Lathe spindle spider internal threads calculation

I just had to bore and re-thread a lathe chuck back plate (2.25"x8). To get the part on size, I measured my spindle over wires, machined a gage from brass (aluminum is fine too) as close to the spindle measurements as I could get, and used this to check the fit as I was working to make sure the backplate would fit the spindle. It is tedious, but it allows you to nail the size right on. Also good threading practice.

Alternatively, if your chuck is a camlock, remove the whole chuck (part still in the jaws), test fit the part, and then reinstall the chuck in the same holes it came from (mark them!). One downside is that test fitting the part will be tricky with the chuck hanging from the back!
 
macardoso said:
I just had to bore and re-thread a lathe chuck back plate (2.25"x8). To get the part on size, I measured my spindle over wires, machined a gage from brass (aluminum is fine too) as close to the spindle measurements as I could get, and used this to check the fit as I was working to make sure the backplate would fit the spindle. It is tedious, but it allows you to nail the size right on. Also good threading practice.

Alternatively, if your chuck is a camlock, remove the whole chuck (part still in the jaws), test fit the part, and then reinstall the chuck in the same holes it came from (mark them!). One downside is that test fitting the part will be tricky with the chuck hanging from the back!
Click to expand...
Surely you jest
 
Jesting about making the intermediate thread gauge?
OR
Jesting about leaving the thread in the chuck while testing thread fit?
 
Mitch Alsup said:
Jesting about making the intermediate thread gauge?
OR
Jesting about leaving the thread in the chuck while testing thread fit?
Click to expand...
Testing a fit with the part still held in the chuck when removed from the spindle, if successful in practice you are far stronger then I have ever been.
Buying or making internal thread plug gauges is a far more elegant solution..
 
Single Point Threading huh? Thread wires and thread mics are fine, cost can put these out reach for some, not to mention unobtainable at 00:40 in the morning. Plugs or ring gauges are not common in home shops, commercial versions are hardened for long life. Workable material is a good choice for any case that does not require certification, just pursuit of desired fit. Machined and stored carefully, they can be sufficiently dependable.

Even on customer parts, ID and OD have simplified solutions, here is my #1 solution and most frequent for ID's, nothing more than ID measuring required, but works in either case.
1] Start with part somewhat longer than finish dimensions.
2] Bore 3 steps at least 1.5 or 2 times longer than thread pitch, ie 16 TPI at ~ .090 to .13 each.
'A' step is just under minor diameter,
'B' step is pitch diameter,
'C' step is just under or equal to major diameter.
3] Mark those with felt tip or layout dye. That reveals tool contact, and corresponding diameter.

When complete, the end chamfer will remove at least the pitch and minor steps, leaving only a quick facing to eliminate the major step.
! Oh, do go on....!,
K, think I will.
A 45° chamfer is correct for overwhelming amount of work. By not matching angle of thread, the runout of starting thread is greatly reduced, and a bit beefier counteracting deformation.
For greater resilience a 'Higbee Cut' (or version of) is recommended. Required in fire hose fittings, and most plug gauges have them too; to protect thread start. Generate this with an endmill, by cutting away the lead until the full thread form is reached, forming a fillet instead of a long disappearing peak.

Higbee cut - Google Search

www.google.com
Here is good info on them along with more thread deburring techniques. Additionally, the start of any finished thread is the most important element, regardless how well done the body is; anyone recall trying to start badly finished sample? Even should it start, it can alter good threads that strip or gall rendering both compromised.
 
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