Rebarreling and Home-Shop Machining

Thursday, July 21, 2005

Edited on August 31 Moved photo 14,15 & 16 from topic above.

Homemade Flycutter Holder to Cut a Long Large Diameter Channel

            I plan to cut about a 1 and 1/8 inch channel in my stainless steel bedding block. The cut needs to be between 8 -12 inches long. I plan to use a long shafted cutter in the lathe spindle to achieve the proper length of cut. See photo 1 - 6. In a previous test, a long shaft, like this, held in a spider on the back of the spindle doubled the rigidity of the spindle. The workpiece will be mounted parallel to the bed in the vise. See photo 7 & 8. Here, also, every bit of rigidity will help. I will raise the work piece up with a couple of small screw jacks to feed the work into the bit. I can level the workpiece carefully on the last few cuts. I’ll post another photo if I succeed later on. I have a little carbide tipped bit that I will use. I have a mock setup of the channeling operation in the last photo.

Test Cut

            I did a test cut with a bit that I had on hand. I turned the lathe in Reverse to have a suitable shape. I made the first cut at 120 RPM at 1/3 inch per minute. I then increased the lathe speed to 640 RPM, but had to decrease speed to .25 inch per minute to avoid chatter. See photo 9. There was a lot less flex in the 1 inch shaft with the higher speed. Possibly a speed in between would be optimum. I also plan to reshape the bit to use it in the forward direction. The holder is more rigid in this direction. It looks like things are going to work fine. I fed upward with a 1/3 turn of the jack’s screws between passes. I found that the smaller machinist clamps were not needed. This sped up raising the bar for a new pass.

Regrind Tip

            I used the Homebuilt Dewalt Tool Post Grinder to Grind a good point on my Carbide Bit. I ground a flat section on the end of the bit to leave a flat ended channel. I angled my 1 inch Diameter stone a couple of degrees to have the bit fall away from the edge just a little. I ground the outside of the bit so that it would fall away from the point a couple of degrees and then fall away from the edge just a bit. This produced a Robust edge for the interrupted cut. The fact that the bit only cuts on a small part of the point increased the contact pressure of the cutter so I could increase my feed rate to 3 inches per minute. This is a 10 fold increase over using the bit as is. I could make good progress now. See photo 10 for a profile of the bit. I could now run the lathe forward which reduced the strain on the setscrews.

Measuring the Radius of the Channel, HS Precision Stock Bedding Block Design

             A lot of people including experienced gunsmiths don’t know how the HS precision bedding block is designed. The radius of the channel is less than the radius of the action. Thus the action isnot intended to seat on the bottom of the channel. Gunsmiths measure this space at the bottom and are alarmed. This space is intended. The action under the pull of the action screws gently wedges along the upper portion of the channel. There are two line contacts up high in the channel rather than full contact. I measured the tendency, if any, to bend the action with a close fitting mandrel inserted in the action and there was none as the action screws were torqued down. Less than .0001 inch (1 ten thousandth). In spite of this gunsmiths have posted that they “see” the action bending when they torque the screws. I could not even measure any bend. What they see is the action lowering itself into the channel, but it does not bend. The bedding block works very well. I have three rifles with this bedding block (no epoxy skim bed) that are averaging .3 inch 3 shot groups after 400-500 rounds.

            I measured the Remington action radius and it was (1.355)/2 inch. I measured the channel radius and it was (1.333)/2 inch. Only slightly smaller to create a gentle wedging effect. The goal of a bedding block is that there be no action to bedding block movement from shot to shot. You do not have to have 100 % contact.

Using a Radius Gage

            A few ways came to mind to measure the radius of the channel. First I have quite a few sockets. My test channel was the channel cut in the forward portion of an HS Precision Stock for a Remington Action. I greased my 1 inch ½ inch drive socket with a medium thick layer of red grease. It left contact marks at the top and bottom of the channel. The socket was 1.33-1.334 inch in diameter so the channel diameter was 1.333.

Using the Chord and Depth Method

            Sockets only come in steps as do radius gages, so I looked for a more general method. I found one in machinist ready reference. A line that touches the circumference of a circle, not on a diameter, is called a chord. If a chord length, c, is known and the distance, m, between the chord and the circumference could be measured then the radius can be calculated.

            I set an extruded aluminum strip about 2 inch long by 1.001 inch wide by .124 inch thick in the channel. I used my depth mike and a .2000 inch ball bearing to measure the depth ,m, from the bottom of the channel to the bottom of the strip. The chord length, c, is the width of the strip. The radius is then calculated with the following formula.

Radius =( (m x m)+ 1/4 (c x c))/(2 x m)

For the HS Precision Stock for the Remington Action

Radius = ((.2369 x .2369) + 1/4(1.001 x 1.001))/(2 x .2369)

Radius = .647

Radius x 2 or diameter =1.294 inch.

            This last figure is close to the diameter of the socket at 1.333 that fit well (.009 inch difference). I can use a thinner layer of grease on the socket and maybe true up my aluminum to get closer results. The measure is sensitive to your measure of m. A dial caliper with its flat tip was not sufficiently precise. You need a ball contact. There are ball contacts with rubberized adaptor to hold it onto your mike available for sale. I used a loose ball from one of these devices.

Using Height of the Cutter

            I measured the radius of my test cut using a thick machinist rule for the chord and a 5/32 ball bearing to measure m. I estimated the ten thousandths place on the depth mike. The ball bearing actually went .0038 inch into the depth mike so I subtracted this from the diameter of the ball and then subtracted the ruler thickness to get m. I measured the rule thickness and width with a ten thousandths reading mike. The diameter of the cut measured 1.231 inch. I then measured the height of the cutter from the bed at the top part of its rotation and subtracted the center height of the lathe. I got 1.238 inch for the expected diameter. The 1.333 diameter socked only left contact marks on the edge of the cut.

Cutting with Coolant

            I continued the cut with coolant and a slightly different cutter shape. See photo 12 and 13. The previous cutter shape produced a smoother cut with higher feed rate possible. There is only a slight difference to the angle between the cutter and the long axis of the one inch shaft.I put a little bit more angle whichcaused more chatter.  The cut, however,  matched the radius of the cutter better with coolant and the more angled tip.  the finish will probably improve when I get the 1.5 inch stock and the cutter quits having to slap the edge of the stock.

Endmill Holders

            The same procedure, in photo 1-6, can be used to make a set of milling cutter holders that will fit in a 3/4 inch endmill holder using 3/4 inch drill rod for stock. Milling bit holders have become fairly inexpensive now, though.

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This entry has 1 comments: (Add your own)

• #1 Comment from robert@... 
  6/9/08 2:20 AM | Permalink

  Very illuminating and well thought out. Thank you for taking the time to share.