Post by springcrocus on Jul 25, 2015 10:22:53 GMT
I've just spent ages trying to purchase a 1/8" keyway broach but could only find sets from DuMont or Steelman which require an extension to the overdraft or some single items from Australia which were attractive until the carriage charge was noticed. After getting back on my chair, I decided to try and make my own broach.
First, the maths. I need to be able to cut the keyway to about 0.064" deep and would like to plane off one thou per tooth. Any more would probably require a press tool of some description and I only have my drilling machine available. Assuming three passes, I need about twenty teeth on the broach and would like to complete a pass in about four inches. I will use a 4mm carbide slot drill to cut the teeth in a length of 1/2" x 3/16" ground flat stock. I don't have 1/2" x 1/8" GFS and I'm not about to buy a piece in case it doesn't work.
First job was to saw off a 7.1/2" length of GFS, load it to the mill table on some sacrificial packing and clock it true to the table. Then the cutter was offered up to the edge of the workpiece until touch-on and then moved in a further three millimeters and locked.
A suitable starting point was chosen about half an inch along the bar and the table locked and the first plunge cut made to create the front form of the first tooth. The table was then wound along six millimeters and the next hole plunged through. This was repeated until I had twenty one holes.
Next, I had to relieve the back of the teeth leaving just the tiniest of flats showing on the tip. I started by repositioning the cutter to 2.5mm in from the face and 1mm nearer the starting end and then plunge-cut the back of the teeth, moving 6mm along each time.
Then I set up my tiny angle plate with the work rest set at ten degrees. The work was clamped to the faceplate with the edge of a tooth in line with the end of the faceplate and a crosscut taken with the 4mm cutter. Then the work was moved to the next tooth position and the process repeated.
The last operation on this edge was to relieve the leading and trailing edges by about five thou to prevent any form of binding after which the broach was removed from the vice.
Now it was time to reduce the thickness of the teeth to 1/8" and for this I mounted the work directly to the table, using the clamping bolts as a fence. I needed to move the work along after milling the first section because my total travel in the x-axis is only 75mm. Three passes each side at ten thou per pass saw the teeth reduced to the correct size.
The last bit of milling was the front-to-back size of the broach. I need about twenty thou difference between the first tooth and the last tooth and this was set up on parallels using feeler gauges to tip the workpiece. I'm aiming to get the major size to about 3/8". Much bigger and the guide bush will be practically cut in half but if I go too small there is more chance of distortion during hardening. Actual size is unimportant as I will make the guide bush and shims to suit.
I have also drilled a hole in the end because I want to hang the broach vertically whilst heating and also when plunging into the clean engine oil, again to reduce the risk of distortion. Before heating, I cleaned the whole thing up with slipstones, tied on the hanging wire and then degreased it.
Then it was time to harden. First, I took the broach gently up to dull red and held it there to soak for a minute or two before carrying on up to cherry. Then it was a straight vertical plunge into the oil, keeping the thing moving gently for a few minutes while it cooled properly. At this point it was glass-hard and I took care not to drop it! It was gently stoned down the sides and back until bright all the way along and then I tempered to about mid-straw for approx 60RC. I could have done it more accurately (about 200° C) in the kitchen oven but senior management may well have objected.
All I need now is a guide bush and shims to fit Britannia's driving and coupled wheels and I'm ready to cut some keyways but I will cover this as an entry to the diary in a few weeks time. I have, however, made a collar from one of my sash weights and used this to test the broach and am happy to say it works a treat. And measuring the depth of this one allows me to correctly calculate the final shim thickness required for the wheels.
And one other tip - if you use a plastic bottle like I did, stand it in another container. When I plunged the broach into the oil, I forgot to allow for the expansion of the oil due to heating and it promptly overflowed all over the bench. Then, to cap it all, I must have hit the bottom of the bottle and the heat in the broach was still enough to melt a small hole in the bottom so I now had leaks at both ends. I must have had a brain-fart when I set that up.
Steve
First, the maths. I need to be able to cut the keyway to about 0.064" deep and would like to plane off one thou per tooth. Any more would probably require a press tool of some description and I only have my drilling machine available. Assuming three passes, I need about twenty teeth on the broach and would like to complete a pass in about four inches. I will use a 4mm carbide slot drill to cut the teeth in a length of 1/2" x 3/16" ground flat stock. I don't have 1/2" x 1/8" GFS and I'm not about to buy a piece in case it doesn't work.
First job was to saw off a 7.1/2" length of GFS, load it to the mill table on some sacrificial packing and clock it true to the table. Then the cutter was offered up to the edge of the workpiece until touch-on and then moved in a further three millimeters and locked.
A suitable starting point was chosen about half an inch along the bar and the table locked and the first plunge cut made to create the front form of the first tooth. The table was then wound along six millimeters and the next hole plunged through. This was repeated until I had twenty one holes.
Next, I had to relieve the back of the teeth leaving just the tiniest of flats showing on the tip. I started by repositioning the cutter to 2.5mm in from the face and 1mm nearer the starting end and then plunge-cut the back of the teeth, moving 6mm along each time.
Then I set up my tiny angle plate with the work rest set at ten degrees. The work was clamped to the faceplate with the edge of a tooth in line with the end of the faceplate and a crosscut taken with the 4mm cutter. Then the work was moved to the next tooth position and the process repeated.
The last operation on this edge was to relieve the leading and trailing edges by about five thou to prevent any form of binding after which the broach was removed from the vice.
Now it was time to reduce the thickness of the teeth to 1/8" and for this I mounted the work directly to the table, using the clamping bolts as a fence. I needed to move the work along after milling the first section because my total travel in the x-axis is only 75mm. Three passes each side at ten thou per pass saw the teeth reduced to the correct size.
The last bit of milling was the front-to-back size of the broach. I need about twenty thou difference between the first tooth and the last tooth and this was set up on parallels using feeler gauges to tip the workpiece. I'm aiming to get the major size to about 3/8". Much bigger and the guide bush will be practically cut in half but if I go too small there is more chance of distortion during hardening. Actual size is unimportant as I will make the guide bush and shims to suit.
I have also drilled a hole in the end because I want to hang the broach vertically whilst heating and also when plunging into the clean engine oil, again to reduce the risk of distortion. Before heating, I cleaned the whole thing up with slipstones, tied on the hanging wire and then degreased it.
Then it was time to harden. First, I took the broach gently up to dull red and held it there to soak for a minute or two before carrying on up to cherry. Then it was a straight vertical plunge into the oil, keeping the thing moving gently for a few minutes while it cooled properly. At this point it was glass-hard and I took care not to drop it! It was gently stoned down the sides and back until bright all the way along and then I tempered to about mid-straw for approx 60RC. I could have done it more accurately (about 200° C) in the kitchen oven but senior management may well have objected.
All I need now is a guide bush and shims to fit Britannia's driving and coupled wheels and I'm ready to cut some keyways but I will cover this as an entry to the diary in a few weeks time. I have, however, made a collar from one of my sash weights and used this to test the broach and am happy to say it works a treat. And measuring the depth of this one allows me to correctly calculate the final shim thickness required for the wheels.
And one other tip - if you use a plastic bottle like I did, stand it in another container. When I plunged the broach into the oil, I forgot to allow for the expansion of the oil due to heating and it promptly overflowed all over the bench. Then, to cap it all, I must have hit the bottom of the bottle and the heat in the broach was still enough to melt a small hole in the bottom so I now had leaks at both ends. I must have had a brain-fart when I set that up.
Steve
