Adjustable device for describing a low angle helix along a shaft

[John McNamaraParticipant @johnmcnamara74883]

Hi

A while back I designed this device with regrinding cutters in mind.

It is fairly simple to make if you have a basic lathe alone but easier if you have a small mill as well.

The CAD sketch below is not a tested working model however it describes the Movements required.

The bottom shaft Could be reduced to a pin if the carriage was supported on a short linear rail and bearing (I think a better solution).
The upper shaft is splined, and is free to rotate in not shown bearings.
The drum with the red wire wrapped around it is spline connected to the rotating shaft. and runs in bearings. The drum at the bottom is free running.

The carriage travel must be a little more than the maximum helix length you require on say a long reamer or milling cutter.

Note the red wire representation both ends are fixed to the carriage.

The key to understanding the to the operation of this mechanism is to first think on how it would work if the Blue slider bar was set parallel to the rotating shaft and the carriage was moved side to side. Nothing! The rotating shaft would stay fixed.

Now consider what will happen if the blue slider bar is set at an angle positive or negative in relation to the rotating shaft. The rotating shaft would rotate slightly creating a CW or CCW helix.

The tensioned wire will be almost backlash free.

TO DO's
TO DO The block that runs on the slider bar Needs to rotate slightly.
TO DO Consider replacing the slider bar with a linear bearing.
TO DO Evaluate the sources of error and correct them as required

Its on my bucket list to build this when the new shed built

Cheers

[John McNamaraParticipant @johnmcnamara74883]

Low backlash.

[Les RileyParticipant @lesriley75593]

I have thought about such things for regrinding cutter flutes.

I came to the conclusion that it would be best done with stepper motors and a CNC control.

However, until I get time to do more, I have bought an air bearing and will do it the old fashioned way!

Les

[Bill Davies 2Participant @billdavies2]

In industry, using cutter grinders, we used a spring-loaded finger against the flute, with the cutter held in a collet in a freely rotating head. I think it would be difficult to work out the helix angle of the cutter.

Bill

[bernard towersParticipant @bernardtowers37738]

Im with Bill on this don't make it any more complicated than it needs to be and I have wound a couple of turns of string round the shank of the cutter with a small weight to load the cutter flutes to the follow finger so as you travel the cutter back and forth it stays engaged with the finger.

[DC31kParticipant @dc31k]

In principle, it has similarity to either a planer spiral attachment (see for example Cincinnatti 'Treatise on planers&#39 or a leadscrew pitch compensating device (in that application the blue rod would be 'wavy' not straight).

What does the coiled wire add that is missing from using a simple drive dog on the rotating shaft, with its free end resting on the inclined rod? Gravity takes care of the backlash.

Maybe someone could comment or confirm: with the 'dog on tilted bar' setup used on a planer, I think there is inbuilt cosine error as the distance from rotation centre to point of contact on the tilted bar varies.

[John McNamaraParticipant @johnmcnamara74883]

Hi All
Thank you for the feedback

I thought about the drive dog on a rotating shaft concept I discarded the idea because the helix would be non linear The radius from the centreline of the rotating shaft to the contact point of the dog touching the inclined plane would change. It would have very limited travel.

I also wanted to be able to rotate the shaft more than a few microns as is needed in a lead screw compensating device. as discussed. I have not seen the Cincinnati treatise on planers, will try to find a copy. does it use gearing?

Apart from a CNC approach, gearing is an obvious choice, However that would require a large time and money investment in gears and a gearbox.

The finger resting on the flute of an existing cutter works fine if you are resharpening. But what if you want to make a new tool from scratch?

A precision cable drive can be made linear in this case 1 to 1, its the angled slide that does the work, with almost no backlash. Under tension it can be made quite stiff. It does not require an existing profile as is used with a tool and cutter grinder. Cable drives are not new A google search will reveal many.

I am sure there ar numerous approaches to this design problem, do any others come to mind?

[Jeff DaymanParticipant @jeffdayman43397]

Look up "rifling bench" for gun making. No need to waste time to re-invent the wheel.

[John PParticipant @johnp77052]

This is one of these type of mechanisms that will do what you want

https://patents.google.com/patent/US2503926A/en

The cnc route is by far the easiest the drawings for my own
machine can be found in at

https://groups.io/g/Quorn-Owners

including an improved text from that which was published
in MEW around 2008 .Shows how to grind tapered endmills
and methods to check the clearance angles.

The only thing that you never need to know when doing this is the
helix angle of the flutes /cutter.

John

[duncan webster 1Participant @duncanwebster1]

If you had a double ended mandrel you could have the blank in one end and another cutter in the other with a tooth rest, then the spiral on the new copies the old. For milling cutters I can't see that the actual spiral angle matters all that much,so no need for infinite variation.

[John McNamaraParticipant @johnmcnamara74883]

Hi Duncan

I wanted infinite variation to enable creating the desired helix angle from scratch. Not only for grinding but milling as well.

I had a Quick look at john P's Patent. It appears it uses rollers on a hardened shaft. I looks like a friction drive? I could not see a positive engagement of the rollers on the shaft? There are commercially available bearing blocks that utilise that principle. Sometimes used in material handling equipment.

I realised that a better arrangement for terminating the wires would be to put them on one pulley not two as shown One over and one under as shown but on the same axis.
And the slider block itself should should pivot on the same axis not in a slot as shown. That eliminates another source of error.

Could this mechanism enable helical gears to be cut with a profile cutter in a small vertical mill?

Mechanisms are fascinating.

[John PParticipant @johnp77052]

Hi John,
This video here uses the same mechanism as in the patent.

John

[John McNamaraParticipant @johnmcnamara74883]

Thanks John P.
Interesting device. The second video he posted shows that it works for both the positive and negative rotation.
There must be a fairly high pressure on the rollers'(s?)s to almost eliminate slipping.

https://www.youtube.com/watch?v=Em_usedaNd0

Edited By John McNamara on 22/01/2023 11:22:47

[Bruce VoelkerdingParticipant @brucevoelkerding91659]

John,

a few decades (?) ago I saw an Advertisement for a "Lead Screw" which relied solely on the concept of 3 Friction Wheels pressed against a hardened "Lead Screw". The "Lead Screw" had no Grooves – it was an exact Cylinder. I can not find a reference to it on the Web now.

What troubles me is with your Design – doesn't your Design rely on Friction between the Wire and the Top Drum to create the Helix (and resist the Milling Cutter's Load ?).

[John McNamaraParticipant @johnmcnamara74883]

Hi Bruce,

Apologies: I was a bit lazy making My CAD sketch, I really should have tidied it up before posting it. The wire cable needs to be clamped to the top drum. in the middle of the turns. This creates a positive engagement.

TODO consider the number of turns required? probably not many, the length of travel and the number of degrees of rotation will need to be evaluated.
If Wire rope is used there is a formula that determines the minimum diameter sheave that can be used for a given wire rope diameter to avoid stress failure. To increase the stiffness of the drive several wires can be run in parallel.

[Graham MeekParticipant @grahammeek88282]

Some years back someone at the Midlands Exhibition had a Quorn or similar with such a variable Helix device set-up for grinding endmills.

Unfortunately I did not take any photographs of this but someone might have done.

From what I can remember it was a very simple and elegant design.

Regards

Gray,

Edited By Graham Meek on 22/01/2023 13:43:17

[Bruce VoelkerdingParticipant @brucevoelkerding91659]

John,

now the ugly (as used in U.S. Design Dept vernacular) part – how do you eliminate the play in the Spline ? This is just a Thought that I have not thought thru thoroughly –

o eliminate the Spline and attach the Wire directly to the Upper Spindle (I assume this is the Work Holding Spindle).

o the Upper Spindle could have a machined Wire Drum.

o the Upper Spindle can not move endwise as that would negate a Tailstock.

o attach the Green Bar to the Carriage holding the Milling Cutter Spindle.

o use a Gear Train from the Spindle to a Lead Screw to move the Wire Carriage the pitch of the Spindle Wire Drum.

[Bruce VoelkerdingParticipant @brucevoelkerding91659]

I worked for General Electric Lighting and had a lot of fun designing mechanical solutions to Lamp Manufacturing Processes. Every Co-Op wanted to do things with Servo Motors, but one quickly realizes in the rough & tough World of Manufacturing that they take a far second place if a purely mechanical Device can be built.

We had mechanical Devices on some Operations that were designed in the 1920's ! All they required were a bit of Lubrication and an occasional new Cam or Cam Roller. A big plus is there is no need to "Home" these devices. Our Operators were of the "Push the Button & Go" type.

[Roger BParticipant @rogerb61624]

Posted by Bruce Voelkerding on 22/01/2023 12:04:56:

John,

a few decades (?) ago I saw an Advertisement for a "Lead Screw" which relied solely on the concept of 3 Friction Wheels pressed against a hardened "Lead Screw". The "Lead Screw" had no Grooves – it was an exact Cylinder. I can not find a reference to it on the Web now.

What troubles me is with your Design – doesn't your Design rely on Friction between the Wire and the Top Drum to create the Helix (and resist the Milling Cutter's Load ?).

I think this is the rolling ring drive you are refering to:

Uhing® – Rolling ring drive: principle, advantages, product catalogue

[John McNamaraParticipant @johnmcnamara74883]

Hi Bruce Voelkerding
To your dot points

o eliminate the Spline and attach the Wire directly to the Upper Spindle (I assume this is the Work Holding Spindle).
Eliminating the spline can be done if the motion is driven at another point in the mechanism.

o the Upper Spindle could have a machined Wire Drum.
Agree and the wire must be attached in the middle of the turns.

o the Upper Spindle can not move endwise as that would negate a Tail stock.
Agree.

o attach the Green Bar to the Carriage holding the Milling Cutter Spindle.
Attach the green bar to the sliding machine table support of the machine (Grinder or mill).

Mount the helix generating device on the work table of the grinder or mill

The green bar mounted on helix generating device, needs to be on its own slide. that runs parallel to the upper work spindle axis. A short piece of linear bearing and a couple of linear bearing slides sources from you know where can be had at low cost. This gets over the spline accuracy problem.

o use a Gear Train from the Spindle to a Lead Screw to move the Wire Carriage the pitch of the Spindle Wire Drum.
I don't think this is necessary.
The motion of the green bar as it slides along will force the work spindle to rotate.
————————————-

How to make it? what will it cost? Travel say 250mm?
A straight shank ER collet chuck for the upper work holding spindle $20-40

HG or EG Series linear bearing set's
For the slide that supports the green angle slide. say 450mm. HG or EG Series Ebay around $35
For the green angle slide. say 300mm HG or EG Series Ebay around $30

Ball bearings or brass bushes for the spindle.
The rest steel mainly from the scrap box.
Lathe and milling work.
——————————

Hi Roger B
I was aware of the Ring Drive
And I also remember a drive being advertised that used external ball bearings as did you.

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