extremely frustrated trying to tap thomson rail

[tmbg]

I got past the problem of being able to drill it. A carbide spotting drill will cut the case easy enough that I can drill it with an HSS drill bit.

However, I haven't had much luck tapping these holes. A couple of them started ok, but I broke the tap. I managed to get usable threads in exactly one hole without breaking the tap, and since then, I can't make a tap even begin to cut threads in any hole. Not just one tap either, brand new taps won't cut.

It doesn't help that I chose the tiniest possible damned screw sizes to use. My 1/4" rails are #2-56, my 3/8 rails are #6-32. I can't make either tap. The one successful hole was a #6.


Any suggestions before I give up and put clearance holes in the rails and put the threads in the support?

[acondit]

Originally Posted by tmbg Any suggestions before I give up and put clearance holes in the rails and put the threads in the support?

That would be my suggestion.

Alan

[DareBee]

Usually we remove the case from the shaft.
Mill or grind a flat that is .050 deep in the area to be drilled/tapped.
It must be .05 deep at all areas where the work is to be done. This usually means the machined off area is much larger than the needed area for tapping.

They can also be EDM tapped.

Going with through holes is definitely the easiest.

[tmbg]

I've cut through the case with a number of different ways. I can cut it with a carbide endmill pretty easily, my carbide 1/4" spotting drill cuts it, I've ground flats on them on a wheel.

On the 3/8" shafts, I can cut the case with a spotting drill and clearly see the softer steel underneath which can be scratched with a prick punch. It drills fairly easily with TiN coated HSS drill bits, but the resulting hole I just can't get a tap started in no matter what I try.

On the 1/4" shafts, even though I can cut the case and drill it, I never can actually see the softer inner metal no matter how deep I drill.


I think I'm gonna give up and through-hole them. My Z car is already drilled for #2 clearance holes, but they're the same size as a #4-40 tap drill, so that'll work out. I should be able to counterbore the 1/4" shafts for #4. My Y gantry is drilled for #6 clearance holes, which is a hair larger than #8 tap drill holes, but I ought to be able to get 50-60% thread depth I'm hoping, and with 6 screws per rail and 1/2" end supports, hopefully it'll be sturdy enough.

[acondit]

Originally Posted by DareBee Usually we remove the case from the shaft. Mill or grind a flat that is .050 deep in the area to be drilled/tapped. It must be .05 deep at all areas where the work is to be done. This usually means the machined off area is much larger than the needed area for tapping. They can also be EDM tapped. Going with through holes is definitely the easiest.

If you can cut the case with a carbide end mill, you could just mill a pocket 0.050" deep with a larger diameter than the tap diameter and then tap. The main idea is to get the tap thread starting below the case hardening.

Alan

[Aplus]

Drill the Case Hardened Shaft a little larger than the clearance hole for the tap. I have used carbide tipped concrete drills and had very good results and they are only a couple of bucks.

[tmbg]

honestly I'm not sure if I can even drill all the way thru on the little tiny 1/4" rails. They seem to be hardened throughout. I cut about halfway through with my spotting drill (works as a counterbore as well), and then tried to drill thru with a carbide #50 drill bit. When it got close to the end, the rail just snapped in half

I'm starting to regret wanting to support these rails.

[quazga]

Firstly, be cautious of generating too much heat as too much heat may warp your rails. =)

I see your dilemma and I've been thinking what you could do to alleviate the problem.

I find it interesting that you were able to get one hole to tap but not the others. Could it be that you heated that hole while drilling to such a degree that you annealed the steel? Who has ideas on this?

Ok, here's what I would do if it were me and I didn't have a million dollars in tools!

To drill holes I would use brass rod turned down to the size I needed and abrasive grit suspended in a cooling fluid.

Or as Aplus said, use his idea. I doubt you'll find masonry bits that small, but you could turn them down with a diamond wheel to the proper diameter.

I'd forget about tapping anymore holes unless you can figure out how you managed to tap the first one. What percentage of thread would you say you were able to cut?

For me I'd just turn down some dowel pins in brass or steel to within 0.002 clearance, drill corresponding holes in the supports and then use epoxy to hold it all together.

If your supports are already drilled with through holes, then you can just grind the threads off your screws and do it that way.

If you were to do it this way, you'd assemble everything with the epoxy and then clamp everything until the epoxy set up.

And I assure you! You will not get it apart! Not without damaging the rails and it will be more than rigid enough being that forces involved with those tiny rails is going to be minimal.

The problem is tho, you'll never get it apart! HAHAHA! =D

You could use heat of course to get it apart, but then that will warp your rails. =)

AWW but there's a way!

Instead of epoxy, you could use a fusible alloy like this:

http://www.rotometals.com/product-p/...nt158alloy.htm

It will go back to a liquid state a low enough temperature that you wont warp your rails.

That's all I can think of at the moment.

Hope you get your rails mounted soon! =)

[tmbg]

funny you should mention that. Last thing I did before I went to bed last night was put the rails on the supports with some epoxy. I didn't use dowels pins, however, and I was a bit concerned that the extremely smooth surface of the rails wouldn't provide enough bite.

I'm about to go downstairs and unclamp them, so wish me luck!

[tmbg]

well, the epoxy didn't hold. I'm not really in a good spot here

not sure where to go from here. May have to rebuild the end caps to fully capture the rails and just leave them unsupported.

[quazga]

aww darn. =/

Yeah those suckers are pretty smooth.

Let's see..... *thinking*

The epoxy will definitely work if it can just have something to adhere to.

Maybe you could do a variation of the dowel pin method. For example, you could cut slots easier than drilling holes. Like if you had a Dremal tool with a abrasive blade and used some cooling you could cut slots say 0.1875 deep and then use sheet metal of the approximate thickness cut like biscuit jointing used in woodworking.

On the other hand, if you went with the end cap method again, you could probably get away with epoxying the support like you just did if you placed the support between the end caps.

Such that the supports would handle force directed into the supports and the end caps would handle force 90 degrees to the supports.

You'll find away. =)

[tmbg]

funny thing is, the epoxy adhered reasonably well to the shafts, it popped cleanly off the ALUMINUM SUPPORTS! They're 80 grit bead blasted, so I figured they'd have a fair amount of bite, but no!

My endcaps already have support built into them, but not axially (they're just thru-holes). I will probably end up rebuilding my endcaps so that they have blind holes. Since I don't want to try to shorten the shafts, that'll probably mean making them out of thicker material (at least one side). I don't want to lengthen the carriage body, because it'd mean remaking it and my leadscrew.