Prepared by: Jim Snowdon
Originator: Rob Hiam
1. Rob Hiam, Mar 21, 2018
When I drill and tap small sizes - 10-16BA I use the recommended drill sizes and then tap with a standard tap. The thread often is loose or the hole tight - do taps and drills vary in tolerances?
I am tapping brass mainly and use saliva as lubricant.
Any advice would be appreciated
Rob H
2. Raymond Walley Mar 21, 2018
Some of the variance is probably due to drill wobble especially drilling by hand or using a handheld mini drill. I use my milling machine with collets to reduce wobble to a minimum. If you want further accuracy then taping using the lathe centres is the way to go. No doubt others will have other ideas too.
Raymond
3. Bob Alderman Mar 21, 2018
Rob
I have found taps to be fairly uniform in size but have found a lot of variation in screws, from loose to will not fit!
Bob
4. Pat Buckley Mar 21, 2018
I think the main problem especially with 12 BA bolts is that metric sized rod is used instead of the correct size imperial. Some years ago the 12BA nuts had a correct thread but 12BA spanners and nut drivers would not fit them but this seems to have been overcome in the last two or three years. And of course the dreaded far east metric 3/16 axles from Hachette part works as well.
Pat.
5. Rob Hiam Mar 21, 2018
The plot thickens
I have bolts of 10Ba that don't fit 10BA tapped holes - too small and too big and two sizes of 10BA taps
6. Jimmy Berry Mar 21, 2018
Ref post #1 -
I believe for brass, paraffin is a good lubricant, though it is a few years now. Also when tapping do you use the correct method, as I notice you do not start with a taper tap, i.e. one turn clockwise, half a turn anti clockwise (to break the swarf) etc, if not then the excess brass could be part of the issue breaking too much off the thread crest.
JimmyB,
7. John Drakeley Mar 21, 2018
Hello Rob & all
To start with have you any means of measuring the Dia. of your 10BA screws. I have measured a selection of my screws both steel and brass and the are all consistent at 1.65mm dia which is slightly under 1.7mm dia this being the major dia. of a 10BA thread. This make me think that what you think are 10BA may not be. While I was measuring the screws I found one which was 2mm so it's possible.
Over the years I must have tapped hundreds of various BA sized holes and can't say I have had any problems so measure what you have and make sure the taps are correct and are what they say they are.
Regards John
8. Rob Hiam Mar 21, 2018
Thanks for advice
Where can I find a table of metric equivalents to convert BA so that I can measure my taps drills and bolts?
Rob H
9. Jim Snowdon Mar 21, 2018
Ref post #8 -
Google. Or more specifically, Google “BA Screw Sizes”.
Jim
10. Peter Reynolds Mar 21, 2018
Ref post #8 -
As Jim says 'Google it'… I found a chart for BA sizes, and the metric hole equivalents for tapping and clearance. Now printed, laminated and in a work bench drawer. Sadly, I'm not at home, so I can't give you the URL directly, but there are charts out there.
PeterR,
11. Simon Dobson Mar 21, 2018
CLAG have a data sheet. http://www.clag.org.uk/bascrew.html
Alternatively, http://www.royalsignals.org.uk/articles/threads.htm
Or https://journeymans-workshop.uk/baandme.php
Advertised tapping drills are sometimes too small for easy tapping, even though they may be “correct”, and using a marginally larger drill might help. I’d very much endorse Raymond’s comment about drilling in a drill press, indeed, I’d tap in one too.
Many BA nuts and screw heads are a different size to that which “should” be used, this is useful if you want a smaller or larger thread or head, but not if you’re trying to find the right nut or spanner with one hand whilst holding it all together with the other.
Best
Simon
12. Trefor Roberts Mar 22, 2018
Ref post #3 -
Yes. I was told by a screw supplier, I can't remember who, that all(?) BA screws come from China nowadays and they often use any bar stock in hand that is near to the correct size.
Apparently this also accounts for the incorrect size of some heads, I have had many that are oversized, which sometimes has caused problems depending on clearances available.
Thinking about it this might cause a problem with head size, but not screw dia. I'm losing it.
Trefor