# Dovetail Tolerance



## Baithog (Feb 16, 2016)

Nothing is ever exact. There is always some variation. A dovetail measuring 6.000000" at one end will not be 6.000000" at the other. How much can a dovetail member taper before it locks up? How much before it gets too sloppy for a hobby level slide?

I am still waiting for some of my tooling to start scraping the X-axis, so I started setting the y-axis up for machining. I'm an 1/8 of an inch or so short on Y travel on my mill/drill unless I take the DRO scale off. So, if I pull the base off the mill, turn it around for the second side, how close do I need to get it?


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## Kernbigo (Feb 16, 2016)

They way you check doveltails is with 2 dowl pins ,and mike accross


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## coffmajt (Feb 16, 2016)

It somewhat depends on how much precision you want in your slide.  I would not want more than .001 in variation from front to back on a dovetail that will be used as a slide, and I don't think that should be a problem for a milling machine -- Jack


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## Kernbigo (Feb 16, 2016)

That is true .001 max


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## 4GSR (Feb 16, 2016)

Baithog said:


> ....snip...... So, if I pull the base off the mill, turn it around for the second side, how close do I need to get it?



You need it running "zero" from end to end when indicating it in, going off the previous machined surface.  When you pick up the cut, leave your cutter about .0005"-.0010" off of the previous cut surface.  Use a sheet of cigarette paper to touch off the milling tool with.  Don't let it cut into the previous cut surface.  Scraping will remove that last .0005-.0010" when fitting.

Ken


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## Baithog (Feb 16, 2016)

That makes me feel better about breaking down the base and managing to get the second side reasonably parallel. I may still remove the glass scale so I can mill it in one setup.

As milled, before scraping, the x-axis table is .ooo4/ft and the saddle is .0005/ft.

The base, as shipped by the manufacturer, is .0004/ft guide side to gibbed side, end versus end. Pretty good, except the flat shinny potions n the middle where it is out .001. The y-axis saddle is is out .003/ft. No wonder the thing never worked right.


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## tertiaryjim (Feb 16, 2016)

After scraping in my lathes cross slide, I found a tight spot. Didn't measure it but it took less than 1 tenth of a thousanths further scaping to clean it up.
The gibbs can only be tightened so much as there must be room for oil but if you have to gap them a few tenths extra because they aren't true, you have that much more
play and less rigidity.
Even with my amateur efforts, the lathe will now produce better finishes. Turning and parting operations are faster and smoother.
I started out scraping in the compound with a reground cutoff blade. The result was so dramatic and the work so difficult, with that blade, that I knew I must get some better tolls to continue.


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## Bill C. (Feb 16, 2016)

Baithog said:


> That makes me feel better about breaking down the base and managing to get the second side reasonably parallel. I may still remove the glass scale so I can mill it in one setup.
> 
> As milled, before scraping, the x-axis table is .ooo4/ft and the saddle is .0005/ft.
> 
> The base, as shipped by the manufacturer, is .0004/ft guide side to gibbed side, end versus end. Pretty good, except the flat shinny potions n the middle where it is out .001. The y-axis saddle is is out .003/ft. No wonder the thing never worked right.



If it was my part I would call the manufacture about their poor inspection practices and see if the will replace this one for a good one.  Just a thought.


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## Baithog (Feb 16, 2016)

> If it was my part I would call the manufacture about their poor inspection practices and see if the will replace this one for a good one.  Just a thought.



I should have done that 7 years ago when I bought it. _I knew I was getting a Chinese project machine, but this one turned out to be worse than normal. It will end up a little CNC mill when I'm done._


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## Baithog (Feb 16, 2016)

tertiaryjim said:


> After scraping in my lathes cross slide, I found a tight spot. Didn't measure it but it took less than 1 tenth of a thousanths further scaping to clean it up.
> The gibbs can only be tightened so much as there must be room for oil but if you have to gap them a few tenths extra because they aren't true, you have that much more
> play and less rigidity.
> Even with my amateur efforts, the lathe will now produce better finishes. Turning and parting operations are faster and smoother.
> I started out scraping in the compound with a reground cutoff blade. The result was so dramatic and the work so difficult, with that blade, that I knew I must get some better tolls to continue.



So what did you use for a straight edge to spot the dovetails? There's no way I can afford a camel back and I doubt anyone would loan me theirs


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## tertiaryjim (Feb 17, 2016)

I machined and scraped a 60 deg. straight edge from a 26" long piece of Durabar. It started out as the oversized 1' x 2" and I hollowed it out to reduce weight.
If the ends are put on parallels it sags three tents in the middle . A camel back wasn't a reasonable option for me either but it would be a much better tool.


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## chips&more (Feb 17, 2016)

Kernbigo said:


> They way you check doveltails is with 2 dowl pins ,and mike accross


Using two dowel pins is a good start. But by no means will it give you the true representation of the dovetails overall dimensional landscape. And this is an interesting topic. I have spent days scraping in dovetails for that perfect fit. Only to find that after a few months of use and re-adjustment of the gibbs that the ends of travel tighten up again. Maybe that perfect fit is not possible and just live with it…Dave.


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## Baithog (Feb 17, 2016)

I thought about that, but have read that it would not be stable and would sag too much. Great to hear that someone has had success doing it. I was thinking that it would help to put it through a self-clean cycle on the oven (~800F) between rough  and final machining to help get some of the stresses out. I will see if anyone in central Florida has a real 18" straight edge when my diamond wheels finally get here. If not, then I'll do the cast bar route.


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## tertiaryjim (Feb 17, 2016)

The piece was very unstable. I would  get it close and it would warp.. Finally had to put it through several sets of heat soaks in the kitchen oven at 550F.
That seems to have given it some stability but it will need to be checked prior to each use.
Was surprised when it showed only three tenths sag over 26". Could have lightened it more.  A shorter bar should have far less problems.
The bar had a tendency to bow down on the ends and go  high in the middle of the 12" long cross-slide. Got better readings when the cross-slide was on edge
so the 2" width of the bar supported itself.
Was using dowel pins in the dovetails to measure them but have yet to build a indicator jig and my mic wouldn't lay in there.
So, had to use the dial calipers.  Poor option but it worked.  Machining a good gib was also a chore.


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## astjp2 (Feb 19, 2016)

I would use a variation of a king way alignment tool and modify it to you application, sue durabar of different diameters and use the locks and rods that are adequately heavy enough to not flex which keeps the measurements accurate to .0001


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## chevydyl (Feb 20, 2016)

I bought an 18" camelback casting from greg dermer I think is his name, 75 bucks plus 18 for shipping. It's styled like a brown and sharpe, except it has a thick flat that's cast with the ability for a dovetail angle. I'm really pleased with it. I have to machine the dovetail edge on it still then will start scraping. It comes stress relieved, it's recommended to rough machine then stress relieve again, then finish machine, the scrape. I'm going to try it without the additional stress relieve we'll see what happens.


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## 4GSR (Feb 21, 2016)

astjp2 said:


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The King Way alignment fixture is a nice tool to have. Rather it is original or home made.  But the way you are using it in your pictures, using the tailstock ways as your datum plane, you may/can get false readings, and you probably are.  Not trying to create an argument here, just saying.  How do you know that the tailstock ways are straight without any wear?  Did you confirm they were straight? or just assume they are thinking they did not appear worn as the carriage ways were?  If you did confirm they were good and straight, how did you verify this?

Ken


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## tertiaryjim (Feb 21, 2016)

After all the work and problems I had to deal with, using durabar, I would suggest that if there is any reasonable way to get a camel-back, do so!
Machining and scraping in a 60 deg straight edge is a big investment in time and labor and a camel-back will be more rigid and better serve you.
_Something I had forgotten is the last time I put it in the oven I decided it wasn't hot enough so I turned on the broiler and let it heat for 1 1/2 hrs before
turning it off and letting it cool in the oven.
Where did this script come from? I don't know how it happened. It just started. Musta hit a wrong buttun _


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## 4GSR (Feb 21, 2016)

I have a 36" straight edge made out of a piece of DuraBar that I bought back in 1997.  It sat outside in the hot South Texas sun for at least ten years before I started machining on it.  Scraped it flat on the two edges two years ago.  Hung it from one end and beat on it like Richard King said to do with a rubber mallet.  Checked it again, still good. I had a local shop I deal with machine the straight edge under my supervision so it would come out straight without too much warpage.  I don't recall the amount I had to scrape to get it straight, but was less than .002" in 36".  I thought that was darn good.  They got it parallel end to end, but missed it side to side, its out about .005".  I can live with that since I'm not depending on it for anything that would require measurement in that plane.  I guess from my experience about cast iron being left out in the sun for a year or so to "season" the iron is true!


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## chevydyl (Feb 21, 2016)

Ken, how would I season iron in alaska lol


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## 4GSR (Feb 21, 2016)

chevydyl said:


> Ken, how would I season iron in alaska lol


Yeah, not funny, isn't it?

Wish i could trade you some of your cold weather for some of our hot weather we get here.

Seriously, one way would be to build a fire in a barbeque pit, you have them up there don't you? and set the piece of cast iron in the bed of coals.  Come back in a couple of days, remove and it should be ready to go.  Of course, I know you guys don't have a supply of Mesquite wood like we do down here.  Coal works too!  Don't let it get too hot, it could melt cast iron!


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## chevydyl (Feb 21, 2016)

I have a wood fired furnace that heats my house, everything really, even the hot tub


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## Baithog (Feb 21, 2016)

I read a Navy report on stress relieving cast iron written back in 1948. They were primarily concerned with heat treating. That probably resulted from the need to get weapons systems out of manufacturing quickly. They did look at the aging effect. They first tried aging indoors at the foundry. Little relief was found. On a hunch that it had more to do with being outdoors, they tried heat cycling for 0F to 220F and achieved minimal reductions, but only cycled a few times. They also tried simulating corrosion by acid etching the surface with some success. One other thing they found was that dumping the mold quickly generated less stress in the castings. Taken together, I would suppose that the stress is injected by differential cooling in the mold. Removal from the sand and environment conditions outdoors allows the metal to relax. The stuff we normally get for machining is a different animal from mold cast iron. Durabar is hot extruded and cooling is slower and more controlled. It should contain less stress to start with. Alaska gets some pretty good temperature swings from winter in the interior to sitting in the sun in the brief summer. Like the Navy, I don't have years to play around with years of sitting it out by the garden, so I'll go with the recommended heat treatment... or at least as close as I can get. The stability of Ken's straight edge is encouraging.

I didn't find any empirical information on ringing the casting, and I've never heard of Ford hanging engine blocks from the overhead and beating on them with big mallets. I will rap my proto straight edge a few times just in case though.

I have enjoyed the discussion, although I still don't know what bridgestone's QA people had for +/- specs on their dovetails... or any of the other manufacturers either.


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## 4GSR (Feb 22, 2016)

Baithog said:


> ....snip....I have enjoyed the discussion, although I still don't know what bridgestone's QA people had for +/- specs on their dovetails... or any of the other manufacturers either.



Each manufacture pretty much establishes the tolerances they want it to be for the machining part of it.  Once that is done, if they specify scraping, then that is a totally different level of specifications.  Most follow Dr. Slenger's (sp) book of accuracy in a machine tool. This is the book you use to determine the result accuracy and from that determine what measure to take to correct by scraping and rechecking.  

For me, if I can get the readings to hold within a .001" total indicator reading or better, I generally call it good.

Ken


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## astjp2 (Feb 28, 2016)

4gsr said:


> The King Way alignment fixture is a nice tool to have. Rather it is original or home made.  But the way you are using it in your pictures, using the tailstock ways as your datum plane, you may/can get false readings, and you probably are.  Not trying to create an argument here, just saying.  How do you know that the tailstock ways are straight without any wear?  Did you confirm they were straight? or just assume they are thinking they did not appear worn as the carriage ways were?  If you did confirm they were good and straight, how did you verify this?
> 
> Ken



Well I had .008 drop at the headstock on the original bed using the least worn ways to measure the carriage ways on, the ones that the carriage rode on were really worn.  I also checked the dovetail ways using the carriage ways to measure from.  The new bed I got from Joe @ Plaza machinery was only worn .006.  Its only a little time to check all of the ways in as many configurations as I could think of and write down the numbers on the bed with a  sharpie.  I can visualize it that way.  I don't have a granite surface plate large enough to measure the bed so I did the best with what I was able to make and the project of making the way alignment tool and actually figure out how to use it.  It was a fun but somewhat time consuming project and slightly expensive buying used indicators and getting someone to fix them.  Sometimes it not the end game, its the path to get there!!!  I am the same way with antique airplanes... Tim


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## 4GSR (Feb 29, 2016)

The easy way to check for wear on any bearing surface on a lathe bed is to check against an unworn surface.  It does not necessarily need to be a way surface.  The areas between the ways are fine to use.  Some people say no, don't do that, in 99% of the cases I've seen over the years, its fine to do so.  I reground a bed using the unworn surfaces as a guide for the sled that held the grinder.


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## astjp2 (Feb 29, 2016)

Whoops, I meant .0006


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## 4GSR (Mar 1, 2016)

astjp2 said:


> Whoops, I meant .0006


Put it back together, call it good, and don't look back!


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