Author Topic: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft  (Read 3685 times)

Offline Stuart

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Sorry for the long title

I am considering a new build and it needs a helix on the crankshaft 1/4 turn

I have the cnc mill and 4th axis is how to do the code


Do I move the x then the a ? As descrete steps

Say 0.1 mm y and a 0.1 degrees ,the repeat until a = 90

Now the above figures except 90 degrees are ball park

Note this will be coded in mach3

Thanks for looking
Stuart
« Last Edit: February 25, 2016, 11:35:16 AM by Stuart »
My aim is for a accurate part with a good finish

Offline jadge

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #1 on: February 25, 2016, 11:24:09 AM »
Been there, done that! It's simple in theory, but tricky in practise. Here's a picture of a worm being machined, using hand written code:



There's no need to take small incremental steps in X and A. For G01 any combination of X, Y and Z values results in the appropriate move in Cartesian co-ordinates with any feedrate value interpreted as mm/min. For G01 with only A any feedrate is interpreted as degrees/min. However, for G01 with some combination of X, Y, Z and A how is the feedrate interpreted? According the specifications it is interpreted as mm/min, even though the toolpath is a helix. So the basic code looks like:

G01 X10 A90 F100

Unfortunately this is where Mach3, or at least the version used by Tormach, falls in a heap. Obviously the feedrate needs to vary with work diameter. The Mach3 screens are confusing on this, talking about 'radius compensation' values. Basically they're nonsense, I never made any sense of them. Mach3 also totally ignored feedrate values in mm/min.

Ulitimately I used G93, inverse time feedrate, for feedrates. Inverse time feedrates are rather bizarre, in that the number given is the inverse of the time taken for the cut in minutes. So a value of 1 means the cut, whatever it might be, will take 1 minute, but a value of 0.2 means that the cut will take 5 minutes. Note also that feedrates using G93 are not modal, so you need an explicit F value on every G01 line.

There's a detailed description of the whole sorry saga of feedrates here:

http://www.model-engineer.co.uk/forums/postings.asp?th=88040

Andrew

« Last Edit: August 27, 2017, 11:17:06 AM by jadge »

Offline Stuart

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #2 on: February 25, 2016, 11:29:03 AM »
Thanks for the info Andrew

As you can gather I am still in the baby steps with gcode, give me c++ or swift I am ok
But this gcode is like going back to asembler

Stuart
My aim is for a accurate part with a good finish

Offline kvom

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #3 on: February 25, 2016, 12:52:32 PM »
In my experience Mach3 coordinates the rotational feedrate to match the linear feedrate in a coordinated move.

In this case the X  move is 1/4 the pitch and the A move is 90 degrees.

Offline jadge

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #4 on: February 25, 2016, 01:05:14 PM »
In my experience Mach3 coordinates the rotational feedrate to match the linear feedrate in a coordinated move.

Quite so; in my experiments Mach3 always co-ordinated the X and A movements, but the rate at which the tool moved, and hence the chip load, bore no linear relationship to the F value supplied.  :(

Andrew

Offline Stuart

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #5 on: February 25, 2016, 01:22:07 PM »
Thanks lads

I think a test piece is in order.

The build in question is a Fenby Corliss and the helix is for the goveneror to alter the valve timing ( i think )

The build is in the planning stage I have located the drawings and castings for it , but as is my way I look at the most difficult part work that out then decide


The code looks dare I say simple it's the way mach3 does its stuff

Stuart
My aim is for a accurate part with a good finish

Offline Jo

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #6 on: February 25, 2016, 01:28:08 PM »
The build in question is a Fenby Corliss and the helix is for the goveneror to alter the valve timing ( i think )

I warned you the governor was interesting  :LittleDevil:

Jo
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Offline Jasonb

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #7 on: February 25, 2016, 01:32:11 PM »
How did Anthony do it as I don't think he has CNC?

I would think it could be done with small movements of a rotary table and equally small movements of the mill then a lick with a fine needle file.

J

Offline gbritnell

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #8 on: February 25, 2016, 01:51:43 PM »
I would think in the 'old days' a universal dividing head would have been used. This is a dividing head geared to the X leadscrew of the mill so when the crank on the dividing head is turned through a series of calculated gears the leadscrew advances the table thus gutting a helix.
gbritnell
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Offline Stuart

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #9 on: February 25, 2016, 02:58:15 PM »
How did Anthony do it as I don't think he has CNC?

I would think it could be done with small movements of a rotary table and equally small movements of the mill then a lick with a fine needle file.

J

He tells all in the EIM article
He made a tube then used masking tape , then a hacksaw , then Swiss files and then loctite it to a smaller shaft along with some sleeves turned the whole lot down to size .

Jo indeed you did you naughty girl putting temptation before me , be you batted your eyelashes at the same time the QRK is 260 according to Jayne

Stuart
My aim is for a accurate part with a good finish

Offline jadge

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #10 on: February 25, 2016, 04:14:47 PM »
Stuart: It's not so much Mach3 as the G-code specification, and yes the code is remarkably simple for what it is doing. The whole program for my worm is based on the quoted single G01 command with slightly varying numbers.

Gbritnall is correct that a universal dividing head and mill would have been used in days gone by. However, it is normal for the table powerfeed to drive the dividing head rather than vice-versa. That said there is a potential problem with short leads. On my setup the mill table leadscrew is 5mm pitch and the dividing head is 40:1. Suppose we need a 10mm pitch helix; that means that the gearing has to increase speed by 20 times, just for it to be decreased by 40 going through the worm and worm wheel. Generally that's not good for the gears. Under those circumstances it might be better to turn the dividing head crank. In the commercial machine shop the problem can be avoided by using a 'short lead' attachment for the dividing head. It sounds fancy, and probably has a price tag to match. But all it does is drive the dividing head spindle directly, bypassing the worm and wheel. I don't have said attachment for my setup, but if I ever need one it is simple to make. It's only a shaft that fits in the end of the spindle and a plate bolted to the mill table to support the other end and then gears to the table leadscrew as before.

Andrew

Offline Stuart

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #11 on: February 25, 2016, 05:16:26 PM »
Ok  to put some numbers to the project


Shaft dia 18mm helix length 37mm 90degrees



So as you see it'd not massive

Stuart
My aim is for a accurate part with a good finish

Offline Roger B

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #12 on: February 25, 2016, 05:17:47 PM »
If I was faced with this I would see if the pitch was possible on my lathe and then use a milling cutter mounted in my Proxxon on the toolpost whilst turning the lathe by hand.
Best regards

Roger

Offline Stuart

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #13 on: February 25, 2016, 05:29:09 PM »
Roger
The max I could do safely is 8 TPI else the gearbox would be under strain if I could it would need to be driven by the feed screw as described in making a quorn

I have a cnc mill with 4th axis so why not use it , will need it for the cams anyway

Thanks for the comment as we all know there are many ways to solve the same problem they did it in the days gone by with the tools they had


Stuart
My aim is for a accurate part with a good finish

Offline cfellows

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Re: Advice please. Hints tips , how to gcode a 1/4 turn helix on a shaft
« Reply #14 on: February 27, 2016, 04:06:27 PM »
Maybe I'm missing something but this seems like a simple, one line g-code command.

G01 X37 A90 F50

That command will move the X-axis 37 millimeters and simultaneously rotate the shaft 90 degrees.  This assumes you have mach3 setup with the 4th axis in degrees where A360 turns the shaft one full revolution.  The F50 specifies a feed rate of 50 millimeters per minute for the fastest moving axis.

Chuck
So many projects, so little time...