Advice for Dove tail Milling First Time

[jlsmithseven]

This is our first time dovetail slot so I am going to go slow. Any advice would be awesome. Thanks guys!

 

[ddickey]

Couldn't you use two different sizes and use the average in your calculation?

 

[Bob Korves]

You can use two different size gage pins for measuring across dovetails as long as you use the same ones for every measurement. I would try to find two that are as relatively close as possible in diameter. The pins will of course need to clear the dovetails for measuring.

 

[jocat54]

I use drill bits--close enough for most work I do.

 

[jlsmithseven]

Couldn't you use two different sizes and use the average in your calculation?

That seems like it might work, thanks for the suggestion. So say I used a .3995 and a .3985 pins (just an example of course). I would just calculate them as a .3980 pin on each side?

 

[jlsmithseven]

Here is my real life example...I need a 1.771 length right. I was planning on using a .2555 and a .2455 gauge pins. I calculated my measurement between calipers for these pins as 1.08798" using a .250 measurement in my calculations. Will this work out OK? I have a .005 tolerance. Thanks guys, any answers before tomorrow would be very helpful because I plan on tackling this first thing in the morning!!!

 

[Bob Korves]

Here is my real life example...I need a 1.771 length right. I was planning on using a .2555 and a .2455 gauge pins. I calculated my measurement between calipers for these pins as 1.08798" using a .250 measurement in my calculations. Will this work out OK? I have a .005 tolerance. Thanks guys, any answers before tomorrow would be very helpful because I plan on tackling this first thing in the morning!!!

What exactly are you measuring at 1.771"?

 

[4GSR]

When I go to measure the location of a dovetail slot. I generally have a sample to take my measurements from using a couple of harden & ground dowel pins of the same diameter. You know, within 0.0005" of each other. If I'm doing something from scratch on a drawing in cad, I generally drop in a couple of circles of a known diameter that represent the dowel pins being used. What ever the cad drawing produces for a dimension, that is what I use to machine the dovetail to. It's ok to show the 1.771" as a reference dimension. I'm just trying to help here.

Your point dimension of 1.771" is impossible in the real world to measure. A 3D laser would be about the only way to measure such a dimension. Oh, the dovetail cutter you use will have a radius on the corner. It may only be R0.001" but its there. Probably turns into a R0.005" once the dovetail cutter is broken in, or much worse. I've knocked off the sharp corners of a dovetail mills to something like R0.030", more than once, too! My new dovetail cutters use a insert that has a R.015" or R.032" on them. They sure help by not creating a stress riser in the sharp corners if the part is going to heat treat afterwards.

But to go back to your question about using two pins of different diameters, no problem as long as you don't change sides the pins are referencing from. Especially if your checking from the side of the tool block to the gage pin. I assume you have a reference tool holder to get your pin measurements from rather than trying to calculate them. The older Machinerys Handbooks have some good information on calculating measurements, lots of trig involved. So much easier to lay out in cad today!

 

[Bob Korves]

I have not done this particular job yet, but I would rough in my first block. making the depth cut dimension (still without the dovetails) slightly smaller to the flat than a factory holder, then machine the dovetails, very little at a time when close to size until it just fits the tool post. Use the tool post as a gage. When they will slide together you want the depth dimension and the gage pin to gage pin dimension to match the factory part. Go VERY slowly until you get a fit that you really like. It should be very similar to the factory holder measurements. Install first the factory holder and then your prototype holder on the tool post and tighten them down solidly. The closer handle should stop in the same clock position for both holders. If not, something is wrong. You especially do not want to see the handle swing farther with your prototype block, that would show a too loose fit. If that happens, make a second prototype to tighter dimensions until it closely matches the factory holder closed handle position. Then make a bunch more just like that one, using the same gage pin and depth measurements of your successful prototype.

 

[mikey]

I'd like to suggest something, may or may not be useful. Look for a straight rod around the shop that is 1/4" in diameter; this will hopefully be large enough to stick out of the dovetails so you can measure between them. You only need two so you can cut something in half and deburr the ends and they only need to be long enough to lodge into the dovetails ... say, maybe 1" or so. Maybe a drill blank?

We know that the base of the dovetail is 1.771" long and the angle of the dovetail is 60 degrees. The constant for a 1/4" rod at 60 degrees is 1.1830" so if you can find that rod and use it, the distance between them should be 1.771 - 1.1830 = 0.588".

I mention this because it is fairly easy to find something that is a consistent 1/4" OD, straight and can be cut in two - printer rod or a drill blank or something like that. It is also useful because most table of constants will be based on a known diameter.

Hope this helps. I'm attaching a pdf file that might be useful for your review.