Y axis in or out 10.4 thou (Depending on which way you have moved the bars.) I am surprised no one else has replied on the trigonometry. A third opinion would help. I did not set out to challenge Neil's calculation. I could see straight away how it would work but at first I could not see how to calculate the spacing so thinking about it resulted in me doing the calculation.

Les.

I feel it might be easier to machine the 60deg groove parallel along the length and then machine the taper on the top face of the block.

A couple more things to think about before making swarf! The accuracy that you are trying to achieve seem to be threatened by the material you choose. It is quite possible that if you use bms, then this is likely to be outside your tolerance. Also,machining two five foot lengths of steel with one 4 or 6 flute cutter is asking a lot. It may be worth considering using a side and face type cutter. It would be really disappointing to achieve your goal, but then find that as the material was not stress-relieved, it has developed distortion across the section,(5 foot is a long way!). Hope this assists your planning,

Roy

Roy,

Thanks. All good points. I'll see if I can find a 1" bore 60 degree cutter at Harrogate; that would seem to be the best option. The saving grace is that the cut is not very deep so an endmill type cutter (or two) may last if I'm forced down that road. Split cane rod makers seem to be obsessive about their tapers, explaining that a thou here or there makes all the difference to the action of the rod. I'm a little bit sceptical, considering the variability of the material that they use, but I do fancy having a rod and reel that I've made for myself.

Thanks everybody, I'll let you know how I get on in due course.

Rod

Hi Rod,

Here's another idea. If you have a spindle shaper or similar, I'm thinking you could cut a sacrificial fence to the required taper on a bench saw and feed this fence along with your cane through a shaper of the correct profile.

Ed.

Hi Rod,
Ed's suggestion has made me think of another possible solution. Instead of machining the bars with a sloping chamfer you may be able to have the same chamfer all the way along the bars but in use have the bars with a tapered gap between them as in Neil's suggested machining method.

Les.

I was at work today, selecting some bar stock for a particular project that I am working on. I was looking at the bars end-on and, after browsing your device last night, another option presented itself, and one that I think is worth considering as it seems quite 'do-able' . Using the 25mm square section as your operating adjustment, select some hexagonal bar, (depending on size), and secure along the top of each square bar with recessed cap- head screws every150mm. With some imagination, mounted side screws will enable you to adjust the hex bar perfectly straight with a good finish and consistent depth. Hope this option is workable.

Roy

I believe the reason for the tapered slot rather than a constant depth is to provide better support for the cane strip along its whole length. In use, the 2 bars are set up to have a taper in the horizontal plane - there is enough flexibility in the bars for the taper to be varied along the length by a few thou through the use of the adjusting screws. The use of these forms is very well established in the rod making world, it is just that I would like to find a method of making them in the home workshop that doesn't involve too much pain. There are more mechanised methods for producing split cane rods but the artisan approach appeals to me (and my bank manager!)

anyway, thanks for your interest guys.

Rod

Hello Rod,

It's me again! If you were to put .006" shim under the second cap-head screw,.012" under the second and carry on until the last screw, you would achieve the desired 1 thou per inch taper. You would be able to machine the top of the hex bar parallel to the bottom of the 1" bar with your 18" travel clamping to the machine bed. Removing the shims and re-fastening directly to the bar would make the assembly more robust. If only I liked fishing, I'd love to make one!

Best of luck,Roy

Edited By Roy M on 28/03/2014 23:58:26

Roy,

Your use of hexagon bar is good with its built in 60 degree groove. Sadly, I already have the square bars. However, you have given me a further idea. One of the problems I've just noticed is that the winding handle on the X axis projects above the table level: This means that I will have to raise the bars on blocks to clear the handle. I can make these raising blocks differing heights to give the taper. The 2 form bars can be fastened together with parallel spacers, say 1/4", so that the cutter is not trying to cut on the point. After each move of the bar, then I just need to adjust the Z axis to touch the previous groove.

Between you and Mick, I think we've come up with the preferred solution for my situation.

Thank you all for your comments, they have stimulated and crystallised my various thoughts on this puzzle

Rod