Thread: Screw Cutting Diameter Allowance

I too would like to know the answer to this. Im guessing the truth is buried somewhere inside 'machinerys handbook'. I guess given its a firm fit in your target bolt hole then you need to machine the next one a bit smaller. Perhaps the 1 - 1.5% target is for precision fit and for general fit its more like 2-2.5%

Not really knowing the class of fit you are after,you could look for an OD of Close .3075" (7.81mm)
Medium .3058" (7.76mm) or Free .3037" (7.71mm).

I find that when buying new bolts , the fit tolerances are all over the place . Seems to be no effective standard in use by the manufacturers. Most of the nuts are rather loose on the bolt . Standards are slipping , like most other things . MIKE

Thanks guys for jumping in on this.
I have spent some time researching the subject, without any singular success, and I am inclined to follow the advice from Pipeclay.
The US has 4 Classes of Fit, #1 the loosest and #4 the tightest.

My Kents Mechanical Engineers Handbook, printed 1946, quotes Class 2 as the general purpose close matching Fit and "represents a high quality of screw thread product".

My reading of the handbook tables has Class 2 for a 5/16 inck 18 tpi maximum OD at 0.3125 in = 7.94 mm and minimum OD = 0.3011 in = 7.64 mm.

I think I will go with the 7.81 mm from Pipeclay.

John.

Well, I made the second screw thread at dia 7.81 mm and it, and the first one, settled in nicely after a bit of burr removal and exercise with lubricant.

The target is a travelling steady for my lathe.
I swapped some Citric Acid for the casting with a Forum member some time ago.
The images show the original casting and the final assemby.

I made:
2 x 7/16 W bolts from 1214 Hex bar,
2 x fingers from 1020 bar,
2 x finger tips from Brass, Loctite into finger,
2 x finger adjusters each:
- 5/16 W threaded 1214 shaft (subject of this thread),
- 1020 head, Loctite onto shaft,
2 x grub screw locking pads from Brass ( dia 4.9 x 2 thick ).

Commercial:
2 x grub screws.

I know the paint is poor, I used a brush with 3 year-old paint purchased with the lathe as a matching colour - it just would not mix properly (that's my excuse).

John.

Sweet, looks good. Have you had a go at using it yet? Im going to start playing with steadies soon to see if I can get the hang of them.

Brendan,
Thanks for the comments.
The steady certainly looks the part on the lathe, but I am not anxious to use it.
I have never used one and, by all reports, they can be a real problem due to burrs on the work and swarf pick-up.
John.