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Worm threading questions

BobM3

Cast Iron
Joined
Feb 23, 2006
Location
Minneapolis
Currently we farm out some small worms for thread grinding. The blanks are heat treated to 50 Rc. There is no roughing out of the threads prior to grinding so I'm guessing there must be a lot of time on the grinder. The thread is .625 PD and the OD is .75. The pitch is .1963 and the length of the worm is 1". Years ago we asked the firm doing them if it would make sense to rough out the threads first but they said it would then be difficult to pick up the thread for finish grinding.

So how would this part be ground?
Multiple passes (it's currently a nice surface finish)?
If multiple passes, a roughing wheel and finishing wheel?
Can this be done with a manual machine?

Thanks,
Bob M
 
Parts get ground in multiple passes. No they don't change the wheel Yes it can be done on a manual machine. Worms have been ground for 60 years.

the problem with picking up the lead is unless you have the lead start from a known feature in the part, like an internal keyway, and it's consistant- the lead gets picked up by looking at where the grinding wheel is contacting the work. A grinding wheel normally flinging off lots of oil into the face of the operator. Much easier to just grind from rough,
than getting a face full of mist every time you chuck a new part. Down side of NOT picking up the lead, is the part THROUGH hardened or just case hardened? You can grind right through the hardened surface if they are not through hard.

Some companies made fixtures for setting up the lead of the part with it mounted on a bench. Not sure if it would be applicable to your part.
 
How "automatic" are manual machines? They won't make the mulitple passes on one automatic cycle I assume. So, like a manual lathe, you'd make one pass then have to move the cross slide in, make another pass, etc?

The worm does have a cross hole. It should be able to be fixtured so the cross hole and the thread have a relationship to each other. Possibly we could then get by with one finish pass on the grinder. What's a typical amount of stock one would need for the finish pass?
 
How automatic do you need? Load the part it grinds it until it's done, and then stops. Now if you have a multistart thread, in a manual machine, this is not an automatic feature. If you have a multi start thread and you're doing 100's at a time, a CNC grinder is the ticket. But say for a two start or four start thread, it's a matter of having the right grinder or accesories. The indexing heads for Ex-Cell-O's are easy enough to use.

For making your blanks if you ran them on a CNC that made the hole ALWAYS match the threads were related then you have to improvise an arbor to use in your thread grinder, that's CLOCKED right. Once the dog driver is located in the right spot, you're off to the races. Fifteen or twenty thousanths for a finish pass.

Subbing out grinding is NOT a bad way to go. There's a whole nuther learning curve involved, with NOT a lot of information readily available. But if you're getting hosed getting your worms ground, than it's certainly understandable. I've been in the same situation. But having a vendor that consistantly gives you a high quality part, is WORTH the cost. I've gone both ways.
 
Thanks for the replies Brian. I am reluctant to get into grinding. Our outside vendor does do a good job but we have guys here sitting around looking for things to do.
 








 
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