This print is a proof-of-concept of a method for making a replacement lens (clear dial cover) for a mechanical dial caliper (see photos). I have included the print files I used, but it should be noted that these parts needs to be customized for specific lens size required , and the thickness of the lens material (plastic sheet) used.

Note : I picked up a large mechanical dial caliper at an auction for a very good price, but the plastic lens cover of the dial was missing and the dial was very dirty. Otherwise, the caliper seemed to working fine. I was able to clean the dial and install a new cover using the method explained here.

Note 2 : Initially I assume the plastic sheet I used was transparent PVC, but after a bit more research I think it might be PETE (or PET) - polyethylene terephthalate. The important point is to use plastic sheet which softens at the temperature of boiling water. The plastic used should have a glass transition temperature (Tg) around 65-80°C.

UPDATE Jan. 9, 2024 : I found a much better method for heating and forming the plastic sheet. The sheet is prepared as noted below, and then the sheet is clamped in the press mold (I used a hemostat) and the whole assembly is immersed in boiling water for about 20 seconds. The assembly is then removed from the boiling water and pressed together immediately to form the plastic sheet (I used some paper towel to insulate my hands from the hot plastic). After forming, either allow to cool or immerse in cold water. This method results in much more even heating, with no clouding of the clear plastic.

Use Instructions

This print is essentially a press mold which is used to press a small piece of clear plastic sheet into a domed lens shape with a “skirt”. For plastic sheet I used a piece of flat PVC sheet salvaged from some food packaging; the sheet had a thickness of 0.44mm.

To use the press mold, cut the clear plastic sheet into an octagonal shape small enough to fit loosely into the mold. I used the octagonal Ring piece as a guide to mark an outline with a felt tip pen, and then simply cut out the piece with scissors.

Place the octagonal blank over the Dome piece, and heat it gently with a hot air gun until it starts to sag; then quickly press down with the Ring piece to shape the lens. I ended up having to heat and press the blank twice to get a fully shaped lens. I also noted that the plastic sheet developed a slight cloudiness from the heating and pressing (I may have overheated), but this did not affect the function too much.

Once the lens is formed, use a scissors to trim away the octagonal excess. Do not cut the sheet flush to the “dome” - leave a small skirt about 0.5mm wide. The skirt will hold the lens under the retaining ring of the caliper dial.

Use a tissues to remove any fingerprints or smudges from the underside of the lens, and then fit it into the caliper dial retaining ring. The plastic lens is thin and flexible enough that your should be able to fit it fully into the retaining ring.

Print Instructions

Print in PETG using the 3mf files provided. I used PETG rather than PLA because of the higher melting point of PETG, but with care PLA might work for this as well. In general :

• 0.2mm layer height
• use variable layer height on the Dome piece; I used a Smooth radius of 3
• 15% rectilinear infill

I lightly sanded the top of the Dome piece, mostly just to remove any burrs.

CAD

https://cad.onshape.com/documents/41d3fb0b50eb2447422b8804/w/5b24728bd65ef406d4f8fe86/e/d6b5a55fa2f1a5be2a446728?renderMode=0&uiState=659c63ad59668219dd37781a