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Machining a Drawbar

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ddickey

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#1
I want to make a new drawbar for my Bridgeport clone.
I was thinking about buying a piece of 7/8" stock but that is a fair amount of stock removal. ~.380" over 18".
The other option is to buy 1/2" 2' long pressing/pinning the larger hex end.
Ideas?
 

Buffalo21

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#3
the drawbar I made for the Rockwell vertical mill, I used a piece of 7/16”-20 B7 hardened all thread and made the nut for the top.
 
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Bob Korves

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#4
Use good steel rod for the drawbar, perhaps something like 1144 stressproof, or even something partially pre-hardened, but not too hard, as the collets are also hardened. The rod needs to be straight. The threads of a drawbar take a load and are tightened and loosened many times over the life of the drawbar. Softer, gummier alloys are not a good idea. If the threads gall or deform you will be frustrated using it. I would not machine the drawbar in one piece, but rather thread or otherwise lock the hex nut to the rod. That could be pinning it, welding nut to rod at the top, using permanent Loctite on threads, or whatever works best for you.
 

BROCKWOOD

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#5
I am just coming around to finishing my drawbar for my mill. I did it in 2 pieces because I had a lot of the small diameter & just a piece for the large nut. All that is left is to machine the mating surfaces. Is there a better way than an interference fit + a pin? Here they are, the 2 pieces waiting for final lathe work.

20170614_191356245.jpg

I sweated most over cutting the threads as my Grizzly lathe / mill combo is a challenge for precision work. Most happy with how the hex came out though! Really happy with the threads as well as they are spot on accurate.

My mill actually came with several drawbars - but the simple versions are not really for my mill as they are too long. This project fits my needs! So, I say, use what you have to make what you need!
 

benmychree

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#6
Best is to thread both ends of the drawbar, thread a extra long nut on the top and pin it, either radially or axially, I have done both, and they work fine. I think using all thread is tacky, not good practice. Using stressproof or 4140 HT is best.
 

ddickey

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#7
What about 01? It's locally available at Fastenal.
I don't have anything long enough otherwise. Would have to order.
I was thinking 1/2" should fit nicely into the spindle then turn down to 7/16 for the threading operation on the end.
The hex part needs hardening, you think the 7/16 threads do also?
 

benmychree

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#8
What about 01? It's locally available at Fastenal.
I don't have anything long enough otherwise. Would have to order.
I was thinking 1/2" should fit nicely into the spindle then turn down to 7/16 for the threading operation on the end.
The hex part needs hardening, you think the 7/16 threads do also?
I would go with 1/2" threads at the top end; I see no need to harden the nut on top, although they do tend to mushroom over with hammering on the end to release the taper, I use an end wrench on it, so the mushroom makes no difference, but do cut a healthy chamfer on top of the nut.
O-1 steel would be better than 1018, but without heat treatment, it is not a lot harder.
 

ddickey

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#9
I suppose a 1/8" spring pin would be to small?
 

benmychree

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#10
It would be OK if used axially, should be larger, 5/32 or 3/16 if used radially.
 

ddickey

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#11
Not sure how you would use axially. how would that negate the twisting forces of tightening and loosening?
Thinking again about using a single piece. It would be good practice using a follower rest plus it would insure you bar is straight.
 

benmychree

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#12
Not sure how you would use axially. how would that negate the twisting forces of tightening and loosening?
Thinking again about using a single piece. It would be good practice using a follower rest plus it would insure you bar is straight.
When doing the pin axially, you just drill the hole in between the threads, from the end, it acts as a key would in a shaft would.
 

ddickey

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#13
Understand now. Thanks
 

Dhector

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#14
This wont be helpful to ddickey but I thought it was interesting to read the post about making a drawbar today, since I just broke mine 2 nights ago and built one this morning.

This is pretty identical to the original, only longer by about 1.25 inches. Anyway, noone in the town I live in had any 3/4 steel hex shaft, or 7/16 steel rod so this is all made from stainless. I had to use 7/16 due to clearances and this was all I could find.

The old drawbar was NOT threaded on the top, just pinned(approx 1/8 pin) I've broke the pin 2 times now, which is why I built a new one. I threaded mine on both ends. Didnt use the lathe(dont know how to thread yet) except for putting the die in the chuck and indicate it, then put the rod on the tailstock and manually turned the chuck to start it. Quit giggling I know there are better ways!:) The first set of threads I did by hand werent exactly straight and I cut them off. These threads now are really straight and the drawbar works fine. Since I made mine a little longer i made the bushing on the left to compensate. Being longer I can possibly repair in the future if need be.

Main question Do you guys see any issues with using the stainless?


drawbar.jpg

drawbar 2.jpg
 

MrWhoopee

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#15
We always made the replacement bar from 7/16" ETD150, then threaded and pinned it into the old nut.

This is the tool you use loosen the drawbar and avoid mushrooming the nut:
WP_20181109_14_40_20_Pro[1].jpg
Made from an old VW lug wrench.
 
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BaronJ

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#16
Hi Guys,

My mill uses an MT3 taper ! I never tighten the drawbar more than hand tight, and I certainly don't have to bash the hell out of the drawbar to get it to release. If you are having to give the drawbar a good bashing then you are over tightening it.
 

BROCKWOOD

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#17
These are the drawbars & drawtube that came with mine. Only the drawtube will draw up properly. I could make bushings to make the others work.


20170427_173515742.jpg
 

Bob Korves

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#18
Main question Do you guys see any issues with using the stainless?
There are lots of varieties of stainless, and they vary considerably from one to another. The biggest issue with stainless with a drawbar, with some stainless, might be galling the threads in the collet. A little Never-seez on the threads may help stop galling. Not a bad idea on any drawbar.
 

Dhector

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#19
There are lots of varieties of stainless, and they vary considerably from one to another. The biggest issue with stainless with a drawbar, with some stainless, might be galling the threads in the collet. A little Never-seez on the threads may help stop galling. Not a bad idea on any drawbar.
Thank you. Its all I could find in town. Obviously I'm new to this and was worried. It seems to work really well so far, but havent used it much. It does thread in very nice I must say, unlike the first set of threads I did by hand. Even without the hex on the top in threaded in tight. I knew that wasnt going to work. Thanks again.
 

ddickey

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#20
Just ordered some split pins and 24" x 1/2" 4140HT from Mcmaster Carr. $9. Didn't think that was to bad.
 

tweinke

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#21
When I made my drawbar I threaded the top and used a ready rod joiner (nut) and Loctite with a pin. Figured if I needed to replace the nut I could drive the pin out apply heat and replace the nut. Its held up good so far.
 

BaronJ

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#22
Hi Guys,

My mill uses an MT3 taper ! I never tighten the drawbar more than hand tight, and I certainly don't have to bash the hell out of the drawbar to get it to release. If you are having to give the drawbar a good bashing then you are over tightening it.
Hi Guys,
After I posted I realised that I hadn't given any indication of what I use to tighten the drawbar on my mill, and "Hand Tight" means nothing without showing the tool that I use.
So this morning I took a picture so you you could see what I use.

10-11-2018-3.JPG
The bottom of that 100 mm long key is a 10 mm square socket.
 
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