Bearing Surface Comparator

[1967spud]

Having had a look round the net for bearing surface comparator (BSC) for a while now and to be honest not found much that floated my boat. Any way after speaking to Davy on here who has been making the odds bits and peices for me for a while now, we decided why not make our own. Watch the vid here then read on....

 

http://www.youtube.com/watch?v=sEj-iFvSC50

 

 

Having had the comparator over the weekend now ive been able to use and test and evaluate the BSC on over 800 7mm Berger VLD's.

 

My finding's are thus

 

Betwean the lowest and highest reading there was over 9 thou difference in bearing surface length, now all these bullets were ALL the same LOT number and came in the packaging.

 

The tips broke down to these groups

 

176 number @ 0.000

256 number @ +0.003

358 number @ +0.006

112 number @ +0.009

 

Now looking at these numbers you will say "0.000 how can that be" but remember these are comparisons of length in relation to each other and NOT actuall physical length's.

 

 

You will ask "why bother with all this BS" i say if the tips bearing surface are this much out, for extreme consistant performance and consitancy its a must do procedure.

 

Discuss!!!!

[John MH]

I have a Tubb BSC, yours is almost a direct copy of his design, however, there is a slight difference in bullet holders. The Tubb version bottom bullet holder Is not sized to sit the bullet on the bearing surface, the bullet sits on it's base. Is not the position that the bearing surface starts the most critical as is will determine the actual bullet jump before it engages in the rifling, knowing the position is, I think, more important that the length. I'll have to go and check the bottom holder again when I get home tomorrow, the base of the bullet may not touch the bottom of the holder.

 

Have you read the Tubb BSC guide?

[Davy]

Thanks for the review Spud, it is always nice to see something I have built being used. Spud and I talked about this a lot and discussed many different design options, he supplied the granite and we both agree it would look nicer and be more stable if the granite was let into a block of wood. Also I will change the angle of the mouth of the comparator top on the next one. Hardest part was finding out how close to the nominal .284" bullet diameter I could get without the bullet disappearing up the hole! (it did on the first couple I made....)

 

Anyway to the theory behind it.

 

I did a very rough measure of a 180 VLD bearing surface and make it around .556" (I haven't built another BSC to confirm this so it is an approximation) this would mean a variance of 1.6% on the bearing surface length if my maths is right.

 

We usually aim to load our powder to +/- .1 grains which on a 50 grain load is 0.2% so why not take the same time on the bearing surface which left unchecked gibes a significantly higher percentage error.

 

Interesting stuff, especially as the Berger's Spud tested were all from the same batch so from the same machine. When Mark first mentioned building one of these I expected to see a variation of a couple of thou max so this is a real eye opener. I have just cobbled up a very rough test on some .308 bullets and the first three gave a difference sixteen thou!

 

Based on this I will be building one for .308/7mm/6.5mm/.244 to check the bullets for the accurate rifles in our house. Not sure my .303's are quite ready for such measurements

[1967spud]

I have a Tubb BSC, yours is almost a direct copy of his design, however, there is a slight difference in bullet holders. The Tubb version bottom bullet holder Is not sized to sit the bullet on the bearing surface, the bullet sits on it's base. Is not the position that the bearing surface starts the most critical as is will determine the actual bullet jump before it engages in the rifling, knowing the position is, I think, more important that the length. I'll have to go and check the bottom holder again when I get home tomorrow, the base of the bullet may not touch the bottom of the holder.

 

Have you read the Tubb BSC guide?

 

in a way John i agree but also you need to think about the drag effect on the rifling of a longer surface compared to a shoter one, once i commence seating my bullets they will be further sorted by base to ogive for seating consistancy.

[John MH]

Anyways, yours is more compact and I like the idea of using a Digital DTI.

[MJR]

Interseting video thanks. How do you vertically align the dti and the bullet holder to ensure you are taking accurate readings with the bullet seated squarely in its holder? Are you going to further batch the bullets with regard to ogive length?

[1967spud]

Interseting video thanks. How do you vertically align the dti and the bullet holder to ensure you are taking accurate readings with the bullet seated squarely in its holder? Are you going to further batch the bullets with regard to ogive length?

 

MJR i work on the principle towards lining it up as to leaving it slightly loose on the mast and spinnning the tip to get a shortest distance measurement working on the principle that the shortest measurement willl be the straightest and visually checking it

[MJR]

Cheers.

[Davy]

Interseting video thanks. How do you vertically align the dti and the bullet holder to ensure you are taking accurate readings with the bullet seated squarely in its holder?

 

MkII will have the bottom comparator base epoxied down in place and I will make a dowel as setting aid to ensure top and bottom are correctly aligned. Half the fun of building such things is working out how to improve them with the next version. I don't do this for a living just to make the things I would either have to pay a fortune for or wait many weeks for delivery from the the USofA and to test my ingenuity. So when Mark rings up and says 'hey how about one of these' I jump at the opportunity.

[MJR]

Davy, i'm not understanding what you mean regarding a dowel as I don't see how a dowel will work. What about machining a flat on the rear side of the vetical rod that the dti mounts on then every time its adjusted vertically it will self centre as the gub screw is nipped up. Once set up as you make them and centred the comparator base as you epoxy it in place all should self centre in use. I know what you mean about half the fun is in the making/designing and very satisfying when you do a job with a tool you have made yourself, good luck with it.

[1967spud]

Davy, i'm not understanding what you mean regarding a dowel as I don't see how a dowel will work. What about machining a flat on the rear side of the vetical rod that the dti mounts on then every time its adjusted vertically it will self centre as the gub screw is nipped up. Once set up as you make them and centred the comparator base as you epoxy it in place all should self centre in use. I know what you mean about half the fun is in the making/designing and very satisfying when you do a job with a tool you have made yourself, good luck with it.

 

I have to agree here that the flat would be better also it easier to tighten up the clamp as well at the same time.

[Davy]

Ah! I see what you mean. I was thinking about how to the initial alignment of the base to the head. Yes I also agree a flat on the back of the bar would make life a lot easier for setting up or change calibres. One thing I keep thinking about is what else you can do with the basic system. You have a base, base block, mast and DTI which in theory is very adaptable. The ability to change the operation with inserts would add versatility. So often we buy a tool that is used for one purpose only and then proceeds to gather dust.

[MJR]

What about an adaptable base with V blocks or bearings to allow the comparator to be used as a run out gauge? Also easy to adapt it to an OAL gauge/comparator.

[Andrew]

I built a similar unit for cast bullet comps many years ago. Because the critical bearing surface on cast bullets is the bore-riding nose, it was set up with a bore-diameter cylinder to match my rifle (.300") and the flat base was put against a ground plate. Bullets were segregated by the variance. When used in conjunction with weight sorting, it seemed to shrink groups. I'm sure it will do the same with jacketed bullets. Nice work! ~Andrew

[tackb]

i would be interested in your results once you get some range time with your sorted bullets , i doubt my scattering of rounds would benefit from this level of precision but i am interested to know if a decent shot gets worthwhile improvement from this sorting ? also i wonder if there is any corellation between this and projectile weight?

[1967spud]

i would be interested in your results once you get some range time with your sorted bullets , i doubt my scattering of rounds would benefit from this level of precision but i am interested to know if a decent shot gets worthwhile improvement from this sorting ? also i wonder if there is any corellation between this and projectile weight?

 

your in luck lol

 

[John MH]

I have a Tubb BSC, yours is almost a direct copy of his design, however, there is a slight difference in bullet holders. The Tubb version bottom bullet holder Is not sized to sit the bullet on the bearing surface, the bullet sits on it's base. Is not the position that the bearing surface starts the most critical as is will determine the actual bullet jump before it engages in the rifling, knowing the position is, I think, more important that the length. I'll have to go and check the bottom holder again when I get home tomorrow, the base of the bullet may not touch the bottom of the holder.

 

Have you read the Tubb BSC guide?

 

 

Having checked my Tubb BSC I now realise that the bullet holders are tapered, therefore the bullet base does not bottom out, as such the set up does allow actual bearing surface comparison.

[1967spud]

Update: Having a wee bit of time to have a further play with this bsc dti i went on to use it in conjunction with a ogive insert from a stoney point tool. My initial goal was to further sort the weight then then beraing surface bullet tips to base to ogive sorted for more uniformity. As you can see i am trying to ring every last bit of deviation out of my reloads, (i'll need all the help i can get to stay with Elwood and 6mmbr) anyway i have done the first 176 tips listed in my groups in the original post. After trawling through all 176 tips there was less than .00015 difference betwean longest and shortest base to ogive measurements. The results were both consistant and to be honest i was surprised how good they were. I am guessing that i probably could have got similar results by only base to ogive sorting in the first place, but by doing it in the sequence i have done two operations in one. All i need to do now is shoot the dam things.