Quorn Tool and Cutter grinder

I am building the Hemingway Quorn Tool and Cutter grinder. A 5/16" x 30 tpi tap and die is needed for the Differential Nut on the Rocking Lever. Has anyone bought these for this job and no longer needs them, and can I buy them from you ?

Michael Malleson.

My rocking lever is tapped 5/16" x 32 TPI as per DHC's book.

Can you elaborate?

The Hemmingway Quorn is an updated and “improved “ version of Prof Chaddocks original, the differential nut is one of these improvements, nothing wrong with the old 32tpi version. Suggest you buy the Quorn book and then you will see what other improvements have been made. Don’t Hemmingway offer the taps and dies under their Proprietary parts listing?

Just checked Hemmingway site and the 30tpi tap and die are listed for sale at around £30 the pair.

I'd jib at £30 for one-off use. The Quorn requires a few single-point screwcutting ops. This could be another.  Alternatively a silver-steel tap could be machined.. The thread is for positioning, not load-bearing, so a truncated thread might help smooth operation and also make for easier machining.

Edited By Clive Brown 1 on 06/05/2023 14:13:38

I can't see this change to 30TPI as an improvement, to me it just seems like a way of generating sales for the special tap and die required.

Mine is tapped 40 TPI and has division marks for precise amounts of adjustment.

Emgee

The OP is making a differental thread, which seems to be a Hemingway alteration, but I agree that the original 40tpi seems to function adequately, unless one wants to work to tenths of a thou. I suppose.

The OP has the benefit of the Hemingway drawings and instructions and could no doubt explain the thinking behind this modification.

It would be nice to know what the other thread of the differential is.
If the rocking lever is the bit I think it is I thought it originally had no scale or index as it wasn't intended to give a specific adjustment just the 'tad' or 'smidgen' we have previously discussed on here,

Thanks for all the helpful replies. The Hemingway version for the rocking levers has two concentric screws making up a differential screw arrangement of 30:32 ratio. When holding the screw, 30 tpi, and turning the nut, 32 tpi, increments of 0.0001" can be made. Hence the need for 30 tpi taps and dies. However, the original Prof.Chaddock arrangement is for a single 32 tpi thread with a locking nut.

So I suppose the choice is mine, with Clive Brown 1's 40 tpi being another option. It all depends on the accuracy I want to achieve.

Michael.

A screw thread is a linear device. I am puzzled by your use of a ratio in conjunction with its operation.

Its name, differential screw, describes its operation. 'Differential' implies 'difference', an additive or subtractive operation. It is not called a quotiential screw, implying a multiplicative or divisive operation.

If you do the maths, 1/30 differs by 1/32 by 1/480, so you are effectively making a 480 tpi thread on the adjuster.

You might ask Hemingway's engineering department what divisions you would need on a dial to convert that 480 tpi into a 1/10000" increment since 1/480 th of an inch is not an integer number of ten thousandths.

The closest integer is 21, and that is hard work to keep track of on a dial. If the dial happens to have 20 divisions, you will be roughly 5% in error every full turn.

The original Quorn design has been around for 40+ years and I have never read or heard any complaints about the 32tpi rocker adjusting screw as originally drawn. The only alteration I would make to the original design is to make the bed bars a couple of inches longer, reason being that if the felt wipers are fitted you can find travel is a bit limited. I fully realise that they were made the size they are because of the standard length of a bar of silver steel is 13 inches but it is also available in metre lengths so it’s easy to just add a couple of inches to the standard sizes.

I'm having difficulty understanding how a differential screw works on the rocking arm. The screw on mine just bears on the rear bar, and the female thread is on the "horn" that clamps to the workhead. I can't really visualise how one can have a differential screw in this application - usually one has a screw that advances through one member with one pitch while screwing into another member, not fixed to the first, with a slightly smaller pitch. Then the effective movement of the second per turn is given by the difference between the pitches. Can anyone post a picture of the arrangement please? I seem to recall seeing a Quorn that was fitted with a standard micrometer thimble on the rocker.

On the other hand it would be easy to have a diff screw on the front bar micrometer, and perhaps more useful.

Well, being non-mathematical I can only go by what is provided in the drawing, both Hemingway and Prof. Chaddocks original, so when I come to do the job I will decide to what accuracy I want to work. I'm not building complex aero engines needing miron level tolerances.

Thanks for all the comments,

Mike.

I used metric equivalents which give metric increments. Can't remember the exact sizes but happy to find out for you if you are interested.

I use a dial indicator bearing on the tool holder to give a readout of movement, rather than scales on the screw head which is much more convenient as well.

To be honest the differential nut arrangement is rarely actually used......easier to just turn the direct nut a tiny amount.

I'm starting to get the picture and it is a potentially useful concept (for something else).
Not using whatever actual diameters used in the Quorn imagine a 32tpi screw going through a suitable bush which itself has a thread of 30 tpi on its outside that goes into the arm.
So if you rotate the screw one turn clockwise and the bush one turn anticlockwise you go forward 1/32 then back 1/30. The difference is (32-30)/(32x30) or 1/480 as given above by DC31k.
Unless you are an anally retentive rivet counter that in this application is near enough to 1 thou.

Since the 30 tpi is a special I am interested in this concept for making my own 31tpi tap instead of 30tpi. Then the equation becomes (32-31)/(32x31) which is 1/992. Provided the screws are accurate that provides something that could be used for very fine control.

To make use easier the two screws could be turned by a gear system simultaneously.

Adjustment to a tenth of a thou or so seems like overkill? Would the spindle bearings, collets and other components of the machine be equal to that in precision? And when would you use such precision in the sharpening of hobby milling cutters etc? Presumably someone came up with the need and so invented the mod to the original design. What was that need? Or was it just overzealousness?

I sat for countless days at a T&C grinder as an apprentice sharpening milling cutters for the toolroom machine shop use and don't remember having a need for that level of precision. (Most days I was so hung over they were lucky to be within a thou, let alone a tenth! )

But if I did want precision control in such a circucmstance, I reckon a 40TPI thread with a large diameter micrometer drum would give enough precision with a lot less faff. Plenty of one-tenth micrometers use it.

Edited By Hopper on 08/05/2023 02:36:55

Edited By Hopper on 08/05/2023 02:37:39

I looked at it slightly differently, thinking the change is for ease of use rather than accuracy:

• A 32tpi thread gives a pitch of 31 thou, so a ⅛ turn of the screw gives a movement of about 4 thou. A bit coarse I'd say.
• 40tpi gives a pitch of 25 thou, so a ⅛ turn gives a movement of about 3 thou. The adjustment is a shade more sensitive, at the cost of a rather fine thread, prone to damage etc, but not impractical. A 60tpi thread feels like a step too far.
• Bazyle's 32/30tpi differential example has an effective pitch of 2 thou, where a ⅛ turn gives a movement of about 2.6 tenths. This is a worthwhile increase in sensitivity, making it easier for clumsy paws to get close to a desired setting. Both threads in the differential are robust, but the builder has to make one, expensive if a tap and die have to be bought specially. It may not be worth the effort.

Another way of improving sensitivity is to increase the diameter of the screw head so that ¹⁄₁₆ or finer movements become easier to do, assuming the thread is well-made enough! Or with a gear, like a 90:1 rotary table.

Dave

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As an aside to this topic, I found it helpful to fit a hard plastic tip to the adjusting screw which thereby rides more smoothly on the back bar when traversing the tool holder.

Doh. silly ne. too much wine at our street party