hello quinn, it is a nice idea, and maybe is cheaper if you craft it yourself
once you start using it, all faces have to indexed pretty parallel to the main vmc table, thus is not critical to have 4 sides, or the section to be a square; it may be a random triangle, etc, but all the perpendicular sections, across it's length, have to be located at same distance from the vmc table ... it may be an idea to finish it on the 4th axis, so to achieve these conditions
even so, this finish operation requires preparatory time, so to be sure that, after you finish it, and remove everything from the table, you can put it all back and achieve same tolerances ... you have to obtain fixture position repetability, thus, when you start machining, be sure that :
... machine is alligned : table not tilted, spindle perpendicular on the table, etc
... 4th axis is spinning arround an axis paralel to X axis ( = reference axis )
... tailstock axis is pretty colinear with the "reference axis"
... so far, if you take down the 4th axis + tailstock, and put them back on the machine, all alignment values have to be pretty equal, otherwise you don't achieve repetability
... now clamp the tombstone, and be sure that this new setup does not change the orientation of the "reference axis"; for example, tailstock may move when clamping, because :
...... tombstone center is forcing it to, because it is not aligned
...... some random plays, random clearances, etc
if these stuff is not checked, then each time you put the tombstone inside the machine, it will be in a different position, and the range will be wide enough to make someone lose time with realigning it every time; even if all is done ok, still the table will be at different positions, but the range will be shorter, and the range value can be predicted if all above steps are done ok
however, let's say that you end-up with the wide - range; if you craft small parts, then the range will get shorter, thus there will be less deviation/part, but overall, it will be the same; for example, if the table is tilted 0.3mm/1000mm, then you can not deliver a long part ( 800mm ) that requires 0.05 paralelism between faces, but you could deliver it if the part would be shorter, like shorter then 100mm
well, i guess you know, is all about tolerances, repetablity expected / desired tolerance, and crafting it with low costs
one more thing :
when you raise a part from the vmc table, you lose rigidity, and you won't be able to machine it at identical specs; if the part will be on that 4th axis tombstone, it won't be like it is securely in a vice on the table, so you shave time with less human/operator intervention between setups, but you lose time with lower specs
so, if you wish for your tombstone to be more rigid, check this tips :
... change aluminium to cast steel ( vibration dampening )
... make it shorter ( less buckling )
... make it thicker ( so put more parts around it's circumference, not across it's length )
* secure it nice to the 4th axis, because, in the end, is all about the rigidity of the 4th axis
can you share the part that you are machining now, and your actual fixtures, for each setup ? who knows, maybe someone will have a nice idea
i hope you find it usefull / kindly