Dowels for locating machine parts 1

[tstanley]

We have an older, large milling machine that has had the ways scraped once, several years ago, and is starting to wear out again. We had the table and ball screw off recently. The ball screw bearings were originally located by tapered dowels. Of course when it was rebuilt, the dowels had to be moved a little bit. The dowel holes are now slightly oval. Now when we put it back together, the dowels need to move again, a little bit.

What is the standard practice in this situation? It seems that going to larger dowels would be one way, but it is tricky to do accurately, and the dowels start to get pretty big as well, but is this the recommended way to do it?

Tom Stanley

 

[sreid]

Taper pins hold parts accurately together and the holes are reamed after the parts are aligned. This also lets you reassemble things accurately. If the alignment needs to be changed, the taper holes are then re-reamed deeper.

 

[grinderguy]

My experience is the same as the first post. We make taper pins for machine tool builders. They are generally hardened and ground, almost always have tapped holes to pull them out. The most common tapers are B&S and metric. The Basic measure for B&S taper is at the large end. The basic measure for the metic is the small end. Look in the Machinery Handbook when you determine which taper you have. I have measured soft common taper pins with tapers that vary by .007. A hardened and ground taper pin varies less than .001.

Robert Setree

http://www.advanced-grinding.com

 

[tstanley]

Thanks guys.

Our machine has the tapered dowels as grinderguy describes. It is just that the machine is now about 14 years old and has been run 7 days a week almost 24 hours a day so its wearing out. As it is wearing out the dowels have to move, and now they don't fit very well. I was hoping that there was a simpler solution as we're not really equipped with the tools to do the job as you describe.

 

[EnglishMuffin]

To enlarge the holes in situ might be difficult - even a drill with a magnetic base is not really rigid enough, since it ideally requires a milling operation prior to the taper reaming. However, it is possible that you could make some special undersize dowels with axial holes in them, and then inject Moglice or a similar Philadelphia Resin product through the hole after assembly, thus taking up the clearance. Those two companies have a lot of experience with this sort of thing, so you might get some further ideas by talking to them.

 

[tstanley]

Thank you.

Those were two helpful names.

Tom Stanley

 

[grinderguy]

We ream tapered holes with hand reamers. They are normally "in stock' items with perishable tool companies. It takes very little skill to ream the holes. Aligning the components in dificult.

Robert Setree

http://www.advanced-grinding.com

 

[unclesyd]

You can rent magnetic base drills that have more than enough rigidity for your job if the holes for the pins holes are accessible. I have a (?Magtech?) that has a 2000 pound hold down force and I know there are larger ones.
I would drill with a taper drill bit and the hand ream to size. You can get any size and length taper pin that you need.
We have made 4' long extensions to get the drill motor above the machine to drill 1" holes in 2" CI on top of 1"CS. All this work and then we had to make barrel nuts to get a wrench to work.