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MilGunsmith
08-28-2015, 10:34 AM
Is there any difference between the accuracy of a sliding v-block rail and a bearing block (Thompson Bearing on shafts) design? Is it just the cost of parts? We are using a rail gun built with Thompson Bearings for ammo testing at work and want to know if that is the best solution.

Dusty Stevens
08-28-2015, 11:31 PM
If you have .0001 of clearance in that bearing what does that equate to at 100 or 200yd? Id say its the best for ammo testing but not for railgun competition.

Jerry W
08-29-2015, 12:27 AM
If you have .0001 of clearance in that bearing what does that equate to at 100 or 200yd? Id say its the best for ammo testing but not for railgun competition.

That was probably a rhetorical question, but it raised my curiosity. A couple minutes with a CAD program provides the answer.

With that .0001" of clearance, 18" from a fixed point, you get .020" at 100 yards.

Jerry

Dusty Stevens
08-29-2015, 11:05 AM
Thank you jerry. I knew it was bad enough to not ever see em on the line.

WilliamA
02-01-2016, 10:57 AM
That was probably a rhetorical question, but it raised my curiosity. A couple minutes with a CAD program provides the answer.

With that .0001" of clearance, 18" from a fixed point, you get .020" at 100 yards.

Jerry

All of that monstrous .0001" clearance can be eliminated with a touch of side-load if you feel it necessary. The weight of the gun platform would keep itself centered on the crown of the bearing guide and the clearance would never come into play.

The majority of the railgun pics that I've seen seem to have an error built right into them by using a pivoting adjustment for windage on a 3 "rail" platform. What I see is the two guides being adjusted laterally with no compensation for the platform traveling in an arc. The result is that the distance between the grooves increases which would cause the guides to then not seat properly. A mere .001" error here would result in an approximate .200" at 100 yards down range and this would vary from shot to shot as the guide/groove "choice" would be random.

I don't shoot bench, have never touched a rail gun and will never build one. I'm just commenting on what I've seen and on the use of Thomson bearings.

W.

Dusty Stevens
02-01-2016, 12:22 PM
Railgun windage doesnt travel in an arc therefore no compensation is needed. It sits on 3 points

WilliamA
02-01-2016, 08:32 PM
Railgun windage doesnt travel in an arc therefore no compensation is needed. It sits on 3 points

From what I've seen the front point is the elevation adjustment and does not move laterally. The rear two points are adjusted laterally for windage. That means the assembly pivots on the front point. Right?

Dusty Stevens
02-01-2016, 11:59 PM
Yes it does. A point on the base sitting in a ground vee under the top

Bill Gruby
02-02-2016, 10:28 AM
Just my opinion but I think he is saying that the lateral move of two of the three points will change the geometry of the triangle created by the three points of the rail. When the geometry is changed, the location of one or more points will change also, but the change is so minute it goes un-noticed. Hope I got that right. I pondered this for a while and concluded it makes no difference.

"Bill"

Bill Gruby
02-02-2016, 11:20 AM
Sorry for the pic but I think this is what William is trying to convey.

WilliamA
02-02-2016, 11:21 AM
Just my opinion but I think he is saying that the lateral move of two of the three points will change the geometry of the triangle created by the three points of the rail. When the geometry is changed, the location of one or more points will change also, but the change is so minute it goes un-noticed. Hope I got that right. I pondered this for a while and concluded it makes no difference.

"Bill"

You understand my observation to a tee but when you consider this thread was "settled" with a .0001" variance beiing too large I can't see how this error would make no difference. I'm sure the shot-to-shot accuracy relies heavily on the two rear points to rest and track in their grooves yet the mere adjustment of, say, .001" windage would increase the groove separation distance by that same amount. Down range that may cause up to .200" MOA just from "wander" from the guide points trying to seat.

Dusty Stevens
02-02-2016, 07:15 PM
The front and one rear sits on a vee. The other rear sits on a flat. You should check one out sometime and see how they work. You move the back end over 1" and its still the same. If you look at one youll see that it doesnt bind like youd think. Sitting on 3 points maybe but the flat on one side of the back takes care of that

Bill Gruby
02-02-2016, 08:55 PM
I kind of figured that the flat side was to compensate. Thank you Dusty. I learn a little more every day.

"Bill"

Dusty Stevens
02-02-2016, 10:22 PM
It does a lot of things- if your width is off by .001 no problem. Automatic temp compensation

WilliamA
02-03-2016, 02:48 AM
The front and one rear sits on a vee. The other rear sits on a flat. You should check one out sometime and see how they work. You move the back end over 1" and its still the same. If you look at one youll see that it doesnt bind like youd think. Sitting on 3 points maybe but the flat on one side of the back takes care of that

That's something that doesn't show in the photos. Thanks for the clarification and correction.

W.