rotary table issue

[tatlerParticipant @tatler]

help

I am trying to cut the spokes for a 1 gauge Green arrow driving wheels.

the wheel size is (old money values sorry) 2.530″ OD ,18 spokes ( so thats 20 degrees on the table) to finish at 0.055″ at the rim to 0.077″ at the hub.

At a loss on how to correctly machine the angle.

everything is centred and i can get the correct number of spaces but the angle has me stumped.

 

joe

[Clive FosterParticipant @clivefoster55965]

Basically you offset the rotary table centre relative to the cutter path to get the spoke width and tweak the actual angle setting to get the taper.

So using round numbers and assuming the cutter is of zero diameter to make it easier to see what’s going on. Think laser cutter I guess. If the spokes were parallel and 0.100″ wide you’d offset the RT centre by 0.050″ to one side and go round cutting one side of the 18 spokes. Move it over 0.100″ so the RT centre is 0.050″ offset the other way and cut the other side of the spoke.

As the spokes are tapered set the offset for the wide end and adjust the angle on the RT so the line of cut is deeper on the narrow end giving the desired taper.

In the real world you have to add a further offset of half the cutter diameter to get the cut line in the right place.

If your trigonometry and geometric visualisation is less than wonderful this is the sort of thing that is likely to net you a whisky bottle size hangover headache without the pleasure of emptying the bottle first.

Playing with CAD makes it all much easier.

Good idea to mock things up on paper with pencil or slender felt tip pen / sharpie in the spindle and a suitable backing board in the RT to support the paper. Draw out something simple, four spokes are plenty, and try to trace over it. When the penny drops after 3 or 4 goes you will wonder what the problem was. But until it does …

The trigonometry involved is fairly straightforward. I imagine Jason or one of the other CAD jockeys can easily show you that. My trig is well up to such things but I found getting the visualisation straight for the moves needed first time through tricky.

Clive

[Andrew CrowParticipant @andrewcrow91475]

The problem you have is that although your rotary table can achieve the spaces, your milling machine needs to have a universal table ie. one that can be set to the angle you require so that the longitudinal movement is at the required angle to the cutter. It can be achieved by calculating the angular offset required so that in your case you centre the rotary table the move the y axis by 11 thou plus .0385(half spoke thickness at the hub) to cut one side of the spoke then cut the other 17. After that reposition for the other side and repeat.

Hope that makes sense.

Andy

[JasonBModerator @jasonb]

You really need to give hub diameter and inside of the rim diameter

But angle wil be approx 0.7deg either way and offset 0.041″ plus half cutter diameter.

Bit of CAD soon gives the numbers but confirm those two diameters or exactly where those widths are taken and the cutter diameter being used and I’ll give it more accurately.

No need for fancy machines, a basic rotary table can do it as that is all I have used in the past

[Andrew CrowParticipant @andrewcrow91475]

Actually I think Clive has got the calculation correct please ignore my drivel. However have you tried Walsall Model Industries they may have some suitable castings off the shelf. Phone number 01922 633718

Andy

[JasonBModerator @jasonb]

Playing with what the ID of the rim may be and how far towards the hub you can get with a 1/16″ dia cutter this is what you will likely end up with. Angle is between 0.6 and 0.7 deg so I would be inclined to use 40mins as it’s an easy number to read off the R/T. But give the numbers I asked for and you can have it exact but will have to wait until the morning.

[Clive FosterParticipant @clivefoster55965]

Andy

If you are following a drawing with all the angles set out as per good industrial practice it is indeed much easier with a universal table that can be given angular offset. Basically set up for parallel spokes then put the taper on using the universal table.

Not much difference when working from scratch tho’. A universal table has its own issues too.

Initial visualisation of what the offset and angle changes do on the real part is the first timers problem until things click.

Wonder how well one of those baby laser pointers would do instead of a pencil or felt pen if using the trace a drawing method to verify your settings.

Clive

PS Nice drawing Jason.

[duncan webster 1Participant @duncanwebster1]

Driving wheels have an oval shaped hub to fit the crankpin. Trying to cut them from solid without CNC is not something I would undertake

[JasonBModerator @jasonb]

As I said no need for universal tables, just put your brain into gear.

Bit of trig or CAD will give the taper angle, Rotate rotary table to that angle, offset according to cutter, cut. Then assuming it is a 90:1 R/T just 5 full turns and cut the next and so on.

At this scale one only needs scribe any hub shape or counterbalance at the rims and then stop the cuttes at the lines. Yes CNC will make it easy but best to have the underlying ability to do it manually. In pravctivce looking at the image I posted the cutter diameter leaves a far larger diameter hub than scale so you can just stick the driving pin where you like.

 

If the OP comes back with the dimensions or better still posts an image of the wheel drawing then I will show how the angle and offset can be worked out but not worth going into detail with assumed sizes.

 

As for the Nice drawing I worked out a hub dia of 0.441″ and assumed an inner rim dia of 2.25″. Until teh OP chips in I’ll leave you to work out how I got the 0.441″

[duncan webster 1]

On duncan webster 1 Said:

Driving wheels have an oval shaped hub to fit the crankpin.

Are all spokes the same (i.e. the same width if projected onto a circular boss at the centre)?

If so, the complication the oval hub adds is that you need to stop the cut at a variable distance from the centre for each spoke gap. That is only 18 more numbers needed to write down (maybe 9 because of symmetry).

If the outer width and inner width are constant, but the length of spoke changes, then the ‘draft angle’ of each spoke varies, the machining is then a lot more difficult and a per-spoke (be-spoke?) calculation is required.

—

Should we mention for clarity that with the offset method, you might do all the left sides first (indexing by 20 degrees to move to the next), then offset in the other direction and do all the right sides? So you are effectively going through 720 degrees of rotation.

From a practical point of view, using the 40 minutes number above (be very sure in your own mind whether that is the included or half-angle), I am trying to picture a way that would minimise chance of error. Do all RT’s have a re-settable dial on them? If not, it might be useful to set the centre of the first spoke to 359 degrees 20 minutes on the RT scale. Then for the first rotation you are lining the RT scale pointer up on 20, 40 degrees.

However, that does not help with the other side. Maybe making some temporary dial markings (in two different colours) would assist. A dry run with a sharpie as the ‘cutter’ would allow you to refine the technique.

[JasonBModerator @jasonb]

The cutter dia linitation leaves an excessively large overscale hub so there may be no need for a lobed hub, just put the pin in the overlarge area.

As for resetting hand wheels, just put a sharpie mark at zero once you have moved the table the initial angle. 5 turns to get you back to the mark.

 

Why would draft angle change? that is related to being able to pull the pattern from the sand so on a different plane to the spoke angles. In this scale I doubt very much if any draft will be included

[JasonBModerator @jasonb]

For th esize being dealt with it would not be hard to produce something like this with just a basic rotary table and mill. 1 setup for the spokes and a couple more simple operations to do the hub and integral balance weight.

I based it on the cast wheels of a larger scale model but with the machined spoke area little more than 2″ across is any more detail needed?

2 1/2 inch gauge LNER 2-6-2 "Green Arrow"

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