repair job: centering a helix

Everyone probably knows how a meat-grinder is constructed. A variable-rate coarse helical feed screw in a cylindrical chamber pushes meat into a perforated plate, and a four-winged flat cutter, driven by the screw, rotates against the perforated plate, nipping off the bits that bulge into the perforations. The cutter has a square (or, surprisingly, rectangular) hole in its center which fits over a mating extension of the screw, which is further prolonged into a spigot or pilot shaft that passes through a center hole in the perforated plate.

The screw-part of the main shaft is inside the chamber, but the driven end (opposite the end that carries the cutter against the plate) is a shaft passing through and supported in a hole in the closed end of the chamber and protruding far enough to carry a hand-crank or pulley to drive it.

The screw is an iron casting, usually heavily tinned. The actual rectangular drive boss and coaxial support spigot on the far end are actually a separate hardened steel part, screwed into the end of the casting.

An over-enthusiastic operator broke this bit off, right in the first thread under the drive boss.

So the repair plan is to extract the stump, tap the hole to a larger size and make a new, stouter drive/alignment end piece. But the question is how to assure that the reassembled strew-plus-extension runs true. You fellows may suggest much better ways than I will describe here.

My first idea was to make a sleeve with concentric OD and ID, fit this tightly over the machined margin of the screw, hold the driven shaft end in a 3-jaw universal chuck, and run the OD of the sleeve in a steady-rest.. Then I thought, simpler to make and split the sleeve and hold it concentric in 4-jaw independent chuck. I had to sneak up on the ID size because I have no easy way to measure the actual OD of the coarse variable thread, and in doing so I discovered that the thread does not have a constant diameter.

So I tossed-out that idea, and decided to put the straight driven shaft part of the thing in the 4-jaw...but how to indicate the thread true? If it were a constant pitch and diameter, and I had change gears for the lathe of that pitch, I could mount indicator in the toolpost and follow the thread with half-nuts closed. But both pitch and diameter appear to vary.

So, what I did .was to put indicator in toolpost, and hold a vee-block against the screw, and place the indicator probe against the flat back of the vee-block. Of course I could not hold the vee-block reproducibly against the screw so I slid and rocked it around and noted the minimum reading in each of the four jaw positions, and adjusted the chuck until they were close.

Of course that does not take into account that the slightly wacky screw may not be coaxial with the shaft, nor the possibility that the common axis of my chuck jaws may not be parallel to the spindle axis, but for boring and threading the hole, for a meat-grinder, I think it will be close enough.

Derision and/or better ideas welcome

I don't have a specific answer for you, but as I am reading this, the first question that pops in my mind is "What is the clearance between the screw and so called "chamber". If we are talking about only a few thousandths, you certainly need to be very careful in your centering approach. But if it is more like 60 to 100 thousandths, then you still need to be somewhat careful, but nowhere near as much.

My wild ass guess is that it is closer to 0.060 than to 0.010. I might be wrong, but either way, that is the very first thing I would try to assess.

Good luck,

Jacques

You are correct Jariou, that the clearance in the chamber is probably ample, but if the spigot on the end wobbles, it will tend to fatigue, and also will wear the bore that the shaft passes through.

Today was a funny day. Started single-pointing the internal thread for the new spigot and it looked WAY too coarse. Had wrong stud gear in train. Fixed that, and kept chasing. Starting to look kinda big, and sample screw still will not start. Apparently I have another wrong gear somewhere (SB 10" with quick-change, 20 T stud gear I am sure is what is supposed to be there, 56 T screw gear maybe not).

So now I have to leave the gears alone and make the stud to fit whatever mystery thread I cut in the bore. They will have to come back to MEEEE for replacement parts FOREVER Bwa-ha-ha-ha!