Reaming Blind Holes?

[Pete W.]

Hi there, all,

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

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.

[Meldonmech]

Hi Pete,

            When making your 1/4 inch dowels drill and tap the end M4 or equivalent, to provide for extraction. A small piece of tube, a large washer and an M4 socket head cap screw will easily remove the dowels when required.
             I have recently fitted a screw cutting gearbox to my Raglan 5inch Lathe.
                                               
                                                          Good Luck   David
 

[DaveH]

Well I'm lost 
Parallel dowels are not supposed to come out - are they?
I thought the idea was for locating, looking at the drawing the two dowels stay in the base (part). The top part comes off with the two screws, the two dowels are a good fit in the top part but stay in the base part, which enables the top part be replaced accurately.  Now if one is going to "knock" them out (the dowels) then what was the point of putting them in.

Or have I got this all confused 
 
DaveH
 

[S. Heslop]

Would a D bit reamer work? As far as I know they arent usually tapered.

[Meldonmech]

Hi Dave,
               To remove the lead screw on a lathe it requires the lead screw support bracket to be removed by sliding it off to the right, which can only be removed by extracting both the screws and the dowels, as the lead screw has to slide through the apron there is not not enough room to remove the bracket with dowels attached to it. As you say this not the way dowels are normally used, but there are cases like this where it is necessary for dissembly, and maintenance.

                                                       Cheers David

[Pete W.]

Hi there, all,

Thank you to everyone who has responded to my post.          

There are a couple of points I'd better clarify:

First of all, this is a hand-tool job.  There's no way I can get the lathe on its side up on the drilling machine table!!

Secondly, the gearbox fitting instructions imply that the dowel holes should be drilled blind, not right through.  The idea that the dowels could be 'knocked out from behind' would require that the holes be drilled right through the front wall of the bed, across the central gulley and through the rear wall of the bed - I don't think Myford intended that.

I don't know if later ML7 & Super 7 production models left the factory with the lead-screw bracket dowelled.  From the way the dowelling procedure is integrated into the gearbox fitting instructions, I'd guess this procedure wasn't done in production on most of the ML7/S7 population.

A member of my family who has a Super 7 with the dowels fitted complained to me that his come loose!            

The rationale for the dowelling mod is now probably lost in the mists of history.  All I can conjecture is that lathes with gearboxes get more powered exercise to their lead-screws and this takes its toll on the security of the right-hand lead-screw bracket.

Regarding David's point, the gearbox fitting procedure requires that the lead-screw be shortened.  In the case of the Super 7, the RH bracket is removed and the lead-screw withdrawn to the right.  In the ML7 case, the RH bracket remains in-situ, the lead-screw retaining nut is removed and the lead-scre is withdrawn to the left.

[lordedmond]

machine reamer will do the job just fine , did the same conversion to my ML7

to answer your question on how to get them out , just drill a hole in the end of the silver steel  dowel that you have made and tap it say 6BA  when you want them out screw a screw in and pull with pliers


Note that is how they are on a factory fitted QC at least thats how it is on my conny


one last point you do not need to remove the lead screw from the lathe just get the hand powered parting off tool out ( hack saw ) and lop it off in situ you will not see it as its hidden

I hope you have the 24 t feed gear for the banjo cluster


STuart

[Lew_Merrick_PE]

Parallel dowels are not supposed to come out - are they?
I thought the idea was for locating, looking at the drawing the two dowels stay in the base (part). The top part comes off with the two screws, the two dowels are a good fit in the top part but stay in the base part, which enables the top part be replaced accurately.  Now if one is going to "knock" them out (the dowels) then what was the point of putting them in.

Dave,

Cylindrical dowel pins are made slightly (.0005/.0002 inch) oversize to press nicely into the part to provide alignment.  However, they can be removed so long as a the press is not too intense.  You can then re-press them several times without having to go to an oversize dowel pin.

The tool & die industry makes special pull dowels that have (A) a groove (usually a spiral) around the OD to break the vacuum of a blind hole and (B) a tapped hole at the center such that you can use a screw and stack of washers to pull the dowel pin.  Commercial dowel pins are case hardened.  So, unless you break through the case, they are a RPITA to drill (and worse to tap).

American industry makes standard press fit and slip fit reamers for inch-sized dowel pins.  Slip fit reamers are usually .0015/.0010 inch oversize.  If you look hard enough, you can find such reamers sized for different materials (LC steel, HC steel, aluminum, etc.) though LC steel is the most common set to find.  They may have disappeared from the market, but they used to make dowel pin reamers for hand usage (as opposed to machine usage).  The difference is that they had between 2X and 1.5X the major diameter ground cylindrically before starting the back-taper (about .002/inch) that reduces friction.

I had to make my own fluted dowel pin reamers for tool steel, aluminum, and magnesium as an apprentice.  It is amazing how lessons of that variety stick with you...

[Pete W.]

Hi there, Stuart,

SNIP

one last point you do not need to remove the lead screw from the lathe just get the hand powered parting off tool out ( hack saw ) and lop it off in situ you will not see it as its hidden

STuart

Thank you for your reply.  Actually, I have a couple of gearbox length lead-screws.  I just need to check that they're the right diameter (5/8" for the ML7?) and then pick the least-worn one.

SNIP

I hope you have the 24 t feed gear for the banjo cluster

STuart

That could be a problem!  I've had this gearbox a few years now and, if I do have the 24T gear, I haven't the slightest idea where it is!!      

Maybe in the kitchen cupboard behind the cornflakes?             

Confession Time:  I haven't drilled and tapped the gearbox fixing holes in the lathe bed yet.  (However, I do have an improved version of the Beeston Myford drilling jig.)

[flutedchamber]

If you pack the flutes of the ream with beeswax when you ream a blind hole the was forces the chips up and acts as it's own lube.  Sounds odd, but works very well.

[holmes_ca]

Hi there, all,

Thank you to everyone who has responded to my post.          

There are a couple of points I'd better clarify:

First of all, this is a hand-tool job.  There's no way I can get the lathe on its side up on the drilling machine table!!

Secondly, the gearbox fitting instructions imply that the dowel holes should be drilled blind, not right through.  The idea that the dowels could be 'knocked out from behind' would require that the holes be drilled right through the front wall of the bed, across the central gulley and through the rear wall of the bed - I don't think Myford intended that.

I don't know if later ML7 & Super 7 production models left the factory with the lead-screw bracket dowelled.  From the way the dowelling procedure is integrated into the gearbox fitting instructions, I'd guess this procedure wasn't done in production on most of the ML7/S7 population.

A member of my family who has a Super 7 with the dowels fitted complained to me that his come loose!            

The rationale for the dowelling mod is now probably lost in the mists of history.  All I can conjecture is that lathes with gearboxes get more powered exercise to their lead-screws and this takes its toll on the security of the right-hand lead-screw bracket.

Regarding David's point, the gearbox fitting procedure requires that the lead-screw be shortened.  In the case of the Super 7, the RH bracket is removed and the lead-screw withdrawn to the right.  In the ML7 case, the RH bracket remains in-situ, the lead-screw retaining nut is removed and the lead-scre is withdrawn to the left.

Pete If I were doing the job I would use Tapered Dowels, if the original line up is not quite spot on, you can relocate and ream a little deeper and you can reset the hole with a tapered reamer, you only need 1/4 inch depth for a good solid dowel location, you can't do that with a parallel dowel, think about it, and it will make sense, and just a light tap on the plate or a small thread in plate for ejection and its off again,

Walter Maisey.......Alberta......Canada

[DavidA]

Dave and David,

My Red Lathe has two taper pins to locate the leadscrew support bearing housing plus two screws to hold it tight to the bed.  The taper pins protude slightly so I can get a hold of them with mole grips and pull them out if I need to remove the housing. As they are for location only, I don't knock the pins in too hard.

Dave.

[Pete W.]

I'm amazed that this thread is still getting replies - thank you all very much.