I promised at the latter end of Numb. 2. to give a more copious account than there I did of making Worms, when I came to exercise upon Printing-Press Spindles; and being now arrived to it, I shall here make good my promise.
¶. 1. The Worms for Printing-Press Spindles must be projected with such a declivity, as that they may come down at an assigned progress of the Bar.
The assigned progress may be various, and yet the Spindle do its office: For if the Cheeks of the Press stand wide assunder, the sweep or progress of the same Bar will be greater than if they stand nearer together.
It is confirm’d upon good consideration and Reason as well as constant experience, that in a whole Revolution of the Spindle, in the Nut, the Toe does and ought to come down two Inches and an half; but the Spindle in work seldom makes above one quarter of a Revolution at one Pull, in which sweep it comes down but half an Inch and half a quarter of an Inch; and the reason to be given for this coming down, is the squeezing of the several parts in the Press, subject to squeeze between the Mortesses of the Winter and the Mortesses the Head works in; and every Joynt between these are subject to squeeze by the force of a Pull. As first, The Winter may squeeze down into its Mortess one third part of the thickness of a Scabbord. (Allowing a Scabbord to be half a Nomparel thick.) Secondly, The Ribs squeeze closer to the Winter one Scabbord. Thirdly, The Iron-Ribs to the Wooden Ribs one Scabbord. Fourthly, The Cramp-Irons to the Planck of the Coffin one Scabbord. Fifthly, The Planck it self half a Scabbord. Sixthly, The Stone to the Planck one Scabbord. Seventhly, The Form to the Stone half a Scabbord. Eighthly, The Justifyers in the Mortess of the Head three Scabbords. Ninthly, The Nut in the Head one Scabbord. Tenthly, The Paper, Tympans and Blankets two Scabbords. Eleventhly, Play for the Irons of the Tympans four Scabbords. Altogether make fifteen Scabbords and one third part of a Scabbord thick, which (as aforesaid) by allowing two Scabbords to make a Nomparel, and as I shewed in Vol. 2. Numb. 2. §. 2. One hundred and fifty Nomparels to make one Foot, gives twelve and an half Nomparels for an Inch, and consequently twenty five Scabbords for an Inch; so by proportion, fifteen Scabbords and one third part of a Scabbord, gives five eighth parts of an Inch, and a very small matter more, which is just so much as the Toe of the Spindle comes down in a quarter of a Revolution.
This is the Reason that the coming down of the Toe ought to be just thus much; for should it be less, the natural Spring that all these Joynts have, when they are unsqueez’d, would mount the Irons of the Tympans so high, that it would be troublesom and tedious for the Press-man to Run them under the Plattin, unless the Cheeks stood wider assunder, and consequently every sweep of the Bar in a Pull exceed a quarter of a Revolution, which would be both laborious for the Press-man, and would hinder his usual riddance of Work.
I shew’d in Numb. 2. fol. 31, 32, 33, 34, 35. the manner of making a Screw in general; but assigned it no particular Rise; which for the aforesaid reason, these Printing-Press Screws are strictly bound to have: Therefore its assigned Rise being two Inches and an half in a Revolution, This measure must be set off upon the Cilindrick Shank, from the top towards the Cube of the Spindle, on any part of the Cilinder, and there make a small mark with a fine Prick-Punch, and in an exact Perpendicular to this mark make another small mark on the top of the Cilinder, and laying a straight Ruler on these two marks, draw a straight line through them, and continue that line almost as low as the Cube of the Spindle. Then devide that portion of the straight line contained between the two marks into eight equal parts, and set off those equal parts from the two Inch and half mark upwards, and then downwards in the line so oft as you can: Devide also the Circumference of the Shank of the Cilinder into eight equal parts, and draw straight lines through each devision, parallel to the first upright line; and describe the Screw as you were directed in the afore-quoted place; so will you find that the revolution of every line so carried on about the Shank of the Cilinder, will be just two Inches and an half off the top of the Shank: which measure and manner of working may be continued downward to within an Inch and an half of the Cube of the Spindle. This is the Rule and Measure that ought to be observ’d for ordinary Presses: But if for some by-reasons the aforesaid Measure of two Inches and an half must be varied, then the varied Measure must be set off from the top of the Cilinder, and working with that varied Measure as hath been directed, the Toe of the Spindle will come down lower in a revolution if the varied Measure be longer, or not so low if the varied Measure be shorter.
There is a Notion vulgarly accepted among Workmen, that the Spindle will Rise more or less for the number of Worms winding about the Cilinder; for they think, or at least by tradition are taught to say, that a Three-Worm’d Spindle comes faster and lower down than a four-Worm’d Spindle: But the opinion is false; for if a Spindle were made but with a Single-Worm, and should have this Measure, viz. Two Inches and an half set off from the top, and a Worm cut to make a Revolution to this Measure, it would come down just as fast, and as low, as if there were two, three, four, five or six Worms, &c. cut in the same Measure: For indeed, the numbers of Worms are only made to preserve the Worms of the Spindle and Nut from wearing each other out the faster; for if the whole stress of a Pull should bear against the Sholder of a single Worm, it would wear and shake in the Nut sooner by half than if the stress should be borne by the Sholders of two Worms; and so proportionably for three, four, five Worms, &c.
But the reason why four Worms are generally made upon the Spindle, is because the Diameters of the Spindle are generally of this propos’d size; and therefore a convenient strength of Mettal may be had on this size for four Worms; But should the Diameter of the Spindle be smaller, as they sometimes are when the Press is designed for small Work, only three Worms will be a properer number than four; because when the Diameter is small, the thickness of the Worms would also prove small, and by the stress of a Pull would be more subject to break or tear the Worms either of the Spindle or Nut.
And thus I hope I have performed the promise here I made at the latter end of Numb. 2. Whither I refer you for the breadth, and reason of the breadth of the Worm.