Milling Curves

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Milling Curves

This topic has 10 replies, 6 voices, and was last updated 15 November 2020 at 21:30 by John P.

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15 November 2020 at 16:47 #507670
Nicholas Hill
Participant
@nicholashill23289
Hello, I have been trying to mill a curved surface, and have found myself in a total pickle!

I am using a Myford with a vertical rotating slide. The concept being that by rotating the slide the curve will be milled.

But…my alignment is all wrong..and I don’t know why.

I have lined everything up with the centre of the spindle – using a scribing point in the chuck, but it keeps producing off centre curves. My only adjustment is the vertical axis of the milling slide – to change the radius.

The photos are of the latest set-up and if I trace the arc…you are talking a variance of 1/8 inch on a surface that is about ½ inch wide!

I assume it is all about geometry..but I’m baffled. If a point lines up with a centre, how can it produce off centre radius??

I am now on my 5^th attempt…and it is not improving..

Any help, would be superb…I’m tearing my hair out..am I fundamentally wrong in the set-up?

PS Its for a mould for a DH88 Comet cockpit section.

Many thanks in advance,

Nicholas
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15 November 2020 at 16:47 #10534
Nicholas Hill
Participant
@nicholashill23289
15 November 2020 at 16:55 #507672
Dave Halford
Participant
@davehalford22513
The arc you need to follow is that of the milling cutter edge not the centre of the cutter. So you are off by the cutter radius.
15 November 2020 at 16:57 #507674
Grindstone Cowboy
Participant
@grindstonecowboy
Dave got in before I could login to reply – yes, agreed, it's the radius of the cutter that's throwing it out.

Rob
15 November 2020 at 17:00 #507675
John P
Participant
@johnp77052
Hi

The relationship between the cutter and the pivot point ie the rotation of the vertical slide is not changed by changing the position of the vertical slide position you are just feeding the work in at the same radius. You would need to make the adjustments to the radius by moving the cross slide.

John
15 November 2020 at 17:39 #507681
Martin Kyte
Participant
@martinkyte99762
Turn a cylinder of the approprate diameter. Slice it in half and stick it to a correct sized rectangular block. Stop faffing about with vertical slides.

regards Martin

PS I think I would have got the woodworking tools out and planed the half round, finishing off with sandpaper.
15 November 2020 at 17:49 #507683
Dave Halford
Participant
@davehalford22513
I must confess I would have whittled it as well, only in MDF which avoids all the grain issues.
15 November 2020 at 18:52 #507696
Nicholas Hill
Participant
@nicholashill23289
Many thanks for the replies.

I appreciate its a bit of a hammer to crack an egg, but I guess the idea was to try milling curves, and as it was in wood, it seemed an ideal trial piece. But..in the three hours or so spent on it…it is not the best choice! So…although this is a very simple example, it is kind of like me learning to tie my laces, before becoming a marathon runner.

After reading the other replies…I am now more confused than ever..So have tried to sketch things out.

I get that the diameter of the cutter is larger than the scribing point..but surely they would have the same centre? So although it wouldn’t be the correct size, it would surely still be parallel? As in it would make a 7/8 inch diameter instead of a 1 inch? I am lining up the diameter to be cut by eye, to the outside diameter of the cutter. The scribing point was purely to ensure things lined up.

Again on the vertical slide, as you offset, that surely just changes the diameter? So if when central everything lines up, if you lower it by an inch, it would create inch radii’s?

My rather poor sketch..with fig 1 set up so everything is in line. Fig 2 being with the verticle slide offset. With in both cases the horizontal axis being the slides axis of rotation.

So to my sketch everything seems to work….but clearly I am wrong! And looking at the logic of the cutter diameter, that seems the answer, but I am missing a stage in the logic…

Thanks again,

Nicholas
15 November 2020 at 19:40 #507703
Clive Brown 1
Participant
@clivebrown1
Am I understanding your operation correctly?

The curve that's generated is controlled by the distance of the pivot bolt on the slide from the cutter periphery.

This in turn is controlled by the position of the wood as held in the vice and the position of the vice in the slide tee-slots.

Moving either the lathe cross-slide or the vertical slide only serves to bring the workpiece into contact with the cutter, it does not affect the curve that is generated.

Edited By Clive Brown 1 on 15/11/2020 19:56:57
15 November 2020 at 20:14 #507709
Dave Halford
Participant
@davehalford22513
I'll paraphrase Clive.

If you place a bog roll where the slide is, why should lifting the bog up or down change the radius of the bog roll it just brings a different part of the roll into contact with the cutter.

Like wise moving the bog roll side ways does not change it's radius, it merely changes the angle of the cutter makes with the bog roll because the pivot point does not change.

I'll let you work out how to reduce the radius now, it pretty much happens every day in most homes.

Edited By Dave Halford on 15/11/2020 20:15:07
15 November 2020 at 21:30 #507725
John P
Participant
@johnp77052
The Myford vertical slide when fitted on the the cross slide unfortunately
does not have the axis of the spindle and the axis of the slide bracket
in alignment ,as it appears you are leaving the fixing screws slack
and are rotating the slide to generate the radius.As can be seen from the
sketch the axis are misaligned by about .330 inch,moving the work set
in the vice on the slide adjusts the position with respect to the axis of
rotation of the slide and therefore the potential radius that could be cut
on the work piece but not the radius that is cut. You can not as described
align the centres of the work and the cutter because of the misalignment
of these two things, the radius that you wish to make is the distance from
the edge of the cutter from the axis of the vertical slide bracket.

As it is if you fitted a 1/4 inch cutter the minimum diameter that you could make

would be about .410 inch.

John
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