That Hard to Find Knock

From Horology magazine, April, 1939

That Hard to Find Knock
By Glen Knauts

The writer is one who places great importance on the correct sound of a timepiece after it has been repaired.  The difficulty we are to deal with should be described as follows: After the timepiece we are to deal with has been carefully cleaned (in a cleaning machine), the balance is in poise, the balance pivots are found to be straight, well formed, highly polished, and fit their respective jewels, the balance is placed in position before the time-piece is assembled, and is found to have the proper end shake, the hairspring is trued in the flat and round, and care is taken to see that the Breguet coil is not rubbing or touching the balance cock at any point or in any position. The balance is then made to vibrate, and we find that the balance comes to a very gradual rest in all positions. The balance is then removed and the timepiece is assembled, the train found to be perfectly free and in perfect order. The pallet fork and bridge are then placed in position, checking for correct end shake in the fork arbor. The balance and cock are now placed in position. At this point the writer makes a series of examinations which he considers to be very important and they are as follows:

First, by the use of a slender feeler, reach into the movement and raise the slot and guard pin of fork as high as their end shake will permit, making sure that the lower balance pivot is resting on the foot endstone. If the guard pin touches the end of the roller jewel, more clearance must be provided and should be accomplished in one of two ways.

The simplest way is to shorten the roller jewel slightly, provided there is proper clearance between the slot and prongs of the fork and the top roller table. If this clearance is not to be had, then it will be necessary to remove the hairspring and roller tables from the balance, set the balance up in the lathe and remove a small portion of the hub of the balance staff. This brings the work to the place where the roller tables will stake onto the staff at a higher level than before and will account for the proper clearance in the parts under discussion. The good mechanic will not stop work in this direction until he is positive that these parts under discussion have proper clearance in the face up and face down positions.

The writer does not consider it permissible to bend the arm of the fork up or down unless he does so with the idea of putting it back to the same level that it originally had when it came from the factory, and he finds many new watches, both Swiss and American, that do not have the proper clearance in the roller and fork assembly as mentioned above.

This completes the first step of examination.  The second of this series of examinations is made after the movement has been completely set up, the dial is off, the mainspring is wound one-half to three quarters of a turn, and the balance will begin to vibrate, provided the timepiece is in beat, and the remainder of the escapement is in proper order. We now stop the balance so that the incoming pallet stone has locked a tooth of the escape wheel. The balance is turned on around so that the roller jewel is well away and out of the fork. We now take the slender feeler again and check the clearance between the guard pin and the safety roller. This clearance should amount to somewhere between .05 to .10 of a millimeter. While the balance is still held in this same position, and the guard pin is pushed over against the guard roller, it must be observed that the incoming pallet stone still engages the escape wheel tooth on the locking surface. If this examination permits the tooth of the escape wheel to advance to the impulse surface of the pallet stone, the repairman should know immediately that adjustment is necessary. Of course, this same examination must be made in regard to the outgoing pallet stone. I t is not the writer's purpose to give a treatise on how to set an escapement if trouble is found in making this second series of examinations, so he will assume that any difficulty that might have been encountered has been taken care of correctly.

Now the balance is permitted to vibrate again, and after making the first few strokes, builds up its constancy and shows a fine vigorous motion. However, on listening to the timepiece, the writer is able to detect a knocking sound in the escapement, and it is around this difficulty that he should like to direct the emphasis of this article. He knows that he is not the only one who has encountered this "knocking" condition because he has talked to other men of high ability who have had similar experiences, and furthermore that the most of this trouble had been found in certain models of certain makes.

The remainder of this article will, therefore, be devoted to the correct method of procedure when this "hard to find" knocking trouble is encountered. The diagnosis of this trouble should be ascertained very carefully and in a systematic manner. First, remove balance from the movement, and remove the hairspring from the balance. Be sure the roller jewel is perpendicular to the roller table. Now put the balance back in the movement without the hairspring and replace balance cock. Turn the balance slowly, and with a high powered loupe determine whether or not the roller jewel has the proper clearance as it comes around past the prong of the fork and enters the slot.

If the proper clearances and depthing are not found, it is well to know that some watch companies make roller tables in assorted diameters, properly gauged, and the repairman should remove the former roller table and replace with another of the proper diameter. However, it is well to say that the trouble is not often found here, so we proceed with another examination.

Again remove the balance from the movement and the pallet fork also. Place the fork in a pallet fork holder so that it can be handled easily. Pick up the balance in one hand, hold the fork in the other and place the roller jewel in the slot of the fork. Does it have any side shake?

If not, it should have several hundredths of a millimeter, and it is well to say that this is where the trouble will most usually be found. We proceed to overcome this trouble by making the slot of the fork slightly wider. Best results can be obtained by polishing the slot of the fork with a very thin steel lap. After these adjustments have been made, the timepiece will not only have an excellent motion, but will also have the correct sound.

The writer has a low percentage of comebacks, and he attributes his success to the fact that he adheres rigidly to the rules of practice as mentioned in this article.