Precise Case Trimming without a Micrometer
Do you have a Wilson case trimmer but can’t afford the $84.00 to add the handy Micrometer upgrade (Sinclair Int’l item 05-4500)? Well Boyd Allen has come up with a clever use of a standard set of calipers that lets you set the cut length precisely within .001″. Just open the jaws of your caliper and put one jaw on the outboard end of the ring (with set screw) that holds the threaded length-adjusting rod. Set the other caliper jaw on the flat face at the end of the threaded rod that contacts the case rim. If you have a sample case set to the correct cut length, use that to set the position of the threaded rod. Then use your calipers to measure that length. This way you can repeat the cut length each time, or adjust the trimmed case precisely in .001″ increments.
If you change from one cartridge to another, just use your calipers to re-set the desired cut length. Alternatively, ReedG notes you can use the inside caliper jaws and measure directly from the end of the threaded rod to the cutter face. That’s a bit trickier, but it measures actual trim length.
It’s actually more accurate and easier to do if you instead use the step measuring function built into the caliper head to get the distance from the end of the screw to the rear of the screw support. If you hold the calipers against the thread and choose a spot on th end of the screw not distorted by the slot excellent repeatability is possible without the slight cosine error using the inner jaws as described.
On the older trimmers (like mine) the “head” of the case stop is larger in diameter than the threads. After the item was published, I noticed that the current trimmers’ case stop “heads” are the same diameter as the threads, making my method impractical. I wish that I had a better understanding of what the previous commenter was writing about. It sounds as though his method might be the way to go with newer trimmers.
Boyd,
I never noticed the casehead stop in the pic being larger than the thread diameter, no wonder I kept getting cosine error. Thanks for pointing that out.
As far as using the step measurement technique with the head of the calipers, open them up more than enough to allow the body and head to rest in contact with the threaded rod sticking out of the support and clamp piece, holding it such that the head is abutted square to the back of this support so that the flat (face opposite that containing the rack) of the caliper body is in full contact with the OD of the threads. Then slowly slide the moving jaw toward the end of the threaded rod until contact is made. This is the std gap measurement method using calipers. It does depend on a repeatable banking surface for the sliding jaw which is why I place the caliper on the threaded rod where the sliding jaw will contact the end of the threaded rod pretty much parallele to the screw slot.
Hope that makes more sense.
This is the same technique you’d use to measure the distance from the casehead to the reference surface on a Wilson case gage for example. That is, the head of the caliper is held square to the casehead and next to the edge of the machined slot with the sliding jaw carefully brought into contace with the reference surface. Touchy but with a bit of practice, repeatable enough. A good test is to use this tecnique to mesure the step created by the machined slot that provides the go/no-go datums.
Pete