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Dovetail Dimensions (Read 3942 times)
SSShooter
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Dovetail Dimensions
Jul 6th, 2011 at 2:16pm
 
Is the dimension of a dovetail the distance, front-to-back, of the dovetail base?
Thanks.
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Re: Dovetail Dimensions
Reply #1 - Jul 6th, 2011 at 3:20pm
 
Nominally, but not always.  So-called 3/8" rimfire scope mounting bases are about, more or less, 3/8" at the narrowest part and closer to 1/2" at the wide part.  Sight dovetails are also nominal...I've seen sights sold as 3/8" that were as tight as .345" across sharp points to as big as .395".   Shocked  Huh

David
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David Kaiser
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Mike_Hunter
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Re: Dovetail Dimensions
Reply #2 - Jul 7th, 2011 at 10:41am
 
Also keep in mind that the DT cut may not actually be .375.  Cutters wear, and I’ve run across a bunch of milling cutters that are not exactly .375,  almost always undersize, some as bad as .365 for a brand new in the box cutter.
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Re: Dovetail Dimensions
Reply #3 - Jul 7th, 2011 at 3:06pm
 
Mike, the Brownells 3/8" sight dovetail cutters are sold as .359", just to be able to make the dovetail fit an undersized sight.  Easier to take two passes than to have to weld a slot or make a sight because your cutter was oversize!

David
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David Kaiser
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Re: Dovetail Dimensions
Reply #4 - Jul 7th, 2011 at 4:22pm
 
David

Not sure I understand the rationale behind that,  if it’s a 3/8 DT cutter it should measure .375 ,  I can understand .375  +0/-.003 as long as it’s noted.  But a 3/8th DT Cutter that’s .016 undersize?    That’s like buying a quarter inch end mill that won’t cut  .250 or a half inch drill that won’t make a .500 inch hole.

Guess they expect you to make multiple passes in the mill and some trial fitting after each pass or spend 15 minutes with a safe sided file to make a sight fit. OR  you can pound the sight hard enough to compress  .016 worth of steel, but it aint gonna be pretty.

That’s why I don’t buy from Brownells, besides they are too expensive .

Since I cut quite a few dovetails, I generally buy the cutters by the dozen.  I had been using  PTG’s DT cutters, but lately ENCO has had their “Import” DT cutters on sale for slightly less than $13 ea.  The ones that they have labeled as Interstate brand are made in Japan, and very good quality, generally .375  +0/.002-.003 that will produce a nice tight dovetail, where it won’t take too much force to move a sight.
I absolutely refuse the buy the China or India MFG cutters or any type.  I’ve seen the size range from +.008 to -.02


V/R

Mike

Mike Hunter
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BP
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Re: Dovetail Dimensions
Reply #5 - Jul 7th, 2011 at 5:35pm
 
Hmm...

Everybody does things a bit differently, which always makes these posts interesting.

My method has always been to make a pass straight across with a small dia standard endmill that is not quite to full depth, and then make the finish cuts to full depth with the dovetail cutter. Always figured this put a lot less stress on the dovetail cutter's fragile teeth.

One other thing - I ran across a prewar copy of an old well known manufactures sheet that shows their barrel dovetail spec had a slight included angle tapering from right to left across the barrel, so the dovetail cuts I make are not a straight-across cut, per the spec.

Probably doesn't matter that much in the grand scheme of things though.

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Re: Dovetail Dimensions
Reply #6 - Jul 7th, 2011 at 6:01pm
 
Mike, the rationale behind the undersized cutter is that the sight makers don't keep to any sort of tolerance for dovetail sizes.  I've had sights from the major makers (Marbles, Lyman, Redfield and Burris) that were sold as having a 3/8" dovetail slide right through a 3/8" dovetail ramp without even slowing down.  I've had the same makers sights and ramps come together with .025" worth of fitting needing to be done on 'em. 

If you've got a handful of sights all the same height and bead diameter and color in stock and need to fit one to the barrel of a .22, it would be lovely if they were all the same dovetail size, but unless you make your own sight blades, they won't be!  So, in this case, either you measure the sight and use an undersized cutter to make two passes and get it right, or you go with the standard cutter and too late find that you've got a dovetail too big for any of the sights in stock.  (I admit, in my younger years I sometimes failed to measure a sight dovetail!)

BTW, I have no connection (now) with the B company.  I use some of their tools because they are appropriate for the job, and I can walk to their office from home so I pay no shipping fees. 

BP, if you measure new sight blades now, you may find some that are tapered, but most will be straight.  If you've got a good fit, a taper is not necessary.   And yes, I rough the cut with a straight end mill, as do most gunsmiths.  Straight end mills are cheap, dovetail mills do get pricey!   Wink 

David
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David Kaiser
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Re: Dovetail Dimensions
Reply #7 - Jul 7th, 2011 at 9:07pm
 
So I’m assuming that “the B Co”  intentionally has cutters made undersized for that reason? 

Dovetail cutters are used for a lot more applications other than gunsmithing, most machinists wouldn’t put up with that type of tolerance in tooling.

Fortunately I generally stick with making Winchester and Colt barrels; believe it or not Winchester held their sight tolerances fairly tight, all of their sights were within .002-.004 pretty impressive considering the tooling that they had 100+ years ago.  And it’s a sad state that modern sight manufacturers decided not to hold tolerances.

Just thinking to myself, right now I’ve got to make ten barrels, a few 86’s, couple 76s and a couple of HW barrels.  Each barrel has at least three dovetails; each dovetail cut requires two operations:  rough cut with a ¼ inch EM and a final cut with the DT cutter.  So for 10 barrels, that’s 30 dovetails, and 60 operations.  No way in the world am I going to hand file 30 dovetail slots to make parts fit.  It’s very seldom that I have to take a file to a DT slot when using original sights, when I do,  it’s generally due to a worn cutter.

V/R

Mike

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Re: Dovetail Dimensions
Reply #8 - Jul 7th, 2011 at 9:49pm
 
Yep, that's why they advertise it as a .359" cutter.  And you are right, most industrial applications are using tighter tolerances than our industry when it comes to dovetails and a lot of other things. 

David
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David Kaiser
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Re: Dovetail Dimensions
Reply #9 - Jul 7th, 2011 at 10:47pm
 
Mike,

One downside is that since the introduction of CNC, it's too easy to add a cutter comp for sloppy toleranced tooling. Makes it rough on the smaller shops that don't have the budget for fancy equipment.


David,

Sounds like a set of male and female sight dovetail go no-go gauges are in order, and maybe Brownells should supply them. Then again, "the B Co" might be afraid of a high number of sight returns, and would avoid the idea like the plague.
       Grin
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Re: Dovetail Dimensions
Reply #10 - Jul 8th, 2011 at 10:20am
 
I'll let you suggest it to them.  After being downsized almost four years ago (after twenty years with them), my relationship with them is limited to purchasing products when necessary.   Roll Eyes

David
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David Kaiser
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Re: Dovetail Dimensions
Reply #11 - Jul 19th, 2011 at 1:59am
 
For what it's worth, I ran into some dimensional differences when installing new front sights on both Browning and the more recent Winchester 1885s made my Miroku, and wrote up a short article on the subject for my web site.  The following is a paragraph from the article.
==============================
If you check with Browning or Winchester, they will likely inform you that the BPCR dovetail slot is 3/8” with 60 degree shoulders and is cut to a depth of 0.090"; all of which are accepted as an industry standard on modern firearms.  But knowledgeable gunsmiths are aware that factory dovetails are slightly oversize on Miroku rifles.  They are also tapered.  In my book on the Browning BPCRs I measured several rifles and reported the range of dimensions.  Nominally a 3/8” dovetail will measure 0.375”, but the right opening of the Miroku/Browning dovetails ranged from 0.380” to 0.384”.  I recently checked the dovetails on several Miroku/Winchester BPCRs and confirmed approximately the same range.  I also measured the left ends and found the width to range from 0.373” to 0.377”, confirming a slight taper.  Therefore, be sure to follow the industry standard process of removing an existing sight by drifting it out from left to right and installing it from right to left.
============================

Wayne
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Re: Dovetail Dimensions
Reply #12 - Jul 19th, 2011 at 7:17pm
 
I also use the Brownells dovetail cutter, and for the very reasoon David mentioned; not all sights are equal. As for go- no go gauges, they wont work unless all the sights are equal.
I usually cut the dovetails for the sights I plan on using, so I use that sight as a gauge for the dovetail I cut. I just make the pass, and gradually open it to make the sight fit. If I make them to a certain standard size, then the one I want might be sloppy. I never cut them until I have the sight in my hand.
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Re: Dovetail Dimensions
Reply #13 - Jul 19th, 2011 at 7:56pm
 
Vall,

Even though some sights have different angles from the more standard 60 degree, the dovetails were usually measured over specified wire sizes, at a specified percentage across the dovetail.

My reason for suggesting go no-go gauges is it might get the manufactures to think about eliminating the slop.
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Re: Dovetail Dimensions
Reply #14 - Jul 21st, 2011 at 10:00am
 
I think you're right for 90% of the sights, but I can get into my collection of sights and try them all into a particular gun, and some will be snug, while others will be either tight or loose. It's just not a guaranteed thing, so I always stick with fitting them to the sight I'm using.
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