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These threads, which for many years gave rise to much speculation as to how they were formed, have at last been found to be integral parts of the "Straasburg inch", which is 1.0511 as compared with the English inch. The lathe-nose thread of 9.45 is therefore only 9 in the Strassburg inch.
The following screw-cutting wheels will cut the 9.45 thread
105/100 x 10/90 or 21/20 x 10/90
The 6 Holtzapffel threads are:
|
Number |
1 |
2 |
3 |
4 |
5 |
6 |
|
English |
9.45 |
13.09 |
16.5 |
19.89 |
25.71 |
36.1 |
|
Strassburg |
9 |
12.5 |
15.75 |
19 |
24.48 |
34.5 |
The wheels 21 and 20 will convert the English lead-screw of 10 t.p.i, as fitted to Holtzapffel slide-rests into the Strassburg inch. To out the Holtzapffel threads calculate for threads in the lower column without regard to the 21 and 20 wheels.
EXAMPLE:
To cut a thread of 12.5 Strassburg use 48 wheel on the mandrel to 60 on the slide rest lead-screw.
48 = 4 x 1 = 4 = 12.5 __ __ __ __ 60 5 10 50
converting to the English inch:
4 x 20 = 8 = 13.125 _ __ __ 50 21 105
Therefore the train will be:
|
Mandrel |
Int.arbour |
Slide Rest |
|
48 |
20 |
|
|
21 |
60 |
Holtzapffel screw threads are a very complicated affair, both deep and shallow threads were made and were only approximate, The subject was very fully discussed in the correspondence columns of "English Mechanics" vol-34, 1881-2:
|
Page 473 |
Ornamental lathe threads |
Dr. James Edmunds |
|
Page 499 |
(Letter to the Editor) |
J.H. Evans |
|
Page 518 |
(Letter to the Editor) |
Dr. Edmunds |
|
Page 544 |
(Letter to the Editor) |
J.J. Holtzapffel and J.H. Evans |
|
Page 566 |
(Letter to the Editor) |
J.H. Evans |
|
Page 589 |
(Letter to the Editor) |
J.H. Evans |
|
Page 611 |
(Letter to the Editor) |
O.J.L. |
|
Vol-35, page 83 |
(Letter to the Editor) |
J.J. Holtzapffel |
For screw-cutting on the ornamental lathe six pitches are available. Unfortunately they are aliquot parts of the Strassburg inch and great difficulty is experienced when replacements or repair is necessary. In the past all such requirements were confined to the maker of the lathe. Today the two great firms of the past are no longer in business as ornamental lathe makers, and it is therefore not possible to obtain from them or anyone else, the parts needed. Had the pitches been those of the English standard there would have been no difficulty.
In order that male and female screws may fit each other correctly, a tapping allowance of the right amount must be made. This allowance as far as the writer is aware has not been computed. For convenience sake therefore, tbe writer has made reference discs of the correct allowance for each of the six pitches. A suitable diameter for the discs is 1-1/4" and they are each turned to the following thicknesses:-
|
Alphabet denotement |
No. |
Threads per inch |
Tapping allowance |
|
C |
1 |
9.45 |
.110 .110 |
|
D & B |
2 |
13-09 |
.077 .080 |
|
F |
3 |
16.5 |
.061 .060 |
|
G & H |
4 |
19.89 |
.040 .045 |
|
I & K |
5 |
25.7 |
.033 .035 |
|
L, N, D |
6 |
36.1 |
026 .025 |
{Ed. note:
Mr. K.J. Fowler was consulted as to the depth of Holtzapffel screw threads and he reported as follows:
"I have been to some trouble to check the Holtzapffel thread depths. My findings listed above show a close similarity to those of G.A. Grace. We have to take into consideration that these screw tools were hand made and I doubt very much that even Holtzapffel could get a closer tolerance than 2/100011 in 2 up and down, also the tools checked had wear owing to use. I suggest therefore that the table given above be adopted as near enough correct."}
