I try to dial everything in to where my .0001 indicator doesn’t move, or as close as possible. When mentioning this to real machinists, they said it isn’t possible to get .0001” tolerances on a shop lathe without temp control, etc. So the question to the machinists that do gun plumbing, what tolerance should I really expect?
Realistic tolerance when building a rifle?
- Thread starter Mram10
- Start date
GeneT
Active member
Well, there's a difference between dialing in to <0.0001" and holding a 0.0001" tolerance. The former can be done, with patience. But holding a tolerance (e.g. machining a diameter to 1.0000" +/- 0.0001" is hard. Very hard. You need to start with climate control, you need to control heat in the workpiece and the tooling, and yeah, it's just not very practical. Stick a bar out a couple diameters from your chuck, turn some wide grooves (say 1/2 a diameter each). Try to get a fine surface finish. Now go back and test each groove with your tenths indicator. Note both the runout and the absolute measurement. *That's* about as good as _you_ can do on _your_ machine.
GsT
jackie schmidt
New member
Any final sizing on parts that are sized to within .001 should be done at room temperature, which we call 78 degrees F.
As an example, when we Machine large bearings, they pick up enough heat to expand .002 to .003 inch.
You simply cool the bearing down before final sizing.
A heavy mist cooler usually works fine.
As was stated, there is a difference between indicating something and sizing something.
As an example, when we Machine large bearings, they pick up enough heat to expand .002 to .003 inch.
You simply cool the bearing down before final sizing.
A heavy mist cooler usually works fine.
As was stated, there is a difference between indicating something and sizing something.
Rubicon Prec.
New member
Keep doing what you're doing.
They are technically pretty much correct, but that doesn't change your work.
They are technically pretty much correct, but that doesn't change your work.
Gene, I check my lathe about once a month with a test bar. I’m not a real machinist so I lack the knowledge to know what to realistically expect.
Jackie, I had no idea bearings would vary that much with temperature! I’m working in a shop with temps from 55-70 degrees depending on when my shop heater decides to kick on...
Rubicon, I definitely will. Thanks. Never ending learning process....
Jackie, I had no idea bearings would vary that much with temperature! I’m working in a shop with temps from 55-70 degrees depending on when my shop heater decides to kick on...
Rubicon, I definitely will. Thanks. Never ending learning process....
jackie schmidt
New member
I am talking about large bearings, often made of some type of bronze allow and larger than 10 inches. If you have 1/4 inch or more to come off, it can easily get to 150+ degrees. That’s enough to make it grow .002 to .003 inch.
Often I will put a mist cooler at the cut, keeping the temp down.
By the way, we have no coolant pumps on any of our lathes. In the end, they tend to be more trouble than they are worth on the type of work we do.
The point is, you have to insure your piece is at room temperatures if you want exact measurements.
Copy. I’ll keep it a big oversized till it cools
Butch Lambert
Active member
So how will you shrink the bore before final cut?
I have a buddy that runs a shop machine shop. He said they will see a difference in their work if someone opens a door on the other side of the shop. I'm guessing their tolerances are way tighter than I could even measure. Heck, my shop is an uninsulated pole barn. It's 23 outside right now so it's 23 in my shop. LOL
jackie schmidt
New member
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MilGunsmith
Member
I think the problem is that everyone buys electronic calipers or DRO's that have readouts to .0001" inches. They then expect the machines to actually do that level of precision. I have had young engineers bring drawings with tolerances of .00001" on them for drilled holes. Part of this is no experience in actually making things, only theoretical work. They have never used old school measuring devices like a vernier caliper. There is also no understanding of how parts need to fit together with clearance for dirt, heat expansion, etc. How many galled barrel threads have we all seen from trying to make the fit too close?
Rubicon Prec.
New member
Can one honestly qualify a +/-.0001” tolerance in a home garage shop with a mic?
I think the .002” window (+/-.001”) stated above is more realistic.
Other than certain chamber diameters, is there a feature you’d toss a barre in the scrap pile if you were at the top/bottom end of that +/-.001” window?
I think the .002” window (+/-.001”) stated above is more realistic.
Other than certain chamber diameters, is there a feature you’d toss a barre in the scrap pile if you were at the top/bottom end of that +/-.001” window?
Cad program tolerancing
Also by default auto tolerancing on cad programs. You can set up the tolerance default on a cad drawing process to insert whatever numbers you want or it will do it automatically from a default program setting. Bam! .0001 tolerance for drilled holes!
Also by default auto tolerancing on cad programs. You can set up the tolerance default on a cad drawing process to insert whatever numbers you want or it will do it automatically from a default program setting. Bam! .0001 tolerance for drilled holes!
GeneT
Active member
I have seen that very thing ambitiously applied across all dimensions in drawings where +/- 0.025 would have been fine just about everywhere. OTOH, there's an argument to be made that expressing a "perfect nominal" makes sense in the age of CNCs that can easily hold tight tolerances. If you're still using number of decimal places to indicate desired precision, you may be getting parts that are closer to your "rounding error" than they are to the 'ideal' dimension, just because the machines can do it. Of course the right thing to do is express a 'perfect nominal' AND an explicit tolerance. But so much work...
I think every engineer should have to spend their first year out of school apprenticing with the type of tradesman with whom they will ultimately work. It certainly won't fix everything, but it'd fix a lot of silliness.
GsT