Workshop Hints
DEAD BEAT ESCAPEMENT PALLET FACE REGRINDING
Written by Robert D. McLaughlin, BSME, MSME
Any part or portion of this article can not be reproduced or copied in any form without the expressed written consent of the author.
The British Horological Institute is very pleased to publish this work by Robert D McLaughlin. The views and ideas expressed in this paper are his alone based on his experiences and has not been modified to show any approved BHI method or system.
I think the way to start this article is to reflect on the numerous times that a clock has come into our shop where, the pallet faces of the deadbeat escapement exhibit deep wear grooves, or have been repaired by an individual who has left the faces rounded. The clock usually will not run in either case. What will be presented here is a method to correct this problem and the methodology to redefine the angles of the entrance and exit pallet faces. The article will also cover how to make and use some relatively simple tooling. The assumption has been made that most of my colleagues have a drill press and a small lathe and this article has been structured to the use of these tools.
In developing this method, two previously written articles, one very practical and the other quite mathematical will be referred to, but there is excellent correlation between the two.
Reference 1: "Restoring the Lift Angles of A Deadbeat Escapement", by Jerome M. Faier, CMC, Horological Times August 1997, pages 24 & 25.
Reference 2: "Studies in the Deadbeat Escapement", A research project by Derwent Mercer, published in the Watch and Clock Journal. (I apologise for the lack of further information, as my copy does not show any publication dates). I do know that this article was funded by the Royal Society of London.
I would suggest that those who are interested obtain a copy of reference 1, as that is the starting point.
Below are some definitions will help.
- PALLET ARM LENGTH (PAL) The distance from the centre of rotation of the anchor to the centreline going through the pallets.
- REFERENCE CIRCLE or Impulse Circle - The circle that is used to define the pallet face angles and can determined from physical measurement of the verge assembly.
Figure 1.
The length of the pallet arm is obtained from the unit called the Anchor Assembly, or Verge Assembly. This is done by direct measurement with a dial caliper .In other words, if we measure the diameter of the verge shaft, the thickness if the pallets, and the overall distance from the outside of the verge shaft to the outside of the pallets, the arm length can then be calculated. See calculation below.
Pallet Arm Length = (Overall Length) - (Verge Shaft Dia/2) - (Pallet Thickness/2)
PAL= (_____________) – (__________/2) – (____________/2)
The pallet arm length and some data extracted from Reference 1 will now be used to determine the Reference Circle.
The following data has been taken from Reference 1. This data is to be used as a given.
1 degree of lift is equal to ¼ of the pallet arm length.
1-½ degrees of lift is equal to 3/8 of the pallet arm length.
2 degrees of lift is equal to ½ of the pallet arm length.
Most standard pendulums use 2 degrees of lift. Heavy pendulums (over 2 pounds) usually use 1-½ degrees of lift. Vienna Regulators typically uses 1 ½ to 1 ¾ degrees of lift.
From the measurement of the pallet arm length and the corresponding selection of the lift, the reference circle or impulse circle can now be determined.
Reference Circle diameter = Pallet Arm Length X (lift factor; ¼, 3/8, 1/2)
RC or IC diameter = (_______________) X (____________)
With the diameter known draw the reference or impulse circle on a heavy piece of card stock. Then add a line tangent to this circle, we can now determine if the unit is a classic deadbeat. The pivot anchor shaft is now inserted into the centre hole of the circle, which has been punched through the card stock. The pallet face angles are now compared to the tangent line. If the assembly is a classic deadbeat, both the pallet faces should match the tangent line at the exact same spot.
Refer to Figure 1. We have shown the tangents that are drawn through the entrance and exit pallet faces, which define the angles of these faces. The next step will be to fabricate a piece of hardware that will be used for grinding these angles. Figures 2 and 3 show the parts that make up the grinding disk assembly. The dimensions were selected to allow for its use in any lathe that is 8mm or larger.
Figure 2
Figure 3
NOTE: If you do not have a lathe that has collets large enough to accept a ¼ inch diameter shaft, the shaft can be centre drilled on each end and the machining can be accomplished between centres. Drill the collar to fit the diameter of the shaft and attach it to the centre of the shaft with Loctite™ Studlock. Next, drill the hole in the disk so that the hub on the collar can be machined to fit the disk. After the Loctite™ Studlock has cured, machine the hub to fit the disk, and attach the disk to the hub. The next step will be to set the assembly up in the lathe and machine both faces so that they are parallel. Once this is completed, the disk is finished as follows;
- Apply double masking tape to one side;
- Use a piece of 400 or 600 grit masking tape, cut out a centre hole to fit over the shaft, and apply over the masking tape.
- Trim the outside to match the disk diameter.
- Repeat this process for the other side. Now the disk is completed.
Since a picture can be worth a thousand words, a picture of how the disk is used to grind the pallet faces would be helpful. Refer to Figure 4. This drawing shows the relationship between the grinding disk-shaft assembly, the verge or anchor assembly, and an item called the anchor platform. The platform has to be attached to the cross-slide of the lathe and it must be set up so that it is perpendicular to the grinding face of the disk. When in use, the grinding disk-shaft assembly is rotated at a reasonable speed by the lathe. The verge assembly is held so that the pallet face is maintained against the grinding disk face with moderate pressure, and the cross-slide is moved in and out by use of the handle on the cross-slide assembly.
Figure 4
PLEASE NOTE. On many verge assemblies the crutch may not be removable, so the pallet faces will be seen as if you are looking at them from the rear of the clockworks.
The next two Figures 5 and 6; will show the verge and pallet face positions with the crutch up, and will define how the anchor platform is fabricated and set up.
Figure 5
The anchor platform is the easiest of all the parts to fabricate and it requires only one set of three holes, for each of the entrance and exit pallets. Referring to Figure 5 the spacing between the alignment pin centreline (APC) and the pivot centre for the entrance pallet is equal to
APC = (Impulse circle Radius) +(Diameter of Alignment Pin/2)
And the spacing for the exit pallet, 
see Figure 6,
APC = (Impulse circle radius) –(Diameter of alignment Pin/2)
Figure 6
Fabrication of the anchor plate is easily done if one has a drill press with an X & Y stage. If not, the alignment pinholes can be drilled by clamping a guide to the table and drilling these holes at the same time. The pivot centre can be located and drilled by using spacers, having the correct thickness and banked against the alignment pins. In practice, the pivot centre should be drilled a few thousandths undersize and then opened with a cutting broach to allow just enough clearance for rotation.
Now that the parts are all fabricated, the set-up should be addressed. The two most important considerations are;
- The platform must be 90 degrees to the grinding face, to insure that the pallet faces will not be ground at an angle.
- The removable alignment pins that define the tangent to the reference circle must have their tangent points in line with the grinding disk face.
Item 1 can be accomplished by using a machinist square and shims at the tool post. Item 2 can be accomplished by clamping a piece of ground stock against the grinding face and at the same time adjusting the anchor platform so that the alignment pins are in contact with the ground stock. This is how one sets the tangent to the reference circle.
Before proceeding to grind the pallet faces be sure to measure the stick-out of each of the pallet faces with reference to their corresponding arm. These dimensions are necessary so that after grinding, both the entrance and exit pallets can be reset to their original dimensions. If the verge has adjustable pallets this is relatively easy to do, but if the pallets are fixed it may be necessary to add material to the faces and then regrind. To add material I have found that hard soldering a portion of a single edge razor blade will work nicely.
The following is a Process Sheet to keep us trouble free.
Measure Stick-out of Pallets
Entrance Pallet; (________________)
Exit Pallet; (_______________)
Diameter of Alignment Pins; (_________)
Diameter / 2; (__________)
Pallet Arm Length; (_____________)
Lift Factor; (__________)
Reference Circle; (PAL X LF): (______________)
Alignment Pin/Pivot Centre Spacing;
Entrance Pallet;
(Ref Circle Radius + Align Pin Dia/2) (____________)
Exit Pallet
(Ref Circle Radius - Align Pin Dia/2) (____________)
This article was written to help the many individuals who have struggled with the deadbeat escapement. My method of going to the basics in it was so that more of my colleagues would have an opportunity to use it. I sincerely hope I have generated a paper that will be of use to many of you.
R. D. McLaughlin
