Gallery, Projects and General > How do I??
Reaming Blind Holes?
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Pete W.:
Hi there, all,

I'm ultra-embarrassed to be asking this - I ought to know the answer!   :doh:   :doh:   :doh:   :doh:   :doh:   :doh: 

But I've totted-up over 70 orbits round the sun without ever having to do this or having the opportunity to see anyone else do it.  So, here goes:

I'm in the process of fitting a Quick Change Gearbox (I know John Stevenson has a different name for them) to my Myford ML7.

Myford recommend adding a couple of dowels to the tail-stock end lead-screw bracket.



The holes in the lathe-bed have got to be a good fit for the dowels so I assumed I'd drill and then ream them.

Then I thought, 'wait a minute!  A reamer has a lead on its front end.  I don't want to drill too deep a hole so how do I get the holes parallel for the whole depth occupied by the dowel?'

Please, what's the answer???
awemawson:
A/ Use a taper reamer and a taper pin

B/ Use a rolled dowel and just drill and tap in with toffee hammer

C/ Drill hole deep enough that lead or reamer is accommodated and use parallel dowel pin
wheeltapper:
Are machine reamers not parallel all the way ( except for a tiny chamfer at the tip ) ?

Roy
vtsteam:
Chamfer the tip of the dowel.


ps. I'd just trial drill a piece of scrap with a new bit and check my dowel pins in that. If the fit was good,I'd just drill. you don't need to ream.

Also, in general if you had a pre-drilled hole that's too loose on a dowel, it's a simple matter to turn a dowel pin to fit. It's easier to make a peg accurate than a hole to fit it. In fact go-no-go gages exist for that reason.

Lew_Merrick_PE:
OK, assuming the material involved has a sufficient shear strength (i.e. not zinc or some other pot metal), drill through everything for the dowel pin 1/32 or 0.75 mm undersize.  Counterdrill to depth using a drill that is .010 to .016 (inches) or 0.25 to 0.4 mm undersize.  The to depth means that the flank of the drill bit goes to a depth equal to the full length of the pressed in dowel pin plus a distance equivalent to the undersize of the counterdrill.  Now when you ream the hole (at half the rotational speed and twice the infeed used for a drill) you will catch on the lip of the counterdrilled hole which should stop your reamer at the correct depth.

The 1/32 or 0.75 mm undersize of the through hole assures that you can get a pin punch through from the back side should you ever need to remove the dowel pin.
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